Design of Single Feed Dual Band Dual Polarized Microstrip Antenna With Defected

Ground Structure for Aeronautical and Radio Navigation Applications

P. R. Prajapati*1, A. Patnaik2 and M. V. Kartikeyan3

Millimeter Wave Laboratory, Department of Electronics and Computer Engineering, Indian Institute of Technology, Roorkee-247667, India1,2,3

Ph: 91-1332-286453, Fax: 91-1332-285368

Email: pravinprajapati05@gmail.com1, apfecfec@iitr.ac.in2, kartik@iitr.ac.in3

Abstract A single feed proximity coupled dual frequency dual polarized microstrip antenna is presented in this paper. A simple back-to-back structure with a common ground plane is used to get dual frequency and directivity at opposite directions. By incorporation of the spiral shape defected ground structure (DGS) in the common ground plane, the realization of circular polarization at lower band is achieved. A dumbbell shape DGS is also incorporated in the common ground plane to reduce lower patch size by 10.4% and to improve impedance matching. The proposed antenna radiates right hand circular polarization at the lower band (1.59-1.61 GHz), which covers aeronautical and radio navigation and satellite application spectrum, and linear polarization at the upper band (2.45-2.52 GHz), which covers fixed mobile radio location and radio determination applications spectrum allocated in India. At boresight, the 3 dB axial ratio bandwidth of 20 MHz at the lower band and a gain of 5.1 dB at the lower resonant frequency and a gain of 6.7 dB at the upper frequency band are achieved. The advantages of using DGS in designing the proposed antenna and comparison of proposed antenna with earlier reported single feed dual band dual polarized antennas are also presented. A laboratory prototype of proposed antenna has been fabricated to cross verify the simulation results. The measured results are in agreements with the simulated ones.

1. Introduction The demand of dual frequency, dual polarized antenna is increasing due to its special characteristics like polarization diversity. In the technique of polarization diversity, single antenna is capable of generating multiple frequency and multiple polarization. The use of dual frequency and dual polarized antenna can make the frequency spectrum doubled and thus increment of channel capacity can be realized. In mobile communication systems and satellite applicationscircularly polarized antenna find applications due to additional design freedom of not requiring alignment of the electric field vector at transmitting and receiving locations. In the past decade, many techniques for the design of microstrip antenna with dual frequency and dual polarization have been proposed [1-7]. A single feed configuration gives advantages like reduction in complexity, low weight and less RF loss. In single feed configuration, realizations of dual polarization by using PIN diodes or micro electromechanical switches have been reported [4, 6]. Use of PIN diodes and MEMS demands the DC biasing network and make the antenna structure complicated. Nowadays, defected ground structure (DGS) has been used by various researchers to improve characteristics of the printed antennas [8-9]. The conducting metal etched off in specific shape from the ground plane known as ‘DGS’, provides wide rejection band covering some frequency range. The main characteristics of the DGS are: (a) the slow wave effect (b) band stop characteristics [9]. This effect produced because of introduction of the effect of equivalent inductive (L) and capacitive (C) components by the DGS. The Microstrip line with DGS has a large electrical length as compared to conventional microstrip for the same physical length and resonance frequency has reciprocal relationship with electrical length. Thus, DGS helps to lower resonance frequency and thus to reduce the size of the antenna. In this paper, back-to-back, multilayered, dual frequency, dual polarized antenna has been proposed . Two DGSs (spiral shape and dumbbell shape) are incorporated to get benefits like realization of dual polarization, good impedance matching, reduction in cross polarization at boresight and patch size reduction. To the best of the author’s knowledge, the first time the realization of dual band dual polarized characteristics of microstrip antenna by DGS has been proposed.

2. Geometry Of The Proposed Antenna

As per Fig. 1, proposed antenna consists of total five printed circuit board (PCB) layers. The top layer has a rectangular patch of size 50 mm × 40.357 mm and the bottom side of fourth layer (L4) has the rectangular patch of size 50 mm × 28.0 mm. The configuration seems back to-back patch antennas having common ground plane having dimension of W × L = 107.5 mm × 78 mm . To excite upper patch, proximity feeding was used.

978-1-4673-5225-3/14/$31.00 ©2014 IEEE

Fig. 1. Cross-sectional view of proposed antenna, Fig. 2. Design of quarter wave transformer (not to scale). Parameters : ε r and tanδ of layers L1, L2, L3, L4 = 2.5 and 0.0017 respectively. ε r and tanδ of layer L5 = 4.4 and 0.0019 respectively. H1, H2, H3, H4, H5 = 1.5748 mm.

