Art wrote:
"I stated "intuitively" since the radiation is axia with respect to the
helix."

A 3 M diameter has about 9.42 M circumference so becomes an effective
axial radiator instead of a radial radiator at the frequency near where
its circumference becomes one wavelength. This frequency is
300/(pi)(diameter) or about 10 MHz. A reflecting surface to make an
axial helix unidirectional needs to be at least as large as the
projection of the helix on the surface of the reflector. Its diameter
should then be about 3 M or more and would be effective only at
frequencies exceeding 10 MHz which is where the helix becomes an
end-fire device anyway.

Best regards, Richard Harrison, KB5WZI