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
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