Jay has added some important notes on the practical side of stacking.
In fact, his disclaimer concerning the switches he sells is probably
not warranted: for stacks, either his or a comparable switching system
is in order.
I want to clarify a couple of points in this connection, but from the
mathematical side of the question. What did I means by nulls in the
last note. And can we confirm that the TO angle for a stack is in fact
higher than for the top beam of the stack when fired alone? And
finally, does the top beam property set change in the presence of a
lower passive beam in the same stack? The answers might be
illuminating, although remember that in a well designed stack, the goal
is to make the passive beam in the stack (when the stack is switched to
something other than both in phase) as operationally insignificant as
possible. In fact, that is achieved when a good stacking height is
chosen for the assembly.
So I set up a stack of identical 4 element Yagis from my collection of
models for midband on 15 meters. The lowest was at 65' up. For this
beam design (about 8.6 dBi forward gain in free space), a separation of
35' yielded nearly maximum forward gain for the pair while preserving a
better than 20 dB front-to-back ratio. The feed Z was about 25 - 28
Ohms.
I place beams at 100' and 135' up. Here are the figures for these
beams when each is the only beam on the tower:
Height Gain To angle Vert B/W 2nd lobe F-B
feet dBi degrees degrees degrees dB
100' 14.27 6.4 6.5 20.0 23.08
135' 14.36 4.8 4.8 14.7 21.92
Now let's stack the beams.
Height Gain To angle Vert B/W 2nd lobe F-B
feet dBi degrees degrees degrees dB
Both 17.33 5.4 5.5 16.5 20.77
135' 14.28 4.8 4.7 14.7 20.81
As you can see, the figures for the 135' only and the 135' w/100
passive are very slightly different, but operationally insignificantly
different in this arrangement. The feed Zs also differ in resistance
by about 1 Ohm (4%).
Moreover, the TO angle of the main lobe is higher for the stack than for
the top beam, either only or w/100' passive. You can repeat this
exercise with innumerable beams in well arranged stacks.
Now compare the "Both" entry with the following:
Height Gain To angle Vert B/W 2nd lobe F-B
feet dBi degrees degrees degrees dB
120' 14.33 5.4 5.4 16.7 20.80
The stack fed in phase is angularly (but not gain) equivalent to a
single beam at 120', about 57% of the distance of separation.
For all of these exercises, I have let the software calculate patterns
to 0.1 degrees so that differences are not washed out, even if they do
not make a big or even a little difference operationally. Since the
two beams in this modeled stack are about 2.15 wl and 2.9 wl up
respectively, differences become small, and comparative modeling
should go to smaller angular resolution in order to see differences
that are at least mathematically significant.
But now let's explore those 2nd lobes I added to the scheme. These
lobes are in fact not down by much relative to the main lobe. However,
between the lowest lobe and the next one up is a null (the one to which
I originally referred) that may be 20 dB or more deep at its pinpoint
extreme. The null is at about 9.8 degrees for the 135'-only operation,
11.0 degrees for the stack, and 13.2 degrees for the 100'-only
operation. On some occasions, the incoming signal angle may be well
into one of these nulls, but never all three simultaneously--because
the nulls are exceptionally sharp.
It is easy to see the importance of having the ability to switch among
arrangements in order to change the null position--or more
operationally, to change the lobe position to catch the signal. A
couple of degrees difference in the lobe angle (either the lowest or
the 2nd) might indeed make a 15-20 dB signal strength difference.
The upshot is that a stack is as likely to miss some action as the top
beam alone or the bottom beam alone, since at a given moment, the
particular path of interest may fall into the null between the first
and second lobes. If you plan a stack, especially one with a top
height well over 2 wl (and here approaching 3 wl), switching capability
may be the key to using the stack effectively.
Further angular fills are possible using the top and bottom beams of a
3-high stack, but the example may suffice to clarify both what I meant
in my original note and part of what Jay was explaining in his.
Essentially the difference between the notes goes something like this:
he would say that the models confirm his experiences, whereas I would
say that his experiences confirm the models. Remember: modeling
software is simply software that does mathematical calculations of
antenna performance based upon fundamental antenna properties as
ultimately derived from relevant basic laws of physics.
Now the usual disclaimer: modeling via NEC is over flat ground with no
bumps or clutter. For other types of terrain, the angles may change,
but the progressions will not. Use either the N6BV or the K6STI
terrain software to translate these figures into those which better fit
your situation.
Since a picture is worth at least 12-14 words, I shall place a version
of these notes as an addendum to one of the stacking entries at my
site, along with an overlay of elevation patterns, so that it is easier
to see the "null and fill" situation made available by switching in
this example. In that same entry are some charts that confirm another
of Jay's notes: there are graphs for stacking separations showing the
range that still keeps the maximum gain of the stack within 0.1 dB of
maximum--and thereby allow one usually to find a separation that also
sustain a good front-to-back ratio--and the higher the gain of the
individual beams, the wider the range of separations that sustain close
to maximum stack gain.
-73-
LB, W4RNL
L. B. Cebik, W4RNL /\ /\ * / / / (Off)(423) 974-7215
1434 High Mesa Drive / \/ \/\ ----/\--- (Hm) (423) 938-6335
Knoxville, Tennessee /\ \ \ \ / / || / (FAX)(423) 974-3509
37938-4443 USA / \ \ \ \ || cebik@utk.edu
URL: http://funnelweb.utcc.utk.edu/~cebik/radio.html
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