La radio es la tecnología de señalización y comunicación mediante ondas de radio . [1] [2] [3] Las ondas de radio son ondas electromagnéticas de frecuencia entre 30 hercios (Hz) y 300 gigahercios (GHz). Son generados por un dispositivo electrónico llamado transmisor conectado a una antena que irradia las ondas y recibidos por otra antena conectada a un receptor de radio . La radio es muy utilizada en la tecnología moderna, en comunicación por radio, radar , navegación por radio , control remoto., teledetección y otras aplicaciones.
En las comunicaciones por radio , utilizadas en la transmisión de radio y televisión , teléfonos móviles , radios bidireccionales , redes inalámbricas y comunicaciones por satélite, entre muchos otros usos, las ondas de radio se utilizan para transportar información a través del espacio desde un transmisor a un receptor, modulando la señal de radio. (imprimiendo una señal de información en la onda de radio variando algún aspecto de la onda) en el transmisor. En el radar , utilizado para localizar y rastrear objetos como aviones, barcos, naves espaciales y misiles, un haz de ondas de radio emitidas por un transmisor de radar se refleja en el objeto objetivo y las ondas reflejadas revelan la ubicación del objeto. En los sistemas de radionavegación como GPS y VOR , un receptor móvil acepta señales de radio de radiobalizas de navegación cuya posición se conoce, y midiendo con precisión el tiempo de llegada de las ondas de radio, el receptor puede calcular su posición en la Tierra. En los dispositivos inalámbricos de control remoto por radio como drones , abridores de puertas de garaje y sistemas de entrada sin llave , las señales de radio transmitidas desde un dispositivo controlador controlan las acciones de un dispositivo remoto.
Las aplicaciones de ondas de radio que no implican la transmisión de ondas a distancias significativas, como el calentamiento por RF utilizado en procesos industriales y hornos de microondas , y los usos médicos como las máquinas de diatermia y resonancia magnética , generalmente no se denominan radio . El sustantivo radio también se usa para significar un receptor de radio de transmisión .
Las ondas de radio fueron identificadas y estudiadas por primera vez por el físico alemán Heinrich Hertz en 1886. Los primeros transmisores y receptores de radio prácticos fueron desarrollados alrededor de 1895-1896 por el italiano Guglielmo Marconi , y la radio comenzó a utilizarse comercialmente alrededor de 1900. Para evitar interferencias entre usuarios, el La emisión de ondas de radio está regulada por ley, coordinada por un organismo internacional llamado Unión Internacional de Telecomunicaciones (UIT), que asigna bandas de frecuencia en el espectro radioeléctrico para diferentes usos.
Tecnología
Las ondas de radio son irradiadas por cargas eléctricas que experimentan aceleración . [4] [5] Se generan artificialmente mediante corrientes eléctricas variables en el tiempo , que consisten en electrones que fluyen hacia adelante y hacia atrás en un conductor metálico llamado antena, [6] [7] acelerando así. En la transmisión, un transmisor genera una corriente alterna de radiofrecuencia que se aplica a una antena. La antena irradia la potencia de la corriente en forma de ondas de radio. Cuando las ondas golpean la antena de un receptor de radio , empujan los electrones en el metal hacia adelante y hacia atrás, induciendo una pequeña corriente alterna. El receptor de radio conectado a la antena receptora detecta esta corriente oscilante y la amplifica.
A medida que se alejan más de la antena transmisora, las ondas de radio se extienden por lo que la intensidad de la señal ( intensidad en vatios por metro cuadrado) disminuye, por lo que las transmisiones de radio solo se pueden recibir dentro de un rango limitado del transmisor, la distancia depende de la potencia del transmisor, patrón de radiación de la antena , sensibilidad del receptor, nivel de ruido y presencia de obstrucciones entre el transmisor y el receptor. Una antena omnidireccional transmite o recibe ondas de radio en todas las direcciones, mientras que una antena direccional o una antena de alta ganancia transmite ondas de radio en un haz en una dirección particular o recibe ondas de una sola dirección.
Las ondas de radio viajan a través del vacío a la velocidad de la luz y en el aire a una velocidad muy cercana a la de la luz, por lo que la longitud de onda de una onda de radio, la distancia en metros entre las crestas adyacentes de la onda, es inversamente proporcional a su frecuencia .
Los otros tipos de ondas electromagnéticas además de las ondas de radio; infrarrojos , luz visible , ultravioleta , rayos X y rayos gamma , también pueden transportar información y utilizarse para la comunicación. El amplio uso de las ondas de radio para las telecomunicaciones se debe principalmente a sus deseables propiedades de propagación derivadas de su gran longitud de onda. [7] Las ondas de radio tienen la capacidad de atravesar la atmósfera, el follaje y la mayoría de los materiales de construcción, y por difracción pueden doblarse alrededor de las obstrucciones y, a diferencia de otras ondas electromagnéticas, tienden a dispersarse en lugar de ser absorbidas por objetos más grandes que su longitud de onda.
Comunicación por radio
En los sistemas de comunicación por radio, la información se transporta a través del espacio mediante ondas de radio. En el extremo de envío, la información que se va a enviar es convertida por algún tipo de transductor en una señal eléctrica variable en el tiempo llamada señal de modulación. [7] [8] La señal de modulación puede ser una señal de audio que representa el sonido de un micrófono , una señal de video que representa imágenes en movimiento de una cámara de video o una señal digital que consiste en una secuencia de bits que representan datos binarios de una computadora. La señal de modulación se aplica a un transmisor de radio . En el transmisor, un oscilador electrónico genera una corriente alterna que oscila a una frecuencia de radio , llamada onda portadora porque sirve para "llevar" la información a través del aire. La señal de información se usa para modular la portadora, variando algún aspecto de la onda portadora, imprimiendo la información en la portadora. Los diferentes sistemas de radio utilizan diferentes métodos de modulación :
- AM ( modulación de amplitud ): en un transmisor de AM, la amplitud (fuerza) de la onda portadora de radio varía según la señal de modulación.
- FM ( modulación de frecuencia ): en un transmisor de FM, la frecuencia de la onda portadora de radio varía según la señal de modulación.
- FSK ( modulación por desplazamiento de frecuencia ): utilizada en dispositivos digitales inalámbricos para transmitir señales digitales , la frecuencia de la onda portadora se cambia periódicamente entre dos frecuencias que representan los dos dígitos binarios , 0 y 1, para transmitir una secuencia de bits.
- OFDM ( multiplexación por división de frecuencia ortogonal ): una familia de métodos complicados de modulación digital muy utilizados en sistemas de gran ancho de banda, como redes WiFi , teléfonos móviles , transmisión de televisión digital y transmisión de audio digital (DAB) para transmitir datos digitales utilizando un mínimo de espectro de radio. banda ancha. Tiene mayor eficiencia espectral y más resistencia al desvanecimiento que AM o FM. En OFDM, se transmiten múltiples ondas portadoras de radio estrechamente espaciadas en frecuencia dentro del canal de radio, con cada portadora modulada con bits del flujo de bits entrante, por lo que se envían múltiples bits simultáneamente, en paralelo. En el receptor, las portadoras se demodulan y los bits se combinan en el orden correcto en un flujo de bits.
También se utilizan muchos otros tipos de modulación. En algunos tipos, no se transmite una onda portadora, sino solo una o ambas bandas laterales de modulación . La portadora modulada se amplifica en el transmisor y se aplica a una antena transmisora que irradia la energía en forma de ondas de radio. Las ondas de radio llevan la información a la ubicación del receptor.
En el receptor, la onda de radio induce una pequeña tensión oscilante en la antena receptora que es una réplica más débil de la corriente en la antena transmisora. [7] [8] Este voltaje se aplica al receptor de radio , que amplifica la señal de radio débil para que sea más fuerte, luego la demodula , extrayendo la señal de modulación original de la onda portadora modulada. La señal de modulación es convertida por un transductor de nuevo a una forma utilizable por humanos: una señal de audio se convierte en ondas de sonido por un altavoz o auriculares, una señal de video se convierte en imágenes por una pantalla , mientras que una señal digital se aplica a una computadora o microprocesador, que interactúa con usuarios humanos.
