A Novel Design of Gap-Coupled Sect oral Patch Antenna for X-band High Frequency Applications

Nagendra Prasad YadavNanjing University of Science and Technology

School of optical and electronicsNanjing, China

nagendra1nagendra@gmail.com

Abhishek KumarDept. of Electronics and Communication

Simla, India

Abstract— Using gap-coupling between the elements, antenna of four sectors by cutting the slots through and through from a circular patch is designed for application in X-band. The bandwidth enhancement along with reduced side lobes is obtained by marginally or fully moving one of the feeding sectoral patches diagonally away. The results are analyzed for different positions of the slot. A bandwidth of 1.2 GHz in X-band along with a reduced side lobe level of -15.0 dB and a gain of 12.0 dBi is obtained. An analysis of mutual coupling and parasitic patch effect is also presented and discussed in this work.

Index Terms— Gap coupling, impedance, bandwidth, antenna gain.

I. INTRODUCTION Narrow bandwidth with low gain and high side lobe level has been a major concern for the optimum use of the microstrip antennas. For circularly polarized antennas, axial ratio bandwidth also needs to be catered for in addition to impedance bandwidth. A number of techniques have been proposed in order to improve the characteristics of antennas such as to enhance gain or widen the impedance bandwidth. Some of them resort the material side by using a thicker substrate or bi-layered substrate [1-3] or reducing the dielectric constant [4]. Gap coupling is one of the techniques being used by the researchers throughout the world [5-9]. Different types of gap coupled antennas have been proposed by the contributors. In some applications, multi frequency operations are required; this can be achieved by gap-coupling [10]. In [11-12] the experiment is performed on gap-coupled circular microstrip antennas and mutual coupling is measured. In contrast to the corporate feed network, gap-coupled method is a good replacement to achieve wide bandwidth with increased gain and reduced side lobe level. Using gap coupled method, the return loss of a mismatched microstrip fed antenna can be decreased significantly [13]. This work analyzes if the gap coupled technique may serve in those cases where the coaxial fed method leads to a low gain patch. There are other methods applying slots in order to improve the performance of a single layer microstrip patch antenna provide motivation for this work. Approaches like rectangular or circular slots take an undisturbed TM10 or higher as an essential starting point [14-15]. Gap coupling in broadband microstrip antennas is therefore discussed in [16] which

describe mostly probe fed constellations. Different multiple different resonance frequencies lead to wider impedance bandwidth and improved gain when losses are low with reduced side lobe level. However, precise general design procedures are not given due to the complex behavior of the parameters.In this paper, we have introduced a novel form of gap coupled antenna in which we divided the circular patch into four sectors through gap coupling. Then, exciting one of the sectors by applying coaxial feeding with increasing gap in between the feeding patch and rest of the other three sectoral patches. This will introduce the effect of coupling in between the three patches. This will result in the increasing bandwidth as we move the feeding patch away from the other three sectoral patches. The simulations are performed with the simulation software CST Studio Suite and for experimental purpose Vector Network Analyzer is used. The simulated and experimentally measured results are in good agreement with each other. A significant comparison is also shown for the excited sectoral patch at a distance of 2 mm up to a distance of 20 mm. The radiation pattern measured is also presented with improved gain and reduced side lobe level. It is clearly shown in the measured results that how the radiation be affected with the introduction of different gap in between the sectoral patches.

