[Amps] Dummy load varying R

[Carl]

That is making the assumption stretch that the measuring instrument is 
correct. As I said earlier the 259 has problems with the digital display 
readout section and we havent heard back yet what the analog VSWR meter on 
it reads.

Carl
KM1H


----- Original Message ----- 
From: "Commander John" <crazytvjohn at yahoo.com>
To: <Amps at contesting.com>
Sent: Thursday, June 09, 2011 4:11 PM
Subject: Re: [Amps] Dummy load varying R


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 at hourglass.com> wrote:


From: Al Kozakiewicz <akozak at hourglass.com>
Subject: [Amps] Dummy load varying R
To: "amps at contesting.com" <amps at 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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