[600MRG] modified MFJ Antenna analyzer failure
Warren K2ORS
k2ors at verizon.net
Thu Mar 21 09:13:19 CDT 2013
John,
A use a Telepost LP-100 digital vector wattmeter with a custom
coupler for LF/MF:
http://www.telepostinc.com/lp100.html
Measures peak and average power and vector impedance.
Antenna analyzers such as MFJ are pretty much useless here as I
get a lot of signal from local AM BC stations.
The LP-100a is not cheap, but its calibrated against a NIST
standard so I really know what's what with the antenna.
73 Warren K2ORS
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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