Computation of Far-Field Impulse Response of an X-band Waveguide Slot Array using the Finite-Difference Time-Domain Method

The waveform in the far field of an antenna array can be obtained using the convolution of the input signal and the antenna far-field impulse response. Since it is very difficult to measure an antenna far-field impulse response, a numerical approach, i.e., the finite-difference time-domain (FDTD) method, is used in this paper to calculate an antenna far field impulse response. It is well known that the FDTD method models electromagnetic problems in both spatial and temporal domains. In conjunction with the time domain near-to-far field transformation, the FDTD method can be used to compute an antenna far-field radiation waveform at any angular location. With the far-field waveform computed by the FDTD method, this paper presents the deconvolution procedure to obtain an antenna far-field impulse response. A 54-element waveguide slot array of X-band navigation radar is used as an example. Its far-field impulse response as a function of the radiation angle is computed using the FDTD method. The far-field waveforms of an FM pulse transmitted by the waveguide slot array are calculated using the convolution of the FM pulse and the array far-field impulse response. The instantaneous frequencies of these far-field waveforms at different radiation angles are examined in details. It clearly shows from analysis results that the instantaneous frequencies at the leading and trailing edges of the received pulse have been distorted. The amount of frequency shift is related to the antenna radiation pattern at different radiation angles