A Compact Hybrid-Fed Microstrip Antenna for Harmonics-Based Radar and Sensor Systems

In this letter, we propose and experimentally validate a compact hybrid-fed microstrip antenna for the harmonics-based radar and sensor systems, which receive the fundamental tone and retransmit the modulated second harmonic. The proposed microstrip antenna is based on a simple single-layered and dual-feed structure, consisting of an inner circular patch operating in the TM110 mode (second harmonic; 6 GHz) and an outer split-ring patch operating in the higher order TM210 or TM310 mode (fundamental frequency; 3 GHz). By varying the feed position and the geometry of the split-ring patch, a specific higher order mode can be excited at the fundamental frequency. Moreover, analytical expressions were derived for calculating the antenna's resonant frequencies. Our measurement results show that the maximum realized gain is 1.17 dB at 3 GHz and is 3.33 dB at 6 GHz, with good impedance matching (vertical bar S-11 vertical bar < -15 dB) and high isolation (< -20 dB).