Gene Fuller wrote:
Have you actually read and understood that article? Corum mentions
several times that everything he reduces to the simple formulas applies
only to quarter-wave resonance conditions.

Yes, a mobile 75m bugcatcher antenna is quarter-wave resonant.
It is clear that you have not taken time to understand the
paper. Figure 2 looks just like a loading-coil, stinger, and
top hat which is 1/4WL resonant. Note that the coil is conceptually
replaced with a length of transmission line and that's exactly
how mobile loaded antennas work. Here are the conditions:

At the feedpoint is a piece of transmission line with a Z0 of
4000 ohms and a VF of 0.02 - physical length to be determined.

Attached to that is a piece of transmission line with a Z0 of
400 ohms and a VF of 1.0. This element is 8 feet long.

The frequency of operation is 4.0 MHz. What physical length
of the 4000 ohm line will cause 1/4WL resonance?

If you can solve that problem, you will understand how loaded
mobile antennas work. Hint: the delay through the 4000 ohm
section is NOT 3 nS.

Look at the author's highlight between equations 31 and 32. Look at the
discussion near equation 47. Look at the discussion following equation
60. Read the entire discussion in section 5.

I have done that, Gene. A 75m bugcatcher coil falls within
the specified test conditions and thus the VF equation should
be within ten percent accuracy.

Note that he does not say the characteristic impedance is a constant
that can be deduced from resonance conditions and then applied to
operating conditions. In fact, he says exactly the opposite.

Yes, and I have never stated otherwise. The approach that works
is to take a 1/4WL self-resonant coil and use only a percentage
*at the same frequency*. The VF and Z0 will remain approximately
the same as long as we don't change frequencies.

Here is what can be done. Take a 75m bugcatcher coil and extend
the number of turns until it is self-resonant at 4 MHz indicating
that the coil is 90 degrees long. Measure the VF of the coil at
the 4 MHz self-resonant frequency. Remove those extra turns and
calculate the new electrical length. Hint: That electrical length
will be nowhere near a 3 nS delay (technically impossible).

"It is worth noting that, for a helical anisotropic wave guide, the
effective characteristic impedance is not merely a function of the
geometrical configuration of the conductors (as it would be for lossless
TEM coaxial cables and twin-lead transmission lines), but it is also a
function of the excitation frequency."

That's true - Z0 and VF change with frequency. The solution is
to measure or calculate the Z0 and VF at the chosen frequency
of operation. Problem solved!

I am suspicious of anyone's motives who says he believes in
an impossible 3 nS delay through a huge loading coil while
dismissing an IEEE white paper that suggests otherwise.
--
73, Cecil http://www.w5dxp.com