II. THEORY

Bandwidth enhancement can be achieved by using closely spaced elements or using parasitic patches. Different approaches have been implemented in order to improve the bandwidth of the antenna such as multilayered structures, single layer multi-resonant designs, aperture coupled designs, etc. The coupling between the elements is a function of the patch size and gap between the patches [17-18]. For a fixed unit cell size, the coupling between the patch elements increases by reducing the spacing between them. However if the size of the unit cell is reduced, a closer spacing between the patch elements is required so as to achieve the same level of coupling for larger unit cell sizes. In this paper, the parasitic patches improve the impedance bandwidth of the proposed antenna design. Only one sectoral patch is excited and the other three patches acts as parasitic patches. These three non fed sectoral patches lead to an increase in the resulting

impedance bandwidth. The impedance bandwidth is found to increase up to a distance of 20 mm from the center of the circular patch. At a distance less than 20 mm, there are two resonant modes. But when the gap is maintained at a distance of 20 mm, the two resonant modes combine and therefore results in the enhancement of the impedance bandwidth. A wideband is obtained at this particular gap, after which the bandwidth starts reducing as the two resonant modes again separates. An analysis of mutual coupling is also presented in this section. In recent times, one of the topics of investigation is how the mutual coupling relates to the antenna radiation properties such antenna gain and side lobe level. The large mutual coupling leads to the reduction in the antenna radiation efficiency, decrease in antenna effective gain, etc but adjacently also creates an effective pattern. Theoretical research has shown that the radiation properties of the printed antennas [19] and mutual coupling between printed dipoles are affected by the radiation fields and the type and the number of surface wave modes propagating within antenna substrate. In order to enhance the radiation properties the antenna structure is divided into four sectoral patches using gap coupling. As the gap between one of the sectoral patches (excited patch) and the other three parasitic patches is increased, the mutual coupling starts reducing. This coupling effect results in the enhancement of the antenna gain, directivity and reduction in the respective side lobe level which is very good for better antenna radiation and thus for wide band applications. And reached a maximum value after which the radiation properties again start degrading due to a large separation between the sectoral patches.

III. ANTENNA CONFIGURATION AND DESIGN

The geometry of the antenna proposed is shown in figure (1) which comprises of a gap coupled sectoral patch antennas. The proposed gap coupled antenna is printed on an FR4 substrate with permittivity of 4.1 and a loss tangent of 0.024. The dimension of substrate when four sectoral patches are unmoved is (50 x 50) mm2 and for moving sectoral patch antenna is (70 x 70) mm2 as shown in figure 2(a). The thickness of the dielectric substrate used is h = 1.59 mm. The circular patch antenna is of radius r = 23.5 mm and is fed coaxially at 9 mm in order to achieve 50 Ω characteristic impedance. For the first case, a double gap of 2 mm each is introduced both vertically as well as horizontally as shown in the geometry. The circular patch after introducing gap both horizontally as well as vertically form four regions namely region-I, II, III and IV. Then the feeding sectoral patch is moved slowly away by keeping the other three sectors unmoved. This feeding sector is moved up to a distance of x = y = 20 mm from the centre at which antenna shows a maximum impedance bandwidth with maximum gain. The prototype of the antenna designed is shown in figure (2). Figure 2(a) shows the top view and 2(b) shows the bottom view of the proposed antenna design.

IV. RESULTS AND DISCUSSINIn order to show the improved performance of the antenna with changing gap between the sectoral patches, it will be compared through simulations and measurements. The proposed antenna structure is simulated using a simulation software CST Studio Suite and experimentally tested using a Vector Network Analyzer at different gaps. As discussed in the earlier section, due to the coupling effect in between the sectoral patches, the impedance bandwidth increases with the increase in antenna gain as we move the feeding sectoral patch away from the other unmoved three sectoral patches up to a certain distance. After which due to larger separation the antenna characteristics again starts degrading.

