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Author Topic: Antennas: Broadcast FM -vs- AM  (Read 6268 times)

Posts: 16

« on: March 05, 2007, 03:55:23 PM »

a young fellow in (a local store) asked me if an
antenna for  FM will work for AM on a radio...
or do the two bands need seperate antennas...
I told him that, typically, a radio will have a  "ferrite-rod/wire-coil" antenna INSIDE the box for AM
and an extendable  whip-antenna for FM...

then he clarifies...  why do cars only have the one-antenna for both  AM/FM ?

well.. I stammred a bit and told him I may have to consult  'Google' on that one...  Wink
I never recalled an internal antenna inside an auto-radio-box...
and would not expect it to get any good reception, under the dash and so near the  ignition...    
my initial thoughts were that  Amp-Mod  -vs-  
Frq-Mod would need basic antenna-design
to be "different"...    so.. any help on this one ?

Posts: 4859

« Reply #1 on: March 05, 2007, 04:03:11 PM »

The car antenna is used for both FM and AM have mid and high impedance amplifiers to couple to a car antenna. A typical car antenna has a 1000-2000 impedance for AM and when you look at the coax from the antenna you will see a very small center conductor.

In a portable radio they use a ferrite rod for AM and a telescopic wip for FM. The Ferrite rod is actually part of the front end amplifier tuning circuit.

Posts: 2527

« Reply #2 on: March 05, 2007, 04:10:08 PM »

Antennas receive RF; how the RF is "modulated" is has no effect on the antenna.  The best joke is HD ready TV antennas.

For insight into the length of auto AM/FM antennas, calculate a quarter wave length antenna for FM and do the same for AM.

Doubling the length of an AM car antenna doesn't accomplish much.


Posts: 4380

« Reply #3 on: March 05, 2007, 04:11:22 PM »

The modulation has nothing to do with it.  AM, or FM the frequency of the band is what is important.  The main reason AM radios have a built-in antenna is so they don't need an external antenna.  The ferrite with a coil of wire works fairly well on the broadcast band for radios that have no amplification. FM radios need a different antenna because the ferrite type wouldn't work well for them.  Car radios are a different animal, they have much more gain and better selectivity and AM and FM work well with the automotive whip.  They certainly don't have ferrite type antennas as the car body acts as a shield.
In the olden times when portable transistor radios first appeared, you would see people with no auto radios hang a little portable transistor radio from the rearview mirror.  This was before boom boxes and mammoth auto amplifier systems and huge speakers.
The little "transistors" barely worked inside cars and their tiny speakers sounded like Alvin the Chipmonk.
73 Allen

Posts: 14444

« Reply #4 on: March 05, 2007, 06:23:27 PM »

The AM broadcast band is 0.530MHz to 1.610MHz while the FM broadcast band is 88MHz to 108MHz. Thats what makes the difference in the antenna requirements rather than the AM or FM modulation types.

A loop stick antenna makes a better antenna for the low frequencies of the AM band than a short whip. Thats why loop sticks are used in portable AM band radios. Short whips make a better antenna for the FM band and so portable FM band radios use a whip. Loop sticks don't work well inside a vehicle and so the car radio compromises by using the same short whip that works well for the FM band on the AM band. The input circuitry of the car radio is designed to provide the proper match for the short whip on both the AM band and the FM band.

Bob  AA4PB
Garrisonville, VA

Posts: 9749


« Reply #5 on: March 05, 2007, 07:00:27 PM »

Antennas are designed differently for different applications. ALL radio signals ALWAYS have a time-varying magentic field and a time-varying electric field. The magnetic field is like the force we see from  magnet, and the electric field is like the force from a comb rubbed through cat fur. Both fields are always present in a radio signal, and both fields must be detected by an antenna.

FM BC is on a high  frequency where wavelength is several feet long, and AM BC is on a low frequency where an equivalent avelength is several hundred feet lon.

A home receiver is used in a room that is generally not enclosed by metal walls.  The lack of metallic walls allows radio signals into the room and they can be heard on a very small antenna if the antenna is properly tuned.

For the AM BC band, the antenna is wound on a small "magnetic-material" rod. You can "tune" the small rod to a low frequency because there is a lot of wire wound on the "magnetic core". As a matter of fact early Zenith radios had an AM and Shortwave coiled antenna they called by a trade name "wave magnet".

For FM, since it is a higher frequency, it is practical to have a larger untuned antenna like a hunk of wire. Some radios even use the power cord as a hunk of wire. The wire is almost self-tuned because it is so long compared to the very short wavelength at the FM band.

A car radio needs an outside antenna. It needs the outside antenna because the car is metal and would shield the signal, especially the low frequency AM broadcast band.

Car manufacturers stick a rod antenna outside the car or use a thin wire antenna in the glass. The radio has a matching system inside that "tunes" the very short antenna to the low frequency of the AM broadcast band.  It also has a different input circuit that accepts the signal from the almost perfectly tuned antenna for the higher frequency FM band. The length of the antenna is nearly perfect for the very short wavelength of the FM band without any special external tuning.

Very few people would want a long whip antenna on a radio in the living room or on a radio you carry around. On a car an antenna inside the car would not work well, and the antenna on the outside or in the glass doesn't matter much to the owner. This is why they use the two different systems.
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