[TowerTalk] Stacks-grounding-impressions
Jay, WX0B
wx0b@arraysolutions.com
Thu, 30 Apr 1998 10:25:38 -0500
L.B and others interested in stacks.
WX0B-Well, I agree with most of Mr. Cebiks comments in his impressions
statement. I also, in the tone of his message looking for more
quantifiable data, would like to contribute.
Since I own 4 stacks and have extesively modeled them as well as
verified by listening and transmitting on them, I feel like I can lend
some crediblity to this thread. And since it also may contribute to my
little business, you should consider anything I say suspicious. But I
would hope that in a whole to generate some discussion among the Stack
owners that will probably verify what I intend to add to L.B.'s start.
L. B. Cebik wrote:
>
> Short note (well, short for me), since the beam builders have been silent.
> Several misconceptions floating on the fringe of the discussion.
>
> 1. Stacks give a big blob of energy, that is, a sort of direct sum of the
> two patterns vertically. Unfortunately, not so. Reason: the individual
> fields add and subtract in the same way that reflected energy from the
> ground does, with attention to phase as well as magnitude. Hence, the
> stack pattern, when both are in use and in phase, is a composite, where
> gain is addition at the expense of null creation, and the effective height
> is that of a single antenna about 2/3-3/4 the up between the two. The
> vertical bandwidth is a function of the effective height, since for any
> height, there will between 0 and 90 degrees elevation be just so many
> lobes and nulls--and that determines the maximum beamwidth
> vertically--with only small adjustments for the antenna structure.
>
WX0B - I agree with the above being basically a vector addition (and
interference) problem. Thats why models work. But unless I
misunderstand his intent he gives the impression that the "expense" of
stacking is null creation and loss of effective height. Yes and No.
Loss of effective hight is possible with very low stacks. But may be
desireable when trying to match the propogation angle. In my 15m 4/4/4
stack the lowest antenna is at 65 feet. I see no loss of effective
antenna height. The upper alone main lowest angle is 6 degrees, both
upper middle Stack is at 6 degrees, and all three are at 7 degrees but
with more gain at 6 degrees than the upper antenna alone. So you dont
have to "sacrifice" the angle. I also dont "create" undesired nulls. I
control my nulls and also my lobes to move the lobes up and down between
the nulls of other combinations. Thereby I observe, and can prove by
use of models that you can effectively achieve more than 15-20db of
effective signal GAIN at a certain arrival angle. Mr. Freemin also
voiced this in a previouse message.
So Stacks if they are switchable can be a tool to move the take off
angles to our advantage.
L.B>
> 2. Stacks are 2 antennas (or more). This makes stack thinking a problem.
> Consider a phased pair of vertically stacked ZL Specials--where every
> element is driven. 1 antenna array. Likewise, although we buy or build
> the component Yagis of a stack individually, once they are a stack, they
> are 1 antenna array. If we switch to top-only or bottom only, we have
> simply altered the array, and the unused antenna must be taken into
> account in figuring the performance of each of these modes. With good
> wide spacing, the difference between top-only and monoband operation at
> the same height is minimal (and hopefully insignificant), but it is not
> zero. Passive elements are parasitical elements if close to the operation
> frequency in resonance, and their effects are calculable, just as with the
> inline parasitical elements that are also passive until driven by the
> field of the driver.
>
WX0B-Spacing is critical in Stacks. I just did an study, (which several
clubs locally have seen) of which one of the results shows that shorter
antennas are actually more critical of stacking distance than longer
boomed stacks. ie. A two element antenna or a short tribander is
unfortunately a more critically coupled antenna. A short antenna placed
too far away from is counterpart will generate strange lobes, including
a first generation lobe being straight up. So you really want to be
closer coupled, and closer spaced not wide spaced to avoid coupling.
Coupling in stack is not so large as to cause problems if for instance
in a three or 4 hight stack you switch out the middle antenna(s). They
are down to 2 and 3 order products. It actually is something I do to
get higher angles in my high stacks. If you switch out the middle
antenna you create major lobes at say 30, 50 degrees. Much like a Both
out of phase (BOP) situation. BOP is a very good way to create the high
angles for a high stack as well.
Since I dont use BOP I need WC4E and K4OJ to chime in here with their
experiences, which I believe to be very good looking at the results of
contests this year.
So mutual coupling of the field created by the unused antennas is not in
my experience a factor.
Also the drive impeadances stay controlled in both the modeling software
and in reality. The stacks being switched through all of their
combinations do not cause the SWR to vary. Which means your amplifier
does not trip out. This does require that you have your stacked
antennas tuned to the same SWR curve.
LB> 3. Frank's stack concerns about the booms: right angle, no
connection
> minimizes energy in the boom; direct connection increases the energy--as
> evidenced by the need to slightly adjust element lengths when connecting
> them to boom as opposed to insulating them. For stacks, the two booms may
> interact, depending upon distance apart, exact lengths, etc. Hence, some
> may experience difficulties; others may not due to the variables involved.
