Cylindrical Conformal Array Project

Rodrigo Cavaco Lourencorodrigo.lourenco@tecnico.ulisboa.pt

Instituto Superior Tecnico, Lisboa, Portugal

October 2015

Abstract

Indoor positioning systems have aroused widespread interest in the scientific community in recentyears. These systems are used for locating objects, people and even for guidance in larger spaces. Inthis work, a literature review of existing indoor location systems and the different technologies andlocation techniques is carried out. This paper presents the design and test of an array conformalto a cylindrical surface to operate in the 5.8 GHz ISM band for possible indoor applications. Theantenna array consists of six coplanar-fed radiating elements, each one covering a 60◦ sector. Aflexible substrate, Kapton R©, has been chosen. The S parameters, as well as the radiation patternsand efficiencies, have been obtained through simulations for two configurations: with only one feedingelement and with two consecutive elements fed. Finally, experimental tests were performed in thelab, with special interest on the reflection coefficient and radiation patterns in several planes. Themeasurement results were compared to the simulated ones, to demonstrate the design performance.

Keywords: Indoor positioning systems, Localization techniques, Conformal array, CPW, ISMBand 5.8 GHz.

1. IntroductionThe location in indoor environments is one of thelatest challenges in the telecommunications sector,with numerous commercial and government poten-tial applications. Although in places where thereis line-of-sight to the satellites, various localizationsystems, called Global Navigation Satellite Systems(GNSS) can provide good location estimates. Incases where there is no line-of-sight, the signalscoming from the satellites are not strong enoughto penetrate most materials. Thus, there are sev-eral solutions based on other existing technologies,and using various types of antennas, as shown infigure 1 [1].

2. State of the Art2.1. Indoor Positioning SystemsIn recent years, the emergence of a greater num-ber of mobile devices and increasingly WLAN’s(wireless local area networks) have led to the de-velopment of indoor location systems based on RFsignals. This trend is likely to continue due tolow costs of technologies and there are no line-of-sight problems. These types of positioning systemsare based on different communication technologies.The most used and investigated technologies are:RFID, Wi-Fi, Bluetooth, UWB and ZigBee.

The system RADAR [2] was the first indoor posi-tioning system based on Wi-Fi technology and was

(a) Based on omnidirec-tional antennas

(b) Based on singular pat-tern directive antenna

(c) Integrated with SAA(sectorized antenna array)

(d) integrated with phasedor smart antennas.

Figure 1: Localization Systems Models. [1]

proposed by a group of researchers from Microsoft.In experiments carried out were used three com-puters as access points and a laptop was used asan object of location. The authors of this systemproposed two approaches to determine the user’s lo-cation. The first approach used a algorithm basedon NNSS (Nearest Neighbor Signal in Space), andthe second using a specific propagation model de-scribed by:

1

P (d)[dBm] = P (do)[dBm]− 10nlog

(d

do

)−

{nW ∗WAF nW < C

C ∗WAF nW ≥ C(1)

While results obtained through the empiricalmethod were superior in terms of accuracy, applyinga propagation model makes it easier to apply thissystem. The precision of RADAR, in this study,was approximately 2-3 meters, which is the typicalsize of a small office.

LANDMARC [3]is an indoor location systembased on RFID technology, low cost and operatesat a frequency of 308 MHz. The basic infrastruc-ture of this system consists of 4 RFID readers, 16active tags with previously known coordinates, anda server responsible for communicate with readersand respective location calculations. The use of ac-tive tags in the system as fixed references is themain difference compared to others based on RFIDsuch as SpotOn [4]. The tests revealed a maximumerror of less than 2 meters and about 1 meter errorat 50 percentile of the data.

The tracking system Ubisense [5] is a Real-TimeLocating System. It is based on UWB technologyand allows to locate people and assets fairly accu-rately. The system Ubisense RTLS is formed bysensors, called Ubisensors, and location tags, calledUbitags, which are carried by people or placed inobjects whose location is verified in real time. Acombination of techniques is used to determine thelocation of Ubitags: Time-Difference Of Arrival(TDOA) and Angle Of Arrival (AoA). This way, itcan locate objects and people moving in real timewith an accuracy of 15 cm in 95% of the readings.

The Topaz system [6], owned by Tadly Co., isbased on Bluetooth technology combined with in-frared use, to provide a better indoor location ac-curacy. The system consists on three main com-ponents namely: location tags, access points andlocation servers. The precison of this system is 2meters in 85% of cases, with a position calculatingdelay between 10 and 30 seconds.

2.2. Conformal AntennasThe use of a conformal antenna to a surface hasaroused interest in the scientific community. Theuse of an antenna array according to a cylindricalsurface is of great value in spectrum monitoring,in use as radar and in indoor localization environ-ments, in order to obtain a full azimuthal coverage.

With respect to this type of antennas, some stud-ies have already been presented in the literature[7, 8, 9]. In the work presented in [7] a metalliccylinder at 22mm distance from the radiation el-ements functions as a reflector and takes effect in

the gain due to the reflection of radiation from theelements in the opposite direction. This papers hasinspired the work presented here.

