I think the message here is that stacking is more complicated than it
looks, and best results are likely obtained by very careful and
extensive modeling. And also that thanks to the variability of
propagation, stacking dimensions don't need to be "ideal" to be useful.
To understand this a bit better, here's my comment to Mike's use of "in
phase" and "out of phase." This is a VERY important concept for us to
understand.
On 4/6/2025 7:50 PM, Mike Fatchett W0MU wrote:
> The models most used had the stacking distance at 30 to 32 ft. It was a
> compromise. Most serious contesters had switches allowing upper, lower
> both, in and out of phase.
Phase is a continuously valued function and is measured in degrees or
radians. It is also a function of time. Phase has no meaning for signals
of different frequencies. The correct word for what hams have
traditionally called "in phase" and "out of phase" is "polarity." The
polarity of a system is either "normal" or "inverse." It's equivalent to
reversing a pair of wires, or of passing a signal through an inverting
stage. It can NOT be accomplished by any delay network, whether
circuitry or transmission line. The vertical directivity of a vertical
stack is the direct result of the phase difference between signals
radiated from the stacked antennas being a direct function of the
difference in time that the two have traveled to reach an observer over
a higher or lower path, and of phase being a continuous variable AND a
function of time.
These fundamentals are WHY stacking relationships between identical
multiband antennas is so complicated. And it's why the design of
broadband antennas is so complex.
A story from my youth. I lived 2-3 miles from a 4-tower AM on 930 kHz
that was non-directional daytime, and with a complex pattern at night.
The pattern was carefully designed by a consulting engineer to produce
strong nulls in the directions of stations already licensed on the same
and adjacent channels when this station was licensed, to protect them
from interference. As I drove through those nulls, I heard distortion
increase as I approached it, the carrier cancel at the null, and reverse
the transition on the other side. All of this occurred within a few
hundred feet. Again, because phase relationships in the signal received
by the radio in my '57 Chevy were changing with distance. BTW -- as a
young EE student, I later worked for that station, and also for a
consulting engineer who designed arrays like that.
73, Jim K9YC
On 4/6/2025 7:58 PM, David Gilbert wrote:
Stacked yagis are only optimized at one frequency, but they are pretty
tolerant of that spacing. LOTS of hams have stacked multi-band
tribanders and they do have worthwhile additional gain on all three
bands. If positioned at the right height they also have the advantage
of at least partially filling in the nulls that would otherwise exist
with a single antenna.
You can verify all of what I just wrote with a modeling program like EZNEC.
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