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I came across a weird case, I have a low power 440MHz repeater transmitter, which I thought to set at about 10 watts. I made this measurement using an accurate offset attenuator and service monitor.

Like usual, adding more lossy coax, more connectors, and greater lengths dropped the power as usual.

However, when I was trying to measure loss through one of the transmit cavities of a duplexer, I was surprised to see an increase in the output power, up to 10.9 watts!

I then connected directly to the little stub of coax which I was connecting between the cavity and measurement system, and the power went up to 12 watts.

Does this mean there is an impedance mismatch somewhere down the line? I understand I am not running the best coax (only LMR240), but I thought it would still all be 50Ω.

VIDEO HERE

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    $\begingroup$ Why not check the impedance with an antenna analyzer or similar? $\endgroup$
    – Phil Frost - W8II
    Commented May 26, 2017 at 18:38

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It has all of the hallmarks of a defective cable.

If the duplexer cavity alone caused the problem, I would attribute it to load impedance. But when you removed the cavity and daisy chained the two coax cables, the result suggests a different problem. Check for bad connections in the cables, mismatched impedances, etc.

You stated your service instruments are solid but if you find nothing else, I would also check the sampling load and its cable to the test set.

BTW - at 70 cm I recommend nothing but low loss, double (or more) shielded cable to improve rejection and lessen desense.

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