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Vivaldi Antenna for MM-Wave Communications

Parent Category: 2014 HFE

By Greg Brzezina, Rony E. Amaya, Aldo Petosa, Langis Roy

1.0 Introduction

The proliferation of portable devices such as smart phones, tablets, and laptops has resulted in an increased demand for mobile broadband wireless, and with the expected growth in the Internet of Things and Machine-to-Machine communications, this increase is expected to exceed the current available capacity. The more efficient use of spectrum and the use of smaller cells will help increase capacity within the frequency bands that are currently allocated to mobile wireless communications. However, this is not considered sufficient to meet the growing demand, and higher frequency bands will also need to be adopted. 

Serious consideration is being given to the 60 GHz Industrial, Scientific, and Medical  (ISM) band by the Wireless Gigabit Alliance (WiGig) (now subsumed by the Wi-Fi Alliance) to develop and promote the adoption of multi-gigabit speed wireless communication technology based on the IEEE 802.11ad protocol, and by industry involved in developing fifth generation (5G) communication systems. These systems will require the development of antenna array technology with beam scanning capability, since high-gain (narrow beam) patterns that can be steered to track mobile devices, or change shape to reduce interference will be needed.  

This paper presents the design of a tapered slot antenna, known as a balanced antipodal Vivaldi antenna, and is seen to be a promising candidate for 60 GHz wireless communications.

2.0 Antenna Technologies

At millimeter-wave frequencies, antenna efficiency is paramount.  Degradations associated with lossy substrates and interconnects, impedance mismatch, integration with transceiver electronics and proximity of nearby materials are severe. Accordingly, antennas are increasingly realized as an integral part of the electronic chip package, as opposed to on the chip itself or as a stand-alone entity. For low cost, high volume products, system-on-package (SoP) design is actually now an absolute requirement.  Relevant antennas for applications at 60 GHz include basic radiators such as dipoles, patches, slots, loops, and inverted F. If higher gain is desired, then Yagi-Uda, horn, and Vivaldi radiators are more appropriate.  It can be shown that the Vivaldi antenna offers several performance advantages that make it particularly well suited to applications in the 60 GHz band. Most importantly, it is inherently broadband and relatively easy to design and fabricate. 

3.0 High-Performance Packaging Options

The adoption of an SoP approach implies that passive components and antennas are implemented as an integral part of the packaging substrate.  As the frequency of operation increases, the electrical performance of the package becomes at least as important as its integration capabilities and overall cost. Table I provides a comparison of high-performance packaging options currently available by presenting their key performance parameters.

1410 HFE antenna table01

The Vivaldi designs presented later use the FerroA6S LTCC process with єr = 5.9 and tan δ = 0.002. Each LTCC tape layer has a post-fired thickness of 97 µm and uses Ag metallization with t = 9 µm and σ = 5.5 x 107 S/m. The high process maturity, excellent electrical performance, good integration capabilities, and low tolerances made LTCC the process of choice for this project.

4.0 Antenna Design

First reported in [1], the Vivaldi antenna is a versatile design since it can be implemented in coplanar, microstrip, or stripline technology. The latter, known as a balanced antipodal Vivaldi antenna (BAVA) was shown by J. Langley et.al in [2] and it offers better isolation and lower losses at high millimeter-wave frequencies. As a result of its stripline topology, the metal stack-up includes three metal layers; the first and third layers being located on the top and bottom surfaces of the substrate, while the second is sandwiched in-between. 

4.1 Calculating Substrate Size and Stripline Width

Such a Vivaldi antenna design can be created by following several steps, as described in [3] and repeated here with some modifications. The first step in the design of a BAVA involves selecting the size of the substrate. A good first approximation is to use a rectangle whose width is half its length, where the latter is at least one full wavelength at the lowest operating frequency. Here, this corresponds to 6 mm when rounded up. If not restricted by the manufacturer, the thickness of the substrate should be chosen to yield a 50-Ohm characteristic impedance with a reasonable stripline width. For the purpose of this article, an LTCC substrate with a relative permittivity of 5.7 and thickness of 0.388 mm was chosen. Now, the classic formula for a stripline’s characteristic impedance can be used and is

1410 HFE antenna EQ 01

After entering the appropriate values, the formula yields a stripline width of 0.135 mm. Based on these results, the remainder of the geometry of the BAVA can be obtained.

