Decoupling Network for Closely Spaced Tx/Rx Antennas Using a Directional Coupler and Transmission Lines

Abstract

This paper presents an efficient decoupling network using a single directional coupler and transmission lines for closely spaced Tx/Rx antennas in single polarized radar applications. The isolation between both antennas can be enhanced by controlling the phase of the Tx leakage signal and the coupling coefficient of the directional coupler. Its structure provides no loss of Tx/Rx paths excepting minute loss due to the coupling of the directional coupler. The Tx leakage is more insensitive to the antenna impedance variation compared with a conventional monostatic system. The design process is as follows: First, closely separated Tx/Rx antennas were designed and their isolation ( ${\mathbf{I}}_a$ ) was extracted. Second, the directional coupler was designed with its coupling coefficient $=\left|{\mathbf{I}}_a\right|$ . Finally, both were connected by transmission lines, of which phases were adjusted for the coupled and antenna-isolated signals with 180° phase difference. The Tx/Rx antennas without and with the decoupling network have 15-dB bandwidths of 23.98–24.43/23.98–24.50 GHz and 23.80–24.36/23.80–24.45 GHz, respectively. Gains of 8.17/8.39 and 8.72/8.73 dBi and efficiencies of 60.56%/60.77% and 63.0%/61.8% in the Tx/Rx antennas without and with the decoupling network were achieved at 24.1 GHz, respectively. The dimension of the proposed antenna is 1.62λ0 $\times$ 2.09λ0 $\times$ 0.0204λ0. The K-band antennas show a good Tx-Rx isolation of 29.3–67.9 dB which was superior at least 9 dB to the conventional antennas.