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Re: [Amps] PEP Wattmeter

Subject: Re: [Amps] PEP Wattmeter
From: Rob Atkinson <>
Date: Fri, 2 Oct 2009 15:27:58 -0500
List-post: <">>

don't hook the scope directly to the feedlne.  you need some kind of
tap.  One quick trick is if you have a good coax switch (one of
sufficient isolation--probably anything not MFJ as I have heard they
have poor isolation) run the amp output feedline into the "antenna"
side of the switch and the dummy load on one of the switched
positions.  switch it through so the amp is going to the dummy load
through that position.   hook the scope to an unused position i.e. if
the dummy load is on 1 put the scope on 4 if it's a 4 position switch.
 The switch makes a quick tap off the feedline.  Drive the amp to some
reading on your wattmeter, say 500 watts since that squared with the
Bird.  With the trace on center, adjust the volts / div. on the scope
so you have a cw (an unmodulated carrier or cw note) trace two
divisions above and below the center.  now you know that pep is 125 w.
per division.  now slow the trace down to a dot slowly moving left to
right and modulate or what ever you want to do to measure the pep when
you are transmitting information and note the no. of divisions the dot
(now a vertical line) extends.  You should be able to compute your pep
based on what you saw with the scope referenced to the 500 watt
measurement.   Obviously this is only as accurate as your wattmeter
which is why I suggested the RF amp meter earlier.  If you don't have
a switch, you can try something else like running coax to the feedline
from the scope and terminating it in a small dipole, each side of
which is coiled around the amp to dummy load coax multiple times.  The
only important thing in a pinch is keeping the scope input protected
but having enough stable RF pickup.   If you have time you can make a
good pickup by putting two UHF jacks, one on each end of a small
project metal box and running no. 10 or 12 solid copper between the
centers, for the feedline, then put a bnc or uhf jack on one side
orthogonal to the copper wire.  attach a small L shaped probe wire to
the BNC jack center to serve as a shielded pickup that isn't touching
the feedline wire  and run a line from the BNC to the scope input.
Make the L that's parallel to the feedline wire about 1/2 inch to an
inch away from it.  It needs to be only 1 or 2 inches long parallel to
the feedline wire.


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