High gain CP aperture-coupled antenna for X-band application.
Antennas with compact structures have found extensive applications in the wireless communication (Xing, Li, and Li 2015). Microstrip antennas with circular polarization (CP) are being popularly used for onboard applications. A broadband circular polarization with lower axial ratio (AR) has always been a subject of interest. Aperture-coupled microstrip antenna is a convenient technique for achieving broad impedance bandwidth (Adrian and Schaubert 1987; Huang, Wu, and Wong 1998, 1999; Karamkar and Bialkowski 1999; Liu, Zhu, and Choi 2017; Pozar and Duffy 1997; Targonski and Pozar 1993; Vlasits et al. 1996). A single-fed aperture-coupled CP antenna with AR bandwidth of 2.5% using cross slot of equal arm lengths (in ground plane) was reported in Vlasits et al. (1996). The authors utilised cross slot of unequal arm lengths in Huang, Wu, and Wong 1999 which resulted in AR bandwidth of 3.91%. CP can also be realised using single-fed aperture-coupled patch on the top with a linear slot at the centre which is loaded with orthogonal V-slots (of equal arm lengths) at either end of the ground plane (Huang, Wu, and Wong 1998) which has resulted in 2.2% AR bandwidth.
Circularly polarised aperture-coupled antenna was investigated using two off-centred orthogonal coupling slots in the ground plane, two separate feeds and a square patch on the top layer (Adrian and Schaubert 1987). In this way, AR bandwidth of 3.5% was achieved. Karamkar and Bialkowski (1999) investigated aperture-coupled circularly polarised antenna using 3-dB branch line power divider at the feed layer and two off-centred linear coupling slots. The utilisation of the branch line power divider resulted in a broad impedance bandwidth and good AR bandwidth. Aperture-coupled stacked patch with optimised inter layer spacing and power divider was explored for wider AR bandwidth (Pozar and Duffy 1997). The variable arm lengths of power divider were used to realise right-hand CP (RHCP) or left-hand CP (LHCP). Series-fed and parallel-fed configurations using power divider were also utilised to generate CP with desired sense (Targonski and Pozar 1993). The concept of reconfiguration ability was studied in Sung, Jang, and Kim (2004) with 2.6% impedance bandwidth. A similar approach was utilised in Yang and Rahmat-Samii (2002) by taking advantage of a combination of single probe and slot-PIN diode, but the AR bandwidth was only 3%. A dual circularly polarised antenna with two L-strip feeds in circular patch was reported in Wu et al. (2008), indicating as much as 4.8% AR bandwidth and about 15-dB interport isolation. In Liu, Zhu, and Choi (2017), impedance bandwidth is enhanced by designing a low-profile aperture-coupled microstrip patch antenna in which TM10 and TM30 resonant modes have been utilised. It is apparent that in the structures presented in Adrian and Schaubert (1987), Karamkar and Bialkowski (1999), Pozar and Duffy (1997) and Targonski and Pozar (1993), higher AR bandwidth and interport isolation could not be achieved without using a power divider. The utilisation of power divider network involves additional space, cost and design complexities, especially in array environment. The other structures (Sung, Jang, and Kim 2004; Wu et al. 2008; Yang and Rahmat-Samii 2002) show some inherent limitation of low AR bandwidth (<5%) and/or inter port isolation.
Taking into account the above concerns, a viable solution is proposed in this article by designing an antenna which is highly desirable for array design in terms of AR bandwidth and desired sense of polarisation. We have explored the development of a high gain circularly polarised aperture-coupled microstrip patch antenna at X-band. The proposed antenna has a circular patch with two diametrically opposite notches which are intended to improve the AR. The ground plane consists of an I-shape coupling slot and is loaded under the radiating patch element. This structure effectively prevents feed line radiation from distorting the radiation pattern. The simple and compact structure of the proposed antenna results in easy fabrication and makes it perfectly suitable for the utilised low-cost systems in wide-band technology. The details of the design procedure along with parametric dependencies have been described and the performance of the antenna has been optimised by successive simulations using Ansoft HFSS. A prototype of the proposed antenna has been successfully fabricated on the substrate, Rogers 4003. Experimentally validated results reveal that the proposed antenna yields about 19% AR bandwidth at each port.
