7) Influence of metallic plates

In this experiment, the degradation of antenna properties was evaluated when the antenna sample board was set close to a metallic plate. The sample board used here was the standard antenna sample as described in section 1. The antenna sample board was attached to the center of a 500mm×300mm sized copper plate (see Fig.7-1) and the copper plate was set on a turn table in the anechoic chamber. The gap between the copper plate and the antenna sample board varied 10mm, 20mm, 30mm, 50mm and 100mm.

・VSWR properties corresponding to the gap length between the copper plate and the antenna sample is shown in Fig.7-2. The VSWR value at 430MHz and the maximum gain dependence on the gap length is shown in Fig.7-3. When the antenna sample was positioned 10mm from the copper plate, the maximum gain of V polarization in YZ plane becomes –30.07dBd, which is inferior to the original by no less than 18dB.
Furthermore, resonance frequency shifts about 13MHz towards low frequency side. By enlarging the gap length from the copper plate the antenna gain recovers dramatically.
・Since the degradation of antenna properties at 10mm distance was so significant, the antenna was retuned in the state of being set 10mm distant from the copper plate. VSWR property shown in Fig 7-4 was the result of optimization. When the radiation pattern was measured in this state, average gain only improved 1.3dB as shown with a red plot in Fig.7-3.





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