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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