4.2 BAVA Geometry

The second step in the design process is to create the layout for the middle metal layer, which includes the stripline feed. The feed has a width defined previously that widens into a flare with an elliptical shape. Other shapes can be used, such as exponential or even linear, but an elliptical taper yields good performance and can be easily created in most CAD tools with only simple boolean operations. Fig. 1 can aid us in visualizing how this is the case. Where, W is the width of the substrate, L is its length, r is the radius of the circle.

1410 HFE antenna 01a

(a)

1410 HFE antenna 01b

(b)

Figure 1 • BAVA feed layout; (a) top view and (b) front view.

It can be seen that only three basic shapes are required to form the layout for this layer; an ellipse for the outer edge of the flair; a circle for the inner edge; and a rectangle for the feed. The rectangle is about half the length of the substrate and centered along its width. The ellipse is centered on the midpoint of the substrate’s left edge and has a major axis and ratio of

1410 HFE antenna EQ 02

Entering the appropriate values yields a major axis and ratio of 3 mm and 0.5225, respectively. The circle is also centered on the substrate’s left edge and a distance away from the horizontal centerline defined by

1410 HFE antenna EQ 03

This makes for a center position located 0.955 mm away from the midpoint of the substrate. And its radius is calculated as

1410 HFE antenna EQ 04

This gives 1.4325 mm when solved. Once these shapes are drawn, the circle is subtracted from the ellipse and the resultant shape is united with the feed rectangle to form the flare (shaded region). There will be some unwanted shapes leftover beyond the edges of the substrate and to the left of the feed line that should be discarded. 

The third step focuses on creating the layouts of the ground flares that reside on the bottom and top of the substrate. The process is the same as before and simplified by the fact that both radiating flares are identical and only mirror images of the feeding flare so no new calculations are necessary. There is some added complexity, however, in creating the ground plane tapers located near the feed point as shown in Figure 2.  

1410 HFE antenna 02

Figure 2 •Ground plane flare design.

A box, outlined in purple, is needed to form the ground plane that borders the feeding edge of the BAVA. Its width is equal to W and its length extends to the centerline of the circles obtained above. Then, the circles are subtracted from the ground rectangle to create a tapered edge that forms a smooth transition between the stripline and radiating flares. All that remains is to unite the radiating section with the newly created transition to form the completed layout of the ground flare (shaded region). 

4.3 HFSS Model and Simulation

The fourth step involves drawing and simulating the design with a 3-D electromagnetic simulator, which only needs to support the basic CAD shapes and operations used in the previous steps. Here, Ansys HFSS is used to verify the accuracy of the design. The completed 3-D model is shown in Fig. 3 with its main features indicated.

1410 HFE antenna 03

Figure 3 • HFSS model of BAVA.

By using an ideal 50 Ω excitation, the impedance bandwidth and radiation patterns shown in Fig. 4 were obtained.

1410 HFE antenna 04

Figure 4 • BAVA simulation results showing (a) impedance match; (b) 3-D radiation pattern; (c) azimuth pattern; and (d) elevation pattern.

From this initial simulation, the impedance bandwidth already covers the entire V-band and validates the selection of a Vivaldi antenna along with the proposed design procedure. The calculated far-field radiation patterns are typical of most Vivaldi antennas and indicate a peak gain of 4 dBi. These results represent a good starting-off point that can be improved upon in many ways. By making use of variables instead of explicit values for the antenna’s geometry parameters, many design variations can be easily generated. For example, enlarging the size of the antenna in either width or length will increase its gain. Another, lesser known method to increasing its gain is to add a dielectric director as described next.

4.4 Gain Enhancement

Vivaldi antennas offer many advantages compared to other topologies but one potential drawback is their low to moderate gain. The example antenna’s gain can be easily increased by adding a dielectric director in the form of a simple extension of the substrate beyond the radiating aperture [4]. The new topology is shown in Fig. 5. By setting the director length, LDD, to 2 mm and keeping everything else unchanged, the gain is increased by 2.5 dBi to 6.5 dBi, which is significant. Further improvements can be made by profiling the height of the director with different steps. 

