# A Compact Broadband Planar Inverted-F Antenna with Dual-Resonant Modes
###### Abstract  
In this paper, a compact broadband planar inverted-F antenna (PIFA) with dual-resonant modes (TM\({}_{1/2,2}\) and TM\({}_{3/2,0}\)) is proposed for 5G applications. By loading a pair of horizontal slots on the patch, the TM\({}_{3/2,0}\) mode can be shifted downward and combined with the TM\({}_{1/2,2}\) mode, leading to a dual-mode operation. Meanwhile, another pair of slots, which are orthogonal to the previous pair, is introduced to improve the impedance matching of the antenna. Moreover, the substrate with a high dielectric constant is used in this design to achieve a compact antenna size. In order to verify the principle and the design method, an antenna prototype was fabricated and measured. Experimental results show that the PIFA has a good performance with a \(-10\) dB impedance bandwidth of 14.5% (3.17-3.67 GHz), a peak gain of 6.23 dBi, a low cross-polarization level of \(-17.3\) dB, and a compact size of \(0.45\times 0.26\times 0.03\ {\lambda_{01}}^{3}\) (\(\lambda_{01}\) is the free-space wavelength at 3.5 GHz). The antenna is predicted to be suitable for 5G terminal applications. plinar inverted-F antenna (P  
In addition to the above techniques, the methods of loading shorting pins [21] and etching slots on the radiating patches [22; 23; 24; 25; 26; 27; 28; 29; 30; 31] are also proposed. By using these methods, resonant modes of the radiation patch can be redistributed, and multiple resonant modes can be merged together to achieve a wide bandwidth. In [21], by inserting several shorting pins, three resonant modes of TM\({}_{1/2,0}\), zeroth-order, and TM\({}_{1,0}\) are merged together to obtain a wide bandwidth. In [22], a PIFA is proposed by etching a series of slots, resulting in two resonances of TM\({}_{0,1}\) and TM\({}_{1,1}\) modes. In [23], by simultaneously loading slots and adding shorting pins, the TM\({}_{3,1}\) and TM\({}_{1,2}\) modes are combined to generate a wide bandwidth of 13.4%. In [24], a triple-mode (TM\({}_{0,1/2}\), TM\({}_{2,1}\), and TM\({}_{0,1}\)) patch antenna is achieved by optimizing the position of the shorting pins and etching two pairs of slots. In [25; 26], with the use of a loading slot and a pair of shorting pins, the resonant frequencies of TM\({}_{0,1/2}\) and TM\({}_{0,3/2}\) modes are reallocated and combined together to achieve an enhanced bandwidth. In [27], by loading a pair of shorting pins, the antenna can achieve a widened bandwidth of 15.3% under dual-mode resonances of TM\({}_{0,1/2}\) and TM\({}_{2,1/2}\) modes. In [28], a novel wideband U-slotted MPA is presented. By loading several shorting vias, the TM\({}_{1,2}\) mode and TM\({}_{2,1}\) mode can be combined together. These methods are simple and clever as they usually keep compact antenna structures. However, most of the designs suffer from deteriorated radiation patterns, such as a tilted mainlobe, a high cross polarization level (\(>\)\(-\)10 dB), the appearance of sidelobe, and so on, partially due to the radiation characteristics of the higher-order modes, and partially because of the loading effect of the shorting pins and slots. Therefore, the patch/PIFA antenna for terminal usage still needs to be improved in terms of both size and radiation performance. In this paper, a compact and wideband PIFA with good broadside radiation patterns is proposed to solve this issue. Here, it is noted that \"good\" means that the principal radiation direction is broadside, and the radiation pattern is symmetrical with a low cross-polarization level of less than \(-\)15 dB and no sidelobe. The antenna operates under the resonances of the TM\({}_{1/2,2}\) and TM\({}_{3/2,0}\) modes, and can cover the 5G n78 band (3.3\(-\)3.6 GHz). Specifically, in this design, a pair of horizontal slots are etched at the edge of the patch, which not only reduces the resonant frequency of the TM\({}_{3/2,0}\) mode, but also improves the radiation patterns of both the target modes.