Las ondas de radio de muchos transmisores pasan por el aire simultáneamente sin interferir entre sí porque las ondas de radio de cada transmisor oscilan a una velocidad diferente, en otras palabras, cada transmisor tiene una frecuencia diferente , medida en kilohercios (kHz), megahercios (MHz) o gigahercios (GHz). La antena receptora normalmente capta las señales de radio de muchos transmisores. El receptor utiliza circuitos sintonizados para seleccionar la señal de radio deseada entre todas las señales captadas por la antena y rechazar las demás. Un circuito sintonizado (también llamado circuito resonante o circuito de tanque) actúa como un resonador , de manera similar a un diapasón . [8] Tiene una frecuencia de resonancia natural a la que oscila. El usuario ajusta la frecuencia de resonancia del circuito sintonizado del receptor a la frecuencia de la estación de radio deseada; esto se llama "tuning". La señal de radio oscilante de la estación deseada hace que el circuito sintonizado resuene , oscile en simpatía y transmita la señal al resto del receptor. Las señales de radio en otras frecuencias son bloqueadas por el circuito sintonizado y no se transmiten.
Banda ancha
Una onda de radio modulada, que transporta una señal de información, ocupa un rango de frecuencias . Ver diagrama. La información ( modulación ) en una señal de radio generalmente se concentra en bandas de frecuencia estrechas llamadas bandas laterales ( SB ) justo por encima y por debajo de la frecuencia portadora . El ancho en hercios del rango de frecuencia que ocupa la señal de radio, la frecuencia más alta menos la frecuencia más baja, se llama ancho de banda ( BW ). [9] Para cualquier relación señal-ruido dada , una cantidad de ancho de banda puede transportar la misma cantidad de información ( velocidad de datos en bits por segundo) independientemente de dónde se encuentre en el espectro de radiofrecuencia, por lo que el ancho de banda es una medida de capacidad de transporte de información . El ancho de banda requerido por una transmisión de radio depende de la velocidad de datos de la información (señal de modulación) que se envía y de la eficiencia espectral del método de modulación utilizado; cuántos datos puede transmitir en cada kilohercio de ancho de banda. Los diferentes tipos de señales de información transmitidas por radio tienen diferentes velocidades de transmisión de datos. Por ejemplo, una señal de televisión (video) tiene una tasa de datos mayor que una señal de audio .
El espectro de radio , la gama total de radiofrecuencias que se pueden utilizar para la comunicación en un área determinada, es un recurso limitado. [9] [3] Cada transmisión de radio ocupa una parte del ancho de banda total disponible. El ancho de banda de radio se considera un bien económico que tiene un costo monetario y tiene una demanda creciente. En algunas partes del espectro radioeléctrico, el derecho a utilizar una banda de frecuencia o incluso un solo canal de radio se compra y vende por millones de dólares. Por tanto, existe un incentivo para emplear tecnología para minimizar el ancho de banda utilizado por los servicios de radio.
En los últimos años se ha producido una transición de las tecnologías de transmisión de radio analógicas a las digitales . Parte de la razón de esto es que la modulación digital a menudo puede transmitir más información (una mayor velocidad de datos) en un ancho de banda dado que la modulación analógica , mediante el uso de algoritmos de compresión de datos , que reducen la redundancia en los datos que se enviarán y una modulación más eficiente. Otras razones para la transición es que la modulación digital tiene mayor inmunidad al ruido que la analógica, los chips de procesamiento de señales digitales tienen más potencia y flexibilidad que los circuitos analógicos, y se puede transmitir una amplia variedad de tipos de información usando la misma modulación digital.
Debido a que es un recurso fijo que tiene una demanda cada vez mayor de usuarios, el espectro de radio se ha vuelto cada vez más congestionado en las últimas décadas, y la necesidad de usarlo de manera más efectiva está impulsando muchas innovaciones de radio adicionales, como sistemas de radio troncalizados , espectro ensanchado. transmisión (banda ultraancha), reutilización de frecuencias , gestión dinámica del espectro , agrupación de frecuencias y radio cognitiva .
Bandas de frecuencia de la UIT
La UIT divide arbitrariamente el espectro de radio en 12 bandas, cada una de las cuales comienza en una longitud de onda que es una potencia de diez (10 n ) metros, con una frecuencia correspondiente de 3 veces una potencia de diez, y cada una cubre una década de frecuencia o longitud de onda. [3] [10] Cada una de estas bandas tiene un nombre tradicional:
Nombre de banda | Abreviatura | Frecuencia | Longitud de onda | Nombre de banda | Abreviatura | Frecuencia | Longitud de onda |
---|---|---|---|---|---|---|---|
Frecuencia extremadamente baja | DUENDE | 3 - 30 Hz | 100.000–10.000 km | Alta frecuencia | HF | 3 - 30 MHz | 100-10 m |
Muy baja frecuencia | SLF | 30 - 300 Hz | 10,000-1,000 km | Muy alta frecuencia | VHF | 30 - 300 MHz | 10-1 m |
Frecuencia ultrabaja | ULF | 300 - 3000 Hz | 1000-100 km | Frecuencia ultra alta | UHF | 300-3000 MHz | 100-10 cm |
Muy baja frecuencia | VLF | 3 - 30 kHz | 100-10 km | Super alta frecuencia | SHF | 3 - 30 GHz | 10-1 cm |
Baja frecuencia | LF | 30 - 300 kHz | 10-1 km | Frecuencia extremadamente alta | EHF | 30 - 300 GHz | 10-1 mm |
Frecuencia media | MF | 300 - 3000 kHz | 1000-100 m | Tremendamente alta frecuencia | THF | 300 - 3000 GHz | 1–0,1 mm |
Se puede ver que el ancho de banda , el rango de frecuencias, contenido en cada banda no es igual sino que aumenta exponencialmente a medida que aumenta la frecuencia; cada banda contiene diez veces el ancho de banda de la banda anterior. El mayor ancho de banda disponible ha motivado una tendencia continua a explotar frecuencias más altas a lo largo de la historia de la radio.
Regulación
Las ondas son un recurso compartido por muchos usuarios. Dos transmisores de radio en la misma área que intentan transmitir en la misma frecuencia interferirán entre sí, causando una recepción distorsionada, por lo que ninguna transmisión puede recibirse con claridad. [9] La interferencia con las transmisiones de radio no solo puede tener un gran costo económico, sino que puede poner en peligro la vida (por ejemplo, en el caso de interferencia con las comunicaciones de emergencia o el control del tráfico aéreo ).
Para evitar interferencias entre diferentes usuarios, la emisión de ondas de radio está estrictamente regulada por leyes nacionales, coordinadas por un organismo internacional, la Unión Internacional de Telecomunicaciones (UIT), que asigna bandas en el espectro radioeléctrico para diferentes usos. [9] [3] Los transmisores de radio deben tener licencia de los gobiernos, bajo una variedad de clases de licencia dependiendo del uso, y están restringidos a ciertas frecuencias y niveles de potencia. En algunas clases, como las estaciones de radiodifusión y televisión, el transmisor recibe un identificador único que consta de una cadena de letras y números denominada distintivo de llamada , que debe utilizarse en todas las transmisiones. El operador de radio debe tener una licencia del gobierno, como la licencia general de operador de radioteléfono en los EE. UU., Obtenida al realizar una prueba que demuestre un conocimiento técnico y legal adecuado de la operación segura de la radio.
Las excepciones a las reglas anteriores permiten el funcionamiento sin licencia por parte del público de baja potencia transmisores de corto alcance en productos de consumo tales como teléfonos celulares, teléfonos inalámbricos , dispositivos inalámbricos , walkie-talkies , los ciudadanos radios de banda , micrófonos inalámbricos , puertas de garaje , y monitores para bebés . En los EE. UU., Se encuentran bajo la Parte 15 de las regulaciones de la Comisión Federal de Comunicaciones (FCC). Muchos de estos dispositivos utilizan las bandas ISM , una serie de bandas de frecuencia en todo el espectro de radio reservadas para uso sin licencia. Aunque se pueden operar sin una licencia, como todos los equipos de radio, estos dispositivos generalmente deben ser aprobados antes de la venta.
Aplicaciones
A continuación se muestran algunos de los usos más importantes de la radio, organizados por función.
Radiodifusión
La radiodifusión es la transmisión unidireccional de información desde un transmisor a receptores que pertenecen a una audiencia pública. Dado que las ondas de radio se debilitan con la distancia, una estación de radiodifusión solo se puede recibir dentro de una distancia limitada de su transmisor. Los sistemas que emiten desde satélites generalmente se pueden recibir en todo un país o continente. La radio y la televisión terrestres más antiguas se pagan con publicidad comercial o con los gobiernos. En los sistemas de suscripción como la televisión por satélite y la radio por satélite, el cliente paga una tarifa mensual. En estos sistemas la señal de radio está encriptada y solo puede ser desencriptada por el receptor, el cual es controlado por la empresa y se puede desactivar si el cliente no paga la factura.
