When multiple antennas, operating at different frequencies, are installed on a single platform where the typical inter antenna spacing is a few wavelengths at the lowest frequency, the mutual coupling between the antennas can be optimized by the suitable selection of frequencies and the separation of adjacent antennas. This paper characterizes the dependency of mutual coupling between monopoles on the frequency and separation of the radiating/interfering monopole as well as on the size and shape of the ground plane. The out of band (off-band) characteristics of the monopoles are studied, and the effect of frequency offset between the adjacent monopoles on off-band mutual coupling is summarized. The off-band mutual coupling is reduced by more than 15 dB when the adjacent antenna frequency is selected to be near the fourth harmonic. In the case of smaller ground planes, better isolation of more than 20 dB is possible at intermediate antenna spacing than at the edges. The effect on radiation pattern of an antenna by the proximity of nearby antennas is also studied. The operating frequency/resonant length of the nearby antenna and the inter antenna spacing are found to be the key factors causing variation in radiation pattern. Lower off-band interfering antenna of bigger size is found to have significant effect on radiation pattern at spacing less than 2λ. Analysis has been carried out using FEKO, whose findings are validated using another software HFSS and measurements.
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