[TowerTalk] Coax lengths for phasing a stack

[Jim Brown]

On Sat,2/7/2015 6:48 PM, Grant Saviers wrote:
> I'm ruminating about what lengths I need for 5L over 5L stacks to 
> equalize the antenna feedpoint phase.  The literature seems thin and 
> the common answer is "make the feed-lines the same length", which I am 
> certain will work.

Yes.

> However, when the stack separations are sufficient, there may be more 
> than 1 wavelength of coax needed to equalize the lengths. e.g. 60' 
> separation on 20m at 0.89 vf (LDF5) is just about 1 wl. It seems to me 
> there is little downside with ham radio modulation systems to phase at 
> 0 (equal lengths) or +/- n * 360 degrees (where n is 1 or 2).  Perhaps 
> some cw waveshaped attack/decay envelopes will change a tiny bit, but 
> that seems like a slight downside.

It's important to understand that PHASE is NOT a two-valued function, it 
is a CONTINUOUS function, measured in degrees, and it is a function of 
frequency and time.  That 1 WL line will only be 1 WL at the frequency 
for which it is cut; either side of that, the drive to the two Yagis 
will be increasingly out of phase.  The difference may or may not be 
enough to matter. The effect would be to tilt the vertical pattern, and 
would be most visible in the nulls. Equal length lines has the advantage 
of making them in phase across the band.

73, Jim K9YC