La radiodifusión utiliza varias partes del espectro de radio, según el tipo de señales transmitidas y la audiencia objetivo deseada. Las señales de onda larga y media pueden brindar una cobertura confiable de áreas de varios cientos de kilómetros de ancho, pero tienen una capacidad de transporte de información más limitada y, por lo tanto, funcionan mejor con señales de audio (voz y música), y la calidad del sonido puede degradarse por el ruido de radio de origen natural y artificial. fuentes. Las bandas de onda corta tienen un mayor alcance potencial, pero están más sujetas a interferencias de estaciones distantes y condiciones atmosféricas variables que afectan la recepción.
En la banda de muy alta frecuencia , superior a 30 megahercios, la atmósfera de la Tierra tiene un efecto menor en el rango de señales, y la propagación de la línea de visión se convierte en el modo principal. Estas frecuencias más altas permiten el gran ancho de banda requerido para la transmisión de televisión. Dado que las fuentes de ruido natural y artificial están menos presentes en estas frecuencias, es posible la transmisión de audio de alta calidad mediante modulación de frecuencia .
Audio: radiodifusión
Radiodifusión significa la transmisión de audio (sonido) a receptores de radio pertenecientes a una audiencia pública. El audio analógico es la forma más antigua de transmisión de radio. La transmisión de AM comenzó alrededor de 1920. La transmisión de FM se introdujo a fines de la década de 1930 con una mayor fidelidad . Un receptor de radiodifusión se llama radio . La mayoría de las radios pueden recibir tanto AM como FM y se denominan receptores AM / FM.
- AM ( modulación de amplitud ): en AM, la amplitud (fuerza) de la onda portadora de radio varía según la señal de audio. La transmisión AM , la tecnología de transmisión más antigua, está permitida en las bandas de transmisión AM , entre 148 y 283 kHz en la banda de baja frecuencia (LF) y entre 526 y 1706 kHz en la banda de frecuencia media (MF). Debido a que las ondas en estas bandas viajan como ondas terrestres siguiendo el terreno, las estaciones de radio AM se pueden recibir más allá del horizonte a cientos de millas de distancia, pero AM tiene menor fidelidad que FM. La potencia radiada ( ERP ) de las estaciones de AM en los EE. UU. Generalmente se limita a un máximo de 10 kW, aunque algunas ( estaciones de canal claro ) pueden transmitir a 50 kW. Emisoras de AM emitidas en audio monoaural ; Los estándares de transmisión en estéreo AM existen en la mayoría de los países, pero la industria de la radio no los ha actualizado debido a la falta de demanda.
- Transmisión de onda corta : las estaciones de radio heredadas también permiten la transmisión de AM en las bandas de onda corta . Dado que las ondas de radio en estas bandas pueden viajar distancias intercontinentales al reflejarse en la ionosfera mediante el uso de ondas celestes o propagación "por salto", las estaciones internacionales utilizan la onda corta para transmitir a otros países.
- Transmisión de onda corta : las estaciones de radio heredadas también permiten la transmisión de AM en las bandas de onda corta . Dado que las ondas de radio en estas bandas pueden viajar distancias intercontinentales al reflejarse en la ionosfera mediante el uso de ondas celestes o propagación "por salto", las estaciones internacionales utilizan la onda corta para transmitir a otros países.
- FM ( modulación de frecuencia ): en FM, la frecuencia de la señal de la portadora de radio varía ligeramente con la señal de audio. La radiodifusión de FM está permitida en las bandas de radiodifusión de FM entre aproximadamente 65 y 108 MHz en el rango de muy alta frecuencia (VHF). Las ondas de radio en esta banda viajan en línea de visión, por lo que la recepción de FM está limitada por el horizonte visual a aproximadamente 30 a 40 millas (48 a 64 km) y puede ser bloqueada por colinas. Sin embargo, es menos susceptible a la interferencia del ruido de radio ( RFI , sferics , estática) y tiene una mayor fidelidad ; mejor respuesta de frecuencia y menos distorsión de audio que AM. En los EE. UU., La potencia radiada ( ERP ) de las estaciones de FM varía de 6 a 100 kW.
- La radiodifusión de audio digital (DAB) debutó en algunos países en 1998. Transmite audio como una señal digital en lugar de una señal analógica como lo hacen AM y FM. [11] DAB tiene el potencial de proporcionar un sonido de mayor calidad que FM (aunque muchas estaciones no eligen transmitir con una calidad tan alta), tiene una mayor inmunidad al ruido y las interferencias de radio , hace un mejor uso del escaso ancho de banda del espectro de radio y proporciona funciones de usuario como guías de programas electrónicos . Su desventaja es que es incompatible con radios anteriores por lo que se debe adquirir un nuevo receptor DAB. La mayoría de los países planean un cambio eventual de FM a DAB. Estados Unidos y Canadá han optado por no implementar DAB.
- Una sola estación DAB transmite una señal de ancho de banda de 1.500 kHz que transporta de 9 a 12 canales de audio digital modulado por OFDM entre los que el oyente puede elegir. Las emisoras pueden transmitir un canal en un rango de velocidades de bits diferentes , por lo que diferentes canales pueden tener una calidad de audio diferente. En diferentes países, las estaciones DAB transmiten en la banda III (174–240 MHz) o en la banda L (1.452–1.492 GHz) en el rango UHF, por lo que la recepción de FM está limitada por el horizonte visual a aproximadamente 40 millas (64 km).
- Digital Radio Mondiale (DRM) es un estándar de radio digital terrestre de la competencia desarrollado principalmente por emisoras como un reemplazo de mayor eficiencia espectral para las transmisiones de AM y FM heredadas. Mondiale significa "mundial" en francés e italiano, y DRM, desarrollado en 2001, es actualmente compatible con 23 países y ha sido adoptado por algunas emisoras europeas y orientales a partir de 2003. El modo DRM30 utiliza las bandas de emisión de AM por debajo de 30 MHz y es El modo DRM + utiliza frecuencias VHF centradas en la banda de transmisión de FM y está destinado a reemplazar la transmisión de FM. Es incompatible con los receptores de radio existentes y requiere que los oyentes compren un nuevo receptor DRM. La modulación utilizada es una forma de OFDM denominada COFDM en la que se transmiten hasta 4 portadoras en un canal anteriormente ocupado por una sola señal AM o FM, modulada por modulación de amplitud en cuadratura (QAM). El sistema DRM está diseñado para ser lo más compatible posible con los transmisores de radio AM y FM existentes, por lo que gran parte del equipo de las estaciones de radio existentes no tendrá que ser reemplazado.
- La radio satelital es un servicio de radio por suscripción que transmite audio digital con calidad de CD directamente a los receptores de los suscriptores utilizando una señal de enlace descendente de microondas desde un satélite de comunicación de transmisión directa en órbita geoestacionaria a 22,000 millas sobre la Tierra. Está destinado principalmente a radios de automóviles en vehículos. La radio por satélite utiliza la banda S de 2,3 GHz en América del Norte, en otras partes del mundo, utiliza la banda L de 1,4 GHz asignada para DAB.
Vídeo: retransmisiones televisivas
La transmisión de televisión es la transmisión de imágenes en movimiento por radio, que consisten en secuencias de imágenes fijas, que se muestran en una pantalla en un receptor de televisión (una "televisión" o TV) junto con un canal de audio (sonido) sincronizado. Las señales de televisión ( video ) ocupan un ancho de banda más amplio que las señales de radiodifusión ( audio ). La televisión analógica , la tecnología de televisión original, requería 6 MHz, por lo que las bandas de frecuencia de televisión se dividen en canales de 6 MHz, ahora llamados "canales de RF". El estándar de televisión actual, introducido a partir de 2006, es un formato digital llamado HDTV ( televisión de alta definición), que transmite imágenes a una resolución más alta, típicamente 1080 píxeles de alto por 1920 píxeles de ancho, a una velocidad de 25 o 30 cuadros por segundo. Los sistemas de transmisión de televisión digital (DTV), que reemplazaron a la televisión analógica más antigua en una transición que comenzó en 2006, utilizan compresión de imagen y modulación digital de alta eficiencia como OFDM y 8VSB para transmitir video HDTV en un ancho de banda más pequeño que los canales analógicos antiguos, ahorrando radio escasa espacio del espectro . Por lo tanto, cada uno de los canales de RF analógicos de 6 MHz ahora transporta hasta 7 canales de DTV, que se denominan "canales virtuales". Los receptores de televisión digital tienen un comportamiento diferente en presencia de mala recepción o ruido que la televisión analógica, llamado efecto " acantilado digital ". A diferencia de la televisión analógica, en la que una recepción cada vez más pobre hace que la calidad de la imagen se degrade gradualmente, en la televisión digital la calidad de la imagen no se ve afectada por una mala recepción hasta que, en cierto punto, el receptor deja de funcionar y la pantalla se vuelve negra.
