I. Introduction
Two-Dimensional discrete-domain infinite impulse response (IIR) beam filters [1] have been proposed for broadband beamforming applications. Such beam filters directionally enhance propagating spatiotemporal (ST) plane waves (PWs) using the concept of 2-D passive network resonance [1]. Here, we propose analog continuous-time 2-D IIR beam filters. This is a mixed-domain method which realizes the analog beam filter in the continuous-time discrete-space (CTDS) domain. The prototype first-order 2-D Laplace transfer function of a 2-D inductance–resistance network [1]–[3] is given byH(s_{x}, s_{ct}) = {R \over R + L_{x}s_{x} + L_{ct}s_{ct}}\eqno{\hbox{(1)}}where resistive termination , is a temporal inductance, and is a spatial inductance [1]–[5]. Here, is the spatial Laplace variable, and is the ST Laplace variable. Note that and are the spatial and ST frequencies (in radians per meter), respectively, and , where is the temporal frequency in radians per second and is the wave speed in meters per second. Transfer function (1) exhibits 2-D resonance along a line-shaped region that passes through the frequency origin in given by (2) and as shown in Fig. 2(a) L_{x}\omega_{x} + L_{ct}\omega_{ct} = 0. \eqno{\hbox{(2)}}