The Basic lessons · Station assembly, practice and safety · B-003-013

Understanding FM: Deviation, Noise, and the Capture Effect

This lesson explains how Frequency Modulation (FM) works, why it sounds clear on VHF bands, and how to set your equipment correctly to avoid distortion. You will learn the specific terms for frequency changes and the rules that keep your transmissions legal and effective.

This lesson is part of SignalPrep Pro. The subsection's 11 exam questions are free to practise: practise B-003-013.

What this subsection asks

• What causes the loud noise heard from an FM receiver in the absence of a signal?
• You are using an FM repeater configured for 5 kHz deviation, but your transmitter is set to 2.5 kHz deviation. What is the consequence?
• What term defines the change in frequency caused by modulation in an FM transmitter?
• What kind of emission would your FM transmitter produce if its microphone failed to work?
• Why is FM voice best for local VHF/UHF radio communications?
• What is the approximate bandwidth of a frequency modulated signal using 5 kHz deviation?
• How is a higher level of the modulating signal represented in an FM signal?
• What modulation method is most closely related to frequency modulation?
• Why isn't FM used as an amateur radio emission mode below 28 MHz?
• Several stations report that your FM transmission is loud and distorted, but on frequency. Which of the following is the most probable cause of the distortion?
• When more than one signal is present, the FM receiver is likely to demodulate only the strongest signal. What is this behaviour called?