- Terrestrial television, over-the-air (OTA) television, or broadcast television – the oldest television technology, is the transmission of television signals from land-based television stations to television receivers (called televisions or TVs) in viewer's homes. Terrestrial television broadcasting uses the bands 41 – 88 MHz (VHF low band or Band I, carrying RF channels 1–6), 174 – 240 MHz, (VHF high band or Band III; carrying RF channels 7–13), and 470 – 614 MHz (UHF Band IV and Band V; carrying RF channels 14 and up). The exact frequency boundaries vary in different countries. Propagation is by line-of-sight, so reception is limited by the visual horizon to 30–40 miles (48–64 km). In the US effective radiated power (ERP) of television transmitters is limited to 35 kW in the VHF low band, 50 kW in the VHF high band, and 220 kW in UHF band; most TV stations operate below 75% of the limit. In most areas viewers use a simple "rabbit ears" dipole antenna on top of the TV, but viewers in fringe reception areas more than 15 miles from a station usually have to use an outdoor antenna mounted on the roof to get adequate reception.
- Satellite television – a set-top box which receives subscription direct-broadcast satellite television, and displays it on an ordinary television. A direct broadcast satellite in geostationary orbit 22,200 miles (35,700 km) above the Earth's equator transmits many channels (up to 900) modulated on a 12.2 to 12.7 GHz Ku band microwave downlink signal to a rooftop satellite dish antenna on the subscriber's residence. The microwave signal is converted to a lower intermediate frequency at the dish and conducted into the building by a coaxial cable to a set-top box connected to the subscriber's TV, where it is demodulated and displayed. The subscriber pays a monthly fee.
Time
Government standard frequency and time signal services operate time radio stations which continuously broadcast extremely accurate time signals produced by atomic clocks, as a reference to synchronize other clocks. Examples are BPC, DCF77, JJY, MSF, RTZ, TDF, WWV, and YVTO. One use is in radio clocks and watches, which include an automated receiver which periodically (usually weekly) receives and decodes the time signal and resets the watch's internal quartz clock to the correct time, thus allowing a small watch or desk clock to have the same accuracy as an atomic clock. Government time stations are declining in number because GPS satellites and the Internet Network Time Protocol (NTP) provide equally accurate time standards.
Two-way voice communication
A two-way radio is an audio transceiver, a receiver and transmitter in the same device, used for bidirectional person-to-person voice communication with other users with similar radios. An older term for this mode of communication is radiotelephony. The radio link may be half-duplex, as in a walkie-talkie, using a single radio channel in which only one radio can transmit at a time, so different users take turns talking, pressing a "push to talk" button on their radio which switches off the receiver and switches on the transmitter. Or the radio link may be full duplex, a bidirectional link using two radio channels so both people can talk at the same time, as in a cell phone.
- Cell phone – a portable wireless telephone that is connected to the telephone network by radio signals exchanged with a local antenna at a cellular base station (cell tower).[12] The service area covered by the provider is divided into small geographical areas called "cells", each served by a separate base station antenna and multichannel transceiver. All the cell phones in a cell communicate with this antenna on separate frequency channels, assigned from a common pool of frequencies.
The purpose of cellular organization is to conserve radio bandwidth by frequency reuse. Low power transmitters are used so the radio waves used in a cell do not travel far beyond the cell, allowing the same frequencies to be reused in geographically separated cells. When a user carrying a cellphone crosses from one cell to another, his phone is automatically "handed off" seamlessly to the new antenna and assigned new frequencies. Cellphones have a highly automated full duplex digital transceiver using OFDM modulation using two digital radio channels, each carrying one direction of the bidirectional conversation, as well as a control channel that handles dialing calls and "handing off" the phone to another cell tower. Older 2G, 3G, and 4G networks use frequencies in the UHF and low microwave range, between 700 MHz and 3 GHz. The cell phone transmitter adjusts its power output to use the minimum power necessary to communicate with the cell tower; 0.6 W when near the tower, up to 3 W when farther away. Cell tower channel transmitter power is 50 W. Current generation phones, called smartphones, have many functions besides making telephone calls, and therefore have several other radio transmitters and receivers that connect them with other networks: usually a WiFi modem, a Bluetooth modem, and a GPS receiver.
- 5G cellular network – next generation cellular networks which began deployment in 2019. Their major advantage is much higher data rates than previous cellular networks, up to 10 Gbps; 100 times faster than the previous cellular technology, 4G LTE. The higher data rates are achieved partly by using higher frequency radio waves, in the higher microwave band 3 - 6 GHz, and millimeter wave band, around 28 and 39 GHz. Since these frequencies have a shorter range than previous cellphone bands, the cells will be smaller than the cells in previous cellular networks which could be many miles across. Millimeter wave cells will only be a few blocks long, and instead of a cell base station and antenna tower, they will have many small antennas attached to utility poles and buildings.
- Satellite phone (satphone) – a portable wireless telephone similar to a cell phone, connected to the telephone network through a radio link to an orbiting communications satellite instead of through cell towers. They are more expensive than cell phones; but their advantage is that, unlike a cell phone which is limited to areas covered by cell towers, satphones can be used over most or all of the geographical area of the Earth. In order for the phone to communicate with a satellite using a small omnidirectional antenna, first generation systems use satellites in low earth orbit, about 400–700 miles (640–1,100 km) above the surface. With an orbital period of about 100 minutes a satellite can only be in view of a phone for about 4 – 15 minutes, so the call is "handed off" to another satellite when one passes beyond the local horizon. Therefore, large numbers of satellites, about 40 to 70, are required to ensure that at least one satellite is in view continuously from each point on Earth. Other satphone systems use satellites in geostationary orbit in which only a few satellites are needed, but these cannot be used at high latitudes because of terrestrial interference.
- Cordless phone- a landline telephone in which the handset is portable and communicates with the rest of the phone by a short range full duplex radio link, instead of being attached by a cord. Both the handset and the base station have low power FM radio transceivers operating in the UHF band that handle the short range bidirectional radio link.
- Land mobile radio system – short range mobile or portable half-duplex radio transceivers operating in the VHF or UHF band that can be used without a license. They are often installed in vehicles, with the mobile units communicating with a dispatcher at a fixed base station. Special systems with reserved frequencies are used by first responder services; police, fire, ambulance, and emergency services and other government services. Other systems are made for use by commercial firms such as taxi and delivery services. VHF systems use channels in the range 30–50 MHz and 150–172 MHz. UHF systems use the 450–470 MHz band and in some areas the 470–512 MHz range. In general, VHF systems have longer range than UHF but require longer antennas. AM or FM modulation is mainly used, but digital systems such as DMR are being introduced. Radiated power is typically limited to 4 watts.[12] These systems have a fairly limited range, usually 3 to 20 miles (4.8 to 32 km) depending on terrain. Repeaters installed on tall buildings, hills or mountain peaks are often used to increase the range, when it is desired to cover a larger area than line-of-sight. Examples of land mobile systems are CB, FRS, GMRS, and MURS. Modern digital systems, called trunked radio systems, have a digital channel management system using a control channel which automatically assigns frequency channels to user groups.
- Walkie-talkie – a battery powered portable handheld half-duplex two-way radio, used in land mobile radio systems.
- Airband – Half-duplex radio system used by aircraft pilots to talk to other aircraft and ground-based air traffic controllers. This vital system is the main communication channel for air traffic control. For most communication in overland flights in air corridors a VHF-AM system using channels between 108 and 137 MHz in the VHF band are used. This system has a typical transmission range of 200 miles (320 km) for aircraft flying at cruising altitude. For flights in more remote areas, such as transoceanic airline flights, aircraft use the HF band or channels on the Inmarsat or Iridium satphone satellites. Military aircraft also use a dedicated UHF-AM band from 225.0 to 399.95 MHz.
- Marine radio – medium range transceivers on ships, used for ship-to-ship, ship-to-air and ship-to-shore communication with harbormasters They use FM channels between 156 and 174 MHz in the VHF band with up to 25 watts power, giving them a range of about 60 miles (97 km). Some channels are half-duplex and some are full-duplex, to be compatible with the telephone network, to allow users to make telephone calls through a marine operator.
- Amateur radio – long range half-duplex two way radio used by hobbyists for non-commercial purposes: recreational radio contacts with other amateurs, volunteer emergency communication during disasters, contests, and experimentation. Radio amateurs must hold an amateur radio license and are given a unique callsign that must be used as an identifier in transmissions. Amateur radio is restricted to small frequency bands, the amateur radio bands, spaced throughout the radio spectrum from 136 kHz to 2.4 GHz. Within these bands amateurs are allowed freedom to transmit on any frequency with a wide variety of modulation methods. In addition to radiotelephony, amateurs are the only radio operators still using obsolete Morse code radiotelegraphy.
