Radio waves are a type of magnetic energy generated by a transmitter and a transmission antenna. The antenna on your car is built to pick up the magnetic energy of the radio waves and send it to the car's radio receiver, which in turn isolates and amplifies the radio waves you want to listen to.
The antenna works on a simple principle. The passage of the radio waves over the antenna's mast produces minute electrical charges. Depending on the length and other details of the antenna design, particular radio wavelengths are captured more easily than others. The design of a car's radio antenna takes into account the differing frequencies of the AM and FM bands to provide good reception with either.
Sending the Signal to the Receiver
It's necessary, of course, for the electrical impulses generated by the antenna to get to your car radio with as little loss of signal as possible. Car antennas achieve this through the use a transmission line. The transmission line consists of a coaxial cable and a connector that plugs into the car radio's antenna jack. Coaxial cable is chosen for its ability to transfer weak signals with a minimum of loss and addition of electrical interference. The coaxial cable in your car is similar to that found connected to the television in your home---it's a two-conductor cable, with an inner copper wire surrounded by an insulating material and an outer shield made of thin copper braiding. Surrounding all of this is a tough, rubberized outer sheath that keeps the cable safe from damage while maintaining flexibility.
For the connection to the receiver, most car antenna transmission lines use a standardized "Motorola" style plug. Some car models employ different connectors, but adapters are available in the event the car radio or antenna is replaced.
Suitability for HD Radio and Other Transmissions
Car radios are designed and built to operate optimally in the AM and FM frequency bands. The advent of the new medium of HD Radio broadcasting has caused some to worry that an additional or new antenna will be required to receive these transmissions, but there's nothing to fear, since HD Radio transmissions are broadcast on standard AM and FM frequencies, "piggybacked" on existing analog transmissions. You do need a special radio to receive HD Radio broadcasts, but the existing antenna on your car will work fine with that radio.
Standard car radio antennas are not designed to operate with some other specialized broadcasts, particularly satellite radio transmissions such as XM or SIRIUS. You'll need a specialized antenna for those services.
- ARRL Antenna Book; American Radio Relay League; 2005
- Photo Credit NL Graphics
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