Ignoring the terrain implications for the moment,
There is no perfect solution with tribanders since optimum for 20M is
not optimum for 15 & 10 ...and so on
try this thought process though
Assumptions
1) Both beams are identical
2) the beam on the top is always at the same (Fixed) height "Z"
3) The lower beam is at (a variable) height "y"
4) Spacing = Z-Y
5) Pick a frequency of interest say 14.200 and a take off angle say 8
degrees
Process
1 )Model it as single 6 element antenna with the top 3 elements at a
height Z and the bottom 3 elements at a height Y
2) Run it through EZENEC/AutoEZ
3) In auto AutoEz run the optimization for MAX GAIN (at a desired take
off angle) while varying Y
4) Run it on all three bands and get a different optimum Y solution for
each band
5) In AutoEz you can then develop a set of 3 curves spacing (Z-Y) vs
gain at the desired take off angle
6) Intuitively from the 3 curves pick a compromise height Y that trades
off gain vs Y using the same Y for all three bands
Dave
NR1DX
On 6/22/2022 4:31 PM, Lux, Jim wrote:
On 6/22/22 12:20 PM, Billy Cox wrote:
Hi Paul,
Below is from pages 12 and 13 of Dean's document, perhaps
this might help to explain what you have observed there?
ACCURACY AND TESTING THE RESULTS
What would I estimate as the “accuracy” of HFTA elevation
predictions? I would say that I would trust the results within
plus/minus 3 dB. In other words, take HFTA results with a grain of
salt. Don’t obsess with changing the height of your antenna by
fractions of a foot to see what happens!
Having said that, now I must state that it is a good idea to compare
elevation patterns in intervals of perhaps 1 foot to assess whether HFTA
is generating reasonably smooth results. Often, the ¼steps used in
the program don’t align exactly and artificial spikes (or holes) can
be created. This is inherent in any ray-tracing program and can only
be eliminated by using extremely small angular step increments —and
doing so would slow down execution even more.
This is an interesting point.. There's a lot more processor HP
available now. There's no reason that one couldn't run 0.1 degree
steps and 30cm increments.
Maybe that's what tools like HOBBIES (mentioned by Jim K9YC?) might
buy you, although from the Wikipedia entry it's more a MoM code and
doesn't deal with far field effects (terrain).
The scattering and diffraction in the far field is a more complex
problem and requires different methods. Jim Breakall, et al., in 1994
"The modeling and measurement of HF skywave radiation patterns in
irregular terrain" in IEEE trans on ant and prop (july 1994)
"The method of moments (MoM) was used in conjunction with the
geometric theory of diffraction (GTD) for predicting the
elevation-plane radiation patterns of simple high-frequency (HF)
vertical monopoles and horizontal dipoles situated in irregular
terrain. The three-dimensional terrain was approximated by seven
connected flat plates that were very wide relative to the largest
wavelength of interest. The plate length along the terrain profile was
the longest possible that still adequately followed the shape of the
path on the azimuth of the elevation pattern of interest and no
shorter than 1 wavelength at the lowest frequency of interest. The MoM
model was used to determine the antenna currents under the assumption
that the terrain was planar (i.e., locally flat) over the distance
pertinent to establishing the input impedance. The currents thus
derived were used as inputs to the GTD model to determine the gain
versus elevation angle of the antennas for HF skywave when situated in
the irregular terrain. The surface wave solution for groundwave was
not included since this does not appreciably contribute any effect to
the skywave far-field patterns at HF in this case. The model
predictions were made using perfect electric conducting (PEC) plates
and using thin plates made of lossy dielectric material with the same
conductivity and relative permittivity as measured for the soil. These
computed results were compared with experimental elevation-plane
pattern data obtained using a single-frequency helicopter-borne beacon
transmitter towed on a long dielectric rope in the far field on a
linear path directly over the antennas. The monopoles and dipoles were
situated in front of, on top of, and behind a hill whose elevation
above the flat surrounding terrain was about 45 m. The patterns of all
of the antenna types and sitings exhibited diffraction effects caused
by the irregular terrain, with the largest effects being observed at
the highest measurement frequency (27 MHz)"
After I do an evaluation for a particular antenna height, I will
often specify an overlay of three heights separated by one foot each.
For example, if you are interested in a single antenna at a height of
80 feet on 14.0 MHz for the K5MA-330.PRO terrain, you might first
compare three heights of 79, 80 and 81 feet, bracketing that height.
The three curves overlaid on each other look relatively smooth,
except there is a 1.4-dB “bump” for the 79-foot height.
Now, run three heights of 80, 79 and 78 feet. Now, the curves for 78
and 79 feet look smooth, but the 80-foot curve has a noticeable dip.
This means that spurious artifacts of the ray-tracing process are
occurring at 80 feet in the program —but these would not occur in the
real world. The solution: don’t use the 80 foot point in the computer
analysis, but you would mount your real antenna at that 80-foot
height if you like the response at 79 or 81 feet.
This gets to the famous quote from Hamming "The purpose of computing
is insight not numbers"
Understanding the limitations and assumptions of the tools is important.
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