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.
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|