One-way voice communication
One way, unidirectional radio transmission is called simplex.
- Baby monitor – this is a cribside appliance for parents of infants that transmits the baby's sounds to a receiver carried by the parent, so they can monitor the baby while they are in other parts of the house. These transmit in FM on 49.300, 49.830, 49.845, 49.860, or 49.875 MHz with low power. Many baby monitors have duplex channels so the parent can talk to the baby, and video cameras to show a picture of the baby, this is called a baby cam.
- Wireless microphone – a battery powered microphone with a short-range transmitter which is handheld or worn on a person's body which transmits its sound by radio to a nearby receiver unit connected to a sound system. Wireless microphones are used by public speakers, performers, and television personalities so they can move freely without trailing a microphone cord. Analog models transmit in FM on unused portions of the television broadcast frequencies in the VHF and UHF bands. Some models transmit on two frequency channels for diversity reception to prevent nulls from interrupting transmission as the performer moves around. Some models use digital modulation to prevent unauthorized reception by scanner radio receivers; these operate in the 900 MHz, 2.4 GHz or 6 GHz ISM bands.
Data communication
- Wireless networking – automated radio links which transmit digital data between computers and other wireless devices using radio waves, linking the devices together transparently in a computer network. Computer networks can transmit any form of data: in addition to email and web pages, they also carry phone calls (VoIP), audio, and video content (called streaming media). Security is more of an issue for wireless networks than for wired networks since anyone nearby with a wireless modem can access the signal and attempt to log in. The radio signals of wireless networks are encrypted using WPA.
- Wireless LAN (wireless local area network or WiFi) – based on the IEEE 802.11 standards, these are the most widely used computer networks, used to implement local area networks without cables, linking computers, laptops, cell phones, video game consoles, smart TVs and printers in a home or office together, and to a wireless router connecting them to the Internet with a wire or cable connection. Wireless routers in public places like libraries, hotels and coffee shops create wireless access points (hotspots) to allow the public to access the Internet with portable devices like smartphones, tablets or laptops. Each device exchanges data using a wireless modem (wireless network interface controller), an automated microwave transmitter and receiver with an omnidirectional antenna that works in the background, exchanging data packets with the router. WiFi uses channels in the 2.4 GHz and 5 GHz ISM bands with OFDM (orthogonal frequency division multiplexing) modulation to transmit data at high rates. The transmitters in WiFi modems are limited to a radiated power of 200 mW to 1 watt, depending on country. They have a maximum indoor range of about 150 ft (50 m) on 2.4 GHz and 50 ft (20 m) on 5 GHz.
- Wireless WAN (wireless wide area network, WWAN) – a variety of technologies that provide wireless internet access over a wider area than WiFi networks do – from an office building, to a campus, to a neighborhood, to an entire city. The most common technologies used are: cellular modems, that exchange computer data by radio with cell towers; satellite internet access; and lower frequencies in the UHF band, which have a longer range than WiFi frequencies. Since WWAN networks are much more expensive and complicated to administer than WiFi networks, their use so far has generally been limited to private networks operated by large corporations.
- Bluetooth – a very short range wireless interface on a portable wireless device used as a substitute for a wire or cable connection, mainly to exchange files between portable devices and connect cellphones and music players with wireless headphones. In the most widely used mode, transmission power is limited to 1 milliwatt, giving it a very short range of up to 10 m (30 feet). The system uses frequency-hopping spread spectrum transmission, in which successive data packets are transmitted in a pseudorandom order on one of 79 1 MHz Bluetooth channels between 2.4 and 2.83 GHz in the ISM band. This allows Bluetooth networks to operate in the presence of noise, other wireless devices and other Bluetooth networks using the same frequencies, since the chance of another device attempting to transmit on the same frequency at the same time as the Bluetooth modem is low. In the case of such a "collision" the Bluetooth modem just retransmits the data packet on another frequency.
- Packet radio – a long-distance peer-to-peer wireless ad-hoc network in which data packets are exchanged between computer controlled radio modems (transmitter/receivers) called nodes, which may be separated by miles, and may be mobile. Each node only communicates with neighboring nodes, so packets of data are passed from node to node until they reach their destination. Uses the X.25 network protocol. Packet radio systems are used to a limited degree by commercial telecommunications companies and by the amateur radio community.
- Wireless LAN (wireless local area network or WiFi) – based on the IEEE 802.11 standards, these are the most widely used computer networks, used to implement local area networks without cables, linking computers, laptops, cell phones, video game consoles, smart TVs and printers in a home or office together, and to a wireless router connecting them to the Internet with a wire or cable connection. Wireless routers in public places like libraries, hotels and coffee shops create wireless access points (hotspots) to allow the public to access the Internet with portable devices like smartphones, tablets or laptops. Each device exchanges data using a wireless modem (wireless network interface controller), an automated microwave transmitter and receiver with an omnidirectional antenna that works in the background, exchanging data packets with the router. WiFi uses channels in the 2.4 GHz and 5 GHz ISM bands with OFDM (orthogonal frequency division multiplexing) modulation to transmit data at high rates. The transmitters in WiFi modems are limited to a radiated power of 200 mW to 1 watt, depending on country. They have a maximum indoor range of about 150 ft (50 m) on 2.4 GHz and 50 ft (20 m) on 5 GHz.
- Text messaging (texting) – this is a service on cell phones, allowing a user to type a short alphanumeric message and send it to another phone number, and the text is displayed on the recipient's phone screen. It is based on the Short Message Service (SMS) which transmits using spare bandwidth on the control radio channel used by cell phones to handle background functions like dialing and cell handoffs. Due to technical limitations of the channel, text messages are limited to 160 alphanumeric characters.
- Microwave relay – a long-distance high bandwidth point-to-point digital data transmission link consisting of a microwave transmitter connected to a dish antenna that transmits a beam of microwaves to another dish antenna and receiver. Since the antennas must be in line-of-sight, distances are limited by the visual horizon to 30–40 miles (48–64 km). Microwave links are used for private business data, wide area computer networks (WANs), and by telephone companies to transmit long- distance phone calls and television signals between cities.
- Telemetry – automated one-way (simplex) transmission of measurements and operation data from a remote process or device to a receiver for monitoring. Telemetry is used for in-flight monitoring of missiles, drones, satellites, and weather balloon radiosondes, sending scientific data back to Earth from interplanetary spacecraft, communicating with electronic biomedical sensors implanted in the human body, and well logging. Multiple channels of data are often transmitted using frequency division multiplexing or time division multiplexing. Telemetry is starting to be used in consumer applications such as:
- Automated meter reading – electric power meters, water meters, and gas meters that, when triggered by an interrogation signal, transmit their readings by radio to a utility reader vehicle at the curb, to eliminate the need for an employee to go on the customer's property to manually read the meter.
- Electronic toll collection – on toll roads, an alternative to manual collection of tolls at a toll booth, in which a transponder in a vehicle, when triggered by a roadside transmitter, transmits a signal to a roadside receiver to register the vehicle's use of the road, enabling the owner to be billed for the toll.
- Radio Frequency Identification (RFID) – identification tags containing a tiny radio transponder (receiver and transmitter) which are attached to merchandise. When it receives an interrogation pulse of radio waves from a nearby reader unit, the tag transmits back an ID number, which can be used to inventory goods. Passive tags, the most common type, have a chip powered by the radio energy received from the reader, rectified by a diode, and can be as small as a grain of rice. They are incorporated in products, clothes, railroad cars, library books, airline baggage tags and are implanted under the skin in pets and livestock (microchip implant) and even people. Privacy concerns have been addressed with tags that use encrypted signals and authenticate the reader before responding. Passive tags use 125–134 kHz, 13, 900 MHz and 2.4 and 5 GHz ISM bands and have a short range. Active tags, powered by a battery, are larger but can transmit a stronger signal, giving them a range of hundreds of meters.
- Submarine communication – When submerged, submarines are cut off from all ordinary radio communication with their military command authorities by the conductive seawater. However radio waves of low enough frequencies, in the VLF (30 to 3 kHz) and ELF (below 3 kHz) bands are able to penetrate seawater. Navies operate large shore transmitting stations with power output in the megawatt range to transmit encrypted messages to their submarines in the world's oceans. Due to the small bandwidth, these systems cannot transmit voice, only text messages at a slow data rate. The communication channel is one-way, since the long antennas needed to transmit VLF or ELF waves cannot fit on a submarine. VLF transmitters use miles long wire antennas like umbrella antennas. A few nations use ELF transmitters operating around 80 Hz, which can communicate with submarines at lower depths. These use even larger antennas called ground dipoles, consisting of two ground (Earth) connections 23–60 km (14–37 mi) apart, linked by overhead transmission lines to a power plant transmitter.
