Abstract
A new kind of circular polarization leaky-wave antenna with N-shaped slots cut in the upper side of substrate integrated waveguide (SIW) is investigated and presented. The radiation pattern and polarization axial ratio of the leaky-wave antenna are studied. The results show that the width of N-shaped slots has significant effect on the circular polarization property of the antenna. By properly choosing structural parameters, the SIW based leaky-wave antenna can realize circular polarization with excellent axial ratio in 8 GHz satellite band.
1. Introduction
Circular polarization antennas have been widely used in areas of satellite communication, mobile communication, radar, remote, and so forth. They can reduce the effects of multipath interference, rain and fog interference, and polarization mismatch in radars and communication systems [1]. There are several kinds of waveguide structures used in design of leaky-wave antennas for generating circular polarization wave, such as the rectangular waveguide with two compound slots cut in top wall [2]. However, for the rectangular waveguide based leaky-wave antenna, it is difficult to integrate with other circuits. Considering the merits of low cost, light weight, low profile, and easy fabrication by PCB or similar processes [3, 4], the substrate integrated waveguide (SIW) is now widely used in design of SIW leaky-wave antennas [5–9]. However, most of the available leaky-wave antennas are designed for radiating linear polarization wave; only a few works concerning circular polarization can be found [10], especially for C band satellite communication (from 7 GHz to 8 GHz).
In this paper, a new kind of circular polarization leaky-wave antenna based on substrate integrated waveguide with N-shaped slots is proposed. Simulation and measurement results show that this kind of structure can realize circular polarization radiation in 8 GHz satellite band by choosing proper parameters for the N-shaped slots.
2. Antenna Design
The substrate integrated waveguide can be equivalent to the conventional rectangular waveguide, where the periodic metallic vias in SIW are considered as the side wall of conventional waveguide. Therefore, the SIW with N-shaped slots proposed in this paper can be equivalent to a dielectric filled rectangular waveguide with N-shaped slots cut in top wall of the waveguide [3].
The geometry of the SIW leaky-wave antenna with N-shaped slots for 8 GHz band is shown in Figure 1. The antenna is designed on a substrate of Rogers Duroid 5880 with a relative permittivity of 2.2 and a thickness of 1.57 mm and is fed by a microstrip line with impedance of 50 Ω from one end. The other end of the antenna is terminated by matching load to maintain a traveling-wave state. The diameter and period of metallic vias are set to 0.8 mm and 1.2 mm, respectively, and the distance between two rows of metallic vias is set to 15.5 mm. According to formulas given in [3], the cutoff frequencies for the dominant and first higher order modes of this SIW are 6.8 GHz and 13.6 GHz, respectively, so for 8 GHz satellite band only TE10 mode can be transmitted in it. There are total 29 N-shaped slots etched along the center line of SIW with a period of 10 mm, and the total length of SIW is 380 mm; these slots form the dominant radiation section of the SIW as shown in Figure 1. The operating frequency is 8 GHz.
As we know, the transverse slot in waveguide wall cuts the current flowing in longitudinal direction, while the longitudinal slot cuts the current flowing in transverse direction. Therefore, only transverse slot or longitudinal slot cannot generate circular polarization radiation. Since N-shaped slot is composed of both transverse and longitudinal slots, it can cut the currents flowing in both longitudinal and transverse directions at the same time. Therefore, N-shaped slots possibly generate circular polarization wave.
As can be seen in Figure 1, the parameters of N-shaped slots in this paper include the length and width of the letter “N” and the length and width of the inclined slot. The circular polarization property of the antenna depends on these parameters of slots.
3. Results and Discussion
3.1. -Parameters and Attenuation Constant
Figure 2 illustrates the -parameters and radiation efficiency of the SIW leaky-wave antenna as functions of width of slots at frequency of 8 GHz; the other parameters of the slots are fixed to mm, mm, and mm. The attenuation constant along antenna and the radiation efficiency can be calculated by the following expressions: where is the length of the radiation section of SIW, is the radiated power, and is the accepted power. It should be noticed that the leakage rate depends on the length of N-shaped slot, so the attenuation constant increases with the length of slot. In addition, the radiation efficiency of antenna increases with total length of radiation section of the antenna but also changes with other parameters of the slots.
3.2. Effect of Structure Parameters on Axial Ratio
Since the period of slots is set to mm, the range of slot lengths should be limited to 2 mm and 7 mm. In order to decrease the intersection between adjacent slots, the length of slot is fixed to mm in this paper.
