Coax transition to annular ring for reduced input impedance at 2.4 GHz and 5.8 GHz

Annular ring patch antennas provide a compact radiating structure in their TM/sub 11/ mode. However, the compact resonant size of the lowest order TM/sub 11/ and TM/sub 21/ modes comes at the price of an input impedance larger than the conventional 50 /spl Omega/ coaxial feed. In the past large matching networks and proximity coupled feeds have been used to reduce the input impedance of small resonant rings. In this paper input impedance matching to an annular ring is demonstrated by feeding a 50 /spl Omega/ coax line through a short microstrip line that is proximity coupled to the antenna. Designs were implemented using the HFSS software package. Measured S/sub 11/ results are presented for designs in both the 2.4 GHz and 5.8 GHz ISM bands. These antennas are designed for a minimum area, and hence in each case a single ring is considered over a truncated square ground plane. These antennas are intended for low profile compact applications appropriate for local wireless networks of interconnected low cost sensors.