The Field Day 40 m Amplifier Meltdown
You are operating a 40 m SSB station during a multi-op event. Your exciter outputs 100 W into a solid-state linear amplifier rated for 1.5 kW PEP output at 50 ohms; you run it at 1 kW PEP. The antenna is a dipole fed with 30 m of RG-213, and the amplifier's built-in SWR meter reads 1.4:1. After 20 minutes of rapid CQing, the amplifier trips on high reflected power, and the SWR meter now reads 3:1. You bypass the amp and the exciter sees 1.2:1. A second operator suggests you simply reduce drive and keep calling, since 'the tuner will handle it.' You have a spare 30 m length of RG-213 and a known-good 1 kW dummy load.
1. Diagnose the fault and decide whether to keep operating through the amplifier.
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Key evidence: the fault is power-dependent. At 100 W (exciter only) SWR is 1.2:1; at 1 kW it is 3:1 and the amp trips. A feedline or connector that arcs or heats under high voltage/current will look fine at low power and fail at high power. This points to the antenna system (connector, balun, or a coax fault) - not the amplifier's SWR meter, since the exciter's reading corroborates a real change. The suggestion to 'reduce drive and keep calling' is wrong: the reflected power and the arc will persist and may damage the amp, the balun, or start a fire in a connector. Do not keep transmitting into the suspect antenna at 1 kW. Substitution is the reliable test: 1) Put the dummy load on the amp output; if SWR stays low at 1 kW, the amp is fine and the antenna/feedline is at fault. 2) Swap the spare 30 m coax; if SWR normalizes, the original coax/connectors are bad. 3) Inspect the balun and connectors for heat, charring, or water. Only resume full power after the cause is found and corrected.
2. If the amplifier is found good and the coax is good, what is the most likely remaining culprit, and how would you verify it without test equipment beyond a VSWR meter?
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The remaining suspect is the balun or antenna connection at the feedpoint (or a high-resistance joint in the dipole). These are the components that see the highest RF voltage and are prone to arcing, corrosion, and moisture ingress. Verification with only a VSWR meter: 1) Measure VSWR at low power (5–10 W) - it will likely look acceptable. 2) Raise power in steps (100 W, 250 W, 500 W) and watch for VSWR to climb or for the meter to become erratic. A VSWR that is low at low power and rises with power is the classic signature of an arcing or heating connection. 3) Physically inspect and re-solder/re-tape the feedpoint and balun; replace the balun if it is a voltage-type that saturates at 1 kW. A current balun rated for the power and a weatherproofed feedpoint are the corrective actions.