High gain and wideband substrate integrated waveguide based H-plane horn antenna

Accepted Manuscript Regular paper High Gain and Wideband Substrate Integrated Waveguide Based H-plane Horn Antenna Anik Ghosh, Kaushik Mandal PII: DOI: Reference:

S1434-8411(18)32626-8 https://doi.org/10.1016/j.aeue.2019.04.005 AEUE 52711

To appear in:

International Journal of Electronics and Communications

Received Date: Revised Date: Accepted Date:

3 October 2018 31 March 2019 9 April 2019

Please cite this article as: A. Ghosh, K. Mandal, High Gain and Wideband Substrate Integrated Waveguide Based H-plane Horn Antenna, International Journal of Electronics and Communications (2019), doi: https://doi.org/ 10.1016/j.aeue.2019.04.005

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High Gain and Wideband Substrate Integrated Waveguide Based H-plane Horn Antenna Anik Ghosh1, Kaushik Mandal2 1 2

Department of E.E.E, Indian Institute of Technology (IIT) Guwahati, Assam, India

Institute of Radio Physics and Electronics, University of Calcutta, West Bengal, India

Abstract A substrate integrated waveguide (SIW) based H-plane horn antenna with high gain and larger operating bandwidth is proposed in this article in a cost-effective way. An air layer is introduced between the top and bottom surfaces to increase the gain by reducing the effective dielectric constant. Extended metal circular patches with horn aperture are adopted to increase the gain of the horn antenna. Metalized via posts are used in front of the aperture to deal with the impedance mismatch issue. Moreover, air-vias are incorporated periodically into the extended dielectric slab to enhance the bandwidth. The proposed antenna is of the dimension 42mm× 18.6 mm×4.8mm. The antenna operates over 20.6-21.2 GHz, 23.0-26.1 GHz and 27.4-28.2 GHz with a measured peak gain of 11.25 dBi at 20.9 GHz. The proposed antenna is fabricated and measured to validate the simulated results and it shows good agreement.

Key words SIW, H-plane horn antenna, high gain, wideband, air gap, metallic post, air vias.

1. Introduction Over the last decade, the substrate integrated waveguides (SIW) shows significant cost improvements over traditional solid rectangular waveguides as they are easily produced with standard PCB fabrication technology. Previously, bulky 3-D transitions such as vertical current probes and fin lines were needed to route from a microstrip line to rectangular waveguides (RWG). It is now possible to fully integrate the transition, connecting planar transmission line, and waveguide on the same substrate [1]. Another attractive feature of substrate integrated waveguides is the large bandwidth they provide, especially in the X-band (8GHz-12GHz), as well as at higher microwave and millimeter-wave frequencies up to 30 GHz. Horn antennas are extremely popular in the microwave region above about 1 GHz. Horns provide high gain, low VSWR, and relatively wide bandwidth. As an additional benefit, the theoretical calculations for horn antennas are achieved very closely in practice [2]. However, substrate-integrated-waveguide (SIW) based horn antennas are not as commonly used as conventional metallic rectangular horn antennas due to their low front-to-back ratio (FTBR)

and impedance mismatch. The planar antenna structures including SIW horn antennas are used in automotive radars and unmanned aerial vehicle applications. In order to achieve an efficient radiation pattern, impedance matching between the aperture of SIW horn antenna and the surrounding media (air) is of great significance. Printed transition is one of the solutions to overcome the impedance mismatching problem [3]. With the reduction of the substrate thickness of a SIW based horn antenna, the impedance of the SIW based horn antenna keeps decreasing. Also, if the thickness of the substrate is too small, the impedance difference between the SIW based horn antenna and the free space becomes significant. This impedance mismatch limits the application of SIW based horn antennas, which are built on thin substrates, and their applications in low frequencies [4-5]. For mm-wave antennas, a wide bandwidth can be achieved with various techniques like a heart-shaped patch, a tapered cavity-backed slot antenna, and angled dipole antenna [6-9]. But it is difficult to achieve wider bandwidth for SIW based H-plane horn antenna. There are various approaches, which are studied in the way to improve the performance of SIW based horn antennas. We have studied that impedance mismatch issue can be decreased by adopting metallic post [10]. The bandwidth can be increased by employing asymmetrical slots based on SIW technology [11] or by adopting perforated air vias [1214]. The gain can be increased by adopting extended rectangular patch [15] or by using ridge SIW [16] or using array [17]. There are also other ways to improve the performance of the SIW based horn antenna [18-20]. In this paper along with some established techniques incorporation of an air layer of thickness 1.6 mm in between the top and bottom surfaces has been conceived to improve the gain by means of decreasing the effective dielectric constant. In addition, three extended metal circular patches with horn aperture are adopted to increase the gain of the horn antenna. Three metalized via posts are used in front of the aperture. These metalized vias deal with the impedance mismatch issue. The proposed antenna is of the dimension 42 mm×18.6 mm×4.8 mm. The antenna operates over 20.6 - 21.2 GHz, 23.0 - 26.1 GHz, and 27.4 - 28.2 GHz. The first two bands cover the K-band (1826.5 GHz) except the band around 22.24 GHz where attenuation due to water vapor is maximum hence it is useful for short-range applications. The microwave domain of this band is used for radar and satellite applications whereas the infrared domain is used for astronomical observations. The third band covers a smaller part of the Ka-band (26.5 – 40 GHz). This band is suitable for radar, space communications, and experimental communications. Ka-band is now considered as the spectrum of the future for NASA communications.

