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Broadband Beam-Forming Circuit Using Microstrip Multilayer Couplers

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Multipath antenna arrays require broadband small-sized beam-forming circuits for their implementation. For Butler matrix circuits implemented by means of directional couplers, the models have been developed of the broadband power tap devices using coupled lines. However, as a rule, the current topologies feature relatively large amplitude imbalance within the working frequency band. In order to reduce the flatness of both the working attenuation and the transient attenuation, a microstrip topology of the multilayer directional coupler is introduced using coupled lines. Distinctive feature of this topology is the uncommon configuration of the coupling aperture. The coupling aperture shape is optimized within the 4-8 GHz frequency band according to the criterion of the minimum deviation of the amplitude-frequency characteristics from the ideal values. Within the octave band, the developed coupler features the flatness of the working attenuation and the transient attenuation of 0.6 dB as well as the phase imbalance equal to 1.2°. The maximum overall size of the coupler topology does not exceed a quarter of the wavelength. A four-beam beam-forming Butler circuit based on this coupler is presented. The results of both electrodynamic simulation of the circuit and the experimental studies of the mock-up produced are also demonstrated. It is determined that they are in good agreement with each other. The experimental values of the transient droop and the working attenuation of the matrix circuit deviate from the level of 6 dB by no more than 2.5 dB. The maximum phase imbalance does not exceed 8° from the nominal value.
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Directional Coupler; Microstrip Topology; Coupling Aperture; Beam-Forming Circuit; Imbalance

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