Radio waves are a type of magnetic energy generated by a transmitter and an antenna. The antenna on your car is designed 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 is working on a simple principle. The passage of the radio waves over the mast of the antenna produces minute electrical charges. Depending on the length and other details of the antenna design, particular radio wavelengths are more easily captured than others. The design of the car radio antenna also takes into account the different frequencies of the AM and FM bands in order to provide a good reception.
It is of course necessary for the electrical impulses generated by the antenna to reach your car radio with as little signal loss as possible. Car antennas achieve this through the use of a transmission line. The transmission line consists of a coaxial cable and a connector that plugs into the car radio antenna jack. Coaxial cable is chosen for its ability to transmit weak signals with minimal loss and additional electrical interference.
The coaxial cable in your car is similar to the one found connected to the TV in your home—a two-wire cable with an inner copper wire surrounded by an insulating material and an outer shield made of thin copper braiding. Surrounding all this is a tough, rubberized outer sheath that keeps the cable safe from damage while maintaining flexibility.
Most car antenna transmission lines use a standardized "Motorola" style plug to connect to the receiver. Some car models use different connectors, but adapters are available in the event that a car radio or antenna is replaced.
Car radios are designed and built to operate perfectly in the AM and FM frequency bands. The advent of the new medium of HD Radio has caused some to worry that additional or new antenna will be required to receive these transmissions, but there is nothing to worry about, as HD Radio transmissions are broadcast on standard AM and FM frequencies, which are "piggybacked" on existing analog transmissions. You do need a special radio to receive HD Radio, but the existing antenna on your car will work fine with that radio.
Standard car radio antennas are not designed to operate with other specialized broadcasts, in particular satellite radio transmissions such as XM or SIRIUS. You're going to need a specialized antenna for those services.
Inside modern shark-fin-style radio antennae on modern cars, you’ll actually find multiple antennae—ones for cell signals, satellite radio, and old AM/FM broadcasts. Inside those boxes are coiled antenna wires, length optimized for whatever EM band they’re intended to pick up.
Most shark fin antennas are actually kind of compromised FM signals — the old school 1/4 wavelength whip antenna is likely to get better reception due to its better gain — but the other antennas it contains (satellite radio, etc.) are now considered more important, as are improved aesthetics, so we're just dealing with it, and for the most part, it's fine.
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