Power measurement and licence compliance on 2 m
An operator builds a 2 m amplifier using a surplus module rated at 100 W output. The licence permits 100 W PEP at the antenna for the class held, but the operator measures 130 W at the amplifier output with a power meter reading into a 50 ohm dummy load. The antenna has 2 dB of feed-line loss and 8 dBi gain. The operator also notices that the meter reads differently when the load is reactive and that the SWR meter shows 1.8:1 on the real antenna. A second station reports splatter on adjacent channels.
1. Is the station compliant? Work through the numbers.
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The licence limit applies at the antenna, so first convert the measured output to antenna power. With 2 dB of feed-line loss, the power factor is 10^(-0.2) = 0.63, so 130 W at the amplifier becomes about 82 W at the antenna. That is within a 100 W limit on its face.
However, three things need checking. First, the meter reading into a dummy load may not be accurate: many meters are calibrated for a particular waveform, and PEP readings on SSB depend on the meter's ballistics. Second, the splatter reports suggest the amplifier is being overdriven or is non-linear, producing intermodulation products and excessive bandwidth even if the carrier power is legal. Third, the 1.8:1 SWR on the real antenna means the amplifier is not seeing 50 ohms, so the power delivered and the stress on the device differ from the dummy-load test.
So compliance is not established by the 130 W figure alone. You must confirm the meter's calibration, check the emitted bandwidth and intermodulation, and measure at the antenna or account correctly for line loss. A station can be under the power limit and still be non-compliant because of splatter.
2. Why does the meter read differently on a reactive load, and what does that imply for the SWR reading?
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A power meter measures voltage and current samples and computes forward and reflected power assuming a 50 ohm resistive system. When the load is reactive, the voltage and current are not in phase, so the meter's directional coupler no longer separates forward and reflected power cleanly; the reading depends on the phase of the reflection and on the meter's directivity. The indicated 'forward' power can be too high or too low, and the indicated SWR can be wrong.
The 1.8:1 SWR reading on the real antenna is therefore only approximate, and the true impedance may have a significant reactive part. For compliance you should measure with a calibrated instrument that reports complex impedance, or use an antenna analyser at the antenna, and you should not trust a simple cross-needle SWR meter for a legal power claim.
The practical implication is that you tune the antenna for a low SWR and confirm with a trustworthy instrument, and you keep the amplifier within its ratings rather than relying on a meter that may flatter the result.
3. What would you do about the splatter reports?
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Splatter on adjacent channels is a sign of non-linearity: either the amplifier is being overdriven by too much drive, or it is running beyond its linear range, or the supply is sagging on peaks. Reduce the drive until the output falls by a few dB and listen again; a clean signal at 80 W is better than a dirty one at 130 W.
Check the supply voltage and current at full PEP, and check the bias if the amplifier is a linear design. Monitor the transmitted signal on a separate receiver with an attenuator, or use a spectrum analyser to see the intermodulation products and the occupied bandwidth. Compare the two-tone test result with the licence's spurious and bandwidth requirements.
If the module is a Class C design intended for FM, it will splatter on SSB no matter what you do; in that case use it only on FM or replace it. Document the measurements and keep the drive at a level where the signal is clean, even if that means well below the nominal 100 W.