2. Design methodology
Figure 1 illustrates the basic configuration of the shared-aperture dual circularly polarised microstrip antenna. The proposed configuration consists of two substrate layers separated by low loss dielectric foam. The top layer of the upper substrate includes a circular patch with two diametrically opposite notches along the periphery for the generation of CP. The back-copper of the upper substrate is etched out. The top layer of the lower substrate is the ground plane and consists of an I-shape coupling slot beneath the radiating patch element. The size and location of the slot have effect on the radiation pattern and, since currents are mostly concentrated at the central part, we locate the I-shape slot at the centre of the substrate. Feed lines are printed in the bottom layer of the lower substrate. The ground plane effectively prevents the feed line radiation from corrupting the desired patch radiation.
For the proposed antenna, 20 mil thick Rogers 4003 has been considered for the upper and lower substrates (with relative permittivity of [[epsilon].sub.r] = 3.55 and dielectric loss tangent of tan[delta] = 0.002). The relative permittivity of the foam spacer is 1.08. The dielectric loss tangent and the thickness of the foam spacer are equal to 0.0022 and 3.2 mm, respectively. In order to achieve a wider bandwidth, lower dielectric constant is a preferred choice. The length of the patch determines the resonance frequency, while the width determines the resonance impedance. Therefore, the dimensions of the circular patch should be decided judiciously for feeding the antenna to achieve a desired resonance frequency as well as desired impedance matching. The impedance bandwidth is determined by the thickness of the dielectric substrate. Along the periphery of the circular patch, two notches are cut at the diametrically opposite points. The dimension of these diametrically opposite notches determines the AR. In the proposed design, the dimension of the notches is kept same in both X- and Y-directions. Successive simulations are performed using HFSS (Ansoft, HFSS (High-Frequency Structure Simulator) 2014) to optimise the slot lengths and width, the dimension of the notches and circular patch, the slot offset and the dielectric gap.
At first, we describe adopted design steps to improve the proposed antenna by four prototypes in Figure 2. The performances of these prototypes are illustrated in Figure 3. Ant 1 consists from a 50 [ohm] microstrip feed line and a rectangular slot of dimensions [X.sub.1] and [Y.sub.2]. The main reason for using this kind of feed line is that microstrip feed lines improve antenna gain by increasing the directivity and reducing the side lobes of the radiation pattern. In order to improve the return loss and AR, two parts are added to the ends of the rectangular slot to achieve an I-shape coupling slot (Ant 2). As seen from Figure 3, the return loss and AR are enhanced compared to Ant 1. In Ant 3, a circular patch is added to eliminate unwanted slot radiations which have negative impact on the characteristics of the antenna. After the addition of the circular patch, return loss has a better band ranging from 8.25 to 10.65 GHz, defined for an [S.sub.11] of-10 dB and the impedance bandwidth is effectively increased. Finally, two notches are cut at diametrically opposite points along the periphery of the circular patch to enhance the AR (Ant 4). As seen from Figure 3, the range of the impedance bandwidth and the return loss are extended from 8.48 to 10.24 GHz and from 8.30 to 11 GHz, respectively.
The parametric analyses of the proposed antenna are presented in Figures 4-7. Figure 4 shows that by varying the radius of the circular patch, the impedance bandwidth get disturbed. Figure 5 shows the variation of AR vs. the dimension of the notches along the periphery of the circular patch. In Figures 6 and 7, return loss of the proposed antenna are illustrated for different values of [X.sub.1] and [Y.sub.1], respectively. It is clear from these figures that we can control the resonance frequency by changing the values of [X.sub.1] and [Y.sub.1]. According to Figures 6 and 7, the normalised slot is fixed at [X.sub.I] = 0.145[lambda], and [Y.sub.I] = 0.082[lambda]. The optimal values for the parameters of the proposed antenna are listed in Table 1. These parameters are as follows:
ARs lower than 3 dB can be achieved for segmentation ratio higher than 19% and better than 1 dB for segmentation ratio higher than 6%.
The normalised offset of the I-shaped slot is fixed at [X.sub.I] = 0.145[lambda], [Y.sub.I] = 0.082[lambda]. The simulated reflection coefficient is illustrated in Figure 11. The simulated 2:1 VSWR bandwidth is about 2.33 GHz (c GHz). The simulated 3-dB AR bandwidth is about 1.7 GHz (8.7-10.4 GHz). Simulated cross-polarisation and co-polarisation patterns at 9 GHz (centre frequency of the simulated impedance bandwidth) are shown in Figure 8.
