Date: Wed, 8 Jun 2011 11:22:11 -0400
From: Al Kozakiewicz <akozak@hourglass.com>
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.
### 66+6j ?? This implies the swr is aprx 1.5:1 What swr did you
measure into the dummy load on 20m? What is the swr on 160/80m.... and
again on 17-15-12-10-6m ? Sounds to me like it will just get worse as
freq goes up....and it's already out to lunch on 20m ! This is typ mfj
'engineering'. Take a simple concept.....then screw it up.
## sure, stray L and stray C will have an impact.... which is why it has to
be designed correctly. Any globar is non inductive. Now if mfj used an
inductive resistor.. [ I doubt it] then it would really be bad on upper hf.
If wide cu strap is used for all internal connections, you will eliminate the
stray L.... and probably be away ahead of the game.
## If worse comes to worse, get a real dummy load. Even the old heath dummy
load... or the mfj 1 gal dummy load will give u an almost perfect 50 ohms and
no reactance up to 10m...and probably higher.
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.
## My hb dummy load has better swr than that mfj unit. Mine consist of 4 x
3" OD thick wall AL tubes welded together. A 200 ohm resistor is inside
of each of the 12" long t 7=ubes. The entire mess sits suspended from the
lid of a 7 gal paint drum. 7-16 female chassis din sits dead center in the
top lid. It's like the heath dummy load..on steroids.
## did u read any reviews on the mfj 300w dummy load. I'm guessing either
[A] you got a defective unit.....or [B] they are all defective due to a
design flaw.
## flash..per e-ham etc. The mfj- 260-C is flat swr to 10m...and only
1.5:1 between 30-650 mhz. Rated for 300w for 2 mins. Rated for 25 w CCS.
It's just a 25 w dummy load..in air.
Later... Jim VE7RF
Any ideas?
TIA
Al
AB2ZY
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