[600MRG] modified MFJ Antenna analyzer failure

[John Langridge]

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