by Nathan M0NXD — What happens when your mic gain is too high on FM
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Deviation Meter
Signal Info
Understanding FM Splatter
FM Deviation: In FM, the carrier frequency shifts proportionally to the audio signal amplitude. The peak deviation is the maximum frequency shift from the carrier.
Allowed Deviation: Wide FM (25 kHz channels, emission 16K0F3E) allows ±5 kHz max deviation. Narrow FM (12.5 kHz channels, emission 11K0F3E) allows ±2.5 kHz. Exceeding these limits causes overdeviation.
Pre-emphasis: All FM transmitters apply +6 dB/octave pre-emphasis from 300 Hz, boosting higher audio frequencies before the modulator. This improves the signal-to-noise ratio after de-emphasis in the receiver, but means high-frequency audio content (sibilants, noise) contributes more to deviation — a key cause of splatter.
Guard Bands: Wide FM occupies ~16 kHz (Carson BW) within a 25 kHz channel, leaving ~4.5 kHz guard each side. Narrow FM occupies ~11 kHz within 12.5 kHz, leaving only ~0.75 kHz — which is why overdeviation on narrow FM is so destructive.
Carson's Rule:BW ≈ 2 × (Δf + f_mod) contains ~98% of the FM signal energy. Sidebands beyond this are typically >20 dB below carrier but can still cause adjacent-channel interference.
Why Splatter Is Bad:Condition 6(2) of the UK Amateur Radio Licence Conditions Booklet (OFW611) requires that radio equipment is “designed, constructed, maintained and used so that its use does not cause any Undue Interference to any wireless telegraphy”. Ofcom’s Amateur Radio Guidance (Section 4.1.1) further states that equipment should be “as stable and as free from unwanted emissions, as the technical development for the radio equipment permits”, and references CEPT ERC Recommendation 74-01 (Table 16, Annex 6) for specific spurious emission limits. Overdeviation causes energy to spill into adjacent channels, interfering with other stations — even at distances beyond 25 km in line-of-sight conditions.
Narrow FM Is More Susceptible: With only 12.5 kHz channel spacing and ~0.75 kHz guard bands, even mild overdeviation causes adjacent-channel interference. The RSGB recommends narrow deviation on 2m FM simplex to match 12.5 kHz channel plans.
Real-World Tips: Set your mic gain so the deviation meter peaks at (but doesn't exceed) the rated deviation with normal speech. Speak across the mic, not directly into it. Even with mic gain set correctly, shouting or holding the mic too close (simulated by the Audio Volume slider) will cause overdeviation. Some operators recommend limiting total deviation to ±4.5 kHz on wide FM to allow margin for CTCSS tones (~750 Hz deviation).