A Compact Omnidirectional Patch Antenna With Ultrawideband Harmonic Suppression

This communication presents an effective approach to design a novel patch antenna with the simultaneous realization of ultrawideband harmonic suppression, omnidirectional radiation, and compact configuration. Initially, the resonant frequency and the input impedance of a short-circuited ring-patch (SCRP) antenna are deeply analyzed. It indicates that the resonant frequency of the desired TM
₀₁
mode could be effectively lowered by properly adjusting the radius of the ring patch. Thus, a compact dimension could be achieved. Meanwhile, an attractive property of a large input resistance ratio between TM
₀₁
and high-order modes is obtained. Then, different from the previous works with the aid of additional harmonic rejected elements, a compact 50 Ω coplanar waveguide (CPW) with the function of an impedance transformer is employed to feed the antenna. After the proper design is executed, the TM
₀₁
mode could be well matched and those unwanted high-order modes are totally mismatched at impedance. The ultrawideband harmonic suppression is thus obtained and additional elements are not required at all. The equivalent circuit model is presented to give a clear insight into its working principle and a unified design procedure is generalized. Finally, two prototypes are designed, fabricated, and tested. The simulated and measured results are in good agreement with each other, certifying the validity of the proposed method. All these results show that an ultrawideband harmonic suppression (7 f
₀
, f
₀
is the operating frequency) has been successfully achieved while keeping the omnidirectional radiation, low profile (0.037 λ
₀
), and compact patch size (0.096 λ
₀
× 0.096 λ
₀
, λ
₀
is the free-space wavelength at f
₀
).