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Re: [Antennaware] cutting phasing lines

To: Allan Greening <>
Subject: Re: [Antennaware] cutting phasing lines
From: John Tait <>
Date: Sun, 07 Sep 2008 15:18:30 +0100
List-post: <">>
Hi Allan.
     I hope the following doesn't come under " Teaching Granny to suck 
eggs".. If so, I apologise.

First of all, you must calculate the Velocity Factor of the cable that 
you intend to use. I would cut a half wavelength of coax, using a higher 
VF than I would expect for that coax.   I think that .8 VF would be safe 
for most, and if you're using good foam dielectric coax or hardline, I'd 
start with .9VF. So.. a half wave  at .8VF is 468/F  * .8 which is 468/ 
1.850 = 253 * .8 = 245ft. 
   Now, get your Antenna Analyser.. Connect a T piece to the antenna 
socket, with a 50 Ohm resistor across one input, and your 245ft of coax 
to the other. Your 245ft is open circuit at the far end.
Find your 1:1SWR frequency nearest to top band. Let's say that it 
measures 1:1 at 1.680 mHz.
    You now know, that 245ft of this particular coax is a half wave (180 
degrees) at 1.680mHz. This information  allows us to calculate the VF as 
follows. 468/ 1.68 mHz = 278.6ft, which is a 1/2 w/l on 1.680 mHz in 
free space.
So.. 245 is to 278.6 as X is to 100, so X = 87.9% = Velocity factor of .879
You would expect to see this .879VF only in good quality foam dielectric 
coax. You'd expect around .68VF for cheap RG58 and RG8.

 The rest is straight forward as you now know the velocity factor of the 

         180 degrees (half wave) = 468/1.85 * .879 = 253 * .879 = 222.4ft
Therefore 1 degree of that coax at 1.850mHz = 222.4/180= 1.23ft
so your 84deg line is 1.23 X 84 =    103.32ft  =  103ft 4ins
and your 71deg line is 1.23 X 71 =  87.3 ft     =    87ft  3 1/2ins

You should always calculate the VF of each new drum of coax, as it 
varies even with the same type from the same manufacturer, unless 
they're from the same production run. I usually find that RG58 can have 
velocity vactors ranging from about .68 to .75 depending on the 
dielectric etc, so unless you know the VF of your coax, it's guesswork.
 I hope this helps.
    Vy 73
      John EI7BA

Allan Greening wrote:
> If using EG213 coax how do I WORK OUT
> 1 If using 84 es 71 degree phasing lines how do i find the length.
> 2 how do I find the cutting frequency
>  on 1.850 Mhz.. for a pair of 1/4 wave verticals  90 ft top loaded??
> tu es 73 vk3pa Allan
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