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[antennaware] Twin-Coax-Cable as single feed line to 1/2 impedance

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Subject: [antennaware] Twin-Coax-Cable as single feed line to 1/2 impedance
From: (Gary Breed)
Date: Mon, 18 Dec 2000 09:22:36 -0500
Billy and all

While parallel feedlines provide new impedance options, remember that
the characteristics of each line do not change -- including loss.

If a 75 ohm cable has a loss of 1 dB/100 ft, the 37.5 ohm line made
of parallel cables will also have 1 dB/100 ft loss.  You have only changed
the impedance.  The benefit in the situation you describe is that matching
the cable impedance to the antenna impedance will reduce mismatch losses
that exceed the intrinsic loss of the cable.

I have used this method to match Yagis, providing a direct feed without
needing a hairpin or gamma match.  1/4-wave lines of parallel 75 ohm cables
will match 28 ohms to 50 ohms (2 el Yagi); parallel 50 ohm lines will match
12.5 ohms to 50 ohms.  You can also get more complicated and use lengths
than 1/4 wave to match to an impedance of some R +/-jX, where the jX
is provided by shortening or lengthening the driven element.

Finally, if you used different cables, be sure they are matched in
length. For example, it is tempting to use 93 ohm RG62 and 75 ohm RG59 to
obtain a 41.5 ohm line to provide both feed and match for your ground plane.
But be careful that RG62 has a higher velocity factor since some of the
dielectric is air, and will be longer than an equivalent length of RG59.

73, Gary

> Greetings Group,
> I would like to have this groups input on something that I have been
> on today.
> Today was not the first time that I've toyed with this idea but I
> the experiments again that I have done a few times with the same results
> before.  There was a question posted on another forum about some ideas for
> low-loss coaxial cable and I offered some information as a POSSIBLE
> This is it:
> The Idea was to use two 72-Ohm Belden cables in parallel to obtain 36-Ohms
> to match a 32-Ohm 1/4 wave ground-plane antenna.  In doing this, the I
> squared R losses would also be 1/2 unless there is something that I am not
> seeing. Since the tests show that it works,  if I qualified the tests
> properly, I would have no doubts except that I have been a Ham and a RF
> engineer for almost 40 years and have never heard of  running two cables
> parallel.

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