[TenTec] OT: SWR & Coax

[Dr. Gerald N. Johnson]

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