Retardation of electromagnetic waves by helices of large diameters

Abstract

This paper is devoted to the study of the wave retardation by helices of large diameters, when the lengthL of one turn of the helix is larger than the vacuum wavelengthλ, i.e.L/λ>1. The dispersion equation of the tape helix wound up on a dielectric cylinder was derived and the dependence of the phase velocity of slowed-down waves on the diameter of helix was calculated. The theory was verified experimentally at a frequencyf=2·385 GHz. Helices were wound up on polyfoam (ɛ=1·04) and plexiglass (ɛ=2·55) cylinders of various diameters. For the helix wound up on plexiglass cylinders it was found that the wave retardation depends on the form of conductor from which the helix is made. The retardation of waves is determined by an effective dielectric constantɛ _ef which is equal either (ɛ _i +ɛ _e)/2 for the tape helix or √(ɛ _i ɛ _e) for the helix made from a wire of the circular cross-section; here ɛ_i is the dielectric constant of inner dielectric rod andɛ _e is that of outer space.