2. Antenna Configuration and design The intention of the authors is to design a SIW based H-plane horn antenna to operate at 25 GHz. The proposed structure is illustrated in Fig.1, and the optimal dimensions are listed in Table 1.

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The 42 mm long proposed antenna consists of three parts, which

are waveguide section (L1), flaring section (L2), and extended dielectric slab (L3). The width of the waveguide is Weff (= 6 mm) and the total width of the antenna is W (= 18.6 mm). In the side view of the antenna, there are three sections- upper PCB layer, air gap, and lower PCB layer. Upper PCB layer (h 1 = 1.6 mm) is a one-sided PCB whose metal part is used as the patch. Lower PCB layer (h 2= 1.6 mm) is also a one-sided PCB whose metal part is used as the ground. In between these two layers, there is an air gap (h a=1.6 mm). Side walls of the H-plane horn are-

Table 1: Parameters of proposed antenna (all dimensions are in mm) Parameter

Value

Parameter

Value

Parameter

Value

Parameter

Value

L

42

ga

3

C

3

hc

3.2

L1

9

da

2

a

2.76

h1

1.6

L2

17.2

g

1.5

R1

2.5

h2

1.6

L3

13

d

0.8

R2

2.108

ha

1.6

W

18.6

gp

3

R3

0.635

Weff

6

Rp

h

4.8

0.15

Fig. 1. Proposed antenna structure, (a) Top view, (b) Bottom view, (c) Side view designed considering the SIW techniques by inserting metallic vias (metal posts) from patch to ground. The vias with diameter d (= 0.8 mm), and height h (= 4.8 mm) are inserted at a gap g (= 1.5 mm) between each other. The air gap is used to reduce the effective dielectric constant, which helps to increase the gain the gain of the antenna. In addition, there are three circular extensions with radius (c) at the ground plane of the horn aperture. These extensions [13] are used to increase the gain of the antenna. In front of the horn flaring there are three metal posts [5] which deals with the impedance mismatch issues. The metal posts in front of the horn flaring are of radius Rp (= 0.15 mm), height (h) and are placed at a gap gp (= 3 mm) in between them. To increase the bandwidth, some air vias

[9] of diameter da (= 1 mm), height (h) are implemented at the extended dielectric section at a gap ga (= 3 mm) from each other. The antenna is excited externally with a SMA connector with a transition pin (probe of the antenna) of length hc (= 3.2 mm) and radius R3 (= 0.635 mm). The outer and the inner conductors’ radius of the connector are R1 (= 2.5 mm) and R2 (= 2.108 mm) respectively.

3. Parametric Studies for Improvement and Optimization of Antenna Performance Some sensitive parameters are investigated to understand their influences on the antenna performance. Effects of those parameters are studied and explained as follows. All the simulations are carried out using FEM-based Ansys HFSS EM simulator.

