A comparison of software- and hardware-gating techniques applied to near-field antenna measurements

It is well-known that antenna measurements are error prone with respect to reflections within an antenna measurements test facility. The influence on near-field (NF) measurements with subsequent NF to far-field (FF) transformation can be significantly reduced applying soft- or hard-gating techniques. Hard-gating systems are often used in compact range facilities employing fast PIN-diode switches (Hartmann, 2000) whereas soft-gating systems utilize a network analyzer to gather frequency samples and eliminate objectionable distortions in the time-domain by means of Fourier-transformation techniques. Near-field (NF) antenna measurements are known to be sensitive to various errors concerning the measurement setup as there have to be mentioned the accuracy of the positioner, the measurement instruments or the quality of the anechoic chamber itself. Two different approaches employing soft- and hard-gating techniques are discussed with respect to practical applications. Signal generation for the antenna under test (AUT) is implemented using a newly developed hard-gating system based on digital signal synthesis allowing gate-widths of 250 ps to 10 ns. Measurement results obtained from a Yagi-Uda antenna under test (AUT) and a dual polarized open-ended waveguide used as probe antenna are presented for the GSM 1800 frequency range.