This seems a easy one. The load is not pure resistance. It has resistance
with reactance which was measured at +6j. Reactance resistance changes with
frequency. a opposite reactance would cancel it out at the frequency of
interest.
john
W9ZY
.... Commander John ....
--- On Wed, 6/8/11, Al Kozakiewicz <akozak@hourglass.com> wrote:
From: Al Kozakiewicz <akozak@hourglass.com>
Subject: [Amps] Dummy load varying R
To: "amps@contesting.com" <amps@contesting.com>
Date: Wednesday, June 8, 2011, 10:22 AM
Maybe not quite amps related, but this is the best forum I know of for RF
expertise ...
Without belaboring irrelevant details, I'm calibrating an LP-100A wattmeter I
just built. With a brand new MFJ 300W dummy load I measure 50.1 ohms with my
old Fluke DMM. The dummy load is connected to the coupler with about 6 feet of
RG-8U. It appears as though the resistance of the dummy load increases with
frequency, to the point where it is 65 ohms in the 20m band and the LP-100 can
no longer compensate.
After fiddling with the coupler with no joy I decide to connect my MFJ-259 to
the cable and verify that the resistance is actually 50 ohms across all the HF
bands. Turns out that it's not. In the 20m band the impedance at the end of
the RG-8 has risen to 66+6j.
This is as simple a setup as I can imagine - a 50 ohm resistor in a metal box
and 6 feet of new 50 ohm coax cable. Tonight I will test again without the
cable if I can find a M-M UHF adapter. But I'm puzzled as to what the issue
could be.
Any ideas?
TIA
Al
AB2ZY
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