For good impedance matching, the quarter wave transformer design (shown in Fig. 2) for feeding and incorporation of asymmetrical slots at patch methods were used. The lower patch was excited by the lower feed line, which is proximity coupled with the lower patch. The width of the lower feed line is 1.26 mm to match 50 Ω impedance of the antenna. To realize single feed, a small hole was created in the L2, L3 and the ground layer and after that the upper feed and the lower feed lines were connected with a metal strip. Here, for getting directivity of different bands at opposite directions, back-to-back structure with two rectangular patches was used. This type of antenna can be used in trans-receivers, which can transmit signals of specific frequency at different direction and at the same time, and also able to receive signals of specific frequency from the opposite direction with different polarization. The lower rectangular patch is responsible for the generation of 2.45-2.452 GHz band and the upper rectangular patch is responsible for the generation of 1.59-1.61 GHz band.

The spiral shape DGS was incorporated under the upper patch in common ground layer. Due to the slow-wave effect of the DGS [8-9], the size of upper rectangular patch reduces by 20% as compared to the without DGS structure. Due to the incorporation of the spiral DGS, the two nearly orthogonal modes are generated at 1.6 GHz frequency. The design dimensions of the spiral DGS were optimized by CST Microwave Studio simulator version 12 [10] such that the orthogonal modes have almost equal amplitudes and near to 90⁰ phase shift, which are the essential conditions to realize circular polarization. The dumbbell shape DGS was incorporated in the common ground plane, under the lower patch, which reduces the size of the lower patch by 11% and improves the impedance matching. The design dimensions and locations of both DGSs are shown in Fig. 3. It was found that using simple rectangular patch design and parallel single feed design, two resonant modes are generated, but due to improper impedance matching, good return loss hasn't been obtained. A good impedance matching was obtained by incorporation of two asymmetrical plus slots at the lower rectangular patch as shown in Fig. 4. Fig. 5 shows that by incorporating asymmetrical plus shape slots in the lower patch, the S11 has been improved from -9.3 dB to -11.57 dB at the lower band and -9.59 dB to -13.89 dB at the upper band.

Fig. 3. Design dimensions and location Fig. 4. Dimensions of the lower Fig. 5. The effect of incorporating of both DGSs. and slot. asymmetrical slots on return loss

of the proposed antenna. To increase the gain and bandwidth at upper band, one more layer of an inexpensive FR4 substrate was stacked above the lower patch. The dimensions of stacked patch (W ×L = 40 mm×30 mm) were optimized such that its resonant frequency is near to that of the lower patch, which helps to enhance the gain and the bandwidth of the proposed antenna.

3. Results and Discussion

3.1. Theoretical and Measured Results

A laboratory prototypes, with five PCB layers, was fabricated.The teflon nuts of 4 mm diameter were used to stack all five PCB layers. 20 GHz Rohde & Schwarz (model 1127.8500ZVM) Vector Network Analyzer (VNA) was used to measure return loss of the antenna.

Fig. 6 (a) shows simulated and Fig. 6 (b) shows comparison of simulated and measured return loss characteristics of the proposed antenna. For S11 < −10 dB, the antenna resonates at two bands : 1.59-1.61 GHz and 2.45-2.52 GHz. The simulated S11 bandwidth of the proposed antenna is about 1.25% at the lower band and 2.82% at the upper band. A reasonably good agreement between simulated and measured result was observed. The small frequency shift is due to fabrication errors and misalignment of antenna layers. Fig. 7 shows the simulated and measured AR characteristics for both bands. It was observed that the proposed antenna has circular polarization (for AR<3 dB) at the lower band (1.59-1.61 GHz) and linear polarization (for AR > 40 dB) at the higher band (2.45-2.52 GHz). Simulated results Fig. 7 (a) shows that at the lower band AR < 0.3 dB at 1.6 GHz and the 3 dB AR bandwidth of 1.2% was achieved, which is suitable for aeronautical and radio navigation and satellite application spectrum allocated in India. However, due to fabrication error, there was a slight shift of resonant frequency and CP characteristics obtained at this shifted frequency band as shown in Fig. 7 (b).

(a) (b) (a) (b) Fig. 6. (a) Simulated and (b) comparison of simulated and measured Fig. 7. (a) Simulated axial ratio characteristics. (b) Measured reflection coefficient characteristics of the proposed antenna. S11 and axial ratio characteristics of the proposed antenna. Fig.8 (a) and Fig. 8 (b) show the simulated and measured gain characteristics of the proposed antenna at the boresight. Simulated results show that at the lower band, measured gain of 5.1 dB and at the higher band, that of 6.7 dB were obtained. The stacked patch located above the lower patch helps to enhance the gain of the proposed antenna at the higher band.The simulation results and measured results showed good agreements with each other. The simulated radiation efficiency vs. frequency characteristics of the proposed antenna shown in Fig. 9. It shows that at the lower band average efficiency of 85% and at the upper band that of 97% have been achieved. At the upper band due to good impedance matching, high radiation efficiency was possible.

