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
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.
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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