[Amps] Dummy load varying R

[David C. Hallam]

Has anyone tested or had any experience with the Rig Expert AA-54 
antenna analyzer?  How does it compare to the MJF 259B?  I recently 
purchased one after deciding on it over the 259?  I like the graphical 
plot feature and the ability to show up to 5 frequencies 
simultaneously.  I don't like the frequency limitation of 54 MHz.

David
KW4DH

On 6/9/2011 7:44 AM, Carl wrote:
> ----- Original Message -----
> From: "Jim Thomson"<jim.thom at telus.net>
> To:<amps at contesting.com>
> 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<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.   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
>
> 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
> again.
>
> Carl
> KM1H
>
>
>
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