Jim,
Sorry to be delayed in answering your note on verticals and GPs, but an
ice storm took out power and we spent Christmas eve and day in a motel
protecting ourselves and several song birds that my YL is rehabilitating.
amazing how 2 days down can put you more than 5 days behind in everything.
Now to verticals and a few--hopefully useful--distinctions.
1. The modeling study on vertical dipoles and their use or non-use of GPs
was intended as a report to show what modeling via NEC (or MININEC) would
suggest about the use of GPs with that type of antenna. Remember that the
antenna is a "complete" antenna, which for my purposes, means that it will
model in free space correctly, relative to applications closer to a ground
system.
2. The study does not apply to monopoles, which--if we think of only the
vertical radiator--are not "complete" antennas in the sense noted above.
They require a completion, which may consist of a GP at right angles to
the antenna, a sloping GP, or a single element 180 degrees in line with
the existing one (thus making a dipole).
3. The length of the vertical element of a given antenna does not
necessaarily determine whether the antenna is a dipole or monopole, since
we may set up a 1/2 wl antenna as a long monopole or as a dipole. Even
the feedpoint may not be decisive. What is decisive is the current
distribution along the antenna. For a ground-mounted 1/2 wl element, if
the current maximum is at the base, then it is working as a monopole; if
the current maximum is at the center, then it is working as a dipole. The
source impedance (minus any matching networks) gives the answer to the
question of when what is which.
4. Elevated ground planes require rather precise symmetry to ensure a
given radiation pattern from a monopole. Even though early modeling of
these systems suggested that we can use minimal radial wires, subsequent
study has shown that the modeler's presumption of symmetry can be
misleading to ham installations where symmetry of a small system is hard
to achieve. The solution is to use more radials so that irregularities
generally wash out. Although a casual system might achieve adquate QSO
results with few radials, 20 radials is not a bad target figure to
compensate for irregularities in pattern. However, an elevated 20-radial
system--like a top hat on the other end of the antenna--may require
refiguring of the radial lengths, since the denser the array of radials,
the shorter they want to be in elevated systems, according to the modeling
I have done.
5. A sloping radial system of rough symmetry effectively achieves
cancellation of horizontally polarized radiation, but NOT of the vertical
radiation component from the system. Hence, the vertically polarized
radiation from the sloping radials becomes part of the overall radiation
from the antenna. If the system is very high--as with a VHF monopole with
a sloping radials system at 30' up--the radiation field will be stronger
at a distance. However, at heights close to the ground, the lower portion
radiation may actually detract from either or both the far field strength
or elevation angle of maximum radiation. (A similar phenomenon applies to
horizontal loops vs. dipoles placed at the same height as the loop top
wire. The horizontally polarized 1 wl loop may show similar or slightly
greater gain, but when the bottom wire is close to the ground, the dipole
placed in the top wire position will show a lower elevation angle of max
radiation and may be the more effective antenna for many operating
purposes.) The are numerous variables involved in determining when the
"lower end" radiation contributes to or detracts from antenna performance,
so I cannot give a general rule of thumb. However, low band antennas with
very shallow radial angles relative to the horizontal plane may usually be
considered as having "flat" GPs.
6. For significantly elevated vertical monopole+radial systems, modeling
in NEC with adequate segmentation can provide reliable guidance in system
design--if you design with reality in mind--knowing where your real
terrain, soil, and other features a. depart from model limitations, and b.
modeling the elements as close to the irregularities of reality as you
can. Excessively simple models may not provide the reliability that a
given installation requires.
7. For buried radial systems, the correlation between models and reality
is being studied in several quarters, and I would personally not offer any
certifications other than a few generalities. Perhaps the most
significant generality borne out by modeling is that many buried radials
are always better than a few with respect to gain and elevation angle,
where a target of 60-120 is close to optimum and lesser radial systems
show gain losses and signal elevations almost independently of radial
length. On questions of the best length, depth, etc. for buried radials
and actual gains to be achieved relative to elevated systems, I have seen
too many conflicting reports to pass along any notes or suggestions with
confidence.
Depending upon the the primary source of experience of many good
investigators into vertical antenna practice, there may even be
disagreement with the most confidently given notes here. However, I hope
the basic distinctions among antenna types and their requirements proves
to be at least a little useful in your planning. As well, any supplements
or disagreements you may find that this note occasions may also add to
your planning storehouse of ideas.
Good luck on the revised installation, and sorry you lost your
"landmass." Happy New Year.
-73-
LB, W4RNL
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