[Amps] Dummy load varying R

[Al Kozakiewicz]

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