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## Re: [Antennaware] cutting phasing lines

 To: Allan Greening Re: [Antennaware] cutting phasing lines John Tait Sun, 07 Sep 2008 15:18:30 +0100 mailto:antennaware@contesting.com>
 ```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 coax.. 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 > _______________________________________________ > Antennaware mailing list > Antennaware@contesting.com > http://lists.contesting.com/mailman/listinfo/antennaware > > > _______________________________________________ Antennaware mailing list Antennaware@contesting.com http://lists.contesting.com/mailman/listinfo/antennaware ```
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