3. Prototype and measurements
Engineering prototype of the shared-aperture dual circularly polarised microstrip antenna has been fabricated using 20 mil Rogers substrates and experimentally evaluated. The photograph of a fabricated prototype is shown in Figure 9.
The different layers of the antenna are schematically shown in Figure 1. The length and width of the top patch is fixed at 40 mm and t is set at 2 mm. Moreover, the ground plane has the dimension of 40 mm x 40 mm. Figure 10 shows the measured reflection coefficient of the proposed antenna. Measured results have the same nature as the simulated results, except the centre frequency which is shifted upward by about 300 MHz.
This up-shift of the resonance frequency occurs because of overetching of the stub length, slot and patch. Achieved impedance bandwidth (shown in Figure 10) (<-10 dB) is about 2.02 GHz (8.65-10.58 GHz). For an ideal circularly polarised antenna, AR should be 0 dB. However, the polarisation of practical antennas is elliptical. In order to determine the AR, the prototype is rotated in its azimuth plane (about the bore-sight) and the relative amplitude is measured. As it can be observed from Figure 11, the linearly polarised transmitting antenna is kept fixed. The AR is measured at both ports over the frequency range 8-11 GHz. The measured AR bandwidth is from 8.48 to 10.24 GHz.
The radiation pattern of the antenna has been measured in compact antenna test range. The transmitting antenna is a linearly polarised horn. Direct measurement of CP pattern could not be performed due to lack of standard reference CP antenna, spinning dipole and other CP measurement facilities. The radiation patterns are measured in two orthogonal cuts (at [phi] = 0[degrees] cut and [phi] = 90[degrees] cut) at the frequency range 8-11 GHz. The measured radiation patterns of the designed antenna at 9 GHz (centre frequency of the obtained impedance bandwidth) are illustrated in Figure 12. The measured 3-dB bandwidth is 54[degrees] at [phi] = 0[degrees] cut and 69[degrees] at [phi] = 90[degrees] cut. The gain of the antenna is 8.7 dBi at 9 GHz. As seen from Figure 12, the proposed antenna has directivity patterns with high gain and the range of the half-power bandwidth (HPBW) is between 30 and 120.
Table 2 compares the properties of the proposed antenna with the structures proposed in Saini and Dwari (2016) and Cai et al. (2015). It is clear from this table that the proposed antenna offers better gain and AR bandwidth with a more compact size.
In this paper, a composite feed circularly polarised aperture-coupled microstrip antenna at X-band with improved characteristics has been presented. The proposed antenna includes a circular patch with two diametrically opposite notches along the periphery which improve the AR. The ground plane effectively prevents the feed line radiation from corrupting the desired patch radiation. The proposed antenna has a simple and compact structure which makes it perfectly suitable for the utilised low-cost systems in wide-band technology and air-borne applications. It can also be effectively utilised in applications where the sense of CP for reception is not known. Measured resonance frequency is about 300 MHz up-shifted from simulated value which is caused by the overetching of the stub length, slots and patch. Achieved impedance bandwidth is about 2 GHz and about 19% for 3-dB AR bandwidth.
No potential conflict of interest was reported by the authors.
Adrian, A., and D. H. Schaubert. 1987. "Dual Aperture-Coupled Microstrip Antenna for Dual or Circular Polarization." Electronic Letters 23 (23): 1226. doi:10.1049/el:19870854.
Ansoft, HFSS (High Frequency Structure Simulator). 2014. v.14. Ansoft Corporation, Pittsburgh, PA.
Cai, T., G.-M. Wang, X.-F. Zhang, and J.-P. Shi. 2015. "Low-Profile Compact Circularly-Polarized Antenna Based on Fractal Metasurface and Fractal Resonator." IEEE Antennas and Wireless Propagation Letters 14: 1072-1076. doi:10.1109/LAWP.2015.2394452.
Huang, Chih-Yu, Jian-Yi Wu, and Kin-Lu Wong. 1998. "Slot-Coupled Microstrip Antenna for Broadband Circular Polarization." Electronic Letters 34 (9): 835. doi:10.1049/el:19980676.
Huang, Chih-Yu, Jian-Yi Wu, and Kin-Lu Wong. 1999. "Cross-Slot-Coupled Microstrip Antenna and Dielectric Resonator Antenna for Circular Polarization." IEEE Transactions on Antennas and Propagation 47 (4): 605-609. doi:10.1109/8.768798.
