I've been using Cecil Moore's (W6RCA) "No-Tuner" with 450 ohm ladder
line for about 7 years along with a K2 / KAT-2 and it has been a
pleasure to use and pretty effective.
It's basically a box of 5 4pdt relays out in the yard that switches
in/out combinations of 16, 8, 4, 2 and 1 foot lengths of 450 ohm ladder
line to the main 450 ohm feedline to get a close match in the shack, and
the KAT2 does the rest. The input to the relay box is fed with RG8X with
a bead balun on the coax right at the box input. The idea is to switch
just enough extra line in to get a current antinode at the antenna
feedpoint for the frequency of operation. For a 80 meter half wave
antenna (with 10 meter fan element tacked on), the main 450 ohm feedline
is about 85-90 feet long. I've worked/confirmed ~140 countries using
this antenna with 12 watts or less (mostly 5 watts). And I'm not much of
a dx fanatic.
73,
Lenny W2BVH
Geoffrey Mackenzie-Kennedy wrote:
On Thursday, November 15, 2007 at 7:00 PM, Goody K3NG wrote:
Using any balun with an unbalanced tuner to feed balanced line can be
bad news (your mileage may vary, standard disclaimers apply). Under
highly reactive loads, a balun used like this can become quite
lossy. There's a couple articles in publications and on the Internet
on this, but Cebik's article is one that comes to mind (
http://www.cebik.com/link/l-bal.html ). It's better to run a real
balanced tuner like a Johnson Matchbox, the MFJ Balanced Line tuner,
or others, if you can.
---------------------------------------------------------
Well put Sir!
As a comment to the List, the usual type of cored current balun
presented with a reactive load might appear not to be introducing loss
because QSOs can be made, but as the members of our QRP fraternity
well know contacts both 'local' and DX can be made using very low
power given the right propagation conditions. Proof of this loss can
be had by increasing Tx power up to the power rating of the balun at
which point the balun could well explode if it has not already at a
lower power, it will certainly becomes hot to touch. As K3NG says your
mileage might vary because the antenna's feedpoint impedance as
transformed by your feeder and seen by the balun might by happy fluke
be non-reactive, feeder length and feeder impedance are factors,
leaving only the R to deal with. If R turns out to be 200 ohms, then a
1:4 balun would work well with a Tx which wants to see a load of 50 +j0.
FWIW I agree with K3NG that the classic balanced tuner such as used in
the Matchbox is the best system to use when feeding a balanced line,
good efficiency and versatile.
73,
Geoff
GM4ESD
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