Question T8A02
From subelement T8 - T8A
What type of modulation is commonly used for VHF packet radio transmissions?
Why is this correct?
FM or PM are the standard modulation types for VHF packet radio because they provide excellent noise immunity and reliable digital data transmission. The digital information modulates the FM/PM signal by shifting between specific audio frequencies (like 1200 Hz for mark and 2200 Hz for space in 1200 baud packet). SSB, AM, and PSK are not commonly used for VHF packet radio due to poor noise performance or incompatibility with typical VHF equipment.
Memory tip
Pattern recognition: VHF/UHF applications almost always use FM or PM for both voice and data. When you see 'VHF' combined with any common amateur application (repeaters, packet radio, APRS), FM/PM is typically the answer. This frequency relationship between bands and modulation types is consistent across many exam questions.
Learn more
VHF packet radio uses FM/PM because most VHF transceivers are designed around FM modulation. The digital data doesn't change the fundamental modulation scheme—instead, it uses audio frequency-shift keying (AFSK) where different audio tones represent digital 1s and 0s. This allows standard FM radios to carry digital data without modification. Understanding this layering concept—digital encoding within analog modulation—helps explain why packet radio, APRS, and other VHF digital modes maintain compatibility with existing FM infrastructure.
Think about it
Why do you think VHF packet radio uses audio tones within FM signals rather than directly modulating the RF carrier with digital data?