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.

This is making no sense to me so I fear we have some sort of semantic
problem. I'm now not sure what you mean by "a radiating aperture of
5.25 feet". "The IEEE Dictionary" says: "In some cases, the aperture
may be considered to be a line." I was assuming that the 5.25 feet
aperture was akin to a line of straight wire 5.25 feet long or a 5.25
foot long (end to end) helical monopole. If that is not the case,
please enlighten me on your definition of "aperture". EZNEC says my
5.25' (end-to-end) physically tall helical monopole is electrically
270 degrees long.

I assumed that 5.25' is the length of a straight wire or the physical
end-to-end length of the helix itself (not the linear length of the
wire). The velocity factor of a helix is a function of the helix
geometry and *varies widely with diameter and turn spacing*. The helix
I designed using EZNEC has a current maximum at the feedpoint, a
current minimum 1/3 of the distance up the helix, a current maximum
2/3 of the distance up the helix, and a current minimum at the end of
the helix. That's 270 electrical degrees any way you cut it because
*there is always 90 electrical degrees between the current maximum and
current minimum in a standing wave*.

The requirement that a 5.25' tall helical monopole has to satisfy to
be 270 electrical degrees long on 26.5 MHz is to have a velocity
factor of 5.25/27.85 = 0.1885 which is a piece of cake. The 5.25' is
the actual end-to-end height of the helical monopole and the 27.85' is
3/4 of a wavelength in free space at 26.5 MHz.

Note that the velocity factor is the distance a traveling wave travels
in the helical medium in unit time compared to the distance a
traveling wave travels in free space in the same unit time.

Richard, this is giving me a headache - what am I missing?
--
73, Cecil, w5dxp.com