This paper discusses a method to evaluate mutual couplings of cavity-backed slot antennas using the FDTD technique. The antenna fed by the short-ended probe is considered, which is investigated as an element of the power transmission antenna, Spacetenna, for the solar power satellite SPS2000. It is found from the FDTD computation on E-plane two- and four-element array antennas that the size of the problem space should be larger for the evaluation of the mutual coupling than for the estimation of the input impedance. Since enlarging the size of the problem space requires a large amount of computer storage, it is not practical for computer simulations. In order to carry out accurate estimations of the mutual coupling with relatively small amount of computer memory, the problem space is extended only in the broadside of the array antenna and in the other directions there are ten cells between the antenna surface and the outer boundary. Computer simulations demonstrate that there are no differences between the results of the proposed problem space geometry and the problem space extended in each direction of the axis coordinate by the same number of cells. Furthermore comparisons of computed and experimental results demonstrate the effectiveness of the approach after discussing how large the size of the problem space is required to estimate the mutual coupling.

To increase the accuracy of a near field antenna measurement system, it is necessary to know radiation characteristics of a probe to detect near field data. Open ended waveguide used as a near field probe in our system was analyzed using Transmission Line Matrix (TLM) method which is a time domain electromagnetic solver. Validity of this analysis has been confirmed by comparison with experimental data and existing theoretical approximation. Frequency dependence of a complex reflection coefficient at the waveguide aperture has been derived and is shown to agree with measured values. The radiation pattern of the open ended waveguide with mounting structure is also calculated. Ripples on both the amplitude and phase patterns are correctly predicted by our simulation. This method can be applied to accurately model the effect of probe antennas to enhance the accuracy of near field antenna range.