Jim,
I do not have data on the particular antennas you are comparing, but a
couple of notes relative to some of your considerations might be useful.
1. Elevated feed on a vertical: The idea of elevating the
feedpoint/maximum current point on verticals is fairly well and long
established in comparison to most (but not quite all) verticals with
maximum current at ground level. Among the advantages are these:
a. Lower noise pick-up, since most man-made noise "hugs" the ground or
only a little above it.
b. Lower losses due to absorption by the nearest ground clutter (house
wiring, shrubs, automobiles, etc.).
c. Lower angle of maximum radiation.
The difficulty in making such an antenna is achieving a reliable feedpoint
impedance when relying on some external structure, such as a sloper fed
against a tower at an elevated point. Various means have been used.
Elevated radials (roof top, etc.) with 1/4 wl vertical have proven to be
an improvement for most users compared to ground mounting. Even low, but
elevated radials (e.g., 8-10' up) have been claimed/shown (status not 100%
certain) to improve vertical performance. GAP uses a variety of
mechanisms within one multiband antenna. This is just to sample the
variety of ways developers of antennas have used elevated high current
points.
2. Capacity hats: This area of antenna design is still subject to much
misunderstanding, mostly because of the continued attempt to apply the LF
and VLF origins of the transmission line antenna analogy to HF. In fact,
the utility of that analogy peters out at about 3 MHz with thin wires and
much lower in frequency with fat wires. There is a little utility program
in the HAMCALC collection (available from VE3ERP--well over 120
calculation programs related to ham needs, with a structured index and a
copy of GW BASIC attached--Murph requests a $5 disk/mail cost cover and
donates everything above costs to the Canadian National Institute for the
Blind amateur radio program) that calculates hats for verticals (or
doubled for dipoles) from 3-30MHz for several basic hat geometries
(squares, hezagons, octagons, with or without perimeter wires). It has
limited accuracy over that wide range because it is filled with correction
factors for wire diameter and frequency, none of which are linear. I
wrote this little program while wrestling with the entire hat question.
The best way to think of a hat (forget the word "capacity") is as a
symmetrical structure at right angles to the main plane of antenna
radiation. It is long enough (i.e., has a certain radius) to complete the
current path requirements to achieve some specified condition, such as
resonance in a driven element, or some other). Because it is symmetrical,
its fields cancel and it does not effectively radiate. This conception
avoids misconceptions attached to the idea of a "capacity hat" in terms of
it being capacitive relative to something else. Nothing else is required
and the hat can be modeled successfully in verticals or horizontal
dipoles, over ground or in free space.
Most hats are at the low current end of an antenna, perhaps to resonate a
vertical 65% or more of true 1/4 wl by adding a horizontal hat to complete
the path so that the feedpoint is resonant. We can continue to shorten
the antenna and increase hat size until the antenna is very short and the
hat very large. We can turn the antenna upside down, feed it high, and
place the hat low: I have seen the design of a 160 meter sloper using
this principle--but remember that the sloper relies on its tower for the
"other side" of the dipole.
Let's turn the vertical right side up and feed it low. The so-called
"image" pole may be in the form of radials that are elevated. The radials
in fact are the same as a hat (although this idea is foreign to those of
us who grew up thinking of elevated radials as an outgrowth of the ground
itself. However, the idea of "ground," as simple as it seems, is actually
a multiplicity of ideas which can be separated. We do this when we
seriously sort out tower paths for RF, DC, lightning/charge build-up, etc.
It is only one more step to sorting reflections that create lobes and
nulls in an antenna's elevation pattern from aspects of ground and radials
that complete the antenna current paths.)
Now the hat of the antenna you are considering appears to be a
nonradiating structure necessary to complete the current path(s) from the
elevated high current point upward, while the remainder of the antenna
does the bulk of the radiation work. Virtually all hats are too small to
affect the angle of maximum radiation: that requires consideration of
reflections from ground plus incidence waves at many wavelengths away from
the antenna. But it may well allow the positioning of the angle of
maximum radiation at as high a point as possible, which may have the
advantages noted earlier.
I do not know if this background will be of any use, but hope it is.
-73-
LB, W4RNL
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