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Re: [Amps] Dummy load varying R

To: "Jim Thomson" <>, <>
Subject: Re: [Amps] Dummy load varying R
From: "Carl" <>
Date: Thu, 9 Jun 2011 07:44:49 -0400
List-post: <">>
----- Original Message ----- 
From: "Jim Thomson" <>
To: <>
Sent: Thursday, June 09, 2011 2:05 AM
Subject: [Amps] Dummy load varying R

> Date: Wed, 8 Jun 2011 11:22:11 -0400
> From: Al Kozakiewicz <>
> 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 air.
> Later... Jim  VE7RF
> Any ideas?
> Al

Using a MFJ to test a MFJ just doubles the error (-;

The early analyzers were poor at reading reactance altho the analog VSWR 
meter is OK.
Even the later 259B has had many factory revisions and not all can be 
upgraded. So now they have a new model 266 to start the process all over 


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