A Low-Profile Antenna With Omnidirectional and Unidirectional Radiation Patterns Over Two Operation Bands

A compact low-profile dual-band antenna with a single circular radiation patch is presented. The circular radiation patch is top-loaded by a metal strip, leading to a zeroth-order resonant (ZOR) mode with monopole-like radiation pattern at the lower operation band, and the ZOR mode is not affected by the position change of the probe feed. Thereby an off-center probe feed can be used to excite a TM01mode with patch-like radiation pattern at the upper operation band, and its performance remains good stability when the size of the metal strip is varying. The equivalent circuit, electric field and current distributions of the proposed antenna are studied to investigate the principles of the ZOR and the TM01modes, and all the results show that the two operation bands can be effectively and independently controlled. As a proof-of concept, a circular patch antenna prototype with low profile of about 0.023 λ0at 2.35 GHz is fabricated and tested. Good agreements between the measured and simulated results are achieved, showing monopole-like and patch-like radiation patterns within 2.28 - 2.40 GHz and 3.30 - 3.60 GHz, respectively, and the average realized gains are about 2.5 dBi and 8.5 dBi for the two operation bands.