[TowerTalk] re; STEPPir ANTENNAs

[Jim Brown]

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.