###### Abstract  
A novel tri-band patch antenna with enhanced bandwidth and a diverse radiation pattern is presented in this study. An off-center probe is introduced to excite a conventional circular patch antenna, and the multi-frequency resonance modes of TM\({}_{01}\), TM\({}_{02}\), and TM\({}_{03}\) with different radiation properties are obtained. Four metal cambered strips are distributed around the circular patch to adjust the impedance matching of the circular patch antenna. In order to widen the impedance bandwidth, a parasitic patch placed above the circular patch antenna is employed in this work. Then, a double-layer patch antenna is manufactured and tested to verify the accuracy of the theoretical analysis and simulated result. The measured results, which are basically consistent with the simulated results, show that the designed antenna can operate in three bandwidths of 1.79-1.81 GHz, 3.74-4.0 GHz, and 4.93-5.44 GHz, with radiation patterns of directional, omnidirectional, and three-beam pointing. The radiation efficiency higher than 85.6% and the maximum gain greater than 9.42 dBi suggest that the investigated antenna is suitable for multi-service scenarios in wireless communication. patch antenna; enhanced bandwidth; diverse radiation pattern; multi-frequency resonance mode; multi-service; wireless communication +
Footnote †: journal: Applied  
TM\({}_{02}\) modes generated by a lower circular patch. Antennas that have a stacked structure are also proposed in [4; 5]. In [4], an artificial magnetic conductor (AMC) characterized by four zero-phases of reflection coefficient is placed on a quad-band coplanar waveguide (CPW) antenna to achieve multi-band and high gain characteristics. Unlike [4], Ref. [5] adopts a dual-antenna design technique. The upper and lower layers of the antenna are, respectively, a printed dipole array responsible for horizontal polarization and a discone antenna responsible for vertical polarization. The measured results show that the two polarization modes of the antenna can work in two bandwidths of 770-980 MHz and 1.69-3.21 GHz, respectively. In [6; 7], two dual-band patch antennas with radiation-differentiated characteristics are presented by employing different radiating elements. However, the maximum gain of these two antennas at the lower band is only 3.1 dBi. Based on multi-resonant modes, two dual-band, dual-polarized antennas are investigated in [8; 9]. However, the impedance bandwidths of the designed antennas are lower than 3.6%. In [10], a rectangular patch loading a branch on one radiating side and etching a rectangular slot on the other side is proposed to generate TM\({}_{01}\) mode and modified TM\({}_{20}\) and TM\({}_{30}\) modes. The measured result shows that the single patch antenna can operate at three bands and the measured patterns are characterized by broadside radiation. A similar method is also applied in [11] to introduce three sets of orthogonal modes, and then three resonance bandwidths with circular polarization characteristics are obtained. In [12], a novel multi-band patch antenna with unidirectional radiation is designed, based on a coupled resonators network. The four frequency bands can be adjusted by changing the coupling strength between the resonators. In addition, the operation bands can be adjusted without changing the shape of the antenna, providing a flexible method of antenna design. However, the measured bandwidths of the antennas in [10; 11; 12] are relatively narrow. By loading two types of slots on the patch antenna, the method of obtaining multi-resonant modes for the antenna and increasing the bandwidth of the antenna is applied in [13]. A composite right/left-handed transmission line technique is also introduced to design the patch antenna with the feature of multi-frequency in [14].