A Compact Wideband Endfire Filtering Antenna Inspired by a Uniplanar Microstrip Antenna

This letter presents a design of a compact wideband endfire filtering antenna. The design starts from an endfire quasi-microstrip antenna, which is unidirectional with a smaller size when compared with a Yagi–Uda antenna. A reflector and a parasitic patch are utilized to introduce two new resonant frequencies to broaden the bandwidth. They also generate two radiation nulls at the lower band and the higher band, respectively. The endfire radiation mechanism can be modeled by a two-element antenna array. The mechanism of these two radiation nulls is explained by the mixed coupling theory. Therefore, the proposed antenna also shows filtering property without any extra circuits. In addition, a capacitive feeding structure is employed to improve the level of the reflection coefficient within the upper stopband. The measured results reveal that the proposed antenna has a bandwidth of 41.9% from 2.00 to 3.06 GHz and a peak gain of 7.3 dBi with an extremely small size of 0.105 λ
²
(λ is the wavelength at the center frequency). Two radiation nulls are observed at 1.78 and 3.19 GHz, respectively. The out-of-band roll-off of the antenna gain achieves 15 dB.