Ian White GM3SEK wrote:
However, it's perfectly clear that Cecil's estimate of "40-50 turns"
needed for a 75m choke is way off.

Wouldn't you say the number of turns depends upon the
diameter of the coil? How many turns would be needed
for 75m self-resonance if one were using RG-58 wound
on a 3 inch diameter PVC pipe?

Using the inductance formula in the ARRL Handbook,
40 turns on a 3 inch diameter form at 4 TPI is
about 32 uH or about 800 ohms on 4 MHz. The
distributed capacitance would lower it even farther.
40 turns on a 3" form would be an absolute minimum
for any high SWR situation on 75m.

Right conclusion about the choke being useless a long way above its peak
resonance - but not the right reason for it. The true reason is the very
simple property of a parallel LC circuit (the inductance of the coil,
resonated by its own self-capacitance) which means that far above
resonance its parallel impedance drops to a very low value. That's what
makes it useless at those frequencies.

Ian, your own graphs show transmission line effects. The fact
that the phase angle does NOT go to -90 degrees and stay there
proves those are transmission line effects. Here's what the
IEEE paper says:

"The concept of coil 'self-capacitance' is an attempt to
circumvent transmission line effects on small coils when the
current distribution begins to depart from its DC behavior."

"There are a great number of formulae for coil self-capacitance.
None are of particular value for quarter-wave helical resonators
anywhere near the 90 degree point."

Your parallel self-resonance *IS* the 90 degree point. I wish
you would take time out to realize that if there were no
transmission line effects, the phase angle would go to -90
degrees and stay there. Please set up a parallel inductor
and capacitor and see for yourself what happens as one
increases the frequency past parallel resonance. The phase
angle is asymptotic to -90 degrees.

The 4.6MHz choke does have series resonances at about 23MHz 33MHz ,but
those cannot be said to affect the performance of the choke in any
practical way, because the choke doesn't have any usable performance at
these frequencies anyway.

Those grapes were probably sour anyway. :-) Ian, a lumped
circuit inductor and parallel capacitance would NOT have
those series resonances. A lumped circuit would go to a
phase angle of -90 degrees and stay there while the impedance
drops inversely proportional to frequency. Seems to me, you have
just admitted that the chokes you wound are exhibiting transmission
line effects just as I predicted. (Except for the VF error I made.)

THE CHOKE DOESN'T HAVE ANY USABLE PERFORMANCE AT THOSE HIGHER
FREQUENCIES *BECAUSE* OF THE TRANSMISSION LINE EFFECTS!!!

Cecil keeps resolutely ignoring the reservation: "within the practical
working range of the choke".

That's circular logic, Ian. I said that series resonant
transmission line effects limit the practical working range of
a coax choke and you disagreed. Now you have proved I am
right with your own measurements on chokes of your own design.

If those actually were lumped inductors and capacitors as you
continue to assert, THE SERIES RESONANCES THAT YOU MEASURED
WOULD NOT AND COULD NOT EXIST!!!

The series resonant transmission line effects are the *CAUSE*
of the practical working range of the choke being limited.

In a parallel LC circuit, as the frequency increases, the
capacitive reactance becomes dominant and decreases inversely
proportional to frequency. The phase angle would be asymptotic
to -90 degrees. None of the measurements look anything like that.
All of the measurements exhibit transmission line effects.

Within the frequency range where the choke has a practically useful
value of impedance, there are no - repeat NO - signs of transmission
line effects. Its behaviour is purely LC, its own inductance resonating
with its self-capacitance.

This will be the forth time I have said this, Ian. If there were
no sign of transmission line effects, the phase angle would go to
-90 degrees and stay there. If the phase angle doesn't go to -90
degrees and stay there, that is *prima facie evidence* of transmission
line effects. Lumped parallel inductors and capacitors don't exhibit
the effects measured by you on the chokes that you wound.

Scale the high impedance points for 75m and the low impedance
bumps due to transmission line effects will occur in the
middle of HF.

Hands-on measurements prove otherwise - the spurious resonances stay
pretty much where they were, above 20MHz.

Uhhhhh Ian, 20 MHz *IS* HF! The transmission line 1/2WL series
resonant effects are occurring in the HF range! That's why the
75m choke doesn't work on 15m!!! 20-30 MHz is 37% of HF.

As stated above, I measured a choke which is resonant at 4.6MHz. It has
over 1000 ohms impedance at 3.5MHz, rising to over 2k at 4MHz, so it
would be a very good performer on both 75m and 80m.

But it wouldn't make a good performer on 15m because of the
series resonant transmission line effects. That was my point
from the beginning. (The mistake I made was forgetting that the
VF of the choke changes with frequency.)

It is ridiculous for Cecil to describe such a choke as "misbehaving and
a very poor design".

This thread has not been about single-band or narrow-band chokes.
We were talking about broadband chokes being able to cover three
octaves of HF. The bunched coil choke is much more narrow-banded
than helical wound chokes thus rendering them virtually useless
for broadband operation. They *misbehave* in broadband (all-HF)
applications. They are a *very poor design* for broadband (all-HF)
applications.
--
73, Cecil http://www.w5dxp.com