Posted by
N2EY on
Feb 21, 2007; 1:37am
URL: http://elecraft.85.s1.nabble.com/Re-wire-antennas-tp444023p444025.html
In a message dated 2/20/07 5:18:23 PM Eastern Standard Time,
[hidden email] writes:
> Even simpler is a 80m dipole fed with balanced line to a tuner for all band
> use. The window line is less costly than coax. A good quality tuner is
> less lossy in multiband use than coax/ tuner balun, etc.. Balanced antennas
> have fewer problems than off center feeds. Balanced line to dipole does not
> need a balun at the antenna.
>
In general, the above is true. But it's not always as simple as it is made
out to be.
First off, the classic lowloss open line numbers we see in the books are
usually
for lines with ceramic insulators and heavy wire - say, #14 spaced 4 inches
with a ceramic spreader every couple of feet. Those numbers don't necessarily
match
those of common "window line" (Twin Lead with holes) and other parallel
lines.
Second, there are all sorts of tuners out there, and they're not all created
equal.
The Ancient Ones used big split-coil balanced tuners, which required a lot of
cut-and-try. And if they were lossy, it was obvious because the coils would
heat up
at medium to high power levels.
The modern single-ended-tuner-with-balun-at-the-output is a different animal.
It can work well with some loads, and be a loss leader in others. At QRP
levels, losses may not result in much heating, either.
I read all sorts of stuff about recommended dipole lengths, but not much
about feedline lengths. But what really matters is the overall system, and what
impedance it presents at the shack end of the line at the frequencies of
interest. A "good" antenna fed with the wrong line can present a shack-end impedance
that is very low, very high, and/or very reactive. The tuner may match it, but
it may not be
very efficient.
One excellent tool we have today that the Ancient Ones did not is antenna
modeling software. Reg Edwards' simple DIPOLE3 gives a good idea of the actual
efficiencies and losses of various dipole/transmission line setups. Free and
easy to use. But it won't tell you how lossy your tuner or balun is.
OTOH, don't let the search for the ideal antenna prevent you from putting up
*something* and trying it out.
73 de Jim, N2EY
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