On Sat, 15 Nov 2003 22:05:45 GMT, "Dale Parfitt"
wrote:

|
|"Paul Burridge" wrote in message
.. .
|
| Hi gang,
|
| I've never had a lot of luck with GDMs for some reason. Even with a
| decent meter, it seems such a drag tuning across such a vast range
| looking for a tiny, easily-missed dip which you have to screw out of
| the meter by forcing the sensing coil so far into the circuit
| concerned you practically break the circuit board. Am I alone in
| finding this potentially invaluable device practically useless in
| practice? Is there a more viable alternative?
|
| p.
| --
|
| First rule is to get a good dip meter- the stuff made for the amateur
|community is very poor- the Eicos, Heath Millen etc. Pick up a Measurments
|model 59. With this meter you can take a 1/2 wave wire- say at 2M and hold
|the meter a couple inches from the center and see a huge dip. Other meters
|don't even respond when held to the wire. Dips on conventional L-C circuits
|can easily be full scale.

Yep. I have two 59s and they are great. I got rid of two Millens.
They are better than anything else but the 59.

As to usefulness, a short war story. Another engineer and I were
working on an AGC problem in the early Phoenix missile i-f amplifier.
Phoenix being a monopulse radar had a three channel receiver with very
tight agc tracking requirments (both gain and phase). The agc voltage
was fed to each of the three channels via feedthru caps in the walls
of a very well shielded and gasketed chassis. Nevertheless, there was
obvious crosstalk.

The other guy said to me, "Wes, do you have a GDO?"

I said, "Sure."

A couple of younger engineers who were watching this asked, "What's a
GDO?"

So we poke the coil of the Model 59 into the chassis and find a nice
resonance at the i-f in the agc wiring. The feedthru capacitance and
wiring inductance were resonating at i-f. We had millions of dollars
worth of test equipment in our lab and I doubt that we could have
devised a test for this without heroic efforts.

The only problem was keeping the GDO hidden from the metrology guys
since they couldn't "calibrate" and service them so they wanted them
gone.

Wes N7WS