Very well said, Jim.
73,
Dave AB7E
On 9/11/2019 3:37 PM, Jim Brown wrote:
On 9/10/2019 4:56 PM, Bob Shohet, KQ2M wrote:
Sure I did my homework ahead of time but my inclination was to build
it and put it up anyway just to see how it performed. I felt better
about it after I modeled it even though I was using software which
was not designed to provide a perfect representation of my qth with
stacking heights over my wildly varying topography in each
direction. And while the antenna on 15 and stacking combos kicked
butt at 109’, a similar stacking proposition for 10 meters with an
antenna at 65’ stacked with others at 100’ and lower heights did NOT
work as the model predicted. In fact the 10 meter antenna at 65’
performed poorly in a stack with all other antenna heights and by
itself in every direction BUT towards Europe.
So “put it up” and try it had great results on 15 and mixed results
on 10, whereas the model which predicted a great stacking pattern in
all directions for both antennas on 10 and 15 also had mixed
results. Ultimately I removed the sidemount @ 65’ for 10 – it was
useless to rotate the antenna at that height for stacking purposes at
my qth – it was never better than another antenna combo. So, no
model is foolproof.
Not quite. This post, and the logic that goes with it, is a great
example of several important concepts/principles.
1) Nearly all real world problems are complex, whether technical,
personal, financial, legal, or political. Wise people know this, and
will reject input that tries to make things too simple.
2) The application of well known fundamental principles to solve a
problem will only be as good as the knowledge of, and the modeling of,
ALL of the variables.
3) Anyone who thinks that "there's a difference between theory and
practice" doesn't know enough about one or the other or both.
4) Nearly all commonly used design equations/formulas are simplified
versions of the full equation. The simplifications are based on
certain assumptions, and if the assumptions are satisfied, the formula
will give a good result.
Example: Commonly used equations for Zo and VF of transmission lines
are simplified, based on the assumption that F is a high number,
leaving F out of the formula, yielding the same value for all
frequencies. But Zo and VF are NOT constant with frequency, and become
increasingly variable at low frequencies. Indeed, VF varies enough
that stubs for 40M and below cut using the simplified formula will
1-2% off frequency.
Example: NEC assumes flat earth and uniform ground characteristics,
and uses ground characteristics in the model. If the ground is not
flat, or if it is not uniform, or even if its characteristics are not
known, or are not entered correctly, NEC will yield some error.
Example: Most hams have conductive elements within their direct field
that can act as parasitic elements of an antenna. Coax from horizontal
dipoles, towers, trees, even wiring in nearby homes can interact with
verticals. If these conductive elements are not known or not
considered, NEC will yield some error. I've posted here several times
the interaction between my tall tower and 160 verticals, and that I
add chokes along the coax from high dipoles to avoid interaction.
Example: HFTA does NOT model interaction of stacked antennas, it only
sums their lobes, assuming that each is a dipole! When a given antenna
is selected (dipole, x-element Yagi) HFTA simply increases the gain.
And the result obtained from the model will only be as good as the
data entered. My terrain is quite irregular -- I don't know how good
the data is nor how closely spaced, and there are limits to the number
of data points.
Example: HFTA can yield errors with some irregular terrain. N6BV has
always recommended running calcs at multiple closely different heights
and azimuths to expose these errors. When I used it to site my
antennas I ran in 10 ft increments to find "sweet spots," then +/- 5
ft. Likewise for azimuths -- I modeled in 5 degree increments to major
directions.
Example: HFTA propagation data is statistical, over long periods of
time, so includes wide variations over the solar cycle(s). AND data
for some bands is interpolated from data for adjacent bands. I think I
recall Dean telling me that 160M data is interpolated.
Example: The combined pattern of antennas in a stack will depend
entirely on the phase response of the combination, including ground
effects. The phase response of an antenna like a Yagi will vary not
only by brand name or generic design, but by every element of the
design, and it varies with angle in both the H and V plane. If we want
to model the behavior of a stack on something as simple as flat earth,
we must enter an accurate detailed model into our design software, and
the model must include accurate details of all matching elements,
phasing lines, and so on!
73, Jim K9YC
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