An accurate way to use a scope is to use a Bird directional coupler which
plugs into a 43. The one I use has -50dB directivity and is used for
spectrum analyzer testing of amps. It was about $20 off Ebay a few years ago
as the great unwashed hordes have no idea what it was.
You can also make your own by modifying any cheap wattmeter and adding fixed
attenuators if needed. Use a Bird or other accurate meter to calibrate and
then use a scope with the proper probe.
Carl
KM1H
----- Original Message -----
From: "Rob Atkinson" <ranchorobbo@gmail.com>
To: <amps@contesting.com>
Sent: Friday, October 02, 2009 4:27 PM
Subject: Re: [Amps] PEP Wattmeter
> Paul,
>
> 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.
>
> 73
>
> Rob
> K5UJ
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