Cecil Moore wrote:
On Mar 29, 5:35 pm, "Richard Fry" wrote:
Quoting from Antenna Engineering Handbook, 2nd Edition by Johnson and Jasik,
page 13-18: "For a normal-mode helix whose dimensions are small compared to
a wavelength, the current distribution along the helix is approximately
sinusoidal."

But John, a helix that is 180 degrees long electrically is not small.
It is electrically double the size of a 1/4WL monopole.


"small" in Kraus's book means "physically" small, not electrically small.


Therefore it is unclear as to the source of
this belief that current would be maximum at the center of "1/2-WL" helix
whose end-end length is 1/4-WL. In reality the current maximum would be at
the base of the radiator, just as it is for a 1/4-wave linear monopole.


Take your NEC helical model and adjust the frequency to approximately
double the resonant frequency and take a look at the current
distribution.



This is no different than taking the "non-helical" antenna and feeding
it at twice the frequency.

I would imagine that the pattern of the helically loaded and the
unloaded will be quite similar at ANY frequency, until you get to where
the *diameter* of the assembly starts to be a significant fraction of a
wavelength.

What might change more is the resistive losses, although I suspect
they'll scale in proportion too. Whether you've strung 10 meters, 20
meters or 30 meters of wire in a physical 10 meter length doesn't change
the *radiation* properties a huge amount.