A. Return loss measurement at different gaps

B. Input impedance measurements at different gaps

C. Radiation pattern at different gaps

A. Return loss measurement at different gaps

The variation of return loss with the frequency is shown in figure (3). Figure (3) shows the return loss variation for the first case when the circular patch antenna is divided into four sectoral patches. The gap introduced in between the four sectoral patches is 2 mm each. One of the sectoral patches is excited by employing a coaxial feed at a distance of 9 mm from the center of the circular patch. At this first step, all the four sectoral patches are unmoved. The antenna resonates in the X-band in the frequency range 9 GHz to 11 GHz. Figure 3(a) shows the simulated return loss variation with frequency and 3(b) shows the experimentally measured variation. The simulated resonating frequencies are 9.4 GHz, 9.7 GHz and 10.4 GHz. At frequency 10.4 GHz, the antenna shows a maximum return loss of -28 dB. The experimentally measured resonating frequencies are 9.7 GHz and 10.4 GHz. At frequency 10.4 GHz, the antenna shows a maximum return loss of -31 dB. But the antenna does not show wideband characteristics. It gives a minor impedance bandwidth of about 1.6 %. As we have given feed to just one of four sectoral patches, the other three acts as parasitic sectoral patches. Due to coupling between the four sectoral patches, it acts as one antenna as a whole. For the next step, in figure 4(a), the excited patch is moved in a direction both horizontally as well as vertically with x = 2 mm and y = 2 mm at an angle of 45 degree. Adjacently the coaxial feed also moved to a distance of 11 mm. Now, the frequency range changes to 9.3 GHz to 10.3 GHz and the antenna shows a dual resonance. The resonating frequencies are 9.4 GHz, 9.7 GHz and 10.3 GHz. It gives an increase in the impedance bandwidth from 1.6 % to 4.2 %. The antenna shows a dual resonance with resonating frequencies at 9.4 GHz and 9.7 GHz with a maximum return loss of -36 dB at frequency 9.7 GHz. When the excited sector is moved further at an angle of 45 degree from the center, the impedance bandwidth goes on increasing up to a gap of 20 mm of the excited sectoral patch with feed at 29 mm. With a gap of 4 mm of the sectoral patch

and feed at 13 mm, the resonating frequencies are 9.5 GHz, 9.7 GHz and 10.2 GHz as shown in figure 4(b). The antenna shows a dual resonance with impedance bandwidth increased to 5.1 % with a maximum return loss of -45 dB at the resonating frequency 9.5 GHz. Similarly moving to 15 mm gap of the sectoral patch with feed at 24 mm in figure 4(c), the antenna shows a dual resonance at the two frequencies 9.5 GHz and 9.7 GHz with an equal return loss of -23 dB at both resonating frequencies. The increased impedance bandwidth reaches up to 6.3 %.The maximum impedance bandwidth is reached when gap of the sectoral patch is maintained at a distance of 20 mm from the center with the coaxial feed at 29 mm. Figure 5(a) shows the simulated resonating frequencies are 9.5 GHz, 9.7 GHz and 10.0 GHz. At these frequencies antenna designed shows a triple resonance with a maximum return loss of -17 dB at the resonating frequency of 9.7 GHz. The experimentally measured results as shown in figure 5(b) are also in good agreement with the simulated results showing a wide band at the same resonant frequency. At this gap the antenna shows a wideband characteristic with the impedance bandwidth increased up to 12.3 %. Thus the results show an increase in the impedance bandwidth 1.6 % to 12.3 % giving a wideband at the excited sectoral patch gap of 20 mm. The designed antenna can be very useful for wideband applications especially at the higher frequencies.By further increasing the gap of the excited sectoral patch from the center of the circular patch antenna in a direction 45 degree, the impedance bandwidth starts reducing. The frequency range is still the same as for other lesser gaps introduced but the triple resonance again changes to dual resonance with the reduction in return loss. The impedance bandwidth reduces to 8.1 % from 12.3 % as shown in figure 4(d). This is because of the coupling effect which results in bandwidth reduction due to larger separation of the excited sectoral patch. Gap in between the patches results in different resonant frequencies in the frequency range 8-12 GHz. At a distance of 20 mm, the antenna shows a wide impedance band, higher gain and low side lobe level. At this particular gap, the different resonant frequencies combine and therefore results in a wide band in the same frequency range in X-band.

