Question T8A12
From subelement T8 - T8A
Which of the following is a disadvantage of FM compared with single sideband?
Why is this correct?
FM receivers exhibit the 'capture effect' where only the stronger of two signals on the same frequency can be demodulated, suppressing the weaker signal entirely. SSB receivers can receive multiple signals simultaneously on the same frequency. This is a fundamental limitation of FM demodulation circuits, not a tuning or quality issue.
Memory tip
Remember that FM's capture effect is actually beneficial in some situations—it eliminates weak interfering signals automatically. But it also means you can't monitor multiple conversations or weak signals like you can with SSB's linear mixing characteristics.
Learn more
The capture effect stems from FM's frequency deviation detection method. When multiple signals are present, the receiver's discriminator circuit locks onto the dominant signal's frequency variations while suppressing others. This same mechanism that provides FM's excellent noise immunity also creates this limitation. SSB's linear detection allows multiple signals to mix audibly, enabling weak signal work and frequency monitoring that makes SSB preferred for DXing and contesting.
Think about it
Why do you think the capture effect might actually be desirable in some amateur radio operating scenarios, despite being listed as a disadvantage?