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At large densities or large distances from the transmitting antenna it is observed that resonance cones, instead of being a single sharp peak, become spatially modulated by a multiperiod interference pattern; this interference pattern can dominate over the phasing imposed by a grill antenna. A simple theory shows that this effect occurs when ωper / c≫1 and that it is caused by electromagnetic terms neglected in the electrostatic approximation.
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Paul M. Bellan (1980) studied this question.
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