Question

What is the necessary condition for a helical antenna to operate in the axial mode?

(a) πD << λ

(b) πD ≈ λ

(c) πD ≈ nλ

(d) πD=0

This question was posed to me in a national level competition.

The doubt is from Frequency Independent Antenna in division Frequency Independent Antenna of Antennas

Answer 1

The correct choice is (b) πD ≈ λ

For explanation I would say: In Normal mode: πD << λ

       Axial mode:  C ≈ λ

      Conical mode: C ≈n λ

D=0 then there is no loop and is no longer helical.