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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: (Clive Hollins)
Date: Sun, 17 Dec 2000 12:07:51 +0000
Yes you can do this- I have used similar arangements
of paralleling cables of the same and different
impedances to match antenna feed point impedences. 
and to match phased arrays.  The only problem you
need to be concerned about is the power handling
capacity of each of the cables.
A couple of 75 ohm cables in parallel will of course
give you 37.5 ohms at both ends, but if you use the
formula Zt= root(Z1*Z2) you can match that 37.5 ohms
to close to 50 Ohms using a lambda/4 length of coax
whose impedaence is 43 Ohms. e.g. a paralleled pair
of 90 Ohm cables-or any combinations that will give
you the same Z.
The classic example of an impedence transformer is
the 1/4 wave length of 75 Ohm  used to match the 100
Ohms of a delta loop to any length of 50 Ohms. 
Tried it once on 28MHz and it worked a treat!

Billy Ward wrote:
> Greetings Group,
> I would like to have this groups input on something that I have been working
> on today.
> Today was not the first time that I've toyed with this idea but I performed
> the experiments again that I have done a few times with the same results as
> 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
> solution.
> 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 in
> parallel.
> I used an MFJ-259-B and measured the Characteristic Impedance of  three
> lengths of Belden 72 ohm cable.
> One of them was Three Feet, one was 12 Feet and the other one was 18 Feet.
> They measured 71.2-, 71.3 -and 71.2-Ohms.  I then measured the velocity
> factor using the "Distance to Fault Mode" to determine the electrical length
> of the cable in inches and and then divided that figure by the actual length
> in inches.
> The Velocity Factor measured at .80, .80 and .79 blinking to .80
> After making up 3 parallel cables by fitting both cables into a single
> Pl-259 at each end just as you would to make a co-phasing cable for CB
> radios, I made the measurements again.  The impedances  were almost exactly
> what I expected at 35.7, 35,7 and 35.4.
> The Velocity Factors were a little further from the single cable figures
> than were the impedances, measuring at 82.3, 82,4 and 82.0.  I  figured that
> this was because the losses from the insulation was divided among the two
> cables causing the Velocity to be just a little faster than one cable.
> I am familiar with Conjugate Matching and realize that there is really no
> practical reason for using this cable arrangement in order to radiate 100%
> of the power delivered by the transmitter less the power dissipated by the
> cable losses..  Also the amount of power saved by halving the losses of
> cable that is already low enough is not worth the time it takes and the
> extra cost of the cable to bother with it.  However, to stir up a discussion
> on the other forum, I offered this idea. So please NO preaching about why I
> do not need to bother with this idea.  It is just a fun thing for
> discussion.
> In your opinions, are there any flaws in this being a viable feed line.
> Billy
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