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Re: Phasing Cables Calculations



----- Original Message -----
From: "John Germanos" <wb9pnu@charter.net>
To: <amsat-bb@amsat.org>
Sent: Tuesday, June 27, 2006 12:20 AM
Subject: [amsat-bb] Phasing Cables Calculations

> I am stcking four 2m yagi's in "H" pattern. I am trying to calculate the
> lenght of the four cables to the 4-way power divider.
>
> I think it goes like:
> 11802.8/freq.Mhz, then divide by two for half wavelength, then multiply by
> velocity factor of cable (for Andrews FSJ4-50B = .81).
>
> Using 144.140mhz, I come up with 33.163 inches of 1/2" heliax per 1/2
> wavelength.
>
> Then I multiply by 3 for convenient cable length, 99.49 inches.
>
> Is this the correct way to calculate the cable lengths?
>
> All help appreciated.
> 73 John WB9PNU

Hi John, WB9PNU

Your calculation is correct and feeding the antenna with any numbar
of  half electrical wavelegth long transmission line the complex impedance
at the antenna connector will appear unchanged at the other end of the
line connected to the generator (TX)

In your specific case your TX will see the same impedance of the antenna
after 6 complete turns on the Smith Chart going clockwise from the load
(the antenna) to the generator (TX)

In any case if  your antenna has SWR because it's complex impedance
differs from  50+j0 ohm than to make a transmission line long any
numbar of electrical half wavelenghts do not change the SWR in the
line nor change the impedance matching to the generator.

In general the only advantage to cut a transmission line any multiple of
1/2 electrical wavelenght long is to get in the shack "about" the same
impedance of the antenna so that using an impedance bridge than the
complex impedance can be measured without to go on top of the tower.

Since we suppose that your 4 antennas are individually mached to 50 ohm
for the lowest SWR than any lenght of coax cable can be used provided
that all 4 runs are cutted with the same lenght.

In your specific case with 4 antenna connected the impedance as seen at
the input of a 4-way power divider 1/4 electrical wavelenght long will be
50/4 = 12.5 ohm and since the output must be 50 ohm than the characteristic
impedance of the power divider must be SQR ( 12.5 x 50 ) = 25 ohm

The most important point here is to have all 4 signal running in phase
from the generator to the load  so that all 4 runs of coax cables must be
cutted exactly with the same lenght including connectors and possibly
using a coax cable coming from the same coil.

Best 73" de

i8CVS Domenico
----
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