# Wideband Circularly Polarized Filtering Hybrid Antenna
## 1 Introduction  
Circularly polarized (CP) antennas are widely used in the wireless communication. Compared with the linearly polarized (LP) antennas, the circularly polarized (CP) antennas give transmitter and receiver more freedom in the orientation, and suppresses multipath interference [1]. A dielectric resonator antenna (DRA) has many striking features such as minute size, flexible excitation and various resonant modes [2]. Some techniques are used to generate the CP radiation for different DRAs, and these techniques are classified into the single and multi-feeding mechanisms [3]. Generally, the multi-fed techniques often need extra complex excitation networks, thus increasing the loss, while the single-fed design is more flexible, with relatively simple structure and low cost. However, the single-fed CP antennas usually have a narrower 3-dB AR bandwidth of only 6% or less.  
Some techniques have been skillfully applied to extend the AR bandwidth of the single-fed CP DRAs. For example, some of the irregular-shaped DRAs were reported [4; 5; 6], such as T-shaped DR, stair-shape DR and trapezoidal DR. A triple-CP-mode stacked DRA with a 3-dB AR bandwidth of 27% is reported in [7]. However, the irregular-shaped DRAs are usually made of solid material, and it is difficult to theoretically analyze such irregularly structure. In [8], a rectangular DR excited by an Archimedean spiral slot owns an AR bandwidth of 25.5% with a gain of more than 5 dBi. In [9], a wide 3-dB AR bandwidth of 41.01% is obtained by using a stair-shaped coupling slot. In [10], a wideband CP magnetoelectric dipole antenna is proposed by using an extended hook shape.  
As the front end of the communication system, the antenna is usually cascaded with filters through the matching circuit to suppress unwanted interference signals. The introduction of a matching circuit not only increases the size of the whole design, but also reduces the gain of the antenna. In order to realize miniaturization and low loss, some filtering antennas with both radiation and filtering characteristics are proposed [11]. Generally, there are mainly two methods to construct the filtering antennas. The first is to use the antenna as the last resonator of the filter, which increases the size and insertion loss of the antenna. The second design is to introduce small modifications into the radiatoror the feeding structure [12; 13; 14], such as loading shorting pins, or parasitic strips on the microstrip line. Since no complex filtering circuits are involved, the performance of the antenna is not affected or less affected. In [14], a U-slot, a defected ground structure (DGS), and parasitic patches are introduced to generate radiation nulls. At the frequencies of radiation nulls, surface currents are concentrated on the parasitic structures. The antenna cannot be radiated effectively, thereby achieving a good filtering response.  
In this paper, a novel wideband single-fed filtering CP hybrid antenna is proposed. The CP field of liquid DRA is excited by a cross-slot. Two radiation nulls are introduced by designing a U-shaped slot and a C-shaped stub on the microstrip line, respectively, by loading four inverted L-shaped metals around the DR to extend the AR bandwidth. The 3-dB AR bandwidth of the proposed structure almost completely overlaps with the 10-dB impedance bandwidth; the effective bandwidth is 38.2%.