Karamkar, N. C., and M. E. Bialkowski. 1999. "Circularly Polarized Aperture Coupled Circular Microstrip Patch Antennas for L-Band Applications." IEEE Transactions on Antennas and Propagation 47 (5): 933-940. doi:10.1109/8.774159.
Liu, Neng-Wu, Lei Zhu, and Wai-Wa Choi. 2017. "A Low-Profile Aperture-Coupled Microstrip Antenna with Enhanced Bandwidth under Dual Resonance." IEEE Transactions on Antennas and Propagation 65 (3): 1055-1062. doi:10.1109/TAP.2017.2657486.
Pozar, D. M., and S. M. Duffy. 1997. "A Dual-Band Circularly Polarized Aperture-Coupled Stacked Microstrip Antenna for Global Positioning Satellite." IEEE Transactions on Antennas and Propagation 45 (11): 1618-1625. doi:10.1109/8.650073.
Saini, R. K., and S. Dwari. 2016. "A Broadband Dual Circularly Polarized Square Slot Antenna." IEEE Transactions on Antennas and Propagation 64 (1): 290-294. doi:10.1109/TAP.2015.2496118.
Sung, Y. J., T. U. Jang, and Y.-S. Kim. 2004. "A Reconfigurable Microstrip Antenna for Switchable Polarization." IEEE Microwave and Wireless Components Letters 14 (11): 534-536. doi:10.1109/lmwc.2004.837061.
Targonski, S. D., and D. M. Pozar. 1993. "Design of Wideband Circularly Polarized Microstrip Antennas." IEEE Transactions on Antennas and Propoagation 41 (2): 214-220. doi:10.1109/8.214613.
Vlasits, T., E. Korolkiewicz, A. Sambell, and B. Robinson. 1996. "Performance of a Cross-Aperture Coupled Single Feed Circularly Polarized Patch Antenna." Electronic Letters 32 (7): 612. doi:10.1049/el:19960459.
Wu, Gao-Lei, Mu Wei, Gang Zhao, and Yong-Chang Jiao. 2008. "A Novel Design of Dual Circularly Polarized Antenna Fed by L-Strip." Progress in Electromagnetics Research 79: 39-46. doi:10.2528/pier07092001.
Xing, Meng-Jiang, Xiao-Zhen Li, and Bin-Hua Li. 2015. "A Novel Coupled Meandered Interval Monopole Antenna for UHF Band." Australian Journal of Electrical and Electronics Engineering 12 (4): 327-331.
Yang, Fan, and Y. Rahmat-Samii. 2002. "A Reconfigurable Patch Antenna Using Switchable Slots for Circular Polarization Diversity." IEEE Microwave and Wireless Components Letters 12 (3): 96-98. doi:10.1109/7260.989863.
Nasrin Yazdanpanah and Yashar Zehforoosh
Department of Electrical Engineering, Urmia Branch, Islamic Azad University, Urmia, Iran
CONTACT Yashar Zehforoosh [??] email@example.com
Received 19 June 2017
Accepted 22 November 2017
[L.sub.F] the length of the feed line [W.sub.F] the width of the feed line [W.sub.sub] the width of the substrate R the radius of the circular patch [X.sub.1] the length of the middle part of the slot [Y.sub.1] the length of the end part of the slot [X.sub.2] the width of the end part of the slot [Y.sub.2] the length of the middle part of the slot h the height of the air layer t the length and width of the notch Table 1. Optimal values for parameters of the proposed antenna. Param. mm [L.sub.F] 23.40 [W.sub.sub] 40 R 4.48 [X.sub.1] 4.8 [Y.sub.1] 2.7 h 3.2 [W.sub.F] 1.7 [L.sub.sub] 40 t 2 [X.sub.2] 0.8 [Y.sub.2] 0.95 Table 2. Comparison of the proposed antenna with some other structures. Antenna Dimension (mm) AR-BW (GHz) Gain (dBi) CP square slot antenna 60 x 60 2-3.7 3.5 Low-profile compact CP antenna 40 x 45 3.49-3.52 6.3 Proposed antenna 40 x 40 8.7-10.4 8.7 Antenna Substrate CP square slot antenna FR4 Low-profile compact CP antenna F4B Proposed antenna Rogers 4003
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|Author:||Yazdanpanah, Nasrin; Zehforoosh, Yashar|
|Publication:||Australian Journal of Electrical & Electronics Engineering|
|Date:||Mar 1, 2017|
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