Space communication
This is radio communication between a spacecraft and an Earth-based ground station, or another spacecraft. Communication with spacecraft involves the longest transmission distances of any radio links, up to billions of kilometers for interplanetary spacecraft. In order to receive the weak signals from distant spacecraft, satellite ground stations use large parabolic "dish" antennas up to 25 metres (82 ft) in diameter and extremely sensitive receivers. High frequencies in the microwave band are used, since microwaves pass through the ionosphere without refraction, and at microwave frequencies the high gain antennas needed to focus the radio energy into a narrow beam pointed at the receiver are small and take up a minimum of space in a satellite. Portions of the UHF, L, C, S, ku and ka band are allocated for space communication. A radio link which transmits data from the Earth's surface to a spacecraft is called an uplink, while a link which transmits data from the spacecraft to the ground is called a downlink.
- Communication satellite – an artificial satellite used as a telecommunications relay to transmit data between widely separated points on Earth. These are used because the microwaves used for telecommunications travel by line of sight and so cannot propagate around the curve of the Earth. As of 1 January 2021[update], there were 2,224 communications satellites in Earth orbit.[14] Most are in geostationary orbit 22,200 miles (35,700 km) above the equator, so that the satellite appears stationary at the same point in the sky, so the satellite dish antennas of ground stations can be aimed permanently at that spot and do not have to move to track it. In a satellite ground station a microwave transmitter and large satellite dish antenna transmits a microwave uplink beam to the satellite. The uplink signal carries many channels of telecommunications traffic, such as long-distance telephone calls, television programs, and internet signals, using a technique called frequency-division multiplexing (FDM). On the satellite a transponder receives the signal, translates it to a different downlink frequency to avoid interfering with the uplink signal, and retransmits it down to another ground station, which may be widely separated from the first. There the downlink signal is demodulated and the telecommunications traffic it carries is sent to its local destinations through landlines. Communication satellites typically have several dozen transponders on different frequencies, which are leased by different users.
- Direct broadcast satellite – a geostationary communication satellite that transmits retail programming directly to receivers in subscriber's homes and vehicles on Earth, in satellite radio and TV systems. It uses a higher transmitter power than other communication satellites, to allow the signal to be received by consumers with a small unobtrusive antenna. For example, satellite television uses downlink frequencies from 12.2 to 12.7 GHz in the ku band transmitted at 100 to 250 watts, which can be received by relatively small 43–80 cm (17–31 in) satellite dishes mounted on the outside of buildings.
Radar
Radar is a radiolocation method used to locate and track aircraft, spacecraft, missiles, ships, vehicles, and also to map weather patterns and terrain. A radar set consists of a transmitter and receiver. The transmitter emits a narrow beam of radio waves which is swept around the surrounding space. When the beam strikes a target object, radio waves are reflected back to the receiver. The direction of the beam reveals the object's location. Since radio waves travel at a constant speed close to the speed of light, by measuring the brief time delay between the outgoing pulse and the received "echo", the range to the target can be calculated. The targets are often displayed graphically on a map display called a radar screen. Doppler radar can measure a moving object's velocity, by measuring the change in frequency of the return radio waves due to the Doppler effect.
Radar sets mainly use high frequencies in the microwave bands, because these frequencies create strong reflections from objects the size of vehicles and can be focused into narrow beams with compact antennas. Parabolic (dish) antennas are widely used. In most radars the transmitting antenna also serves as the receiving antenna; this is called a monostatic radar. A radar which uses separate transmitting and receiving antennas is called a bistatic radar.
- Airport surveillance radar – In aviation, radar is the main tool of air traffic control. A rotating dish antenna sweeps a vertical fan-shaped beam of microwaves around the airspace and the radar set shows the location of aircraft as "blips" of light on a display called a radar screen. Airport radar operates at 2.7 – 2.9 GHz in the microwave S band. In large airports the radar image is displayed on multiple screens in an operations room called the TRACON (Terminal Radar Approach Control), where air traffic controllers direct the aircraft by radio to maintain safe aircraft separation.
- Secondary surveillance radar – Aircraft carry radar transponders, transceivers which when triggered by the incoming radar signal transmit a return microwave signal. This causes the aircraft to show up more strongly on the radar screen. The radar which triggers the transponder and receives the return beam, usually mounted on top of the primary radar dish, is called the secondary surveillance radar. Since radar cannot measure an aircraft's altitude with any accuracy, the transponder also transmits back the aircraft's altitude measured by its altimeter, and an ID number identifying the aircraft, which is displayed on the radar screen.
- Electronic countermeasures (ECM) – Military defensive electronic systems designed to degrade enemy radar effectiveness, or deceive it with false information, to prevent enemies from locating local forces. It often consists of powerful microwave transmitters that can mimic enemy radar signals to create false target indications on the enemy radar screens.
- Radar altimeter – a specialized radar on an aircraft that measures the altitude of the aircraft above terrain by bouncing a radio beam off the ground surface and measuring the time for the echo to return.
- Marine radar – an X band radar on ships used to detect nearby ships and obstructions like bridges. A rotating antenna sweeps a vertical fan-shaped beam of microwaves around the water surface surrounding the craft out to the horizon.
- Weather radar – A Doppler radar which maps weather systems and measures wind speeds by reflection of microwaves from raindrops.
- Phased-array radar – a radar set that uses a phased array, a computer-controlled antenna that can steer the radar beam quickly to point in different directions without moving the antenna. Phased-array radars were developed by the military to track fast-moving missiles and aircraft. They are widely used in military equipment and are now spreading to civilian applications.
- Synthetic aperture radar(SAR) – a specialized airborne radar set that produces a high-resolution map of ground terrain. The radar is mounted on an aircraft or spacecraft and the radar antenna radiates a beam of radio waves sideways at right angles to the direction of motion, toward the ground. In processing the return radar signal, the motion of the vehicle is used to simulate a large antenna, giving the radar a higher resolution.
- Ground-penetrating radar – a specialized radar instrument which is rolled along the ground surface in a cart and transmits a beam of radio waves into the ground, producing an image of subsurface objects. Frequencies from 100 MHz to a few GHz are used. Since radio waves cannot penetrate very far into earth, the depth of GPR is limited to about 50 feet.
- Collision avoidance system – a short range radar or LIDAR system on an automobile or vehicle that detects if the vehicle is about to collide with an object and applies the brakes to prevent the collision.
- Radar fuze – a detonator for an aerial bomb which uses a radar altimeter to measure the height of the bomb above the ground as it falls and detonates it at a certain altitude.
- Radar speed gun – A handheld Doppler radar used by traffic police to measure the speed of vehicles to determine if they are obeying the local speed limit. When the officer points the gun at a vehicle and presses a trigger, its speed appears on a numeric display. Speed guns use the X band or Ku band.
Radiolocation
Radiolocation is a generic term covering a variety of techniques which use radio waves to find the location of objects, or for navigation
- Global Navigation Satellite System (GNSS) or satnav system – A system of satellites which allows geographical location on Earth (latitude, longitude, and altitude/elevation) to be determined to high precision (within a few metres) by small portable navigation instruments, by timing the arrival of radio signals from the satellites. These are the most widely used navigation systems today. The main satellite navigation systems are the US Global Positioning System (GPS), Russia's GLONASS, China's BeiDou Navigation Satellite System (BDS) and the European Union's Galileo.
- Global Positioning System(GPS) – The most widely used satellite navigation system, maintained by the US Air Force, which uses a constellation of 31 satellites in low Earth orbit. The orbits of the satellites are distributed so at any time at least four satellites are above the horizon over each point on Earth. Each satellite has an onboard atomic clock and transmits a continuous radio signal containing a precise time signal as well as its current position. Two frequencies are used, 1.2276 and 1.57542 GHz. Since the velocity of radio waves is virtually constant, the delay of the radio signal from a satellite is proportional to the distance of the receiver from the satellite. By receiving the signals from at least four satellites a GPS receiver can calculate its position on Earth by comparing the arrival time of the radio signals. Since each satellite's position is known precisely at any given time, from the delay the position of the receiver can be calculated by a microprocessor in the receiver. The position can be displayed as latitude and longitude, or as a marker on an electronic map. GPS receivers are incorporated in almost all cellphones and in vehicles such as automobiles, aircraft, and ships, and are used to guide drones, missiles, cruise missiles, and even artillery shells to their target, and handheld GPS receivers are produced for hikers and the military.
