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.
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.
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).
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.
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.
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
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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