RF Power Measurements

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From: Craig LaBarge (74740.3166@compuserve.com)
Date: Sat Feb 11 1995 - 19:47:57 EST


Well, I made some progess on my power measurement discrepancies. (Remember
the two watches??) A friend of mine (N3LA) let me borrow a Bird Thruline
wattmeter and a Boonton 92EA RF voltmeter to play with. After some
experimenting, I found that the rather large discrepancy stemmed from the fact
that my MFJ wattmeter reads high and my RF voltage probe reads low!

I connected the Bird wattmeter in series with the MFJ cross-needle wattmeter
in my antenna tuner. I found that the MFJ was giving significantly higher
readings than the Bird meter. Unfortunately, the only slug I had for the Bird
was a 50 watt unit. Thus, resolution at the very bottom of the scale wasn't
all that great. But at least I knew the MFJ *was* reading high.

Next, I took some comparative measurements using the Boonton RF voltmeter and
my Heath RF probe/DVM across the dummy load. My DVM read consistently lower
than the Boonton, but was always well within 10 percent. This was encouraging!
Since the Boonton is an accurate lab-quality meter, I decided this would be the
"watch" I would go by.

While I was set up, I took readings with both meters on each band at
approximately 1 watt and 5 watts (as indicated by the Bird wattmeter). So now
I have some idea of the error in the Heath probe so I can come up with a fudge
factor if I really want to be fussy. I also used the Boonton to set the output
of my 40-40 for about 950 mW (which is how I got started with all this in the
first place). :-)

So it looks like my MFJ rigs aren't really putting out 4 - 5 watts after all.
It's closer to 3 - 4 watts. But, I guess MFJ uses their own wattmeters to
calibrate their transceivers! :-) The other thing I found out is that when I
thought I was making contacts with my 40-40 pumping 1.5 watts into my
rainspout, I was actually running QRPp! Not too shabby!

73, Craig WB3GCK

P.S. Speaking of rainspouts, I had my first gutter-to-gutter QSO last week on
30 meters with N4EJK in Georgia. He was running 60 watts into his gutter
(wow!). Still looking for that first 2-way QRP gutter-to-gutter QSO...


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