Question T8A04
From subelement T8 - T8A
Which type of modulation is commonly used for VHF and UHF voice repeaters?
Why is this correct?
FM (Frequency Modulation) and PM (Phase Modulation) are the standard modulation types for VHF and UHF voice repeaters because they provide clear, noise-resistant audio quality suitable for local communications. AM has poor noise performance, SSB is used for weak-signal work over long distances, and PSK is a digital mode. FM/PM maintain constant amplitude while varying frequency or phase to encode audio information.
Memory tip
Look for frequency ranges in the question - VHF/UHF typically means local/regional communications where FM excels, while HF suggests long-distance work favoring SSB. The application context (repeaters vs. weak-signal) is your key pattern recognition tool.
Learn more
VHF/UHF repeaters use FM/PM because these modes excel at local coverage with good noise immunity. A typical VHF repeater FM voice signal occupies 10-15 kHz bandwidth. The constant amplitude characteristic of FM signals makes them ideal for repeater infrastructure, where consistent signal levels help maintain reliable automated operation. This contrasts with SSB's 3 kHz bandwidth, which is better suited for weak-signal work.
Think about it
Why do you think repeaters specifically benefit from FM's constant amplitude characteristic compared to the varying amplitude of AM or SSB signals?