[TowerTalk] Coax lengths for phasing a stack

[john at kk9a.com]

Phase in antenna stacking is not that critical, you can be out 20 degrees
and not see much difference in the pattern. Using equal lengths of feedline
is so much easier and in most systems it is not cost prohibitive. According
to my modeling software even if the feedline to one antenna is 100 WL longer
the pattern does not change. Does this seem correct?

John KK9A




To:	towertalk at contesting.com
Subject:	Re: [TowerTalk] Coax lengths for phasing a stack
From:	Jim Brown <jim at audiosystemsgroup.com>
Reply-to:	jim at audiosystemsgroup.com
Date:	Sun, 08 Feb 2015 10:23:01 -0800

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