Gary,
I have read with great interest your researches into the 160 m vertical
dipole, sloping GP, and flat GP. What follows are some thoughts that may
or may not be of interest or use.
First, let's compare the hat (type 2) vertical with the flat (type 3) 1/4
wl w/gp vertical.
a. In free space, where there are no reflections, the hat vertical has a
larger length of high current than the 1/4 wl w/gp. Hence, for straight
out (tangential) radiation, we would expect higher gain relative to the
standard (dBi) than from the 1/4 wl w/gp.
b. Over ground, the hatted vertical has some interesting properties
relative to the way in which you set up the project. Since the antenna
feedpoints--1/4 wl down from the top--are the same, we should look at the
radiation originating below that point. For the flat vert with gp, there
is none, since the elements are horizontal and symmetrical, with the
radiation cancelling. This is not true of the hatted vertical. There is
very significant radiation originating from the vertical region of the
antenna below the feedpoint. The ultimate pattern of the antenna, as
viewed in an elevation pattern, is the combination of all radiation from
all segments, with an angle of maximum radiation that results from
combining all of the incidence and reflection components. Since we have
radiation from the lower portion of the antenna, I would expect two things
to happen.
1. The net T-O angle would be higher than without the lower region
radiation components.
2. The vertical beamwidth is likely to be greater than for the 1/4 wl
w/gp, resulting in lower gain.
As I look at your numbers, I see indications (although not sufficient data
to confirm or rebut these expectation) of these phenomena. In fact, the
progession of hatted data appears to be a progression that is relatively
smooth in its curve.
c. The sloped radial (type 1) antenna is quite interesting. As you drop
the height, the radials increase their angle relative to the top wire.
With any angle other than true vertical, the wires show what can be
analyzed as a combination of horizontal and vertical radiation, with the
vertical growing weaker and the horizontal stronger as the angle increases
relative to the top vertical wire. For each step. the horizontal
radiation cancels. The vertcal portion, by growing weaker, would have a
lesser role to play in the reflection components of the overall field over
ground than the lower portion of the hatted antenna. Hence, depending
upon height and the consequent angle of the sloping radials, the take-off
angle would be somewhere between the value for the hatted antenna and the
1/4 wl w/gp antenna. The flatter the radials, the closer to the flat gp
version; the more vertical the radials, the closer to the hatted vertical.
Note that the surface area of the sloping radials is n times the top wire,
where n is the n umber of radials. since all wires are #12, I would
expect the current sum of the four bottom wires to be slightly higher than
the current on the top wire even if the junction is at true electrical
center. This current ratio can upset some very simple calcuations we
might otherwise perform on the situation.
d. If you were working with higher altitudes for the antennas, I would
also suggest a check of secondary lobes as places where power can go other
than beamwidth and ground loss. This would be important if someone
replicates this experiment for 40 meters, for example.
e. The maximum gain differential, although appearing very distinct, is
still about 1/2 dB or less than 0.1 S-unit. Hence, someone might
legitimately and without loss of significant operational advantage
opt--based on mechanical or other reasons--for the hatted version of the
antenna.
f. Average ground is in fact one of the lowest gain grounds over which a
vertical antenna system can be placed. At heights close to the surface,
in most cases, only something like the "very poor" category (c=0.001 s/m;
d.c.=5) is worse. You may wish to repeat the runs over ground ranging
from very poor through very good and look for variations, including some
possible peaking of values at intermediate points along the way. This
would likely require closer increments between runs, which might make the
experiment more complex than you want to get into.
g. I concur that more radials in the models are useful to try, although
since you are using a wholly elevated system, You might make the increases
modest for each replication.
I hope something in these notes will be useful to you.
-73-
LB, W4RNL
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