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The influence of RF absorbers in the vicinity of an antenna is investigated in order to find the minimum size for an ultra-small anechoic box necessary for measuring the reactive-near-fields of small rod array antennas. We use the return loss at the port of the antenna measured by changing the separation distance between the antenna and the absorber to evaluate the influence. We find that an antenna's performance is not influenced by nearby absorbers even if the absorbers are placed within the reactive-near-field region of the antenna when the absorbers are properly chosen. A prototype ultra-small anechoic box with outer dimensions of 5 λ is fabricated based on a reactive-near-field probing technique. To evaluate the proposed anechoic box, we compare a 1-GHz bandwidth return loss obtained from the ultra-small anechoic box with that obtained from a large anechoic chamber. They show good agreement and the maximum difference is only 0.28 dB. We also compare far-field radiation patterns obtained by calculating the reactive-near-fields measured in the ultra-small anechoic box with those obtained directly from far-field pattern measurement to confirm that the proposed ultra-small anechoic box is effective for use in measuring and characterizing the rod array antenna performance.