Dual-frequency and broadband circular patch antennas with a monopole-type pattern based on epsilon-negative transmission line

Dual-frequency and broadband circular patch antennas based on epsilon-negative (ENG) transmission line (TL) with omnidirectional patterns are presented. The ENG TL is achieved by placing mushroom structures along its circumference. Compared to the conventional circular patch antenna whose fundamental operation mode is TM ₁₁ mode, the fundamental operation mode of the new antenna is TM ₀₁ mode with a monopole-type radiation pattern. With careful design, two omnidirectional modes, TM ₀₁ and TM ₀₂, can be excited simultaneously. Through this excitation, a dual-frequency antenna with monopole-type pattern can be realized. Frequency ratio can be tuned by changing the number of mushroom unit cells. A broadband antenna can then be implemented by optimizing the frequency ratio. The performances of the proposed antennas are studied numerically and experimentally. The measured and simulated results are in good agreement.