1410 HFE antenna 05

Figure 5 •Gain enhancement with a dielectric director.

 

4.5 Antenna Launcher Modelling

In reality, this antenna would be measured with the use of a coaxial V connector and a custom metal launcher as used in [5]. The launcher consists of two parts: the pedestal and the holder. The purpose of the pedestal is to keep the antenna level and aligned with the V-connector feed pin secured in place by the holder. At high-millimeter-wave frequencies, the size of the launcher becomes comparable to a wavelength and it acts as a reflector; in effect boosting the gain of the antenna under test. To estimate its influence, including a representation of the launcher is necessary. A complete mechanical model can be used but often only a simplified representation is sufficient. Both cases are shown in Fig. 6. 

1410 HFE antenna 06a

(a)

1410 HFE antenna 06b

(b)

Figure 6. Measurement launcher models; (a) full; and (b) simplified.

When the simplified model of the launcher is added to the simulation, the gain is increased by 0.7 dBi at 60 GHz. Knowing this value can help improve the agreement between simulations and measurements. 

5.0 Conclusion

In summary, a design procedure has been presented for creating Vivaldi antennas suitable for applications in the 60 GHz ISM band. The design starts by selecting the size of the antenna, which is on the order of one wavelength long and half a wavelength wide at the frequency of operation. A stripline topology is used to take advantage of its lower loss and inherently superior isolation compared to other methods. Then by using elementary geometry and basic Boolean operations, the three required flared metal layouts can be created. A technique to enhance the gain is also shown along with a model of a custom launcher. Both are incorporated with an example design and simulated with Ansys HFSS. The results prove the usefulness of the Vivaldi antenna and the proposed design procedure.

References

[1] P. Gibson, “The vivaldi aerial,” in 9th European Microwave Conference, 1979.

[2] J. Langley, P. Hall, and P. Newman, “Balanced antipodal Vivaldi antenna for wide bandwidth phased arrays,” in Microwaves, Antennas and Propagation, IEE Proceedings, vol. 143, no. 2, 1996, pp. 97–102.

[3] G. Brzezina, L. Roy, and L. MacEachern, “Planar antennas in LTCC technology with transceiver integration capability for ultra-wideband applications,” IEEE Transactions on Microwave Theory and Techniques, vol. 54, pp. 2830–2839, June 2006.

[4] G. M. Brzezina, R.E. Amaya, D. Lee, K. Hettak, J. Sydor, L.Roy, “A 60 GHz System-On-Package Balanced Antipodal Vivaldi Antenna With Stepped Dielectric Director (BAVA-SDD) in LTCC,” in 41st European Microwave Conference (EuMC), 2011, pp. 547-550.

[5] G. Brzezina, R.E. Amaya, A. Petosa, L. Roy, “Broadband and compact Vivaldi arrays in LTCC for 60 GHz point-to-point networks,” in Wireless and Microwave Technology Conference (WAMICON), 2014, pp.1-5.

About the Authors

Dr. Greg Brzezina holds a Ph.D. degree from Carleton University in Ottawa, Canada and has over 10 years of experience in the design of antennas. He is currently a research engineer at the Communications Research Centre Canada and an adjunct Professor with the Department of Electronics at Carleton University. He can be reached at: greg.brzezina@crc.gc.ca,

Dr. Rony E. Amaya received his Ph.D. Degree from Carleton University in Ottawa, Canada. He has previously worked at Nortel Networks and Skyworks Solutions. He is currently a research scientist with the Communications Research Centre Canada and an adjunct Professor with the Department of Electronics at Carleton University. His research interests include circuit design, system integration, packaging and integrated antennas operating from S-band to E-band.

Dr. Aldo Petosa is a senior research scientist at the Communications Research Centre Canada.  His research interests include: antenna arrays, microwave lenses, dielectric resonator antennas, and reconfigurable antennas. He is author or co-author of over 180 journal and conference papers, and has written two books on antennas.

Langis Roy is a professor of electrical engineering in the Department of Electronics at Carleton University.  His research interests are in microwave electronics, integrated active antennas and high-performance heterogeneous packaging for a broad range of wireless and opto applications. He has co‐authored over 100 scientific papers with his research group and holds three patents on system-on-package designs.  He can be reached at lroy@doe.carleton.ca.

 

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