[TowerTalk] Determining coax length?

[Jim Lux]

On 11/12/13 8:06 AM, john at kk9a.com wrote:
> I also cut the cables the same length before installing the antennas by
> laying them flat on the ground. I build identical antennas and use the
> same model balun. When connecting the baluns it is very important to
> connect them identically (center of the coax to the same side of the
> driven element for each beam).
>

There have been reports of the velocity factor varying substantially 
within a single spool of coax (other than a "once per turn on the 
spool").  It's not a parameter that is tightly controlled by the mfr.

If you have a lot of wavelengths, it can add up.  Say you've got 200 
feet of coax and you're phasing at 10 meters, so the coax is a bit over 
6 wavelengths long.  A 1 % change in velocity factor is 22 degrees.

Mind you, I think that other factors will probably have a bigger effect 
on the relative phasing (the self and mutual Z of the antennas are 
probably not that well controlled, as elements wave around in the wind, 
etc.)

I would put the thing up and try it.  If the performance didn't seem 
right, I'd stick short lengths of coax in one side or the other to shift 
the relative phasing.    What it's going to do is move the positions of 
the nulls in the various configurations, and I think that's generally 
quite "site dependent" in any case.