A Reconfigurable Patch Antenna With Linear and Circular Polarizations Based on Double-Ring-Slot Feeding Structure

This article presents a general design method for the aperture-coupled linearly polarized (LP) and circularly polarized (CP) reconfigurable antennas. A novel double-ring-slot structure is developed as an improved aperture-coupled feeding network. First, an LP antenna integrated with filtering radiation performance is investigated. The double-ring-slot feeding structure not only realizes a bandpass-type filtering response with two deep radiation nulls at passband edges but also ensures wideband antenna impedance matching. Second, by coupling an extra microstrip line to the double-ring slot as a virtual input port, a CP antenna is developed. Finally, by inserting a p-i-n diode as the switch to control the ON-OFF state of the virtual input port, an LP-to-CP reconfigurable antenna is realized. Since no intricate components, such as filter resonators or reconfigurable circuits, are involved, the proposed reconfigurable antenna features a very simple structure and high radiation efficiency. To reveal the design procedure, the working mechanisms of the LP filtering antenna and CP antenna functions are studied and presented individually. For demonstration, the proposed antenna is fabricated and measured. When it operates as an LP antenna, it realized a 35.3% measured impedance bandwidth, 8.2 dBi flat in-band gains, and more than 15 dB out-of-band radiation suppression level, whereas, for the CP antenna operation, 12.6% measured axial ratio (AR) bandwidth is obtained.