3.1. Design Steps Design steps have been illustrated in Fig. 2. The stepwise changes in reflection coefficient (S11) are summarized in Fig. 3. Initially, a simple SIW based H-plane horn antenna is designed with an air gap in between the upper and lower surface and with only one circular extension in front of the horn aperture. This is shown in Fig. 2(a) and mentioned as Design-A which operates over only a single band 24.21- 26.4 GHz. In the next step for the better impedance bandwidth, three metallic posts are inserted at the horn aperture and it has been mentioned as Design-B as shown in Fig. 2(b). After that instead of a single circular extension, three symmetrical circular extensions are incorporated for better gain. This is mentioned as Design-C as shown in Fig. 2(c). These three extensions operate at different frequencies and those frequencies merge to give broader bandwidth. Finally, eighteen symmetrical air-vias are incorporated in the extended dielectric section for better gain and bandwidth and it has been termed as the proposed design as shown in Fig. 2 (d). Overall performances of different design steps are compared in Table 2. The diameter and the gap between the metallic vias are calculated using the equations (1) and (2) as described in [1]. d < λg/5

(1)

g ≤ 2d

(2)

Where, d = diameter of the metallic vias, g = gap between the metallic vias and λg is the guided wavelength.

Fig. 2. Evolution of proposed antenna (a) Design-A, (b) Design-B, (c) Design-C, and (d) Proposed

Design steps

Table 2: Performance comparison of different design steps Result Peak gain Operating band (bandwidth)

Design-A

10.25 dBi at 24.6 GHz

24.21-26.4 GHz (2.19 GHz)

Design-B

10.73 dBi at 21.2 GHz

21.01-21.44 GHz (0.43 GHz), 23.43-26.46 GHz (3.03 GHz), 27.92-28.25 GHz (0.33 GHz)

Design-C

10.75 dBi at 21.2 GHz

21.01-21.42 GHz (0.41 GHz), 23.43-26.48 GHz (3.05 GHz), 27.92-28.25 GHz (0.33 GHz)

Proposed design

11.75 dBi at 21.3 GHz

21.00-21.44 GHz (0.44 GHz), 23.45-26.52 GHz (3.07 GHz), 27.95-28.29 GHz (0.34 GHz)

Fig. 3. Stepwise improvement of reflection co-efficient (S11)

4.2. Air-gap (ha) Variation The height of the air-gap (ha) in between the two dielectric layers has been varied to set the optimum value and it is summarized in Fig. 4. If the height of the air-gap is reduced to 1 mm then the antenna gives a lower gain of 11.33 dBi at 20.2 GHz over an operating range 19.85-20.49 GHz, 22.48-23.00 GHz and 24.81-27.23 GHz. Again if the height of the air-gap is increased to 2 mm then also the antenna gives a lower gain of 11.48 dB at 21.7 GHz over an operating range 21.50-21.81 GHz, 23.86-26.56 GHz and 27.97-28.17 GHz. The proposed antenna with h a =1.6 mm

Fig. 4. Variation of S11 characteristics with the variation of the air-gap air-gap provides a higher gain of 11.75 dBi at 21.3 GHz over a relatively broad operating range 21.00-21.44 GHz,23.45-26.52 GHz and 27.95-28.29 GHz. Now for the ha =0 mm), that means there is no air gap in between the top and bottom substrate and the height of the antenna as well as all the vias have been reduced to 3.2 mm. This design provides a wide bandwidth with poor impedance matching at the higher frequencies and a very low peak gain of only 7.6 dBi at 17.5 GHz where impedance matching is better. Though the operating band is quite large, we step forward from this design due to its very low gain and poor impedance matching performances. Hence, with the introduction of an air gap of 1.6 mm the proposed antenna exhibits much higher peak gain of 11.75 dBi at 21.3 GHz, and also operates over multiple frequency bands with better impedance matching. The width of the air-gap plays important roles in impedance matching and gain performance, as the overall dielectric constant, antenna and waveguide dimensions are changing with it. Air-gap (ha) variation changes the height of the antenna as well as the height of the vias hence the antenna characteristics, also changes quickly. Without any air gap, the antenna provides wideband but, the gain is very poor. For the other three different values of h a, the S11 plots are showing a similar kind of nature but it changes quickly due to the combine effect of the change in dielectric constant, antenna dimension, waveguide dimension, and vias height. So, the introduction of the air gap in between two substrate layers is justified.

