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@hourglass.com>
To: "amps@contesting.com" <amps@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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