Advanced Butler Matrices With Integrated Bandpass Filter Functions

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Abstract
A novel class of Butler matrix with inherent bandpass filter (BPF) transfer functions is presented in this paper. The Butler matrix is the fundamental network to split and recombine the signal in multi-port power amplifiers, however, to suppress spurious frequencies generated by the amplifiers or to provide near-band rejection in order not to interfere with other transmission/receiving bands separate filtering is often required. Here, the traditional power division and phase distribution of the Butler matrix are included together with filtering selectivity into one single device based only on coupled resonators. An analytical synthesis procedure of the coupling matrix for 2 k ×2 k networks is presented here for the first time. The proposed solution has shown significant advantages in terms of size reduction compared to the traditional baseline consisting of a distribution network plus a bank of BPFs. The synthesis and design of a 2 × 2, 180 ° hybrid coupler at 10 GHz and a 4 × 4 Butler matrix with an equal-ripple four-pole Chebyshev bandpass characteristic centred at 12.5 GHz with 500-MHz bandwidth are described, confirming the synthesis technique proposed. Two models of the 4 × 4, one built with additive manufacturing and the other with milling, are also presented and compared. Experimental measurements are in good agreement with both simulations and theoretical expectations.

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