As an example, I use to work on some navigational
transmitters with power output of a few hundred watts.
There was a test jack for sampling the RF output and you
hooked a scope to the jack. The gotcha was when you
hooked the coax cable to the transmitter first instead of
the scope. If you did that, the transmitter went down
within seconds.
This is rot! On a transmitter with even a few watts output the
RF probe coupling would be -10dB or more, so even an open
or short on this would give a return of -20dB. This is so
small it can be ignored, in fact many antenna systems aren't
that good.
As a termination a scope is a very poor match for any low Z
RF source it's impedance being 1Mohm or more so it
wouldn't matter if you plugged it in or not. Also most test
gear can't absorb any ammount of watts for any length of
time, so if the port was a high power snif it would have to be
terminated at the port with a high power attenuator which
gives a good match regardless of it's terminating Z.
The open circuit at the other end of the coax got
reflected back to the transmitter as a low
impedance and detuned it, and the monitoring circuits
would detect the detuning and pull the plug.
This depends entirely on the wavelength length of the
coax. Did you use an exact odd multiple of 1/4 wavelength
coax every time?
can you remember the make/model of these transmitters?
Lee.
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Received on Sat Mar 09 2002 - 11:55:25 GMT