Design and Implementation of a Planar Slot Antenna for SSR

A useful, low cost, low complexity, and one layer microstrip slot L band antenna for secondary surveillance Radar (SSR) application is presented. The printed circuit feed network is a rat-race divider to create sum and difierence ports and sixteen unequaled power dividers to satisfy the Chebyshev distribution. The feed network has an equally length parallel structure that create constant group delay in all output ports and wide frequency bandwidth. The suggested structure is a one-layer slot array of 2 £ 8 slot elements that are fed with a microstrip corporate feed network. To improve total frequency bandwidth the slots are designed with unequal lengths. A substrate with low permittivity is used to decrease the side lobes. A 2:1 VSWR bandwidth of 9.4% is achieved with the presented geometry in simulation to cover the 1030§10MHz transmit and the 1090§10MHz receive frequency band. The antenna gain for the sum pattern is 18.5dB and 19dB at 1030MHz and 1090MHz, respectively with a 3dB beam width of 52 - in elevation and 9.5 - in azimuth. The difierence has a i19dB null at the peak of the sum pattern and an acceptable pattern against the sum pattern for SSR applications. Furthermore, the antenna has no need of the back flll antenna to eliminate the back lobe, which is critical for SSR applications. Its low complexity structure makes it appropriate for many radar applications. 1. INTRODUCTION The Secondary surveillance radar (SSR) mechanism is based on sending a pair of signals at the frequency of 1030MHz for interrogation and receiving a pair of signals as an answer at 1090GHz (1). SSR antennas are built in the form of planar or cylindrical array (2). Re∞ector and multi-layer antennas are the most common SSR antennas in the literatures that have some disadvantages like complexity and being bulky (3,4). One layer microstrip array antennas which is proposed in this work, are the useful antennas in mass production because of their advantages like easily printed, light weight and low cost which makes them suitable for radar applications. However they have some disadvantages like narrow bandwidth and high side lob level. To enhance the bandwidth, many attempts have been done in microstrip fleld. One method of increasing the bandwidth for the microstrip antennas is using stacked elements (5). (6) proposed a stacked patch array of 1 £ 8 elements with 14dBi gain and the SLL of i14dB, which is not enough low to provide the Side Lobe Suppression (SLS) for the SSR (1). In addition using stacked element increase the complexity of the system. In this paper a slot array antenna for the secondary surveillance radar application is presented. The antenna structure against the stack patch antenna is very simple. The fed slot antenna could increase the bandwidth in addition of reducing the antenna size. However, the fed slot antennas produce back radiation. To omit the back-lobes in the microstrip antennas, a metallic box is used behind the antenna. This technique is used to enhance the performance of the proposed antenna against with standard SSR antenna. The feed network has an equally length parallel structure that create constant group delay in all output ports and so cause wide frequency bandwidth. At flrst the performance of the SSR is described. Then the proposed antenna parts such as: single element, the sum pattern, the difierence pattern and the feed network is presented. Finally the simulation results using in 3D EM simulation software High Frequency Structure Simulator (HFSS) are presented. The substrate that is used in the design is Rogers RT Duroid5880 with the dielectric constant of 2.2 and the thickness of 0.787mm. Also the conductor cladding is about 17"m. 2. ANTENNA CONSIDERATIONS