214 Designing a Phased Array Antenna Using Antenna Magus and CST STUDIO SUITE Antenna synthesis lets engineers investigate many potential designs and produce antennas that fit the specifications quickly. For effects that cannot be accounted for analytically such as edge effects and mutual coupling, full-wave 3D simulation can complement synthesis tools and allow designs to be checked and fine tuned. This article explores the synthesis of an antenna array, using a phased array satellite communications antenna as an example. The traditional approach to antenna array design is as follows: first, the engineer searches textbooks and journals for antenna types and array layouts which fulfill the basic requirements and, using equations and design curves, produces a viable blueprint. However, this would only be an approximation of the final antenna array performance is influenced by edge effects, coupling between elements and interactions with the feed and platform, among other things. For this reason, the engineer would then build prototypes which can be tested in the lab and, by trial and error, produce a final design that fits the specifications. This can be a slow, expensive process, especially if multiple prototyping stages are needed. Using antenna synthesis and electromagnetic simulation, the design flow can be accelerated, the range of designs investigated can be broadened, and the number of prototypes needed can be reduced. Specification Operating frequency Value Polarization RHCP Beam properties Steer angle Radome size 1518 MHZ 1675 MHZ Electronically steerable point beam with nominal gain of 12 dbic and a maximum of 17 dbic across a majority (85%) of the spherical coordinate system, with SSL of approx. 13 db. 36 azimuth, 18 elevation. 1 mm x 3 mm x 5 mm To demonstrate this workflow, this article will show how a planar phased array was designed using Antenna Magus, an antenna synthesis tool, and CST MICROWAVE STUDIO (CST MWS), a fullwave 3D electromagnetic simulation program. Ground plane measurement system Standard A-ring 781 ground plane, 2.4 m x 1.6 m section of a 3 m radius cylinder The antenna in question is for satellite communications on commercial aircraft, and operates in the L-band (1518 1675 MHz). The main beam of the array is electronically steered between 36 azimuth and 18 elevation to maintain connection with the satellite as the aircraft moves. Feed type Single or dual coupled feed System impedance 5 Ω Table 1: Summary of given design specifications
Selecting the array element The operating bandwidth of an array is mainly determined by the impedance bandwidth of the individual array elements. The process of selecting what type of element would be best suited to the given requirements, such as bandwidth, impedance, feed network and substrate, is accelerated using Antenna Magus. Antenna Magus includes a large database of over 25 antenna types. These include detailed information and synthesis algorithms drawn from measurements and published antenna designs. The antennas can be searched by keywords so that the most appropriate types of antenna for a problem can be easily identified and selected. (all elements excited equally and in phase). Both S 11 and S 21 are changed dramatically in an infinite array environment, due to strong mutual coupling between the elements. The effect of the mutual coupling will worsen when applying a feed network using different excitations to steer the main beam. This would cause more of the radiated energy to leak into adjacent elements as the squint angle increases [1], making accurate and efficient electronic steering impossible. -1 S1,1 [magnitude in db] In this case, the elements should be planar and have a bandwidth of 1%. Therefore, we search in Antenna Magus for planar and moderate bandwidth (between 1% and 7% bandwidth). This produces 27 results, of which the circular EM coupled patch is a good fit in terms of bandwidth. Infinite array Single element However, this antenna type is not circularly polarized. Adding circular polarization to the list of search terms produces few elements that are directly suitable for using in an array. Instead, to produce a suitable element, we can combine antenna types. A search for circular polarization and planar shows that using a dual feed with patch antenna produces the desired polarization. Combining these results in a dual fed EM coupled circularly polarized circular patch shown in Figure 1. The models exported from Antenna Magus can be used to easily generate the new structure in CST MWS. The new structure simulated in CST has an impedance bandwidth of 13.4 % below, an S 11 of -1 db and maximum gain of 7 dbi at the centre frequency. To investigate how well this element works as part of an array, the model is exported to CST MWS and simulated using the transient solver (T-solver). The periodic boundary condition allows an infinite array to be simulated in order to