Design of dual-band cavity-backed slot antenna loaded with spurline

A design of dual-band cavity-backed slot antenna loaded with a spurline is presented for GPS and S-band radar applications. In the design, a very thin cavity (0.017 in thickness at 1.575 GHz) is used to achieve unidirectional radiation. The dual-band responses of the antenna are excited by the slot resonance at 1.575 GHz and waveguide transition effect at 2.4 GHz. Reasonable impedance bandwidths are obtained as 1.6% (26 MHz) at 1.575 GHz and 8.4% (203 MHz) at 2.4 GHz. The proposed slot can also be loaded with a spurline to adjust the antenna centre frequency and the phase of the radiated field. A switch using a PIN diode is deployed with the spurline. It is found that the spurline and the corresponding bias circuit for the PIN diode have little effect on the antenna radiation. Two proposed slots are combined to form an antenna which is right-hand circularly-polarised at both 1.575 and 2.4 GHz. A phase delaying system consisting of delay line and the spurline is designed to achieve 90° phase difference at both frequencies. The measured axial-ratio bandwidths are 1.5% (24 MHz) at 1.575 GHz and 4.5% (110 MHz) at 2.4 GHz, which are sufficient for practical applications.