# How do VHF antennas compensate for change in frequency in Frequency Modulation?

I would like to understand how antennas specifically, compensate for variability in the frequency while receiving FM waves in VHF. The way FM works is that the receiver system - receiver and antenna locks on to a central frequency to receive a signal. Now, once the central frequency is locked how does the antenna vary its \lambda to receive the modulated wave ( at the new frequency)? Or does this antenna have a frequency tolerance? E.g. λ * 0.95 to λ * 1.05. The extra 0.05 accommodates the variation in frequency due to modulation?

Reference to how FM works: How does FM station have fixed frequency when FM modulation changes the frequency?

Edit: Specified the 2m Band.

• Are you speaking of the 88-108 MHz FM broadcast band, or something else? Please answer by editing your question. Aug 9, 2021 at 23:05
• @MikeWaters I actually meant in the frequency modulation in the 2m band. My apologies for the confusion. And thanks for point it out. I realised that I had FM instead of VHF in my question. I edited that too. Aug 9, 2021 at 23:14

Most VHF+ antennas are not that narrow banded. They usually have a low enough Q.

Thus, there is usually no need for a tunable antenna.

Also, antennas are generally not any different regardless of the modulation type.

All modulated signals, not just FM, have a bandwidth. The antenna has a bandwidth, which must be wider than the modulation of the signal you are using.

Note that receive bandwidth and transmit bandwidth may be different, but generally it is actually difficult to make an antenna with a bandwidth so narrow (for either) that it would be a problem with FM.

• Note that any type of modulated carrier has a non-zero bandwidth, so this result applies to all of them. Spread spectrum is possibly the extreme example. Aug 10, 2021 at 23:53

Or does this antenna have a frequency tolerance?

This is closest, but the antenna, itself, does not actually have a “frequency tolerance” (more usually called a bandwidth). Rather, the antenna's properties, including impedance/SWR, radiation pattern, and losses, vary with frequency, and the range of frequencies over which you can transmit are mainly determined by your transmitter's ability to cope with varying impedance of the antenna — i.e. how high a SWR it can transmit into.

If you read the documentation for an antenna, it may specify a frequency range, but that will always mean something like “the SWR will be no higher than [x] between frequency [a] and frequency [b]”. If you choose a higher acceptable SWR, the range becomes wider.

However, this is more relevant for tuning range than for the ability to transmit FM — typical antennas will have usable SWR over a much wider frequency range than any individual signal's bandwidth or FM deviation. Consider this: if they didn't, then you couldn't change your transmit carrier frequency without modifying the antenna, since a meaningful change of transmit frequency is one which is wider than an individual signal's bandwidth!

(Some types of antennas amateurs use, such as the “magnetic loop” antenna, have much narrower bandwidths to the point of requiring retuning — changing the antenna impedance to be favorable at the desired frequency — for frequency changes within a band. These antennas have their own tuning mechanisms designed for frequent use, unlike dipole or vertical antennas where the tuning mechanism is usually “cut to the correct length”.)

• While this isn't wrong, it is easy to make an antenna that has little or no impedance change over a very wide bandwidth. Also, antennas like the small loop can have an extremely narrow bandwidth, and must be tuned as the radio changes frequency. But for the general case, with most antennas, this answer is right. Aug 10, 2021 at 2:56
• @user10489 Thanks for pointing that out. I've updated my answer to mention the loop case. Aug 10, 2021 at 3:29

For reception, a random piece of wire will be a usable antenna, although an antenna tuned to the desired frequency will be more efficient. I am using Amateur 2 meter band (144 - 148 MHz) antennas to receive marine VHF signals in the 156 - 158 MHz range with no problems.

For transmission, properly tuned antennas are much more important, but most antennas will work efficiently over some range of frequencies - 2 meter antennas should work well over the full 144 - 148 MHz band.

• In other words, the receive bandwidth is typically much larger than the transmit bandwidth. Aug 10, 2021 at 22:53
• Hello Peter, and welcome to ham.stackexchange.com! Aug 11, 2021 at 0:09
• I don't know if this matters, but the 16kHz bandwitth of a 2m FM signal is about 0.01% of the transmission frequency. So, the antenna would have to be very hi-Q to have problems receiving. Aug 11, 2021 at 0:53
• @user10489 it is the receiver that is much more tolerant to changing antenna properties compared to transmitter. In most cases the transmitter will be quite unhappy with SWR=5, up to and including a permanent failure. One usually keeps SWR below 2 or even below 1.5-1.7 . OTOH, a receiver can work at SWR=10.0 with only few dB of its sensitivity lost. Aug 11, 2021 at 16:38
• @fraxinus: that's a different viewpoint than I was considering. The receive bandwidth of a small loop, while still larger than its xmit bandwidth, is not much larger. The cut off is very sudden, and you don't even receive static outside of that. The receiver has nothing to do with the antenna bandwidth at least in that case. Aug 11, 2021 at 19:51

Antennas dont actively compensate for FM deviation. Antenna Q is designed for the antenna application. The bandwidth or Q of communications antennas is fixed by the design to accomodate the range of frequencies expected. Of course tradeoffs are made between cost, and efficiency of the antenna. Besides, the width of the 2 meter band (144 to 148 mhz) is far wider than the FM deviation bandwidth used on the Amateur Radio Services, so its a non problem.

This might be of interest to you. A question on calculating antenna Q was posed here:

Is it possible to calculate the Q factor for an antenna type, given a frequency?