Acknowledgements

can be observed between the three measurements. The small differences between the data directly acquired in FF and the transformed data are attributed to the lack of application of

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

. The gray

Figure 17. Comparison of the main cuts of the normalized amplitude of the AUT: (a) <sup>φ</sup><sup>¼</sup> <sup>0</sup><sup>∘</sup> and (b) <sup>φ</sup><sup>¼</sup> <sup>90</sup><sup>∘</sup>

shaded areas indicate the valid margin of the NF-FF transformation [23].

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

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

probe correction techniques during the NF-FF transformation (Figure 17) [1].

5. Conclusion

264 Holographic Materials and Optical Systems

required.

proposed.

This work has been partially supported by the Ministerio de Ciencia e Innovación of Spain/ FEDER under projects TEC2014-55290-JIN (PortEMVision) and TEC2014-54005-P (MIRIIEM); by the Gobierno del Principado de Asturias through PCTI 2013-1017 GRUPIN14-114 and by grant LINE 525-002; and by the Academy of Finland through DYNAMITE project. The authors would like to thank M.Sc. Luis Díaz for his help with 3D modeling.
