FM Bandwidth

The Technician question pool (2022-2026) asks you to identify FM bandwidth:

T8A09: What is the approximate bandwidth of a VHF repeater FM phone signal?

A. Less than 500 Hz B. About 150 kHz C. Between 10 and 15 kHz D. Between 50 and 125 kHz

This question gets at an important characteristic of FM signals, so let's consider it carefully. Let’s start by picking apart this question for interpretation and definitions. Then we’ll get to the particulars of the correct response options.

Bandwidth: The question is asking about bandwidth. The bandwidth of an RF signals is the range of frequencies (RF spectrum) used to carry information. It is a range of radio frequencies transmitted or received for which the power is not zero. An RF signal typically utilizes a substantial range of radio frequencies to carry information such as a set of audio frequency signals representing an operator’s voice.

When you push-to-talk and speak into the microphone your FM transmitter emits a range of several thousand hertz of different frequencies and not just that singular frequency value to which you have tuned the transceiver. That displayed frequency value may be considered a reference value called the carrier frequency, and with FM the emitted signals will vary in frequency both higher and lower than that carrier frequency value by several thousand hertz. The full range of the frequencies emitted, as determined by the highest frequency value minus the lowest frequency value, is the bandwidth.

FM signal frequency deviates higher and lower than the displayed carrier frequency value to determine the bandwidth of the FM signal. Bandwidth is the range of frequencies used in an RF signal.

For example, suppose you are tuned to the 2-meter FM phone band carrier value 146.520 MHz and you make a call, stating your call sign. Suppose as a result that the FM transmitter emits signals representing the modulated audio of your voice from 146.526 MHz down to 146.514 MHz. The bandwidth of the signal is 146.526 – 146.514 = 0.012 MHz, or 12 kHz.

VHF Repeater FM Phone Signal: The question specifies a “VHF repeater FM phone signal.” The VHF portion indicates operations on the 6-meter, 2-meter, or 1.25-meter bands. Beyond the fact that FM phone signals are commonly used on the three VHF amateur bands this information is mostly irrelevant. The fact that it is a repeater signal is also irrelevant. The fact that it is phone mode is very relevant! Let’s see why.

The frequency deviations of the FM signal are determined by the amplitude of the modulating signal (audio signal for phone mode). Note how RF frequency is higher with +V modulating signal values and lower with -V modulating signal values.

With FM the amplitude of the modulating signal determines the magnitude of the frequency deviations from the carrier frequency. For phone modes the modulating signal is the audio signal generated by the microphone, and the amplitude of these audio signals represents their power. The power of the audio, and hence the amplitude of the audio signal, is determined by the combined effects of the loudness of your voice into the microphone and the microphone’s audio amplifier circuit.

The bandwidth consumed by an FM phone signal becomes wider as the modulating audio amplitude becomes greater, as happens when the sound of the operator’s voice is loud.

The upshot of these effects is that as you speak louder into the microphone and increase the amplitude of the modulating audio signals, the FM frequency deviations from the carrier value increase. As the frequency deviations increase your transmitter emits a broader range of frequencies. That is, your FM signal has greater bandwidth. So, scream into your microphone and your bandwidth gets very wide (within some limits), or whisper into the microphone and your signal is of much narrower bandwidth. Remain completely silent and your bandwidth drops to almost nothing. If your screaming results in the modulating circuit exceeding normal FM bandwidth limits, you are over-deviating and your signal may be distorted or cause interference to adjacent phone channels in the band. However, most modern transceivers have nice RF limiter circuits that help to avoid FM overdeviation in transmissions. (However, distortion will also be imposed by over-driving the microphone amplifier, "clipping" the waveforms of the audio signals that are modulating the FM signal.)

FM Bandwidth: From the discussion above you can see that the bandwidth of an FM phone signal will vary from moment to moment depending on the loudness of the operator’s voice. This is controlled by speaking distance to the microphone and the loudness of the voice. The amplification setting of the microphone will also impact the resulting amplitude of the audio signals fed into the FM modulator circuit, and most radios provide for operator adjustment of the amplifier to help obtain good modulation for a variety of voice characteristics. Simply asking your fellow hams on the air how your FM audio sounds is the best way to judge your own need to increase or decrease your voice volume or speaking distance to the microphone.

The typical amateur radio FM signal bandwidth varies from about 10 kHz to 15 kHz as a result of the characteristics of FM transmitter engineering for amateur radio use. A good estimate of an FM signal’s bandwidth can be obtained using Carson’s Rule. Carson’s Rule is a simple calculation using the transmitter’s engineered peak frequency deviation value and the highest modulating frequency (highest audio frequency in the phone mode case). Carson's Rule is covered in the General License Course.

The answer to Technician Class question T8A09, “What is the approximate bandwidth of a VHF repeater FM phone signal?” is “C. Between 10 and 15 kHz.”

-- Stu WØSTU