[Amps] Dummy load varying R

[Robert Groh]

No real component is ever (well, almost never) 'pure' - there are always 
'strays' creeping around - series inductance, shunt capacity, etc.  


For example, a 1 inch piece of 16 gauge copper wire has an inductance of about 
0.018 uH which is about 1.6 ohms at 14 MHz.  So even a nice, pure 50 ohm 
resistor with a 1" wire connected will be 50+j1.6 at 14 MHz.  


And an actual '50 ohm load' will have internal parallel and series capacity and 
inductance to further confuse the matter. Oh, and don't forget tolerance. Your 
load is 50 ohms nominal plus or minus something.  And you will probably find the 
manufacturer specifies a certain VSWR for the load which means that the input Z 
can vary anywhere inside that VSWR circuit.  


And let's complicate things even more - you say you measure a certain R and X.  
How accurate is your measuring instrument?  I doubt if it is very accurate 
itself.  And is your measuring instrument in calibration?  Probably not.

All that being said, I know from 50+ years of RF design experience that trying 
to tie all of this down is quite difficult .... and, probably, for ham use, 
unnecessary. 


So, to answer your question (a bit) - yes, it can be difficult. 

73
Bob, WA2CKY

P.S. The addition of the cable will change things but should just (for that 
short a cable) just shift the impedance while having very little effect on the 
actual VSWR.  







________________________________
From: Al Kozakiewicz <akozak at hourglass.com>
To: "amps at contesting.com" <amps at contesting.com>
Sent: Wed, June 8, 2011 10:22:11 AM
Subject: [Amps] Dummy load varying R

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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