Transmission Lines at Very High Radio Frequencies

TLDR: In this paper, the optimum design of the line, whether used as a low-loss inductive or capacitive reactance or to give high selectivity or high impedance as a resonant line, is analyzed for both parallel-wire and concentric lines, and curves are included showing radiation resistance, selectivity, input impedance, optimum values of spacing, conductor radius, etc.

Abstract: Radiation resistance, although ordinarily neglected, is actually of dominant importance in determining the selectivity factor Q and the input impedance Z s for both parallel-wire and concentric lines at high radio frequencies, and therefore materially changes the optimum design of the line, whether used as a low-loss inductive or capacitive reactance or to give high selectivity or high impedance as a resonant line Accurate design equations for maximum selectivity and for maximum impedance are developed in this paper for both parallel-wire and concentric lines, and curves are included showing radiation resistance, selectivity, input impedance, optimum values of spacing, conductor radius, etc It is shown that for maximum Q the optimum ratio of spacing to wire radius for parallel-wire lines is D/r = 6186, and the ratio of outer conductor radius to inner conductor radius for concentric lines is b/a = 422, as compared with values of about 36 for both ratios when radiation resistance is neglected For maximum impedance corresponding values are D/r = 2096 and b/a = 143, as compared with values of 8 and 92, respectively, predicted neglecting radiation resistance Moreover, Q and Z s are not proportional to D and b, as indicated in previous analyses; instead definite values of D and b give maximum Q and slightly larger values give maximum Z s , and even a small departure from the best value produces a large decrease in Q or in Z s