I. Introduction
Transmit-receive (TR) phased arrays can operate in half or full duplex modes. When transmitting in half duplex, as done in pulsed radar systems, a switch is used to route the signal from the final stage power amplifier (PA) to the antenna, reducing the available receive-mode time [1]. Alternatively, a diplexer can be used between the antenna and TR module where the array transmits in an up-link frequency band, which is either closer or farther separated from the receive down-link band. In this case, the spectrum is not used efficiently and the diplexer can be relatively large and expensive, e.g. in cell phones [2]. Full duplex, or simultaneous transmit and receive arrays, usually require circulators, which are bulky, lossy and limited in bandwidth. Additionally, circulators have limited isolation which can be degraded when the impedance of the antenna changes during scanning [3]. Another approach, considered in this paper and illustrated in Fig. 1 is to use switches in each TR module, but use a fraction of the array elements for transmitting, while the remaining elements receive [4]. This digitally-scanned array architecture is reconfigurable in terms of number of T and R elements.