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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: (Billy Ward)
Date: Sun, 17 Dec 2000 05:12:21 -0000
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 
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 

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 

In your opinions, are there any flaws in this being a viable feed line.


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