Isolation improvement for x-band FMCW radar transmit and receive antennas

In this research, improvement on antenna isolation for frequency modulated continuous wave (FMCW) radar by adjusting side lobe level (SLL) and air gap distance is presented. The radar has two separate and parallel antennas: transmit and receive antennas. Low SLL will introduce high antenna isolation. For achieving a good performance, the required SLL is set to a minimum of 13 dB to produce high isolation and maximum reflection (echo) on the antenna main lobe. The Chebyshev method was applied for improving the main lobe efficiency, which is by modifying incoming power distribution on each patch. The patch shape was rectangular and this shape is modified by inserting slots to change polarization direction from vertical to horizontal polarization. Antenna feeding is performed by using the proximity method that aims to reduce radiation effect from transmission lines. This results in low interference to the main antenna radiation. The SLL for Chebyshev method is about -30dB and, for Uniform distribution, the SLL is about -13dB. The antenna development was started from defining its configuration, design and simulation, and finalized by optimization. From the simulation results, Chebyshev method gives low SLL and higher isolation in comparison to that of uniform method. For the Chebyshev antenna array with a distance of 9cm, the value of S12 equals -77.18048 dB and S21 equals -77.18053 dB. While for uniform antenna array with 13cm air gap, the S12 equals 75.864015 dB and S21 equals -75.86401dB. Lower values of S12 and S21 show higher antenna isolation. From the experiments, it was shown that the air gap distance is not linearly related to the isolation level. There is a certain distance where the optimum isolation value can be achieved. For the Chebyshev antenna array, it was shown that the simulation and measurement results are very similar.