I've been very pleased with my MFJ-974, a true balanced tuner. My
measurements of feedline current with RF ammeters indicate that
overall it is as efficient as my home brew link coupled tuner with
plug in coils. I've used the MFJ tuner with several combinations of
feedline lengths and radiator lengths, and I've never failed to get a
perfect match on all bands from 80 m to 10 m.
I'd like also to mention that Dipole3.exe is an excellent, free, and
very easy to use program for modeling a dipole fed with balanced
line. You can play with different values of feedline length and
radiator length, as well as many other parameters, to determine
efficiency and impedance at the transmitter end of the feedline. One
source of Dipole3.exe is http://www.smeter.net/software/dipole3.exe .
Dick, K0KK
On Nov 15, 2007, at 1:00 , Goody K3NG wrote:
vf
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.
Don Wilhelm wrote:
Curt,
There seems to be a misconception among hams that when using 450
ladder line, a 4:1 balun is the best thing to use. While that may
be true in some cases, in many other cases, a 1:1 balun will serve
better. It all depends on the feedpoint impedance of the feedline
at the shack end, and that has little relationship with the fact
that 450 ohm line is being used - i.e. the feedpoint impedance is
*not* the characteristic impedance of the transmission line in
most cases. The feedpoint impedance looking into the shack end of
a multiband antenna can vary from extremely low to extremely high
- it all depends on the length of the antenna and the length of
the feedline.
--
Blog: http://thek3ngreport.blogspot.com/
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