Far field extrapolation in the boresight direction from near field samples to improve the signal integrity in time domain measurement

Antenna pattern measurements in time domain can provide broadband frequency domain pattern data. In some applications there is a need to know the transfer function of an antenna in a certain frequency range of interest in the boresight direction. In this case one transmits a time domain waveform from the antenna under test (AUT) and receives the signal from a known receive antenna. From the received signal and the receive antenna transfer function it is possible to determine the transfer function of the AUT. The radiated far field exhibits spherical wave behavior in the form exp (-jkr)/r, where r is the distance between the two antennas and k is the free space wavenumber. If the spacing r does not satisfy the far field criterion, the radiated field varies with r in the form of a series in inverse powers of r (J. E. Hansen, Spherical Near-Field Antenna Measurements, Institution of Engineering and Technology, 1988), thereby resulting in signal distortion. An extrapolation technique was presented for determining the gain and polarization of an antenna from measured field amplitudes only at reduced distances at a single frequency (A. C. Newell, R. C. Baird, P. F. Wacker, IEEE Transactions on Antennas and Propagation, vol. 21, no. 4, July 1973). A recent study focused on this problem for a monopole antenna using an averaging technique in the intermediate distances (R. D. Tamas, L. Babour, A. Danisor, and G. Caruntu, Proc. IEEE International Workshop in Antenna Technology, 2010). We found that their method is not suited to apertures and other extended antennas.