Cecil Moore wrote:
Jim Kelley wrote:
Richard Harrison wrote:
"Since the velocity with which the signal propagates along the
helix wire approximates the velocity of light if the frequency is not
too low (caveat is unimportant, see footnote in book), the axial field
due to the signal advances with a velocity that is very closely the
velocity of light multiplied by the ratio of helix pitch to helix
circumference."

Fig. 7-19 is certainly interesting.

Cecil owes you a fruit basket I think. ;-)

Unfortunately, I must disagree (very slightly) with Kraus.
Using Kraus' concepts *verbatim*, the delay through a coil
would be the same whether the wire is coiled up or straightened
out (if I understand correctly what he is saying).

On my web page at w5dxp.com/current2.htm I have a 30 turn
coil with a diameter of 6" causing a 38 degree phase shift
at 3.8 MHz. If the coil were straightened out, it would be
about pi*6"*30 = 565 inches or 47 feet. Since a wavelength
is about 259 feet at that frequency, 47 feet would be about
65 degrees. So Kraus' rule-of-thumb is off by about 70%.
His VF would be about 0.009 where the actual VF is more
like 0.106. 65 degrees of wire doesn't replace 65 degrees
of antenna. In this case, 65 degrees of wire replaces
38 degrees of antenna. The "missing degrees" are in the
impedance discontinuity between the coil and stinger.

There is an interaction between turns that increases the
VF of the coil so there is a very tiny grain of truth in
what Tom says. The interaction between turns increases the
coil VF from Kraus' 0.009 to the actual value of 0.016
but certainly not all the way to 1.0 as W8JI asserts.

Kraus may have been off by 70% but W8JI is off by 6000%
so it seems that Kraus was still a lot closer to the
technical truth that W8JI ever was.

Cecil,

I don't have the third edition of Kraus' antennas book, but I do have
the second edition. He does not make that simplified statement in the
second edition. He has equations and charts showing how the Vf changes
with the dimensions of the coil and the wavelength. He also references a
paper by Chu and Jackson that is now about 60 years old. In that paper,
the authors show that the Vf increases dramatically as the relative
wavelength becomes longer with respect to the coil dimensions.

73,
Gene
W4SZ