calculate the effect of mutual coupling between individual array elements. Figure 2 shows S-parameter characteristics of the single candidate element as well as the same element in an infinite array Frequency / GHz Figure 2: S-parameter results comparing the performance of a single element (blue) to an infinite array (red). A shielding cavity was then added to the antenna element using the modeling tools in CST MWS. By placing the element inside a cavity, the effect of mutual coupling is reduced. However, this also results in a reduction in performance bandwidth. S-parameter results in Figure 3 show a decrease in impedance bandwidth. This would mean that the antenna would no longer meet the given requirements. Information about wide-band planar antennas in the Antenna Magus database indicates that stacked patch antennas are able to achieve wider bandwidth than single patch elements. This increased bandwidth is realized by including an additional patch element and substrate with a slightly lower resonant frequency above the existing structure. Loose coupling between the bottom and top patches results in wider impedance bandwidth [2]. Using this principle, a new array element - a dual fed EM coupled circularly polarized circular stacked patch, can be designed by combining 3 antenna topologies from Antenna Magus, as illustrated in Figure 4. + = a Circular EM coupled patch wider bandwidth b Dual fed patch circular polarisation Figure 1: Combining two Antenna Magus elements to form the new Dual fed EM coupled circularly polarized circular patch (Transparency is used to show feed lines and substrates). 2
optimization in CST MWS, the element was quickly tuned to ensure it fits the design requirements. S1,1 [magnitude in db] -1 With shielding Without shielding Frequency / GHz S1,1 [magnitude in db] S-Parameters [magnitude in db] -1-1 -35-4 With shielding Without shielding -45 Frequency / GHz Figure 3: Addition of a shielding cavity around individual array elements reduces impedance bandwidth (marked with arrows). S1,1 (single element) S1,1 (infinite array) S2,1 (single element) S2,1 (infinite array) Title Figure 5: S-parameter results of the Dual fed EM coupled circularly polarized circular stacked patch. (a) S 11 including (red) and excluding (blue) the square shielding cavity. (b) S 11 and S 21 including the shielding cavity, in isolation (dashed traces) and in an infinite array (solid traces). The second candidate antenna was then exported to CST MWS to be designed and optimized. Some adjustments to the feed were made to improve the real and imaginary impedances. This was done by increasing the feed line width to reduce the reactive component of the input impedance from 1 Ω to the desired Ω, while increasing the resistive component from 4 Ω to 5 Ω. The T-solver was again used to simulate the structure. By combining the design guidelines provided in Antenna Magus with the sensitivity-based Figure 5(a) shows the S 11-parameter simulation of the second candidate antenna with and without the shielding cavity. When comparing the bandwidth performance of the first and second candidate elements (excluding the cavity shielding) we see that the second candidate element has a bandwidth of 2%, 6.5% more than that of the first candidate. By adding the shielding cavity, the bandwidth is reduced to 14.2%. The effect of mutual coupling is again investigated using CST MWS. S-parameter results in Figure 5(b) show + + = Dual microstrip-edge-fed circularly polarised rectangular patch Circular stacked pin-fed patch EM-coupled Circular Patch Figure 4: 3 topologies in Antenna Magus combined to design a new Dual fed EM coupled circularly polarized circular stacked patch. 3
that due to mutual coupling, the impedance bandwidth decreased further to 11%. This means that the array still meets the 1% bandwidth specification. The final circularly polarized array element model was exported from CST MWS back into Antenna Magus. The CST model can be added to a template in the Antenna Magus datastore and then used in much the same way that the native antenna models can. Designing the array layout Once we have a suitable element, the array synthesis tool in Antenna Magus can be used to design and synthesise the full array which complies with the physical design specifications. Under the Layout Information tab, we set up the requirements of the array: 36-element array, consisting of 4 rows of 9 elements; Villeneuve taper with an SLL of 14 dbi and an element spacing of half a wavelength. The farfield pattern can be imported into Antenna Magus by using the Save As Source option under Farfield Properties in CST MWS to produce a.ffs file which can be used by Antenna Magus. Using this information, Antenna Magus calculates the distribution matrix for the exciting the elements to produce the desired radiation pattern. Beam steering is accounted for by adjusting the desired main lobe direction Antenna Magus