5. Conclusion

Indirect off-axis holography is a method that allows for phase retrieval of an unknown field from amplitude-only acquisitions. This technique has been widely employed for antenna measurement and diagnostics for which phase acquisition is challenging, especially at high frequency bands, where very accurate positioning and high environmental stability are required.

Several modifications such as the modified hologram technique and the use of synthesized reference waves have been discussed, in order to overcome known disadvantages of the conventional technique regarding the required sampling rates or the spectral overlapping issues. Nevertheless, even with these modifications, indirect off-axis holography exhibits some limitations, and thus, three novel methods developed in order to overcome them are proposed.

Two of the presented techniques employ mechanical shifts, the first one to avoid the use of phase shifters and reduce the cost of the measurement system, and the second to control the position of the image terms in the same way that it is controlled with synthesized reference waves but with radiated reference fields. This enables to apply synthesized reference-like techniques in high frequency bands. The last technique is an extrapolation of the conventional technique employed for efficient phase retrieval of broadband antennas in which the phase is retrieved point-by-point in the acquisition plane and simultaneously for all frequency bands, by filtering the hologram in the TD instead of the k-space. Table 1 summarizes the main advantages and disadvantages of the conventional and novel indirect off-axis techniques employed for antenna metrology.

Experimental validation has been presented for each of the proposed methods with very good agreement with the reference results obtained from acquisitions performed directly with amplitude and phase.


Table 1. Main features of the presented indirect off-axis holography techniques for antenna metrology.
