# Wideband Circularly Polarized Filtering Hybrid Antenna
## 3 Working Principle  
CP field can be excited by a cross-slot with different lengths. As shown in Figure 2, the design process of the proposed antenna is demonstrated by comparing three reference antennas. Ant.1 is a cross-slot-fed liquid DRA, Ant.2 is with four rotationally symmetric inverted-L metals fed by a cross-slot, and Ant.3 is obtained by combining DRA and the inverted-L metals. The simulated AR results of the three antennas are compared in Figure 2d. As seen from the figure, the Ant.1 generates two AR minima at 1.46 GHz and 1.8 GHz, respectively. Furthermore, Ant.2 generates another CP minimum at 2 GHz due to the rotationally symmetric structure. The Ant.1 and Ant.2 were combined, and the two structures were skillfully adjusted to make the AR minima at adjacent frequency, thus the overall AR bandwidth can be extended. It is worth mentioning that the CP fields generated by the inverted L-shaped metals have the same sense of polarization with the fields generated by the cross-slot.  
In order to investigate the CP characteristic of the proposed antenna, the electric field distribution on the y'oz and x'oz plane are shown in Figure 3, where the x' and y' are along the laying direction of the cross-slot. It can be confirmed that the TE\({}_{111}^{\prime}\) is excited at 1.44 GHz, and meanwhile, the electric field distribution at 1.7 GHz is shown in Figure 3b at the x'oz plane corresponding to the mode of TE\({}_{111}^{\prime}\). The orthogonal TE\({}_{111}^{\prime}\) and TE\({}_{111}^{\prime}\) mode are resonated at a different frequency due to the cross-slot with unequal length. Thus, the  
Figure 2: Configurations of the reference antennas. (**a**) Ant. 1. (**b**) Ant. 2. (**c**) Ant. 3. (**d**) ARs.  
CP at first AR minimum is achieved due to the \(\text{TE}^{{}^{\prime}}_{111}\) and \(\text{TE}^{{}^{\prime}}_{111}\) mode, while the second AR minimum is achieved due to the short slot with \(\text{x}^{\prime}\)-polarization and \(\text{TE}^{{}^{\prime}}_{111}\) mode.  
Figure 4a,b gives the simulated AR with a different length of short slot and the inverted L-shaped strip, respectively. As seen in the Figure 4a, the short slot has serious effects on the second AR minimum, but a slight effect on the first AR minimum and no effect on the third AR minimum. This is because the length of the short slot has little influence on the \(\text{TE}^{{}^{\prime}}_{111}\) mode, thus having a slight effect on the first AR minimum. The third AR minimum is introduced by the rotationally symmetric inverted L-metals, so the length of the short slot has no effect on the third AR minima. Figure 4b shows that when the length of the inverted L-shaped metal increases, the frequencies of the third AR minimum gradually decreases, while the other two AR minima are hardly affected.  
To investigate the mechanism of the radiation null of the proposed filtering antenna, a parametric study is performed. As shown in Figure 5a, with increasing the length of the C-shaped strip, the lower radiation null is shifted to the lower frequency, while the upper radiation null remains unchanged. The influence of the U-shaped slot etched on the feedline is studied, as shown in the Figure 5b, with increasing the length of the strip, the upper radiation null is shifted to the lower frequency, while the lower null is not affected.  
Figure 4: Simulated ARs for different length of (**a**) the cross slot. (**b**) the inverted L-shaped metals.  
Figure 3: Simulate the E-field of proposed antenna. (**a**) y’oz at 1.47 GHz. (**b**) x’oz at 1.7 GHz.