3.3. Horn-flaring Variation Horn-flaring angle is another important parameter, with the variation of this angle along H-plane; the S11 characteristics of the antenna differ as shown in Fig. 5. For the reduced flaring angle of 22 °, it provides a gain of 10.61 dBi at 21.3 GHz and operates over the frequency bands 21.12-21.54 GHz, 23.52-26.56 GHz, and 28.05-28.31 GHz. If the horn-flaring angle is increased to 31.5° then it gives a gain of 10.65 dBi at 23.5 GHz and operates over the 20.98-21.40 GHz, 23.25-26.42 GHz, and 27.90-28.19 GHz bands. Whereas the proposed design with an optimum flaring angle of 26.2° provides a higher gain of 11.75 dBi at 21.3 GHz with almost same operating bands.

Fig. 5. Variation of S11 characteristics with the variation of the horn-flaring

4. Measured Results and Discussions In order to realize the improved performance of the proposed antenna, the dimensions of geometrical parameters are chosen from the parametric study of the antenna as described in section 2. The proposed antenna is fabricated for experimental verification. Fig. 6 shows the measured and simulated frequency response of the reflection coefficient (S11) for the proposed antenna. The S11 is measured by using a vector network analyzer. The measured impedance operating ranges, defined by S11 ≤ -10 dB, is 20.6-21.2 GHz, 23.0-26.1 GHz, and 27.4-28.2 GHz. The comparison shows that the measured result reasonably agrees with the simulated result. However, the measured result displays slightly difference at operating frequencies. The extra dielectric part kept at the two sides of the antenna for the alignment may cause dielectric loss and the low-cost SMA connector contributes to conductor loss in the practically implemented antenna may be responsible for the slight mismatch between the measured and simulated S 11 plots. The photos of the fabricated prototype are shown in Fig. 7.

Fig. 6. Comparison between simulated and measured S11 characteristics

Fig. 7. Fabricated prototype of the proposed antenna (a) Top view, (b) Bottom view (Length), (c) Bottom view (Width), and (d) Side view. The simulated and measured E-plane and H-plane far-field radiation patterns at 20.9 GHz, 23.2 GHz, and 27.7 GHz are shown in Fig. 8(a), Fig. 8(b), and Fig. 8(c), respectively. At all frequencies, H-plane patterns are directed towards the end-fire direction. The radiation pattern should be in the end-fire direction (900) as the proposed antenna is H-plane horn antenna. It should be tilted at 900. However, the patterns are tilted in between 600 to 750. This tilt in radiation pattern occurs due to the extended dielectric slab in the antenna [17-20]. Also, the SMA connector feeding is not at the center, due to that there is a back reflection occurs inside the cavity wall. Due to this reflection the radiation pattern becomes tilted from its regular uniform position. The fabrication tolerance due to lots of shorting pins and the open air radiation pattern measurement environment accuracy may be responsible for the slight mismatch between the measured and simulated radiation patterns.

Fig. 8. Comparison between simulated and measured E-plane and H-plane radiation patterns at (a) 20.9 GHz, (b) 23.2 GHz, and (c) 27.7 GHz.

Fig. 9. Comparison between the measured and simulated peak gain of the proposed antenna. The simulated and measured peak gain of the proposed antenna is shown in Fig. 9. The maximum realized peak gain is 11.75 dBi at 21.3 GHz (simulated), and 11.25 dBi at 20.9 GHz (measured). Fig. 6 shows that around 25 GHz there is a little notch hence the impedance matching, is poor. As a result, the gain of the proposed antenna has been fall sharply around 25 GHz. Comparison of the proposed antenna with some related previous works is summarized in Table 3. Length and height of the antenna [10] are smaller but it provides much lower gain and narrow bandwidth in comparison to the proposed one. In [12], bandwidth is wide but it provides much lower gain and also its length is quite longer than the proposed one. In [13] the antenna dimension is bigger than the proposed antenna. Its bandwidth is higher but theTable 3: Comparison of the proposed antenna with some previous works Relative works

Dimension

Gain

Operating Range (GHz)

[10]

27 mm×18 mm×1.6 mm (approx)

5.8 dBi at 16.4 GHz

16.25-16.5 GHz

[12]

60 mm×10.6 mm (approx)

9.5 dBi at 22 GHz

16-24 GHz

[13]

42 mm×18.6 mm×3.15 mm

10.1 dBi at 22.7 GHz

21.8-22.2 GHz, 24.0-24.3 GHz, 26.5-26.7 GHz, 27.4-27.6 GHz

[14]

