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Re: [TenTec] TT 238A transmatch mods for 6 meters

To: tentec@contesting.com
Subject: Re: [TenTec] TT 238A transmatch mods for 6 meters
From: "Dr. Gerald N. Johnson" <geraldj@storm.weather.net>
Reply-to: geraldj@storm.weather.net,Discussion of Ten-Tec Equipment <tentec@contesting.com>
Date: Sat, 08 Dec 2007 22:33:04 -0600
List-post: <mailto:tentec@contesting.com>
On Sat, 2007-12-08 at 09:27 -1000, Ken Brown wrote:
> > Anyone try any mods for a TT 238A transmatch to get it to work on 6 meters?
> >
> >
> >   
> Probably possible, but not desirable. Consider the number of unused yet 
> inductively coupled and shorted turns on the inductor. Better to have an 
> antenna with low SWR on the feedline. If you are using open wire 
> balanced feedline (the only kind you would want to use on six meters 
> with a high SWR) then you would want to use a balun between the 50 ohm 
> coaxial system and the balanced feeders that is optimized for six 
> meters. Not one that is designed for MF/HF. If you must use a tuner on 
> six meters, build one dedicated to six meters. Otherwise efficiency is 
> bound to be poor.
> ARRL has had a few articles about six meter tuners in older handbooks. I 
> cannot recall ever seeing a bandswitched tuner that covered six meters 
> and any other band. There is probably good reason for that. Look at the 
> components in the handbook tuners for six meters, and ask yourself 
> whether it seems reasonable to use the sized inductor and capacitor in a 
> HF tuner for six meters.
Without looking this minute, I think the coil in one of my 6m tuners has
8 or ten turns 1.5" diameter. #12 bare spaced out about 1 to 1.5" long
with a half inch gap in the middle. Tuned with a double 25 pf butterfly
capacitor. Input link coupled with two turns #12 insulated. Open wire
feeder tapped 2 or 3 turns out from the center gap on each side. With
the balanced capacitor, its not good or necessary to ground the center
tap of the coil. A dual 25 or 35 pf variable would work as well at 6m.

As an alternative to the variable link, the link could be fixed and have
a variable capacitor, probably about 50 pf in series with the ground
side of the coil.

Make it twice that size for 10m, and 1/3 that size for 2m.

And there have been 6m tuner designs in ARRL Handbooks and antenna
handbooks at times.

73, Jerry, K0CQ

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