then produces new distribution matrices for simulating these arbitrary beam directions in CST MWS. Figure 6 shows the synthesised array layout with phase distribution and two synthesised array patterns, for a scan angle of θ = 3 and ϕ = 1 (left) and for a scan angle of θ = 5 and ϕ = 7 (right). The distribution matrix is exported from Antenna Magus, and imported into CST MWS using the array wizard feature to create the final 36-element array. This matrix contains the individual excitations for each element to produce the squint angles defined in Antenna Magus. A separate matrix is produced for each squint angle. Using these, we can carry out simulations of the final array using the T-solver. Figure 6: Array synthesis results from Antenna Magus showing: Distribution matrix, imported array element and synthesised patterns for scan angles of θ = 3 and ϕ = 1 (left) and θ = 5 and ϕ = 7 (right). 4
The results of these simulations are shown in Figure 7. The difference between the Antenna Magus and CST simulated pattern results arises from the fact that Antenna Magus does not compensate for corner and edge elements or mutual coupling between elements. It assumes the same radiation pattern for all elements. As expected, the difference is more noticeable as the squint angle increases, because here the edge and corner elements have a greater effect on the radiated pattern. Finally, the installed performance of the design can be investigated. By adding the antenna array model to a standard ARINC 781 ground plane, or even a full aircraft model, the effect of the plane body can be taken into account to ensure that the antenna still behaves as expected when in situ. Several approaches exist for calculating installed performance: the asymptotic solver uses a raytracing method to quickly calculate scattering effects, the integral equation solver uses surface meshing to model electrically large objects efficiently, and the general time domain solver performs a full-wave simulation. ϴ: 3 Φ: 1 ϴ: 5 Φ: 7 Figure 7: Layout and radiation pattern of 36 element L-band array designs for squinted patterns of (a) θ = 3 and ϕ = 1 and (b) θ = 5 and ϕ = 7, simulated in CST MWS. Figure 8: (a) An aircraft model with the antenna model and radome installed (highlighted). (b) A farfield for the array including the effect of the fuselage. 5
Conclusion This article illustrates the advantage of using Antenna Magus to assist with fast first order array synthesis and closing the loop, using a full wave simulation tool like CST MICROWAVE STUDIO. An L-band planar array was designed for use in mobile satellite communication, using Antenna Magus to select a promising topology, and CST MWS to optimize and verify the design. Bandwidth was improved by using a stacked patch element, and unwanted mutual coupling was reduced by adding shielding cavities to each element. The final design met the given specifications. Further investigation can be done to realise the physical feed network and to further reduce mutual coupling and element port coupling. For example, this array design uses the same element for centre, side and corner elements. Using specially designed elements along the edges can further improve the array s performance at higher squint angles. Robert Kellerman Magus (Pty) Ltd Stellenbosch, South Africa www.antennamagus.com Magus (Pty) Ltd develops Antenna Magus, a software tool aimed at simplifying the antenna design process. The database of antennas and antenna arrays may be explored to find the most suitable design options. These may then be designed to meet specific system criteria, and subsequently exported as fully-parametric, ready-to-run CST MICROWAVE STUDIO models that seamlessly integrate into the broader system design workflow. Antenna Magus can be used in a wide variety of industries, including, but not limited to, telecommunications, mobile devices, aerospace, satellite, automotive, radio astronomy and defence. Antenna Magus is available through CST sales channels. www.antennamagus.com [1] C.A. Balanis, Modern Antenna Handbook, John Wiley & Sons, New York City, pp. 135 137, 2 September 211. [2] Rod B. Waterhouse, Design of Probe-Fed Stacked Patches, IEEE Transactions on Antennas and Propagation, vol. 47, no. 12, December 1999. CST Computer Simulation Technology AG Bad Nauheimer Str. 19, 64289 Darmstadt, Germany Phone: +49 6151 733, info@cst.com For more information, please visit: www.cst.com Trademarks CST, CST STUDIO SUITE, CST MICROWAVE STUDIO, CST EM STUDIO, CST PARTICLE STUDIO, CST CABLE STUDIO, CST PCB STUDIO, CST MPHYSICS STUDIO, CST MICROSTRIPES, CST DESIGN STUDIO, CST BOARDCHECK, PERFECT BOUNDARY APPROXIMATION (PBA), and the CST logo are trademarks or registered trademarks of CST in North America, the European Union, and other countries. Other brands and their products are trademarks or registered trademarks of their respective holders and should be noted as such. CST 214 6