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From: Hardie@MIT-MULTICS.ARPA (Maj. Douglas Hardie)
Newsgroups: net.ham-radio
Subject: RF Probes
Message-ID: <1647@brl-tgr.ARPA>
Date: Sun, 22-Sep-85 19:39:16 EDT
Article-I.D.: brl-tgr.1647
Posted: Sun Sep 22 19:39:16 1985
Date-Received: Wed, 25-Sep-85 10:03:22 EDT
Sender: news@brl-tgr.ARPA
Lines: 31

I was reading the Hints and Kinks in May 85 QST, article on Match Your
RF Probe to Your Meter.  There were several things I believe misleading
and possibly wng in the article and the associated Editor's comments.

The calculations given for a 11M or 10M meter are fine, except that the
meters I am familiar with have a 1M resistor in the DC probe.  Since
these RF probes replace the DC probe, the meter no longer has a 11M
impedance.  Hence it seems that one calculates the proper resistance for
the dividor using the 11M for the meter, but then must add 1M to
compensate for the missing probe resistor.  When I do that for a 11M
meter, I get the probe resistor of 5.556M for which I believe there is a
standar 5.6M resistor that should be close enough.  The Heathkit probe I
have that was "designed" for my meter ignores the 1M resistor and uses a
4.7M divider resistor also.

Also the orientation of the diode in fig 1 seems to imply that the probe
will put out a positive DC voltage.  However, I find that it puts out a
negative voltage.  My Heathkit probe has the diode reversed and a quite
differnet explination of its workings.  It works, and no amount of
playing with fig 1 or 2 made it put out a positive voltage.

The voltage divider (fig 3) in the Editor's comments I have tried and
found very unsatisfactory.  The capacitor C is effectively accross the
bottom resistor in the dividor.  Hence the division ratio is complex and
frequency dependent.  I tried to make that scheme work with two fixed
resistors to give a 1/10 ratio and found that the same power output into
a dummy load on 80 meters gave a very different probe voltage than on 10
meters.  If I remember right, it was about a 50% variation.  I never
found a completely acceptable way to solve the divider problem, but ran
across an article which recommended making a wire twist capacitor across
the top resistors in the dividor to compensate.


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