10 Mar
2002
10 Mar
'02
8:10 a.m.
On Sat, 9 Mar 2002; "Davison, Lee" <Lee.Davison(a)merlincommunications.com>
wrote:
> 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!
No it's not. I know of a couple technicians who goofed and caused
it to happen.
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.
I agree with what you say here, even for the vintage of the
transmitters. They were built in the 40's & 50's. It was
even fascinating to see how the technology worked. To modulate
a portion of the RF with low frequency (ranging from 30 to 90 hz)
there was two RF coupling loops with a specially designed fan
blade spinning between the loops. This changed the amplitude
of the RF coupled from one loop to the other.
You can probably imagine the fascination of some technicians
when we swapped out this vintage tube type monsters with
state-of-the-art-neato solid state units. Got questions from
the some of the old timers 'Duh, it taint got no moving parts, how
does it modulate the carrier?'.
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.
Because of the vintage of the equipment and operating in the
VHF band (108-118 mhz), we had special scopes that that had
a special RF input section that could handle a few watts. This
was what we used to check the modulation pattens and the like.
> 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?
Yes, it was required for use with the scope.
can you remember the make/model of these transmitters?
Nope, I have slept since then. They were all special designed for
the government. In fact when they were replaced with solid state
units (1980's), we had orders to destroy the old ones. They did
not want them on the surplus market where they might show up in
operation later.
I do remember now, one model was built by Wurlizter (sp?), yep, the
electronic organ folks.
And now, I would like to apologize to the list. While this meets (and
exceeds greatly) the 10 year guideline, it would not by any stretch of
the imagination 'compute'. I was simply trying to point out the need
for proper termination of a circuit, regardless of where the circuit is
and what it does. Forgive me.
Mike