[Amps] Dummy load varying R

[Roger (sub1)]

On 6/9/2011 2:05 AM, Jim Thomson wrote:
> Date: Wed, 8 Jun 2011 11:22:11 -0400
> From: Al Kozakiewicz<akozak at 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.

A 14 or 16" globar becomes inductive rather rapidly due to it's physical 
length as you go up in frequency.

>   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.
>
>
The Heath and other copies stayed close to being non reactive up through 
their range due to the very small size of the resistor which was oil 
cooled and inside a coaxial sleeve open on both ends to aid oil flow 
(IIRC).  I used one recently to check out a 2-meter KW, although it was 
time limited at full output.
>
> 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.

As I see it the "dummy load" is not nearly as simple as we think they 
are...Yes the things are pretty much resistive  for low power loads like 
the "cantenna", but those are limited power and time.  As soon as you 
start going higher power the dummy load can get complicated.  Those big 
globar resistors  work fine on 160, but by the time you reach 20 meters 
they are no longer purely resistive. Their length becomes a noticeable 
part of a wave length.  That means they require compensation.

My commercial dummy load which is rated 1 KW continuous and time limited 
up to 5KW had to be compensated.
I'd have to dig up the e-mails, but I believe it was all posted here on 
"amps" as some one worked out the requirements and the manufacturer now 
incorporates those values in the commercial version.

There are "flat pack" resistors from Mouser and others than run about 
200 to 300 watts IIRC. I think they are about 2X 2 or 3 X 3 inches.   4 
of these can be combined in a square on a heat sink, or water cooled 
heat sink and work quite well at least up through 6 meters.  With the 
heat sinks out the mechanics gets a bit trickier, but I believe these 
could be combined to go to the legal limit with forced air cooling and 
beyond with water cooling and copper heat spreaders to the heat sinks.
> ##  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.
They wouldn't be the only one and mine was much larger than the MFJ.  It 
just needed some compensation to get to 5=50, X=0 throughout it's useful 
range.

73

Roger (K8RI)
> ##  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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