- Radio beacon – a fixed location terrestrial radio transmitter which transmits a continuous radio signal used by aircraft and ships for navigation. The locations of beacons are plotted on navigational maps used by aircraft and ships.
- Very High Frequency Omnidirectional Range (VOR) – a worldwide aircraft radio navigation system consisting of fixed ground radio beacons transmitting between 108.00 and 117.95 MHz in the VHF band. An automated navigational instrument on the aircraft displays a bearing to a nearby VOR transmitter. A VOR beacon transmits two signals simultaneously on different frequencies. A directional antenna transmits a beam of radio waves that rotates like a lighthouse at a fixed rate, 30 times per second. When the directional beam is facing north, an omnidirectional antenna transmits a pulse. By measuring the difference in phase of these two signals, an aircraft can determine its bearing (or "radial") from the station accurately. By taking a bearing on two VOR beacons an aircraft can determine its position (called a "fix") to an accuracy of about 90 metres (300 ft). Most VOR beacons also have a distance measuring capability, called distance measuring equipment (DME); these are called VOR/DME's. The aircraft transmits a radio signal to the VOR/DME beacon and a transponder transmits a return signal. From the propagation delay between the transmitted and received signal the aircraft can calculate its distance from the beacon. This allows an aircraft to determine its location "fix" from only one VOR beacon. Since line-of-sight VHF frequencies are used VOR beacons have a range of about 200 miles for aircraft at cruising altitude. TACAN is a similar military radio beacon system which transmits in 962–1213 MHz, and a combined VOR and TACAN beacon is called a VORTAC. In 2000 there were about 3000 VOR beacons worldwide, but this number is declining as aviation switches to the RNAV system that relies on Global Positioning System satellite navigation.
- Non-directional beacon (NDB) – Legacy fixed radio beacons used before the VOR system that transmit a simple signal in all directions for aircraft or ships to use for radio direction finding. Aircraft use automatic direction finder (ADF) receivers which use a directional antenna to determine the bearing to the beacon. By taking bearings on two beacons they can determine their position. NDBs use frequencies between 190 and 1750 kHz in the LF and MF bands which propagate beyond the horizon as ground waves or skywaves much farther than VOR beacons. They transmit a callsign consisting of one to 3 Morse code letters as an identifier.
- Emergency locator beacon – a portable battery powered radio transmitter used in emergencies to locate airplanes, vessels, and persons in distress and in need of immediate rescue. Various types of emergency locator beacons are carried by aircraft, ships, vehicles, hikers and cross-country skiers. In the event of an emergency, such as the aircraft crashing, the ship sinking, or a hiker becoming lost, the transmitter is deployed and begins to transmit a continuous radio signal, which is used by search and rescue teams to quickly find the emergency and render aid. The latest generation Emergency Position Indicating Rescue Beacons (EPIRBs) contain a GPS receiver, and broadcast to rescue teams their exact location within 20 meters.
- Cospas-Sarsat – an international humanitarian consortium of governmental and private agencies which acts as a dispatcher for search and rescue operations. It operates a network of about 47 satellites carrying radio receivers, which detect distress signals from emergency locator beacons anywhere on Earth transmitting on the international Cospas distress frequency of 406 MHz. The satellites calculate the geographic location of the beacon within 2 km by measuring the Doppler frequency shift of the radio waves due to the relative motion of the transmitter and the satellite, and quickly transmit the information to the appropriate local first responder organizations, which perform the search and rescue.
- Radio direction finding (RDF) – this is a general technique, used since the early 1900s, of using specialized radio receivers with directional antennas (RDF receivers) to determine the exact bearing of a radio signal, to determine the location of the transmitter. The location of a terrestrial transmitter can be determined by simple triangulation from bearings taken by two RDF stations separated geographically, as the point where the two bearing lines cross, this is called a "fix". Military forces use RDF to locate enemy forces by their tactical radio transmissions, counterintelligence services use it to locate clandestine transmitters used by espionage agents, and governments use it to locate unlicensed transmitters or interference sources. Older RDF receivers used rotatable loop antennas, the antenna is rotated until the radio signal strength is weakest, indicating the transmitter is in one of the antenna's two nulls. The nulls are used since they are sharper than the antenna's lobes (maxima). More modern receivers use phased array antennas which have much greater angular resolution.
- Animal migration tracking – a widely used technique in wildlife biology, conservation biology, and wildlife management in which small battery-powered radio transmitters are attached to wild animals so their movements can be tracked with a directional RDF receiver. Sometimes the transmitter is implanted in the animal. The VHF band is typically used since antennas in this band are fairly compact. The receiver has a directional antenna (typically a small Yagi) which is rotated until the received signal is strongest; at this point the antenna is pointing in the direction of the animal. Sophisticated systems used in recent years use satellites to track the animal, or geolocation tags with GPS receivers which record and transmit a log of the animal's location.
Remote control
Radio remote control is the use of electronic control signals sent by radio waves from a transmitter to control the actions of a device at a remote location. Remote control systems may also include telemetry channels in the other direction, used to transmit real-time information of the state of the device back to the control station. Unmanned spacecraft are an example of remote controlled machines, controlled by commands transmitted by satellite ground stations. Most handheld remote controls used to control consumer electronics products like televisions or DVD players actually operate by infrared light rather than radio waves, so are not examples of radio remote control. A security concern with remote control systems is spoofing, in which an unauthorized person transmits an imitation of the control signal to take control of the device. Examples of radio remote control:
- Unmanned aerial vehicle (UAV, drone) – A drone is an aircraft without an onboard pilot, flown by remote control by a pilot in another location, usually in a piloting station on the ground. They are used by the military for reconnaissance and ground attack, and more recently by the civilian world for news reporting and aerial photography. The pilot uses aircraft controls like a joystick or steering wheel, which create control signals which are transmitted to the drone by radio to control the flight surfaces and engine. A telemetry system transmits back a video image from a camera in the drone to allow the pilot to see where he is going, and data from a GPS receiver giving the real-time position of the aircraft. UAVs have sophisticated onboard automatic pilot systems that maintain stable flight and only require manual control to change directions.
- Keyless entry system – a short-range handheld battery powered key fob transmitter, included with most modern cars, which can lock and unlock the doors of a vehicle from outside, eliminating the need to use a key. When a button is pressed, the transmitter sends a coded radio signal to a receiver in the vehicle, operating the locks. The fob must be close to the vehicle, typically within 5 to 20 meters. North America and Japan use a frequency of 315 MHz, while Europe uses 433.92 and 868 MHz. Some models can also remotely start the engine, to warm up the car. A security concern with all keyless entry systems is a replay attack, in which a thief uses a special receiver ("code grabber") to record the radio signal during opening, which can later be replayed to open the door. To prevent this, keyless systems use a rolling code system in which a pseudorandom number generator in the remote control generates a different random key each time it is used. To prevent thieves from simulating the pseudorandom generator to calculate the next key, the radio signal is also encrypted.
- Garage door opener – a short-range handheld transmitter which can open or close a building's electrically operated garage door from outside, so the owner can open the door when he drives up in his car, and close it after he leaves. When a button is pressed the control transmits a coded FSK radio signal to a receiver in the opener, raising or lowering the door. Modern openers use 310, 315 or 390 MHz. To prevent a thief using a replay attack, modern openers use a rolling code system.
- Radio-controlled models – a popular hobby is playing with radio-controlled model boats, cars, airplanes, and helicopters (quadcopters) which are controlled by radio signals from a handheld console with a joystick. Most recent transmitters use the 2.4 GHz ISM band with multiple control channels modulated with PWM, PCM or FSK.
- Wireless doorbell – A residential doorbell that uses wireless technology to eliminate the need to run wires through the building walls. It consists of a doorbell button beside the door containing a small battery powered transmitter. When the doorbell is pressed it sends a signal to a receiver inside the house with a speaker that sounds chimes to indicate someone is at the door. They usually use the 2.4 GHz ISM band. The frequency channel used can usually be changed by the owner in case another nearby doorbell is using the same channel.
Jamming
Radio jamming is the deliberate radiation of radio signals designed to interfere with the reception of other radio signals. Jamming devices are called "signal suppressors" or "interference generators" or just jammers.[15]
During wartime, militaries use jamming to interfere with enemies' tactical radio communication. Since radio waves can pass beyond national borders, some totalitarian countries which practice censorship use jamming to prevent their citizens from listening to broadcasts from radio stations in other countries. Jamming is usually accomplished by a powerful transmitter which generates noise on the same frequency as the target transmitter.
