Input Impedance of a Probe Antenna in a TEM Cell

The input impedance of a probe antenna exciting a transverse electromagnetic (TEM) cell is analyzed via a variational formulation. The resulting impedance is shown to consist of two distinct terms; an ordinary rectangular waveguide contribution and a gap perturbation. Numerically generated curves for both are given and suggest that a simple algebraic approximation for the input impedance should normally suffice. The resistive portion is found to be proportional to the square of the probe length, while the reactive portion is largely capacitive. These results should enhance the use of probes inserted in TEM cells either to excite or to measure fields.