On Mar 31, 10:58*am, "Richard Fry" wrote:
A normal-mode helical with a radiating aperture of 5.25' is not a "270
degree" radiator on 26.5 MHz. *It is a ~ 51 degree radiator on that
frequency.

Sorry, I just noticed you are talking about physical length rather
than electrical length. Do you agree that the helical is 270 degrees
long *electrically* because there are two current maximum points and
two current minimum points on the helical antenna that is 5.25 feet
long?

FP-Imax-////////////////////-Imin-////////////////////-
Imax-////////////////////-Imin

The Imax points are 3.5 feet apart. They are not very far apart
compared to wavelength (~0.1WL) but they are far enough apart to raise
the take-off-angle by 6 degrees for my particular helical according to
EZNEC. With everything else being equal, when a 5.25 foot helical
antenna has more than one current maximum point on the antenna, it
will raise the take-off-angle by an amount correlated to the
percentage of a wavelength spacing between the two current maximum
points.

Conclusion: What you have said seems to be a fact for antennas with
only one current maximum. The presence of two (or more) current
maximum points on the antenna modifies the take-off-angle according to
the laws of radiation physics which is demonstrated by NEC using the
method-of-moments algorithms. A 5.25' end-to-end helical is not the
same as a "~51 degrees radiator on 26.5 MHz" when it has two (or more)
current maximum points separated by, e.g. 0.1WL. The two take-off-
angles are 20% different just as they should be.
--
73, Cecil, w5dxp.com