Angle of arrival estimation using hybrid rat race coupler for X-band applications

Abstract

The conventional approaches to estimate the angle of arrival (AOA) have the limitations of hardware complexity, and where algorithms are used to optimize the estimation, there is the associated computational cost. This work presents a simplified approach in which the sum and difference of the received signals using a hybrid rat race (HRR) coupler forms the basis for the estimation of AOA. A 2 × 1 array designed to operate at 10 GHz is fed with quarter wave transformer impedance matching technique and is integrated with HRR coupler. The integrated array is evaluated for AOA estimate in both the azimuth and elevation planes. The transmit antenna foot print of 0° to ± 90° is considered. Result of the study indicates that the designed HRR coupler works optimally, exhibiting minimal reflection loss at each port, reasonable isolation between ports and acceptable phase difference at the sum (Σ) and difference (Δ) ports. The array exhibits a resolution capacity of ± 55° in the azimuth plane and ± 48° in the elevation plane, with an RMS error of less than 5° in both the planes. Beyond the estimated AOA, the array exhibits very high values of RMS error. Typically, in the transmit antenna footprint of ± 80°, the RMS error is as high as 42.5°. An important observation from the study is that the RMS error is independent of the position of the radiators in space, geometry of the array elements, and mis-orientation in the planes of the array. For a specific transmit antenna footprint of + 15° in the azimuth plane, the gain is optimal compared to that in the elevation plane. The proposed design is easy to implement, without the complexity of scanning or phased array. In addition, the absence of algorithm of any type eliminates the computation cost issue that is common in estimators based on adaptive beam forming and smart antenna.