Overdeviation on a Coastal Station's VHF Working Channel
A small marine public coast station operates a 50 W VHF base station licensed for 50 W output. Boaters in the harbour complain that whenever the coast station transmits on channel 22A their conversations on the neighbouring channel break up, and one reports hearing the coast station faintly two channels away. A technician connects a service monitor to the transmitter through a 30 dB pad and a dummy load. With a 1 kHz tone at full audio input, peak deviation reads 6.8 kHz. The microphone gain control is at maximum and the internal compressor threshold has been set to the top of its range. With no modulation the carrier measures 157.1004 MHz against an assigned 157.1000 MHz, and the transmitter delivers 60 W into the dummy load.
1. How much must the deviation be reduced, and why does excessive deviation cause interference on other channels?
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Peak deviation must not exceed 5 kHz. At 6.8 kHz the peak is over by a factor of 6.8/5.0 = 1.36, which is 20 log₁₀(1.36) ≈ 2.7 dB of audio gain reduction - and that is only the minimum, because a steady 1 kHz tone at full input is a laboratory condition. Speech peaks, the compressor's attack time and a loud talker will push deviation higher in service, so aim for about 4–4.5 kHz on the tone so that speech peaks stay inside 5 kHz.
Why it splatters: an FM signal's occupied bandwidth grows with deviation. Carson's rule gives roughly 2(Δf + fm).
- At 5 kHz deviation and 3 kHz maximum audio: 2(5 + 3) = 16 kHz, which fits the marine VHF emission design and the 25 kHz channel spacing.
- At 6.8 kHz: 2(6.8 + 3) ≈ 19.6 kHz, and the sidebands and noise skirt extend beyond even that, into the adjacent channel.
That is precisely what the boaters describe - the neighbouring channel breaks up whenever you transmit. The fix is a deviation alignment, not a power adjustment: set the mic gain and compressor threshold for correct peak deviation, check the audio response across 300–3,000 Hz, and re-measure at normal speaking level with a modulation meter.
2. Evaluate the carrier offset and the 60 W reading. Which of the three findings actually explains the complaints?
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Carrier offset: 0.4 kHz at 157.1 MHz is 0.4/157.1 ≈ 2.5 parts per million, comfortably inside the frequency tolerance for VHF marine equipment. Record it and move on - it is not causing the trouble and chasing it wastes time that belongs to the deviation alignment.
Output power: 60 W against a 50 W licensed output is about 0.8 dB over. Turn the transmitter down to 50 W or below. The extra 0.8 dB buys almost nothing in range - VHF coverage is limited by antenna height and terrain, not by the last watt - and it is not worth a notice of violation or an enforcement letter.
Deviation: this is the fault that explains the complaints. A signal occupying 19.6 kHz in a 25 kHz channel plan, with sidebands spilling into the adjacent channel, will be heard by receivers that are working perfectly. It also explains the report of being heard two channels away, since the higher-order components of an over-deviated signal extend well beyond the Carson estimate.
Order of work: confirm deviation into the dummy load first (that isolates the antenna), set the limiter and mic gain, then re-check on the real antenna, and finally confirm harmonics and spurious outputs with the service monitor while still on the dummy load.
3. What operating decisions should the station make while the fault exists, and what should be logged?
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Interim operation: do not keep transmitting a signal outside its technical limits on a working channel and assume it will be tolerated. If the station's procedures and licence allow it to stay on air, reduce microphone gain enough to bring peak deviation inside 5 kHz as a documented interim measure, and log that reduced setting. If the deviation cannot be brought inside the limit, take the transmitter off air for everything except distress and urgency traffic, arrange a replacement or loan set, and tell the licensee. Interference to channels used by other stations is not the complainants' fault, and it is not a receiver problem just because only other people notice it - confirm the report with a second receiver before dismissing it.
Logging: every complaint (time, channel, what was heard, who reported it), every measurement (deviation, carrier frequency, forward and reflected power), every adjustment and the time the station returned to normal service. If a complaint escalates to the licensing authority, that log plus the service-monitor printouts are the difference between 'we found and fixed it' and an unresolved enforcement matter.
Also check the antenna and its joints while the transmitter is on the dummy load. Overdeviation is the likely cause here, but a corroded antenna joint or a failing PA can generate out-of-band products that would explain a report of being heard two channels away.