2
$\begingroup$

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

$\endgroup$
  • 1
    $\begingroup$ Why not check the impedance with an antenna analyzer or similar? $\endgroup$ – Phil Frost - W8II May 26 '17 at 18:38
2
$\begingroup$

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.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.