Just remember when you are using open-wire feedline that you are not
matching the feedline to the feed-point impedance of the antenna and
therefore you are running a high swr on the feedline. That's no problem for
the open-wire line since it has quite low loss compared to coax. But if you
use a balun of any ratio and switch to coax in the middle of the feedline,
you are also likely to run a high swr on the coax as well, and that could
present a problem, at least in higher loss and perhaps also in high voltage
arcing over the coax if you are running high power. It would be better to
run the open-wire line all the way to the tuner in your shack if at all
possible.
I've been using open-wire fed antennas since 1952 with excellent results.
There are no particular lengths of feedline that are magic. I use whatever
length is necessary to fit between the antenna and the shack. The impedance
presented at the transmitter end of the line will be a function of the
termination impedance at the antenna and the electrical length of the line
of course. A Smith Chart would be a good aid in figuring out what that
impedance will be if you are concerned about it. So long as the impedance
at the shack is within the matching capabilities of your tuner there, you
probably won't care exactly what it turns out to be. If your tuner can't
match it, one thing you can do is to add an additional length of open wire
feedline between the tuner and the line coming into the shack. A quarter
wavelength would be a good first try, although something considerably
shorter might work. That will change the impedance and likely put it into
the range of your tuner.
Jim, W8KGI
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