[TowerTalk] cutting phasing lines

[jimlux]

vk3pa at vk3pa.com wrote:
> Gdy all, my Q is I need to cut phasing lines for 7.050 Mhz of 84  
> degrees es 71 degrees! ok So far,  now 360 degrees = 1 wave length, so  
> 0.5 wavelrngth (=180 degrees) would be 468/freq /vf es / by 180 would  
> be 1 degree so x by required degrees  eg 84 es 71  ok ! so now how do  
> I convert 84 es 71 degrees to a frequency so I can cut the lines WITH  
> my antenna bridge rx vector analyst autek VA1?? OR AM i  WAY OFF IN MY  
> THINKING!! 73 es tnx Al vk3pa
>


I tend to use metric for this, since 3E8 m/s is a nice round number.

7.050 MHz = 42.553 meters
84/360 * 42.553 = 9.93 m
71/360 * 42.553 = 8.39 m

Multiply those lengths by the velocity factor of your coax (e.g. .80 or 
whatever)

But, to cut it with your antenna analyzer...

Say you want it to be 74 degrees at 7.050 but 90 degrees (e.g. it looks 
like a short) at the test frequency.  That means your test frequency 
needs to be higher (e.g. more degrees for the same physical length)..

7.050 * 90/74 = 8.574 MHz... That is, a piece of coax that is 90 degrees 
at 8.574 MHz will be 74 degrees at 7.050.


So, if you've got 80% VF, cut that coax to around 6.7 meters (80% of 
8.39m).. hook up the analyzer, and start trimming til it reads shorted 
at 8.574.