The characteristic impedance and phase velocity of a shielded helical transmission line

This paper presents the results of a theoretical and experimental investigation of the properties of a shielded helix when used as a transmission line. Since such a structure is dispersive and because of the presence of both TM and TE modes it cannot be said to have a characteristic impedance in the true sense. However, such an impedance is defined for low frequencies in terms of a distributed capacitance and inductance. These parameters, the capacitance and the inductance are determined in the case of infinite wave length for round wire helices. The equations are developed for a uniform dielectric inside the shield and for two dielectrics, one inside the helix and the other between the helix and the shield. The analytic results are then shown to agree very well with the experimental results obtained by the author.