40mm×37mm×4.3mm

8.5 dBi at 21 GHz

17.7-26.7 GHz

[15]

42 mm×18.6 mm×3.15 mm

10.1 dBi at 22.7 GHz

21.8-22.2 GHz, 24.0-24.3 GHz, 26.5-26.7 GHz, 27.4-27.6 GHz

[17]

40mm×14mm×2.5mm

9.7 dBi at 27.5 GHz

27-28 GHz

42 mm×18.6 mm×4.8 mm

11.25 dBi at 20.9 GHz

20.6-21.2vGHz, 23.0-26.1GHz, 27.4-28.2GHz

Proposed

gain is much lower than the proposed antenna. The dimension of [14] is almost same (with bigger width) and bandwidth is higher but it also provides much lower gain than the proposed antenna. The dimension of the proposed antenna is almost same (with bigger in height) as [15], but its gain and bandwidth both performances are better than that. The antenna [17] shows lower gain as well as narrow bandwidth with almost same dimension. Instead of FR4, if low loss costly RT/Duroid 5870 substrate is used, then this same design provides much higher gain of 15.33 dBi at 25.6 GHz, 11.88 dBi at 29 GHz and 13.77 dBi at 32.2 GHz over operating ranges 25.42-25.76 GHz, 28.86-29.22 GHz and 31.85-32.63 GHz. Due to the non availability of this costly substrate, the design has been simulated and fabricated using the available low cost FR4 substrate.

5. Conclusion In this paper, a SIW based H-plane horn antenna with improved gain and bandwidth is proposed, fabricated and validated with the experimental data. This antenna is able to provide a high gain of 11.25 dBi at 20.9 GHz and operates over the frequency ranges 20.6 - 21.2 GHz, 23.0 - 26.1 GHz, and 27.4 - 28.2 GHz. The proposed antenna exhibits better gain and bandwidth in comparison to the most of the antennas of its kind. Higher gain performance is achieved by adopting an air-gap in between two dielectric layers. Metallic extensions in front of the horn aperture and the air vias at the extended dielectric part are useful techniques to enhance the gain further, while the metal posts with horn aperture deal with the impedance mismatch issue successfully. Stable directional radiation patterns and acceptable gain, as well as bandwidth, make this antenna attractive candidate in the field of SIW based H-plane horn antenna.

6. Acknowledgments The authors would like to acknowledge Prof. Partha Pratim Sarkar, Department of Engineering & Technological Studies, University of Kalyani, West Bengal, India, for providing measurement facilities

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Anik Ghosh was born in Kolkata, West Bengal, India, 1994. He received his B. Tech degree from Adamas Institute of Technology under affiliation of formerly known West Bengal University of Technology in Electronics and Communication Engineering and M. Tech degree in Radio Physics and Electronics from the Institute of Radio Physics and Electronics, University of Calcutta in 2016, 2018 respectively. He is currently pursuing his Ph. D. from Indian Institute of Technology (IIT) Guwahati, Assam, India since January 2019. His current research interests include the Microwave Antenna and Circuits, SIW based Antenna and Filter, Micro-strip Antenna etc. Kaushik Mandal received his B. Sc degree in Physics (H), B. Tech and M. Tech degree in Radio Physics and Electronics from the University of Calcutta, in 2001, 2004 and 2006 respectively. He received his Ph. D. (Tech.) from the University of Kalyani, in July 2014. From 2016 he is working as an Assistant Professor in the Institute of Radio Physics and Electronics, University of Calcutta, West Bengal, India. He has authored or co-authored over 30 internationally refereed journal and conference papers. His current research interests include the characterization and application of DGS, antennas for RF energy harvesting system, SIW integrated microstrip antenna, and performance enhancement of microstrip antenna using frequency selective surface (FSS). Dr. Mandal is a co-recipient of the IEEE TENCON 2017 Best Paper Award (in the track ‘Antenna’). He is an active reviewer of IEEE Transactions on Antennas and Propagation, IEEE Antennas and Propagation Magazine, IET Microwaves Antennas & Propagation, Progress in Electromagnetics Research (PIER) Journal, Microwave and Optical Technology Letters, and AEU - International Journal of Electronics and Communications. Dr. Mandal is a senior member of IEEE.