[600MRG] Fwd: Re: modified MFJ Antenna analyzer failure

[pat bunn]

-------- Original Message --------
Subject: 	Re: [600MRG] modified MFJ Antenna analyzer failure
Date: 	Thu, 21 Mar 2013 09:22:40 -0400
From: 	pat bunn <pbunn at patbunn.com>
To: 	John Langridge <jlangridge at sbcglobal.net>



John,

I have had very good luck with my Rig Expert 30. It was also very
helpful when designing the original matching for the vertical.

I have blown the FETs out one time and it was due to my own stupidity. I
entered 447.2 instead of 474.2 KHz into the signal generator and started
my beacon. By the time I realized what I had done,( about 30 seconds)
the fets were history. But at $2.50 each , it's not a disaster.

I do need a power meter SWR bridge on the antenna feed all the time, so
if anyone has a good design, I'd like to try it. I bought a real nice
large NOS Simpson 200 uA meter at Charlotte to build something along
that line.

Pat
N4LTA


On 3/21/2013 9:05 AM, John Langridge wrote:
> 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
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