Al Kozakiewicz wrote:
> 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?
Sure, try 6 OTHER Ohm meters on the load and the cable.
--
Ron KA4INM - Every HAM should have at least one desk MIC,
it's the perfect place to hang the cans when
CW is "arm chair copy."
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|