ken.d.brown at hawaiiantel.net
Sun Feb 1 02:00:21 EST 2009
> A G5RV fed with open wire feedline and a tuner will work just as well as a
> regularly configured G5RV.
Sounds like a dipole fed with open wire feed line to me. What makes this
> The flat top wire does the radiating and it is
> the same length in either configuration so the results are the same. The
> advantage of the G5RV was that no tuner was required for the bands it was
> designed for. That one higher frequency 'problem' band is the cause of all
> the modifications and adjustments already described. The designer of the
> G5RV did not include that band in his basic design.
> The advantage of a
> simple dipole, resonant length or not, fed open line is a perfect 1:1 on
> any frequency on any band and you don't have to use coax anywhere.
Huh? A perfect 1:1 what? Not SWR. A dipole fed with open wire line has a
fairly high SWR on most frequencies. The losses with high SWR on open
wire line are very low compared to with coax. It will only be a perfect
1:1 match on a few select frequencies where the antenna feed point
impedance equals that of the open wire feed line. That will be great for
a radio designed to work into the open wire feed line impedance. Not too
many of those rigs around these days. With most rigs you'll probably
want an impedance matching device, and a balanced to unbalanced
transformer of some sort, maybe a combination all in one box, such as a
Johnson Matchbox or one of the newfangled balanced tuner products on the
market these days.
Even if you choose a particular length of open wire line, such as a half
wavelength on one frequency, so that the source (radio) sees the feed
point impedance of the antenna, there is still a high SWR on the open
wire transmission line. The source is just connected to a special place
on the line, a half wavelength from the load.
One problem with many tuner and balun combinations is that the balun is
at the high SWR side of the impedance matching system. Baluns are most
often designed to work with resistive loads, and don't always work as
well with reactive loads. A better way is to have the balun on the low
SWR side of the tuner, between it and the source (radio), so that the
balun sees a resistive load, when the tuner is tuned right.
Another option is to use a folded dipole, which will have an impedance
near that of open wire line, and you can have a low SWR on the open wire
line. If you choose or build the feed line to the right dimensions, you
can actually have a 1:1 match between a folded dipole feed point and the
You don't really need a low SWR on the open wire line though, because it
has such low loss even with high SWR.
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