So Reg, would you advise using a (4 to 1) balun with the open wire on one
side, and a short run of coax, say 20 feet or so on the other to the antenna
tuner? Would this work well? Does one need to 'make' the open wire feeder,
or is the commercial stuff you buy ala ham coax suppliers good enough?

regards, Jerry

"Reg Edwards" wrote in message
...
I plan on constructing some simple dipoles. For fun, I have been
modeling
them using EZNEC. It seems the "common" practice is to feed the antenna
with
50 ohm cable. The antenna feed point impedance is usually ~70 Ohms.
Would
it
make more sense to feed a dipole with 75 ohm TV coax to keep the SWR low
on
the cable and have the mismatch at the radio? If I plan on using an
antenna
tuner, would this setup be preferred over 50 ohm feed cable? Does it
really
make any difference?
===============================

Tom you are quite correct. The lowest loss on a transmission line occurs
when the line impedance is matched to the antenna feedpoint impedance. So
a
standard 75-ohm line is a better match to the centre of a simple 1/2-wave
dipole which is around 70 ohms when at exact resonance.

Furthermore, the attenuation of a 75-ohm TV line with the same overall
diameter is less than a 50-ohm line. Especially if the 75-ohm line does
not
have solid polyethylene insulation but is partially air spaced. The two
effects, between them, may result in a noticeable increase in signal
strength on the 10m band if the feedline is very long, say 150 feet or
more.
For short feeders there will no practical difference between the two
impedances.

But coax of any impedance limits use of an antenna to frequencies at which
it is 1/2-wave resonant and 3/2-waves resonant.

To take advantage of a tuner you must use a balanced high impedance
feedler
such as 450-ohm ladder line or 600-ohm open-wire line. For the same weight
of line the higher the impedance the better. Line loss can then be
reduced
to a negligible amout. Less than either 50 or 75-ohm coax at any
frequency.

But the big advantage of the high impedance line is that its very low loss
applies to ALL HF bands from 160m to 10m. The antenna does not have to be
resonant at any frequency. It can be of random length and work all bands
if
it is long enough at the lower frequencies. The high-Zo feedline can also
be of random length. Best to bring it all the way from antenna into the
shack near the tuner.

You will never have a better, all-round, all band antenna than a high
dipole
fed via 600-ohm line, overall dipole length as long as you can fit into
your
back yard.
----
Reg, G4FGQ