> For a monobander, with no other boom with which to interact, connection or
> not may be made on other grounds (pun intended).
WX0B-All of my monobanders are on about .5 wavelength booms. Like it or
not thats the way the manufacturer made them. So this would seem to me
to be the worst case senerio for coupling energy from a stack to say the
next antenna which is turned 90 degress.
I have modelled it and find no unusual currents in the boom and have
listened to the effect of turning upper antennas, middle antennas, and
lower antennas not only 90 degrees but 180 degrees to the main stack.
No problems are noted and indeed the stack performes very well using the
turned antenna. Gain in two directions is there and wild couplings which
surely would be visible in SWR are not there in both the models and in
reality.
I was very concerned for the situation where from Texas running JA and
Sout America/caribean at the same time. Which is the case in the late
afternoons here. My concern was that pointing any single antenna in the
stack 180 degrees out would very much cancel the stack operation. To my
suprise and one which we have verified many times the Stack works just
like you would think. You get the benifit of the two high stack on
Japan, and the other antenna has gain in the South america direction as
a single yagi would. No cancellation of the stack gain is noticed. And
the whole pattern looks like a very bad yagi design, in other words
terrible front to back performance. Which in this case is desireable.
One very important test was to use the upper and lower antennas on Japan
and turn the middle one to South America (180 degrees out) The high
lobes appear in the pattern to JA just like they do when you switch out
the middle beam. A very well behaved and predictable outcome. And SWR
remains constant.
>
LB> 4. Noise from wind, rain, other atmospheric ionization and
molecular
> alignment has not been fully quantified in anything that has passed my
> way. Any references would be appreciated. Here, like many other areas of
> antennas and propagation, we need to keep good records if our accumulated
> experience is to contribute to good general practice and not be merely
> anecdotal. Such good records were the beginnings of the modern
> improvements in the understanding of propagation phenomena--not the
> isolated stories, but careful records of what was observed under what
> conditions.
>
WX0B-I like this one. Rain Static in a stack. This may be anacdotal
but several of us heard it including very experienced guys lik N5NJ, who
was running EU on 20m at the time. We had terrible sizzling static
appear before a rain storm hit. I was seemingly being carried in the
wind before the storm. And we have heard it hear many times dureing
contests. It can get so bad here in Texas that you cannot hear the
stations calling. 20-30 over S9. Bob, N5NJ pulled out the headphone
plug so we all could hear. I told him to listen while I switched out
the top beam out of the stack. Gone. Back to running. It happens all
the time here just befor or during highly charged storms. For some
reason the highest antenna in a stack picks up most of the static. All
of my beams have grounded elements directly bolted to the boom. Except
the 40s.
Anyone else notice this?
LB> However, be careful about indirectly grounding parasitical elements,
in
> the manner we often use for driven elements (e.g., a 50K resistor or
> equivalent). Many parasitical elements carry lighter currents and
> relative to a zero-level boom may have a higher impedance--and 50K may not
> effect RF isolation, if that is what we want. The effect may be
> unintentional detuning. If grounding parasitical elements is desired,
> then the entire antenna should be tuned up for that arrangement, without
> assuming that the dimensions for the insulated element arrangement are
> satisfactory. Sometimes they will be, sometimes not.
WX0B - I do not have any experience with using resistors to ground
insulated elements.
LB> In many matters related to antennas and propagation, we are still a
ways
> from precise quantification. The key is learning which ones and not
> throwing out what we can well calculate just because we cannot precisely
> quantify everything. For example, I sometimes encounter remarks like "I
> just don't trust modeling." In one sense, that is unjust, since it
> discards an entire technique of design and analysis without qualification.
> Treated as a limited and well-specified introductory remark, however, it
> might be the beginning of an illuminating presentation, since I do not
> trust models either. However, my distrusts are very specific and
> documented. Likewise, one can say "I distrust range tests." Again, I do
> to, but only where the limitations have been documented and we can specify
> the range of distrust. I do not distrust them in general to the degree of
> discarding the entire enterprise. And, yes, it is tough work figuring out
> what to trust and what to suspect, and those categories change with each
> increment of knowledge we acquire.
>
> So my recommendation is that everyone interested in antennas keep
> excellent observational records (not long after-the-fact impressions) so
> that the aggregate of wide spread amateur experience can contribute to the
> process of making more precise those areas where we can't yet do it.
>
> Soap has dissolved and the box broken, so time to end.
>
> -73-
>
> LB, W4RNL
WX0B- Thanks LB for getting me fired up once again on Stacks. I need to
write that article and use my plots to get some of this information out.
Now my box has fallen apart, who will take this and carry on?
Jay, WX0B
>
> 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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| ARRAY SOLUTIONS | Jay Terleski email: wx0b@arraysolutions.com |
| WX0B | http://www.arraysolutions.com |
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