Hey Roger,
Most cb radios which this is used on will tune for a conjugate match at 36
ohms. If they do not, with the addition of an "Antenna System Tuner"
commonly called an antenna tuner, the conjugate match can be obtained
bringing the system into resonance, whereby all available power from the
transmitter will be absorbed by and radiated by the antenna with exception
of the power lost by the normal cable db/100ft losses. I am familiar with
the series circuit that you mention but losses are
ADDED.
>From: "Roger D. Johnson" <n1rj@pivot.net>
>To: Billy Ward <billydeanward@hotmail.com>
>Subject: Re: [AMPS] Parallel coaxial feedline For 1/2 Characteristic
>Impedance
To: <antennaware@contesting.com>
>Date: Wed, 20 Dec 2000 16:10:41 0500
>
>Hi Billy...
>
> The parallel 72 ohm cables are a close match for the ground
>plane but what are you going to do at the input end? I was
>wondering if you had heard of series section matching. This
>will allow a perfect match using only 72 and 50 ohm cable.
>
>73, Roger
>
>
>Billy Ward wrote:
> >
> > Greetings Group,
> >
> > I would like to have this group's 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
> > lowloss coaxial cable and I offered some information as a POSSIBLE
> > solution.
> >
> > This is it:
> >
> > The Idea was to use two 72Ohm Belden cables in parallel to obtain
>36Ohms
> > to match a 32Ohm 1/4 wave groundplane 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 MFJ259B 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 71.2Ohms. 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
> > Pl259 at each end just as you would to make a cophasing 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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