Far-field antenna patterns determined from infrared holograms

We describe a technique based on optical holography which allows determination of the amplitude and phase of an unknown antenna on a near-field plane from amplitude-only measurements. We measure the interference pattern between the fields radiated by the antenna under test (AUT) and a known reference field. The reference field is produced by a standard gain horn radiating at an angle to the AUT and positioned so that the peak of the radiation occurs approximately at the same place as the peak from the AUT. The fields are detected by a resistive screen which absorbs some of the incident energy and heat as a function of the electric field intensity distribution. We use an infrared camera to record the temperature distribution caused by the interference of microwave energy radiated by the reference and the AUT. Data are processed using an enhanced algorithm based on conventional holography for recovery of the complex near field. The new algorithm allows elimination of the spurious images commonly present in optical hologram readouts using illumination of the hologram with the reference wave.