So I replaced the transistor and connected a 5.5Ω in series with a 100nF cap across the transformer. I then used the nanoVNA to look back to the collector from the output port. Varying the collector voltage from zero, and then 8.8v to 16v showed almost no shift in the return loss.
However, adjusting the bias for 2A of collector current really shifted the return loss curve.
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| Idle current - bottom curve 2Amps top curve |
Given the collector voltage was largely unchanged, the transistor capacitance from the collector to base and emitter would have been relatively constant. So the only explanation left is the current changed the transformer's characteristics.
Next step is to replace the transistor yet again (smoke escaped) and try a larger transformer.
73's
Richard
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