Jim,
There are no good matchboxes on the market for balanced lines. Those
that use the ferrite core baluns are asking for abuse without proper
system analysis. The Viking suffers from bandwidth limitations as
correctly mentioned by your respondent. Not a good choice for 160M.
I missed your original query. XM and Ten-Tec both make good KW
tuners. They use L networks which inherently have the best bandwidth
and lowest loss. Neither handle balanced lines any better than any
other tuner on the market for the reason above.
If you are only going to use it on 160M, why not MAKE one? Most
antennas at 160M are low impedance anyway so you only have to carry
current rather than big voltages - as long as you stick to an
L-network. I have a great tuner on my inverted L. 8 turns #12 house
wire, three 820pF surplus mica transmitting caps padding a dual 360pF/
section 500V BC variable. The antenna base impedance is about 20 ohms
and is 70 feet from the shack. I didn't want to mess with a tuner
outside, so I feed it with a length of half inch 75 ohm TV hard-line.
The loss here is less than 0.1 dB. I measured the impedance with a
simple noise bridge, calculated the needed values with the formula
from the handbook, scrounged in my junque box, and was on the air with
Mr. Alpha in an hour.
By the way, building a tuner in a shielded box is a waste of time if
the antenna is fed with coax. You get a lot more rf radiated into the
shack from the antenna than from a puny little matchbox. In my shack,
I have a "one antenna one matchbox" policy. All are homebrew. None
cost more than $20 to build but it depends on where you get your
parts. The total cost is still cheaper than any single commercial
tuner you can buy. It sure makes changing bands easy.
For balanced lines, you still need to know the impedance at the tx
end. The only reason you are using balanced line is to keep the
system losses low. If you can get it close to 200 (+/- 100) ohms
or 50 +/- 25 ohms, by changing length of antenna or feedline, use
a core balun. If not, you can build a balanced tuner from
schematics in any Handbook - the older the better. When I used
one, I built it on the wall where the feeders came in. Once tuned
and coax hooked up, any tweaking for band edge SWR was done with a
small L-net at the operating position.
Speaking as one who has designed and sold more than a few products
for the amateur and commercial markets, it hurts me to hear people
complaining of the faults in matchboxes which are, in truth, due to
applications outside their design capabilities.
Dick Frey, P.E. 9A/K4XU in beautiful downtown Zagreb
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