[TenTec] OT: SWR & Coax

[Denton]

Ok....well that pretty much explains it all! My Palstar BT1500A is looking 
more attractive all the time.....
----- Original Message ----- 
From: "Dr. Gerald N. Johnson" <geraldj at storm.weather.net>
To: <tentec at contesting.com>
Sent: Tuesday, July 08, 2008 8:39 AM
Subject: Re: [TenTec] OT: SWR & Coax


> On Mon, 2008-07-07 at 21:32 -0700, Denton wrote:
>> I am using a 230 ft horizontal loop up about 30 ft fed with 600 ohm 
>> homebrew
>> ladder line.
>> I have a revamped johnson matchbox, a TT 238A tuner and am currently 
>> running
>> a Palstar BT1500A balanced L antenna tuner.
>> All 3 had about the same field strength from 80 thru 30 meters. From 20
>> meters thru 10 meters The Palstar and TT has about the same field 
>> strength
>> readings with the matchbox showing less rf field strength.
>> Both the Palstar and 238A are far easier to tune (lower Q I assume) than 
>> the
>> matchbox.
>> The matchbox and Palstar show the best balance with the 238A with its
>> internal 4 to 1 balun showing a bit of feeder imbalance especially on 10
>> meters.
>> I used a field strength meter almost directly underneath one leg of my 
>> loop
>> to determining readings for field strength and used the same meter taking
>> readings equidistant...about an inch or so...on either side of the 
>> feeders
>> to determine balance criteria.
>> I suspect the 4 to 1 balun in the 238A tuner simply isn't up to snuff as 
>> far
>> as balance is concerned...and I would suspect it's efficiency on anything
>> above barefoot power.
>> I haven't made much sense about the matchbox not having much field 
>> strength
>> readings on the higher feqs...any guesses? too small coil wire size 
>> perhaps?
>
> Probably more a case for the matchbox of too much tuning capacitance
> demanding it run with a high loaded Q on the higher frequencies. It
> always has the differential caps across the coil plus the tuning
> variable. The differential caps make it very handy at setting the
> impedance ratio, but also limit its matching range. A high loaded Q
> means lots of circulating current in the tuned circuit and so lots more
> loss from that circulating current and makes the tuning more critical.
> E.g. harder to get to the match.
>
> 73, Jerry, K0CQ
>
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