B. Input impedance measurements at different gaps

The input impedance curves for different gaps are shown in figure (6). Figure 6(a) shows simulated input Impedance (in ohms) variation with frequency (in GHz) when the four sectoral patches are unmoved and figure 6(b) shows the experimentally measured input Impedance (in ohms) variation with frequency (in GHz). Figure (7) shows simulated input impedance (in dB) variation with frequency (in GHz) when the excited sectoral patch is moving for different gaps. Figure 8(a) and 8(b) shows the simulated and experimental input Impedance (in ohms) variation with frequency (in GHz) for the excited sectoral patch at a gap of 20mm respectively. The results show a good impedance matching at different resonating frequencies in the frequency range from 9 GHz to

11 GHz. The coaxial feed is given at a calculated distance in order to achieve 50 ohms characteristic impedance. When the four sectoral patches are unmoved, at frequency 9.7 GHz and 10.4 GHz, good impedance matching is obtained as shown in the figure below. At the resonating frequency of maximum impedance bandwidth with a gap of 20 mm of the excited sectoral patch from the center, a good impedance match of 50 ohms is obtained at the resonating frequencies of 9.7 GHz and 10.0 GHz. The simulated results are in good agreement with the experimentally measured ones. The impedance matching at different frequencies for different gaps is therefore presented.

C. Radiation pattern at different gaps

The radiation patterns for different gaps at the resonant frequency are shown in figure (9). The polar plots at the central resonant frequency of 10 GHz are therefore plotted at different gaps. Figure 9 (a) shows polar plot at frequency 10 GHz for the antenna structure when the four sectoral patches are unmoved. The plot is shown for Directivity Abs (Phi = 0) between Theta degree vs. dBi. At frequency 10 GHz in the main lobe direction 25.0 degree, the antenna gain is 7.8 dBi. Within 3 dB angular width in the direction 23.4 degree, the side lobe level is -7.8 dB. When the gap of the excited sectoral patch is increased by 2 mm that is x = y = 2 mm, the radiation properties starts improving with the increasing impedance bandwidth as shown in figure 9(b). The side lobe level reduces to a good extent. In the main lobe direction 12.0 degree, the antenna gain is 7.1 dBi within 3 dB angular width in the direction 35.6 degree. The side lobe level is reduced to -9.1 dB which shows improvement in the radiation characteristics. In figure 9(c), for the gap of the excited sectoral patch increased to 4 mm that is x = y = 4 mm, the radiation properties again improve with the increasing impedance bandwidth. The side lobe level reduces to a good extent. In the main lobe direction 7.0 degree, the antenna gain is 8.6 dBi within 3 dB angular width in the direction 41.5 degree. The side lobe level is reduced to -15.2 dB which shows improvement in the radiation characteristics. Similarly, when the gaping is increased to 15 mm, the radiation properties again show good response. The antenna gain in the main lobe direction 1.0 degree increased to 10.3 dBi. And for 3 dB angular width, the side lobe level is further reduced to -14.4 dB in the direction 40.7 degree as in figure 9(d).The maximum antenna gain is obtained when the excited sectoral patch is placed at a distance of 20 mm from the center of the circular patch antenna that is for x = y = 20 mm. At this distance, the antenna shows maximum antenna gain for the resonant frequency 10 GHz. In figure 9(e), in the main lobe direction 0.0 degree, the antenna gain obtained is 12.0 dBi. The side lobe level is reduced to a good extent that is, for 3 dB angular width in the direction 35.0 degree, it is reduced to -15.0 dB.

