Cross-coupling between open-ended coaxial radiators

A solution is given for the problem of cross-coupling between a pair of open-ended coaxial cables, excited in the TEM mode and terminating in a ground plane. New integral and series representations are derived for the mutual admittance of the TEM modes, and useful low-frequency and far-field approximations are found. Computed results are given for several examples of cables radiating into lossless dielectric media. Measured data is given for a pair of cables radiating in air and computed results agree very well over a wide frequency range. These results show the coupling coefficient increasing initially with frequency, in contrast to waveguide radiators where the coupling coefficient mainly decreases with frequency; the levels attained are significant, particularly above the cut-off of the TE ₁₁ mode. Cross-coupling is shown to be greatest when the inner-to-outer conductor ratio is about 0.53. The solution should be useful for analysing arrays of coaxial horns and obtaining the permittivity and permeability of materials.