Some questions about antennas:
Why are AM & FM antennas so different?
Is a VHF antenna the same as a FM antenna?
Can you tune an antenna to pick up one station better than the rest?
(Can you tell my cable is out?)
Why are AM & FM antennas so different?
Antennas are tuned to the wavelength of the radio signal they are meant to receive. In general, the length of an antenna element is some fraction of the wavelength of the RF signal, such as 1/4, 5/8, or 1/2. The lower the frequency, the longer the wavelenth. AM broadcast signals are in the range of 530 to 1700 kHz, while FM broadcasts range from 88 to 108 Mhz, so AM waves are about 160 times longer than FM waves. FM antennas can be a simple, short dipole-type antenna, similar to a set-top TV antenna. In theory, an AM antenna could be too, but that would mean elements 160 times the length of the ones on your TV. To make the antenna smaller and more manageable, a suitable length of wire is wrapped in a loop of many turns, either in an air-core or on a ferrite rod.
Is a VHF antenna the same as a FM antenna?
They are. VHF TV and FM radio brodcasts are within the same range, so a set of TV rabbit ears can receive FM signals fairly well.
Can you tune an antenna to pick up one station better than the rest?
Sort of. You can tune the length of an antenna to be exactly 1/4, 1/2 or 5/8 wave and pick up a weaker station, but in general, a stronger signal will be picked up better, and a very strong signal can be picked up even if the antenna length is way off. If you collapse the antenna on a portable radio completely, you’ll still be able to tune in strong FM stations.
AM and FM are two different modulation (ways of modifying a wave in order to carry signal). It just so happens that broadcast AM is done at a lower frequency (longer wavelength) than broadcast FM. AM is ~300 M and FM is ~3 M.
It is possible to us AM on higher frequencies and FM on lower. Aviation uses AM at frequencies just above broadcast FM.
Note that the video signal on your TV is AM modulated (well, vestigial side band, but that is beyond the scope of the question)
A common antenna is about the same length (or 1/2 or 1/4) the wavelength of the signal. Since 300 M antennas are not practical AM antennas are typically coiled up. (to oversimplify).
It is possible to pick up frequencies outside this, but the antenna doesn’t work as well. So yes, it is possible to tune an antenna for a particular station. Thats why rooftop antennas are typically a bunch of different length tubes. Each length picks up a channel and nearby channels (again, simplifying)
VHF is IIRC, 30 to 300 MHz (~10 to =~1 M). So FM is VHF. Broadcast FM is in between channels 6 and 7 on broadcast TV.
Note that cable TV channels ARE not the same as broadcast TV channels frequencywise.
Brian
who again emphasises that he simplified some things and therefore introduced some inacuracies
Antennas are complicated things. They are almost a complete electronic circuit by themselves. Putting it as simply as I can, it’s a piece of wire that sticks up in the air in the hopes that some electromagnetic and/or electrostatic waves will cut into it, thereby inducing a voltage along the length of the wire, which results in a tiny current that is fed into a high gain RF amplifier.
AM and FM antennas are different because of the spectrum of frequencies they are made to receive. The FM broadcast band is 88-108 MHz and AM is 535-1705KHz. An AM antenna is actually several loops of wire, although it may be disguised to appear as only a single loop. The loops provide higher inducatance which is required to tune to the lower frequencies of the AM spectrum. FM antennas manage to get enough stray inductance from a single length of wire. So although an FM antenna looks bigger, it is “electrically” much smaller than an AM loop antenna.
A glance at the formula for resonant frequency will reveal the relationship between inductance, capacitance and frequency. I don’t feel like fiddling around with the symbol font, so just look here.
VHF is the broad spectrum between 30MHz-300MHz. UHF starts there and goes up to 3GHz. So a VHF antenna has to be able to tune to a broader range of frequencies and should be made with as flat a response (treat all frequencies with the same gain) as possible. It would not be hard for a VHF antenna to bring in an FM radio station, but FM broadcast stations have only a 200KHz bandwidth, so the tuning circuitry would have to be very selective to be able to process it and reject the adjacent stations.
You can tune an antenna to pick up one station better than another. This is exactly what you are doing when you spin that tuning knob or press the tuning button.
If I understand correctly, the “coil” type AM antennas are designed to pick up the magnetic field component of the EM wave, not the electric field.