Further increasing the gap of the excited sectoral patch from the other three sectoral patches leads to a degradation in the radiation properties of the antenna design proposed. The antenna gain reduces to 9.7 dBi and the side lobe level increased to -11.1 dB at the resonant frequency of 10 GHz as can be seen in figure 9(f).Therefore the above discussion of the results verify that by introducing gap in between the circular patch antenna, the antenna characteristics improved nicely to a good extent as the gap leads to coupling effect in between the sectoral patches. This coupling between the parasitic patches results in the enhancement of the impedance bandwidth of the antenna. Further in the proposed antenna design, as the gaping goes on increasing the antenna characteristics further improve. With the increase in the impedance bandwidth, the radiation characteristics such as antenna gain increases and the side lobe level decreases. For better antenna radiation, the antenna gain should be good enough with the side lobe level as low as possible. As the patch separation increases the coupling becomes much smaller as compared to conventional circular patches. So the antenna characteristics improve up to a particular distance. But after that, due to large separation the impedance bandwidth as well as the antenna gain starts reducing and the side lobe level starts increasing which is not good for better antenna radiation. For large separation or thicker substrates, the surface waves would become more important, and then a lowering of the mutual coupling would indeed occur through a reduction in the surface-wave excitation. The gap in the sectoral patches results in different resonant modes with a low impedance bandwidth. As the gaping increases, these different resonant modes start combining with the improvement of antenna characteristics. At a certain gap of 20 mm, as discussed above in the results, the resonant modes match and combine to provide a wide impedance bandwidth. At this particular gap, the other antenna characteristics such as antenna gain, side lobe level and back radiation improve and give better results. Further increase in the gap leads to impedance bandwidth reduction because the resonant modes again start separating into different resonant modes. Subsequently the radiation properties of the proposed antenna design also degrades with the lowering of antenna gain and increase in the side lobes.

V. CONCLUSION

In this paper, a novel form of gap coupled sectoral patch antenna based on moving excited patch has been presented. The excited sectoral patch is moved slowly away from the other three sectoral patches and the variation in the antenna characteristics is therefore shown in the results. Due to coupling in between the sectoral patches, the impedance bandwidth increases up to a certain distance and the antenna proposed results in the wideband characteristics with perfect impedance matching. With this the radiation characteristics are also improved with the enhancement of antenna gain and

subsequent reduction in the side lobe level. The gap coupled antenna proposed is very helpful in high frequency wideband applications. This antenna can also be very handful in pervasive wireless applications.

VI. REFERENCES

[1] C. A Balanis,., Antenna Theory: Analysis and Design, Wiley, 2005.[2] G Kumar,. and K. C. Gupta, “Broadband Microstrip Antennas Using Additional Resonators Gap Coupled to the Radiating Edges,” IEEE Trans. Antennas and Propagation, Vol. 32, No. 12, 1984.[3] T. N Chang,. and J. –H. Jiang, “Enhance gain and bandwidth of circularly polarized microstrip patch antenna using gap-coupled method,” PIER 96, 127-139, 2009.[4] M., Manteghi, “Wideband microstrip patch antenna on a thick substrate,” Antennas and Propagation Society International Symposium, Vol. 2008, 1-4, 2008.[5] R Yang,., Y. Xie, D. Li, J. Zhang and J. Jiang, “Bandwidth enhancement of microstrip antennas with metamaterial bilayered substartes,” Journal of electromagnetic waves and applications, Vol. 21, No. 15, 2321-2330, 2007.[6] Chang, F. S., K. L. Wong and T. W. Chiou, “Low cost broadband circularly polarized patch antenna,” IEEE Trans. Antennas and Propagation, Vol. 51, No. 10, 3006-3009, 2003.[7] Aanandan, C. K., P. Mohanan and K. G. Nair, “Broadband gap coupled microstrip antenna,” IEEE Trans. Antennas and Propagation, Vol. 38, No. 10, 1581-1586, 1990.[8] Lee, R.Q., K. F. Lee and J. Bobinchak, “Characteristics of a two layer electromagnetically coupled rectangular patch antenna,” Electronics Letters, Vol. 23, No. 2, 1301-1302, 1987.[9] Nishiyama, E., M. Aikawa and S. Egashira, “Stacked microstrip antenna for high gain and wideband,” IEE Proc.-Microw. Antennas Propag., Vol. 151, No. 2, 143-148, 2004.[10] Wood, C., “Improved bandwidth of microstrip antennas using parasitic elements,” IEE Proc.-Microw. Antennas Propag., Vol. 127, No. 4, 231-234, 1980.[11] P., T Kumar,. Chakravarty, G. Sing, S. Bhooshan, S. K. Khah and A. De, “Numerical computation of Resonant Frequency of gap coupled circular Microstrip antennas,” Jounal of Electromagnetic Waves and Applications, Vol. 21, No. 10, 1303-1311, 2007.[12] A Kandwal,., T. Chakravarty and S. K. Khah, “Circuital method for admittance calculation of gap coupled sectoral antennas,” Microwave and optical technology letters, Vol. 54, No. 1, 210-213, 2012.[13] C. S Lee,. and V. Nalbandian, “Impedance matching of a dual- frequency microstrip antenna with an air gap,” IEEE Transactions on Antennas and Propagation, Vol. 41, No. 5, 680-682, 1993.[14] J.-Y Sze,. and K.-L. Wong, “Slotted rectangular microstrip antenna for bandwidth enhancement,” IEEE Transactions on Antennas and Propagation, Vol. 48, No. 8, 1149-1152, 2000.[15] W.-W Chai,., X.-J. Zhang, and S.-G. Liu, “Wideband microstrip antenna array using U-slot,” PIERS Online, Vol. 3, No. 7, 1085-1088, 2007.[16] K. P Ray,., S. Ghosh, and K. Nirmala, “Compact broadband gap-coupled microstrip ntennas,” IEEE Antennas and Propagation Society International Symposium 2006, No. 9-14, 3719-3722,2006.