Considering the conditions for generating circular polarization wave, the range of slot widths is set to 2 mm and 8 mm. Figure 3 illustrates the effect of width of N-shaped slots on the axial ratio, while mm and mm, from which we can see that the axial ratio is influenced significantly by the width . In order to find the reason behind this phenomenon, the amplitudes of the two components and of far field for two cases of mm and mm are given in Figure 4. It can be found that when mm, the amplitude of almost equals that of , but for mm is much smaller than . In addition, the increasing of width of N-shaped slots will cut more current filaments; therefore, will increase and ratio of will decrease; this results in a significant change in the axial ratio.
(a)
(b)
While the width of N-shaped slots is fixed to mm, the axial ratio as function of length is given in Figure 5; the length of is fixed to 5 mm. Because the width of two vertical slots is set to 0.5 mm in this paper, the N-shaped slot structure keeps unchanging when horizontal length of inclined slots increases from 4 mm to 5 mm; this corresponds to the straight segment in the bottom of the curve. From Figure 5, it can be found that when the length of the slots is larger than that of the inclined slot , the axial ratio increases sharply. On the other hand, the axial ratio of the antenna increases gently, when the length of inclined slot is larger than .
Figure 6 illustrates the electric field distributions of the N-shaped slot for parameters of mm () and mm (), respectively, from which we find that the field component for mm is much larger than that of mm. The reason is that more current filaments are cut by the left and right edges of the two vertical slots and inclined slot when mm (), as can be seen in Figure 6. Figure 7 shows the axial ratios of the antenna in main beam direction for different widths of inclined slot. It can be found that changing of width has little impact on the axial ratio.
(a) mm
(b) mm
3.3. Antenna with Optimum Circular Polarization Property
According to the above analysis, properly selecting parameters of N-shaped slots can give minimum axial ratio in main beam direction of the antenna for a specific frequency band. From our work, it is found that when the parameters of N-shaped slots are selected to be mm, mm, mm, and mm, the axial ratio reaches its minimum value of 0.443 at 8 GHz. The photograph of the fabricated antenna corresponding to these parameters is shown in Figure 8. The measured and simulated -parameters of the antennas are illustrated in Figure 9. It can be found that the simulated and experimented return losses of the antenna are lower than −10 dB from 7.6 GHz to 8.2 GHz. The differences between simulated and measured results are probably due to the errors of fabrication and measurement. Meanwhile, the measured is a little bit smaller than the simulated one, because the loss of feeders and dielectric substrate are not considered during the simulation.
The axial ratios of antennas for different widths in main beam direction as functions of frequency are given in Figure 10. It can be found that, for mm, the simulated axial ratio is less than 3 dB in the band from 7.6 GHz to 8.2 GHz, and the measured axial ratio is less than 5 dB in this band. When the slot width is increased to 6 mm, the axial ratio of the corresponding antenna is larger than that of the antenna with mm; this phenomenon can be explained by Figure 4. The comparisons of axial ratios of the antenna with N-shaped slots and the antenna with H-shaped slot given in [9], in band from 7.6 GHz to 8.2 GHz, are also given in Figure 10. It can be found that the axial ratio of antenna with N-shaped slot is lower than that of antenna with H-shaped slot at specific frequency band.
Figure 11 gives the simulated and measured radiation patterns at 8 GHz, when the antenna is fed from left side. It can be found that the antenna generates right-hand circular polarization wave; the simulated gain is 8.93 dBi, and the gain for the left-hand circular polarization wave (cross polarization) of this antenna is −1.36 dBi, which is about 10.3 dBi lower than that of RHCP; measured gain is lower than the simulated result. The gain of the antenna in [9] for H-shaped slots is also given in Figure 11, which is 0.4 dBi lower than that of the antenna with N-shaped slots.
It is found from analysis that when the antenna is fed from the right side, the left-hand circular polarization (LHCP) radiation will be generated, as shown in Figure 12. The gain for the LHCP wave is 10.82 dBi higher than that of RHCP wave. The axial ratio of the antenna fed from right side is given in Figure 13. It can be found that the range for axial ratio less than 3 dB is 56.5° and 63.5°.
4. Conclusion
In this paper, a novel kind of circular polarization leaky-wave antenna with N-shaped slots based on substrate integrated waveguide is proposed, and the axial ratios for different parameters and frequencies are given and discussed. From the simulated and measured results, it is found that this SIW based leaky-wave antenna can realize circular polarization radiation in the 8 GHz satellite band, and it maintains the desirable properties of low profile and light weight and is easy to integrate with planar circuits.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
This work was supported in part by the National Nature Science Foundation of China under Grant 61331002 and in part by the National Program on Key Basic Research Project under Grant no. 2013CB328903.