Cecil Moore wrote:
Jim Kelley wrote:
Presumably there is a lower limit to the number of turns the coil
would have to have, or an upper limit to the pitch angle, in order to
behave as described - a helical sheath. Tesla coils usually have at
least a few hundred turns wound closely together, and often operate at
wavelengths considerably longer that 75 meters. One could easily
argue that 30 turns do not a Tesla coil make, in which case Eq. 32
would not apply.

Dr. Corum says that it behaves as a helical sheath when
it is electrically longer than 15 degrees (0.04WL). The
frequency doesn't matter - just the electrical length.

:-) And obviously it's electrical length depends on Vp, which depends on
whether it behaves as a helical sheath.

Note that the title of the paper is: "RF Coils,
Helical Resonators and Voltage Magnification
by Coherent Spatial Modes". "Tesla coil" does not
even appear in the title.

Are you trying to imply that the paper isn't about Tesla coils?

In Dr. Corum's paper, take a look at "Figure 2, A
capacitively tuned distributed resonator" and tell
us how it differs from a 75m mobile antenna with a
top hat.

Hopefully you're not serious. Because, borrowing from Richard Clark,
it's a 'cartoon'.

The 1/4WL self-resonant frequency for a 75m Texas
Bugcatcher coil has been measured at ~6.5 MHz where
it is known to be electrically 90 degrees long. Why
does anyone have a problem with it being electrically
40 degrees long on 4 MHz?

Maybe it is. I happen to think that because of its simplicity, it's an
attractive notion. But it's not clear to me that the article applies to
coils with these parameters, and I haven't seen any (reputable)
empirical evidence to support it.

In "Fields and Waves ...", by Ramo and Whinnery, the
analysis of a helical sheath assumes an infinitely
long helical sheath for the purpose of eliminating
reflections. Does that ring a bell? Hint: The current
on a standing-wave antenna cannot be used to measure
phase shift or delay. Yet, that is exactly what w8ji
and w7el tried to do.

What do you suppose Corum^2 meant when they wrote "Experimentally, the
wave velocity and velocity factor may be measured by determining the
axial length of the standing wave pattern on the helical structure"?

I once turned my 75m mobile Texas Bugcatcher system
into a Tesla coil. I had a latch to which I could
connect the top ball of the antenna when I needed
to lean it over for more clearence. I was at a hamfest
at night and had forgotten I had the antenna latched
down. I started transmitting and my friend told me
I was drawing a two-inch arc from the tip of my antenna
to the pickup body. It was indeed "Voltage Magnification
by Coherent Spatial Modes".

No offense, but some of their work seems aimed squarely at the 'Art
Bell' crowd. Describing constructive interference as "voltage
magnification" is an example. It's as if they were publishing in the
19th century.

ac6xg