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This paper presents a novel method for cross-band scattering cancelation in closely-spaced antennas inspired by absorptive/reflectionless filtering principles. Prior-art approaches that combine a bandpass-filter (BPF) unit and an antenna in multiple-input multiple-output (MIMO) systems can improve cross-band isolation between the antenna ports. However, such procedure is ineffective in suppressing cross-band scattering, which leads to radiation-pattern distortion. This is because in closely-packed antennas each radiating element behaves as an external source for the others, so that it induces currents into them. To overcome this problem, a method originating from absorptive/reflectionless filtering techniques is reported here and applied to a dual-antenna scenario. In complementary-diplexer-based reflectionless BPFs, an absorptive bandstop-filter-(BSF)-type branch loaded by a resistor is utilized to absorb the out-of-band RF power not transmitted by the main BPF channel. By using this concept in a first patch-type antenna and owing to the reciprocity principle, the RF coupled energy or induced currents in the first antenna due to the surrounding antenna can be absorbed. Thus, radiation-pattern distortion for the surrounding antenna is avoided. Equivalent-circuit, simulated, and measured results are provided to show the effectiveness of this engineered scattering-suppression method, which is believed to be promising for application in cross-band multi-antenna systems.

Scattering Suppression of Closely-Packed Cross-Band Patch Antennas Using Absorptive/Reflectionless Filtering Techniques