(a) (b) Fig. 8. Simulated and measured gain of the proposed antenna at both the bands Fig. 9. Simulated radiation efficiency vs. frequency characteristics of the proposed antenna. The simulated vector current distribution on the surface of the proposed antenna at 1.6 GHz shown in Fig. 10. From the Fig. 10, right hand CP wave in +z direction can be observed.

(a) ωt = 0⁰ (b) ωt = 90⁰ (c) ωt =180⁰ (d) ωt =270⁰

Fig. 10 Simulated surface current distributions of the proposed antenna at 1.6 GHz at four different time instants.

A brief comparative study of design and development of single feed dual band dual polarized antenna is given in table I. The proposed antenna gives more gain as compared to [1, 4, 5]. In the proposed design, the technique used to design dual band and dual polarized antenna is incorporation of simple DGS at a common ground layer and asymmetrical slots at the lower patch, which is a very simple technique as compared to techniques proposed by [4] and [7] to realize dual band and dual polarization.

Table I. Design and development of single feed dual band dual polarized antennas: A comparative study.

Ref. no. and year → [1] 2000 [2] 2001 [4] 2008 [5] 2008 [7] 2012 Proposed

Antenna Parameters↓

Size of patch (L × W ) mm2

55 × 34 60 × 40 Circular patch,Radius = 20 mm 36 ×36 20 × 35

L1 ×W1 = 40.357 × 50,

L2 × W2 =28 × 50

Polarization Horizontal

(H) and Vertical

CP and LP H, V, LP and CP, Total 4 polarizations

Elliptical and LP H and V CP and LP

Axial Ratio Bandwidth - Not

mention

Band 1 = 55MHz Band 2 =

110 MHz - - 20 MHz

Techniques used Slot antenna Slot

antenna PIN diode and

RF choke

Incorporation of minor slot

near with major slot

PIN diode

Back to back square patch antenna with slot and dual

DGS

Gain

1.54 dB

Not

mention

6.3 dB

3 dBi at 1st band, 4 dBi at

2nd band Not mention

5.1 dB at 1 s t band, 6.7 dB at

2nd band

4. Conclusion

A stacked dual band dual polarized microstrip antenna using two DGSs has been proposed and successfully implemented. The spiral DGS on a common ground layer helps to excite two near orthogonal degenerate modes for CP radiation and asymmetrical plus shape slots helps for impedance matching. The proposed antenna can provide 3 dB axial ratio bandwidth of 20 MHz and 10 dB return loss bandwidth of 20 MHz at the lower band and 70 MHz at the upper band. A parametric study of the effect of the DGS on performance parameters of the proposed antenna had also been carried out. This work demonstrates that the DGS can be successfully used in antenna design to realize dual polarization, to improve impedance matching, to reduce the size of the patch and for the improvement of the cross polarization level at LP band.

5. References

1. Sona O. Kundukulam, Manju Pauson, C. K. Anandan, P. Mohanan, K. Vasudevan, “Dual band dual polarized compact microstrip antenna,” Microwave and Optical Technology Letters, vol. 25, no. 5, June 2000. 2. Manju Pauson, Sona O. Kundukulam, C. K. Anandan and P. Mohanan, “New compact dual band dual polarized microstrip antenna,” Microwave and Optical Technology Letters, vol. 29, no. 5, June 2001. 4. Rui Hung Chen and Jeen Sheen Row, “Single feed microstrip patch antenna with switchable polarization,” IEEE Trans. on Antennas and Prop., vol. 56, no. 4, Apr. 2008. 5. Sarin V.P. , Nisha Nassar, Gijo Augustine, P. Mohanan, C. K. Anandan, K. Vasudevan, “A dual band dual polarized microstrip antenna for WLAN applications,” IEEE Int. Symposium on Antenna and Prop. Society, 2008, pp. 1-4. 6. Medeiros C., Costa J. R., Fernandes C. A., “MEMS reconfigurable stacked antenna for WLAN applications,” IEEE Antennas and Propagation Society International Symposium, pp. 1-4, 2008. 7. M.H. Amini, H.R. Hassani, S. Mohammad Ali, “A Single feed reconfigurable polarization printed monopole antenna,” 6th Europian Conf. on Antennas and Prop. (EUCAP), pp. 1-4, 2012. 8. A. Arya, A. Patnaik and M. V. Kartikeyan, “Back to back combined single feed proximity coupled antenna with dumbbell shaped DGS,” J. of Electromagnetics Analysis and Applications, vol. 3, pp. 43-46, 2011. 9. D. Guha, Y. M. M. Antar, Microstrip and Printed Antennas new trends, techniques and applications, 1st edition, Wiley Publication, U. K., 2011. 10. CST Microwave Studio Simulator, version.12.