[17] , R. P Jedlicka., M. T. Poe, K. R. Carver, “Measured mutual coupling between microstrip antennas,” IEEE Trans. Antennas and Propagation, Vol. 29, 147-149, 1981.[18] P Nayeri,., F. Yang and A. Z. Elsherbeni, “Bandwidth improvement of reflectarray antennas using closely spaced elements,” Progress in electromagnetic research C, Vol. 18, 19-29, 201, R. R 1.[19] Ramirez. and F. De. Flaviis, “A mutual coupling study of linear and circular polarized microstrip antennas for diversity wireless system,” IEEE Transactions on Antenna and Propagation, Vol. 51, No. 2, 2003.

List of figures:Figure-1: Geometry of antenna designFigure-2: Prototype of antenna designed

(a) Front view

(b) Bottom view

Figure-3: Return Loss (in dB) variation with frequency (in GHz) when the four sectoral patches are unmoved

(a) Simulated return loss vs. frequency

(b) Experimental return loss vs. frequency

Figure-4: Simulated return Loss (in dB) variation with frequency (in GHz) when the excited sectoral patch is moving

(a) For gap = 2mm

(b) For gap = 4mm

(c) For gap = 15mm

(d) For gap = 22mm

Figure-5: Return Loss (in dB) variation with frequency (in GHz) for the excited sectoral patch at a gap of 20mm

(a) Simulated return loss vs. frequency

(b) Experimental return loss vs. frequency

Figure-6: Input Impedance (in ohms) variation with frequency (in GHz) when the four sectoral patches are unmoved

(a) Simulated return loss vs. frequency

(b) Experimental return loss vs. frequency

Figure-7: Simulated input impedance (in dB) variation with frequency (in GHz) when the excited sectoral patch is moving

(a) For gap = 2mm

(b) For gap = 4mm

(c) For gap = 15mm

(d) For gap = 22mm

Figure-8: Input Impedance (in ohms) variation with frequency (in GHz) for the excited sectoral patch at a gap of 20mm

(a) Simulated return loss vs. frequency

(b) Experimental return loss vs. frequency

Figure-9: Radiation Pattern Directivity Abs (Phi = 0) between Theta degree vs. dBi

(a) When the excited sector is unmoved

(b) For gap = 2mm

(c) For gap = 4mm

(d) For gap = 15mm

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