[TowerTalk] "experts" on loading towers on low bands

[jimlux]

On 9/10/19 10:12 AM, Jim Brown wrote:
> On 9/10/2019 7:57 AM, John King via TowerTalk wrote:
>> who are so highly educated and possessing the "ultimate factual truth 
>> on the subject of feeding a tower" to "pooh pooh EVERY other hypothesis
> 
> The various methods of feeding a tower are not hypotheses -- that is, 
> ideas or concepts to be proved. Rather, they are applications of the 
> fundamental principles of physics -- our understanding of how things 
> work, developed as a result of scientific study over hundreds of years, 
> the results of which are studied in an organized fashion, and written 
> down so that others can learn from, apply, and even study further to 
> learn about the subject in question.
> 
> Sometimes statements or recommendations made here (and elsewhere) are 
> the result of erroneous understandings of the fundamental principles. 

I think, often, questions (or disputes?) arise because of a specialized 
or oversimplified conceptual model that works for a large subset of 
cases, but breaks horribly when moving past the (poorly documented) 
validity boundaries.

Look at all the explanations of why a resonant dipole (defined as where 
the reactive term of the feedpoint impedance is zero) isn't exactly half 
a wavelength long - This result is derivable at some length from 
Maxwell's equations if you care to do it, but that derivation doesn't 
provide a conceptual simplicity.  It's Much easier to talk about "end 
effects" or some idea of parasitic C, even if that's not really what's 
going on.

Likewise various ways to describe why 2 element Yagi antennas have gain, 
whether it's actually a "reflection" from the reflector, or mutual 
coupling producing an appropriately phased field radiated from the 
longest element.  In this specific example, there's a confusion because 
there *is* an apparent connection to a dipole in front of a screen, or a 
dipole in front of a "reflector".  The "reflection" analogy works great 
for a metallic screen - you can imagine a propagated wave reflecting off 
a mirror - the fact that at the core, that reflection is actually 
because of currents induced in the reflecting plane is a bit less obvious.

Similarly, there's all kinds of explanations of why particular feed 
methods work - some are useful for helping empirical design and 
adjustment with limited test tools - some are useful for treating the 
network mathematically - this is especially so when the feed network 
itself radiates.


If I could give an example of a poor conceptual model that has had 
amazing sticking power over the decades it's the "quarterwave" 
explanation for tesla coil secondary resonance.  As it happens, for 
coils of conventional sizes, the length of the secondary winding wire 
happens to be close to 1/4 wavelength at the self resonant frequency of 
the coil.  But it's coincidence.
You can model a Tesla coil secondary very accurately as a collection of 
lumped elements with distributed L and C. (and you don't need a very 
large collection, 1 L and 1C gets you within 5%)

But still, there are "propagation" related explanations attempting to 
show that there's a traveling slow wave propagating along the length of 
the secondary which is some sort of LC transmission line.  Yeah, you can 
make that model work, but it doesn't give good insight into what's 
actually going on in the TC secondary (energy transfer from L to C and 
back again, with sparks growing when the energy is in the C)

Similar, all the discussions about loading coils in vertical antennas.