US Federal law prohibits the nonmilitary operation or sale of any type of jamming devices, including ones that interfere with GPS, cellular, Wi-Fi and police radars.[16]
Scientific research
- Radio astronomy is the scientific study of radio waves emitted by astronomical objects. Radio astronomers use radio telescopes, large radio antennas and receivers, to receive and study the radio waves from astronomical radio sources. Since astronomical radio sources are so far away, the radio waves from them are extremely weak, requiring extremely sensitive receivers, and radio telescopes are the most sensitive radio receivers in existence. They use large parabolic (dish) antennas up to 500 meters (2,000 ft) in diameter to collect enough radio wave energy to study. The RF front end electronics of the receiver is often cooled by liquid nitrogen to reduce thermal noise. Multiple antennas are often linked together in arrays which function as a single antenna, to increase collecting power. In Very Long Baseline Interferometry (VLBI) radio telescopes on different continents are linked, which can achieve the resolution of an antenna thousands of miles in diameter.
- Remote sensing – in radio, remote sensing is the reception of electromagnetic waves radiated by natural objects or the atmosphere for scientific research. All warm objects emit microwaves and the spectrum emitted can be used to determine temperature. Microwave radiometers are used in meteorology and earth sciences to determine temperature of the atmosphere and earth surface, as well as chemical reactions in the atmosphere.
Etimología
The word "radio" is derived from the Latin word "radius", meaning "spoke of a wheel, beam of light, ray". It was first applied to communications in 1881 when, at the suggestion of French scientist Ernest Mercadier, Alexander Graham Bell adopted "radiophone" (meaning "radiated sound") as an alternate name for his photophone optical transmission system.[17][18] However, this invention would not be widely adopted.
Following Heinrich Hertz's discovery of the existence of radio waves in 1886, a variety of terms were initially used for this radiation, including "Hertzian waves", "electric waves", and "ether waves". The first practical radio communications systems, developed by Guglielmo Marconi in 1894–5, transmitted telegraph signals by radio waves, so radio communication was first called "wireless telegraphy". Up until about 1910 the term "wireless telegraphy" also included a variety of other experimental systems for transmitting telegraph signals without wires, including electrostatic induction, electromagnetic induction and aquatic and earth conduction, so there was a need for a more precise term referring exclusively to electromagnetic radiation.
The first use of radio- in conjunction with electromagnetic radiation appears to have been by French physicist Édouard Branly, who in 1890 developed the coherer detector, which he called in French a radio-conducteur.[19] The radio- prefix was later used to form additional descriptive compound and hyphenated words, especially in Europe. For example, in early 1898 the British publication The Practical Engineer included a reference to "the radiotelegraph" and "radiotelegraphy",[20] The French text of both the 1903 and 1906 Berlin Radiotelegraphic Conventions includes the phrases "radiotélégraphique" and "radiotélégrammes".
The use of "radio" as a standalone word dates back to at least December 30, 1904, when instructions issued by the British Post Office for transmitting telegrams specified that "The word 'Radio'... is sent in the Service Instructions".[21] This practice was universally adopted, and the word "radio" introduced internationally, by the 1906 Berlin Radiotelegraphic Convention, which included a Service Regulation specifying that "Radiotelegrams shall show in the preamble that the service is 'Radio'".
The switch to "radio" in place of "wireless" took place slowly and unevenly in the English-speaking world. Lee de Forest helped popularize the new word in the United States—in early 1907 he founded the DeForest Radio Telephone Company, and his letter in the June 22, 1907 Electrical World about the need for legal restrictions warned that "Radio chaos will certainly be the result until such stringent regulation is enforced".[22] The United States Navy would also play a role. Although its translation of the 1906 Berlin Convention used the terms "wireless telegraph" and "wireless telegram", by 1912 it began to promote the use of "radio" instead. The term started to become preferred by the general public in the 1920s with the introduction of broadcasting. (the word broadcasting originated with the agricultural term meaning roughly "scattering seeds widely".)[23] British Commonwealth countries continued to commonly use the term "wireless" until the mid-20th century, though the magazine of the British Broadcasting Corporation in the UK has been called Radio Times since its founding in the early 1920s.
In recent years "wireless" has gained renewed popularity as a more general term for devices communicating using electromagnetic radiation, either radio waves or light, due to the rapid growth of short-range computer networking, e.g., wireless local area networks Wi-Fi, and Bluetooth, as well as cell phones, to distinguish these uses from traditional "radio" communication, such as broadcasting.
Historia
- SeeHistory of radio, Invention of radio, Timeline of radio, History of broadcasting
Ver también
- Outline of radio
- Electromagnetic radiation and health
- Radio quiet zone
Referencias
- ^ "Radio". Oxford Living Dictionaries. Oxford University Press. 2019. Retrieved 26 February 2019.
- ^ "Definition of radio". Encyclopedia. PCMagazine website, Ziff-Davis. 2018. Retrieved 26 February 2019.
- ^ a b c d Ellingson, Steven W. (2016). Radio Systems Engineering. Cambridge University Press. pp. 1–4. ISBN 978-1316785164.
- ^ Kraus, John D. (1988). Antennas, 2nd Ed. Tata-McGraw Hill. p. 50. ISBN 0074632191.
- ^ Serway, Raymond; Faughn, Jerry; Vuille, Chris (2008). College Physics, 8th Ed. Cengage Learning. p. 714. ISBN 978-0495386933.
- ^ Balanis, Constantine A. (2005). Antenna theory: Analysis and Design, 3rd Ed. John Wiley and Sons. pp. 10. ISBN 9781118585733.
- ^ a b c d Ellingson, Steven W. (2016). Radio Systems Engineering. Cambridge University Press. pp. 16–17. ISBN 978-1316785164.
- ^ a b c Brain, Marshall (2000-12-07). "How Radio Works". HowStuffWorks.com. Retrieved 2009-09-11.
- ^ a b c d "Spectrum 101" (PDF). US National Aeronautics and Space Administration (NASA). February 2016. Retrieved 2 December 2019. Cite journal requires
|journal=
(help), p. 6 - ^ "Radio Regulations, 2016 Edition" (PDF). International Telecommunications Union. 3 November 2016. Retrieved 9 November 2019. Cite journal requires
|journal=
(help) Article 2, Section 1, p.27 - ^ Baker, William (2020). "DAB vs FM: The differences between analogue and digital radio". Radio Fidelity online magazine. Retrieved 14 September 2020.
- ^ a b Brain, Marshall; Jeff Tyson & Julia Layton (2018). "How Cell Phones Work". How Stuff Works. InfoSpace Holdings LLC. Retrieved 31 December 2018.
- ^ "Ground infrastructure". Russian Satellite Communications Company.
- ^ "UCS Satellite Database". Union of Concerned Scientists. 1 January 2021. Retrieved 21 May 2021.
- ^ "What jamming of a wireless security system is and how to resist it | Ajax Systems Blog". Ajax Systems. Retrieved 2020-01-18.
- ^ "Jammer Enforcement". Federal Communications Commission. 2011-03-03. Retrieved 2020-01-18.
- ^ "radio | Origin and meaning of radio by Online Etymology Dictionary". www.etymonline.com. Retrieved 2020-05-24.
- ^ "Production of Sound by Radiant Energy" by Alexander Graham Bell, Popular Science Monthly, July, 1881, pages 329–330: "[W]e have named the apparatus for the production and reproduction of sound in this way the "photophone", because an ordinary beam of light contains the rays which are operative. To avoid in future any misunderstandings upon this point, we have decided to adopt the term "radiophone", proposed by M. Mercadier, as a general term signifying the production of sound by any form of radiant energy..."
- ^ "The Genesis of Wireless Telegraphy" by A. Frederick Collins, Electrical World and Engineer, May 10, 1902, page 811.
- ^ "Wireless Telegraphy", The Practical Engineer, February 25, 1898, page 174. "Dr. O. J. Lodge, who preceded Marconi in making experiments in what may be called "ray" telegraphy or radiotelegraphy by a year or two, has devised a new method of sending and receiving the messages. The reader will understand that in the radiotelegraph electric waves forming the signals of the message start from the sending instrument and travel in all directions like rays of light from a lamp, only they are invisible."
- ^ "Wireless Telegraphy", The Electrical Review (London), January 20, 1905, page 108, quoting from the British Post Office's December 30, 1904 Post Office Circular.
- ^ "Interference with Wireless Messages", Electrical World, June 22, 1907, page 1270.
- ^ "broadcast | Origin and meaning of broadcast by Online Etymology Dictionary". www.etymonline.com. Retrieved 2020-05-24.