Topband: 160 metre vertical with 'top loading'

[Dan Zimmerman N3OX]

W4EF writes:

My suspicion is that some taper does occur in air wound loading coils at
> HF frequencies, but that the amount of taper doesn't follow the simple
> rule that it equals the amount of taper that would occur in the length
> of straight radiator replaced by the inductor. I think the degree of
> taper depends on the velocity of EM wave propagation through the coil
> and to some extent on the amplitude and phase of the displacement
> current from the inductor to ground (It wouldn't surprise me if there is
> an interdependence between the EM wave velocity and the magnitude and/or
> phase of the displacement current). These two quantities are probably a
> function of the length and form factor of the inductor.



Yep, and I think this issue is quite interesting.  There's this paper by
Corum and Corum, Reference [1] on ON4AA's inductor calculator page, that
attempts to quantify the physical form-factor dependence of
EM propagation characteristics on a helix.

ON4AA's page is here, with some good information on the topic:

http://hamwaves.com/antennas/inductance.html

ON4AA's calculator uses the formulas of the aforementioned paper (among
others) to make some non-trivial predictions about how coils behave.  I
would like to test these specific predictions quantitatively.


> That said, it
> would not surprise me in the least that for inductors that are
> physically small relative to the overall radiator length, the amount of
> current taper across the length of the inductor is negligible (i.e.
> current at the top and current at the bottom are for all practical
> purposes the same). I could be wrong, of course. That I will readily
> concede.
>

If the Corum sheath-helix model is a good description of what's going on,
the velocity factor of these coils can be surprisingly low in some cases.
The #18AWG 10 turn per inch, 2 inch diameter miniductor that was the subject
of much scrutiny should have a velocity factor around 4% at 3.8MHz.  The
coil modes can therefore have very short spatial wavelengths, changing
things a lot over the length of the coil.

According to the model, a coil shows a lot of  dispersion and the
characteristic impedance of propagating fields is very high (thousands of
ohms) so measuring the "velocity factor" is considerably more difficult than
it would be for a chunk of ordinary transmission line.  To act as a simple
delay line, the coil must be terminated in its characteristic impedance,
which is itself a strong function of frequency.  ON4AA's calculator makes
the model pretty accessible, giving the characteristic impedance and the
axial propagation factor Beta with fields going like E = E0*exp(i*Beta*z),
with "z" the distance along the coil.  The coil mode wavelength is lambda =
2*pi / Beta.  It's about 3.5 meters for the coil mentioned above.

I've used some rather skeptical language above ("if the model is correct,
according to the model," etc.)  This is not because I think the Corums'
model or ON4AA's  implementation and incorporation of extensions (like some
of G3YNH's work: http://www.g3ynh.info/zdocs/magnetics/part_1.html)  are
wrong.   It's simply because I haven't verified the calculation in detail
nor have I been able to test the predictions to my satisfaction.    I think
the models have predictive power and I think quantifying current taper is
potentially one of those things.  But any observed current taper depends
BOTH on coil mode effects and on external influences, including the antenna
particulars.

I think it would be most useful for the community if we can focus some
testing on specific predictions from proposed mechanisms for current taper.
 There are a lot of possibilities, some of which are inherent to the coil
and antenna setup, and some of which are plausibly dominated by measurement
perturbations.  I'm particularly interested in this sheath helix model for
propagation on a coil.  There are several rather non-trivial things it
predicts.  One important one is the sequence of successive coil resonances.
 In some sense (one that may make a certain Texan a bit unhappy with the
language I'm using), the model is intended to be a self-consistent wave
description of the "self-capacitance" or "inter-turn" capacitance of the
coil.  The coil is linked together in a complex way by the electromagnetic
field around it... so you can't just draw in little fixed capacitors in
between adjacent turns.  But the electric field is always there when there's
a time varying magnetic field.

The sheath helix model is **not** only valid near self-resonance, but
self-resonances are one of the easier ways to test the model.  I have not
yet had satisfactory results in this area, but I haven't even come close to
doing anything I would consider conclusive.
I've just dabbled with a couple ideas while thinking about other ways to
test.

 I think this is a really interesting problem.   I don't generally make much
noise about it because it will take a lot of work on my part to convince
myself one way or the other about what's happening (pure slow coil modes.
vs. pure external perturbations vs. BOTH or "other"), and I have seen many
discussions go sour.  So mostly I stay out of it, because I don't feel like
I have much meat to contribute.   But I think some of this is news to some
people, and it seems worth mentioning the possibility that there's rather
slow EM propagation on some "typical" coils.

I think it's funny and a little telling that the best piece of experimental
evidence I can find that the sheath helix predictions work is on a page
purporting to show the absence of such effects.  But that's a story for
another time, **after** I know that it's possible to reproduce the result I
think I see there.

K2AV says:

If two responsible experimenters get opposite results in what on
> surface are identical experiments, THERE IS A REASON, which means
> THERE IS AN OPPORTUNITY TO DISCOVER AND ADVANCE


That's exactly how I feel about it.   It seems that a couple other parties,
(not yet present here), feel strongly like they need to protect the ham
community from the dangers of wrong thinking instead.  Real, useful
skepticism requires thinking the wrong thing and the right thing
simultaneously for a long time while you're testing or digesting the results
of tests.

73
Dan