Concerning shunt fed verticals, I shunt feed my (very) short 48 ft HDBX for 260
m. It’s all I had and modeling showed that the shut would have to go to the top
of the tower; I couldn’t figure out how to model my 40CD2, DW3, and KLM KT-34A
that are on it along with the 8 ft mast. So I built it and then tried to
measure the feed impedance with an MFJ analyzer because it’s what I could get
hold of. There’s a 1000 W AM BC station about 4 mi away that caused the
analyzer troubles. I had to take a shot at the sign of the reactive component.
I lashed together an L-network based on what I’d found and it worked. I then
built a motor-drive for a vacuum variable.
I was worried about high voltages at the top, and so slowly increased power
input, looking for any signs of racing across as much of the 160 m band as I
could match. Up to 1500 W I had no problems.
As for modeling, it has limits. As a meteorologist, I know how meteorological
models work. I can assure you that they are never exact because there is no
closed form solution to the Navier-Stokes equations. Thus we must be satisfied
with finite difference, or finite volume approaches. What’s more, we cannot
model all the processes that occur, which requires parameterizations for water
substance and turbulence closure scheme, all of which are approximate.
In theory, we know how it all works, but in practice modeling the process at
fine enough spatial and time scales (think molecular) is beyond anyone and any
computational facility. Thus, my often irritating tag line about theory and
practice.
Kim N5OP
"People that make music together cannot be enemies, at least as long as the
music lasts." -- Paul Hindemith
> On Sep 11, 2019, at 5:51 PM, David Gilbert <xdavid@cis-broadband.com> wrote:
>
>
> 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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