[600MRG] modified MFJ Antenna analyzer failure

[John Langridge]

Let me begin by saying thank you to all of you who have considered my problem 
and offered solutions, both here on the list, directly, or via phone calls.  It 
truly is appreciated and I think others will find the information very useful as 
well.

In the short term, I have taken Pat's advice and contacted MFJ to see what might 
be done.  If I had to send it in for repair and realignment (the latter part is 
what I really don't want to fool with!), my mod would easily be removed or I 
could take my chances when they take things apart.  It may be that they can put 
me in contact with a tech and sell me the parts and I can be done with it.  THAT 
would be the path of least resistance and none of it is the end of the world.

Going forward, I also need to come up with an SWR meter solution and a number of 
you have included some really good recommendations.  Thanks again.  I will keep 
you posted on how that progresses and any schematics that I have, I will 
certainly forward them on as a number of direct respondents are as interested in 
a solution as I am.  Additionally, Robert mentioned this Impedance bridge and I 
think I will watch the surplus market and ham fests for what might turn up.  
Options are always good. 


Until this is resolved, I will cautiously start out my sessions very QRP, 
peaking the base current and watching the variables I have mentioned in a 
previous email before increasing power.  If a FET goes pop, I have 25 more ready 
to be installed.

Thanks again and if there are other comments and recommendations waiting in the 
wings, I would be very happy to hear them!  


73,

John KB5NJD/WG2XIQ




________________________________
From: John Langridge <jlangridge at sbcglobal.net>
To: 600m research group <600mrg at w7ekb.com>
Sent: Thu, March 21, 2013 8:05:57 AM
Subject: [600MRG] modified MFJ Antenna analyzer failure

Hey guys,

At my station, part of my SOP is to adjust my antenna system before and during a 

session using my modified MFJ259B.  The mod is the one detailed by KL7UW and I 
have found that its measurements are "in the ball park" compared to known 
accurate meters that I have borrowed in the past.  Last night, I had a failure 
with my analyzer and it has been suggested that the detector has finally 
popped.  50 ohms loads are now 39 ohms and reactance changes very little from 0 
even when I know it is much higher.  I have always been very careful to make 
sure the analyzer is not connected to anything when transmitting and that was 
the case again last night.  I am hoping someone knows a solution to this problem 

short of sending the box back to MFJ.  I suspect my mods, which are all crammed 
inside the chassis, will get wrecked during repair or they may not even want to 
work on it given it has been modified.  


While I try to find a solution, I am wondering what methods you guys are using 
to ensure a match.  How do you know in a pinch that its 50 ohms at the feed 
point or do you worry about it?  I think we can all agree that a matched system 
works best and given my low grant ERP, I don't want to waste anything and think 
that careful attention has contributed to my success thus far. Minor variations 
probably do not matter too much and the data supports that fact and my FETs and 
current limited power supply have never blown apart as the result of an untimely 

mismatch but I exercise an overabundance of caution so I am not experiencing 
downtime on a regular basis.  I have been pleasantly surprised at just how much 
abuse the FET's I use can take, however.  


Here are a few of my options as I see it:

I can used the remote control variometer to peak antenna current and I know 
where it should be but I also have seasonal variations due to foliage.

I know about how much current my PA draws for a given voltage when everything is 

working.

I probably have a forward power meter that will allow me to peak the output 
using the variometer, perhaps peaking that in conjunction with the base 
current.  Accuracy would not be important on the meter but it may change the 
system when I take it out of line (it shouldn't but I've seen it happen before).


long term I need to look a mod for an SWR meter.  There was one floating around 
at one point for a simple heathkit meter mod.  anyone have the pdf or a link for 

it?  I have looked for it in the past but never found it and admit its probably 
right in front of my nose.

I hear good things about the scope match option.

adding a motorized coil to my shunt match would allow me to handle minor 
seasonal or weather-driven variations...all of these are something to think 
about in the future...

Any additional ideas or guidance would be appreciated.

thanks!

John KB5NJD/WG2XIQ
_______________________________________________
600MRG mailing list
600MRG at w7ekb.com
http://w7ekb.com/mailman/listinfo/600mrg_w7ekb.com