How Do TV Antennas Work?by Timothy Smithee
Cable TV offers hundreds of channels and other services but many television signals, including HDTV signals, are still available over the air for free. All you need to receive them is an antenna. A little knowledge about how antennas work will help you choose the right antenna.
Broadcast television signals are carried on radio waves generated by applying an electrical signal to a transmitting antenna and received by antennas that detect the radio waves. Whether the signal content is standard television or HDTV television, the radio wave itself is the same as radio waves used for everything from wireless phones to Wi-Fi. Radio waves are a form of electromagnetic radiation, like light, heat and microwaves. All radio waves travel at the speed of light.
Wavelength and Frequency
Radio waves are typically described by their frequency. Think of a wave at the ocean: the frequency is the number of crests (or cycles) in one second. This unit of measure is called hertz after Heinrich Hertz, a German physicist who experimented with radio waves. The wavelength is the space between the crests. Wavelength times frequency equals velocity. Since the speed of radio waves is a constant (the speed of light), if you know frequency you can determine wavelength (and vice versa). Knowing the wavelength is helpful for using the correct antenna. A general rule of thumb is lower frequencies have longer wavelengths and require larger antennas.
Television signals are carried on several different ranges of frequencies. Each channel in the VHF and UHF bands has its own specific frequency, and they are grouped follows. Channels 2 through 4: 54 to 72 MHz Channels 5 and 6: 76 and 88 MHz Channels 7 through 13: 174 to 216 MHz Channels 14 through 83 (UHF): 470 to 890 MHz The wavelengths of television channel radio waves are about 1 foot to 18 feet. This does not mean an 18-foot antenna is required to receive channel 2. Ideal antenna lengths are slightly less than half the wavelength desired.
Any piece of metal can be affected by radio waves. An antenna is designed to be particularly sensitive to radio waves at a specific frequency or range of frequencies. Both indoor and outdoor antennas contain a variety of metal elements in different sizes to detect the range of television signals. Outdoor antennas tend to be larger for better reception of the large low-frequency waves. Radio waves are directional and television signals are transmitted as horizontal waves, so ideally antennas should be horizontal and aimed at the source. Electronics in a tuner detect and isolate the specific frequency desired and extract the signal information from the carrier wave.
Many antennas are advertised as suitable for or optimized for HDTV. This is marketing hype. The antenna receives radio waves, and whether those waves include HDTV content is irrelevant to the reception of the signal. However, while analog content will partially display when the signal is weak, HDTV content will not display if the signal is too weak to provide enough information. A larger or better placed antenna may bring in enough information to display the HDTV content. Make sure that an antenna includes elements for UHF frequencies if the HDTV signals you hope to receive are on channels 14 to 83. Some older TV antennas are designed to receive VHF signals only.
Signal amplifiers are built in to some antennas and also available as standalone units. These may be required if one antenna is connected to several televisions or there is a long cable run between the antenna and the receiver. Amplifiers will boost a weak signal, but interference may also be boosted. Radio waves in the frequencies for television are limited to line of sight and cannot travel more than about 80 miles. Moving an antenna to a higher location and aiming it at the transmitter may give better results than adding a signal booster.
- link Denny's Antenna Service: TV Antenna Range and Reception
- link Georgia State University - HyperPhysics: TV and FM Radio Band
- link United States Army Signal Center: How Radios Work
- link University of California at Berkeley - Seti at Home: Electromagnetic Radiation
- link NASA Science Mission: Anatomy of an Electromagnetic Wave
- photo_camera Ryan McVay/Photodisc/Getty Images