(a) Antenna array (b) Single element

Figure 2: Antenna array for MIMO radar [7]

3. Simulations

Printed antennas are widely used in manufacturinglow cost antennas and consists, in its simplest form,on an printed metal element, named patch, placedon a dielectric substrate. After careful consider-ations about the various substrate materials andtheir characteristics, a Kapton polyamide film hasbeen chosen. As the film sample used in the an-tenna construction contains only a metallic layer,implementing a CPW feeding is possible.

Figure 3: Single element model

The model of the reference element shown in 3together with SMA connector. In the design stepsseveral sizing studies were conducted to achieve theexpected results, including the parameters that bestdescribe the behavior of an antenna: reflection coef-ficient, operating band, radiation patterns and alsoradiation efficiency and total. Using a software tool,it was also possible to optimize the parameters withthe Classic Powell algorithm, wherein the initial pa-rameters already provide a good estimate of the tar-get, and therefore these variations are smaller.

To obtain the dimensions shown above, it wasnecessary to measure several parameters, namelythe side of the triangle patch, the radius of thesemicircle and the spacing between the feed line

2

Figure 4: Dimensions of the single element (inmilimeters)

and the ground plane. In order to tune the an-tenna to the desired band and with the desired ra-diation characteristics, different scans of these pa-rameters were performed. Various techniques ap-plied in this work, were presented in other publica-tions ([10, 11, 12, 13, 14]) in order to increase theoperation band, to select the main frequency, etc.

This section presents the results obtained fromthe different simulations in free space, using CSTMicrowave Studio software, based on the dimen-sions of the single reference element shown in figure4. The reflection coefficient module, S11, demon-strate the mismatch of the antenna and it is de-fined by the ratio of the reflected power, Pr, andthe incident power, Pi, in the antenna.

|S11| = 10 log10

Pr

Pi(2)

The graph obtained of the reflection coefficientfor different frequencies in the UWB band is shownin figure 5.

Figure 5: Reflection coefficient of the single element

After a cautious analysis of the results in Fig-ure 5, it can be stated that the reference element

is adapted to frequencies between 3 and 8 GHz. Itcan be seen that there is a resonance in the 5.8 GHzISM band, which is intended to be used. The radi-ation pattern is a graphical representation or func-tion that describes the spatial radiation propertiesof an antenna in the far field of the antenna. Theradiation efficiency is defined as the ratio betweenthe power absorbed by the antenna and the powerthat is actually radiated. The total efficiency, on theother hand, takes into account the effects of malad-justment and antenna dissipation. The results forthe reference element are presented in table 1, to-gether with the maximum antenna gain realized inthe XZ horizontal plane at 5.8 GHz.

Radiation Efficiency [%] 99.36

Total Efficiency [%] 98.95

Realized Gain 2.291

Table 1: Efficiencies and Realized Gain

After having dimensioned the reference element,the next phase of the design project included thesimulation of folding six elements on a cylindricalsurface, and placing a reflector within the structure,as illustrated in figure 6.

(a) 1

(b) 2 (c) 3

Figure 6: Antenna array

It was chosen a cylindrical structure for easy in-tegration of the antenna in objects with the sameshape and it has three symetric planes. Using a

3

structure with a circular periodicity makes similarresults for all constituent elements of the array. Theintroduction of a concentric cylindrical reflector isintended to reflect in the opposite direction the ra-diation and consequent increase of the gain in thedirection desired, in each sector of 60◦. The reflec-tor is made of aluminum foil. A polystyrene modelwas used to fill the space between the elements andthe reflector and the inner reflector. These mate-rials were chosen because they facilitate the con-struction of the prototype, and expectedly do notsignificantly influence the antenna performance.

First, simulations with a test signal in one excitedport were performed. Representative plots of thereflection coefficient S11 and Sij are shown in figures7 and 8. It should be noted that due to symmetry,S21 and S61 are identical, as well as the S31 and S51,thus the figures include only a subset of these.

Figure 7: S11 Parameter

Sij

Figure 8: Sij Parameters

A key objective of this work is the development ofelements capable of radiating in a 60◦, so when in-tegrated into a cylindrical structure of six elements,each element should cover only one sector. The re-sults of the simulated model are in line with this ob-jective as shown in figure 9. In the horizontal planeXZ, figure 9, was obtained the 3dB beamwidth of61.7◦, with the main lobe at 0◦. The magnitude ofthe main lobe is 08.21 dB. In figure 9c, it shows theradiation pattern in the vertical plane YZ. It wasalso possible to obtain simulated data on efficienciesin this configuration. The results are presented intable 2

Another added feature is the possibility of com-bining two consecutive ports to cover an intermedi-ate sector of the radiation elements. The fact that

(a) 3D Farfield

(b) XZ Plane (c) YZ Plane

Figure 9: Simulated radiation patterns with oneport fed

Radiation Efficiency [%] 99.05

Total Efficiency [%] 98.20

Table 2: Array efficiencies

there is a reflector within the structure allows thereflection of incident waves coming from the twoports. For simulation purposes, it is excited port 1and 2 with a signal with the same amplitude andphase. In figure 10 is presented the radiation pat-tern related to this setting, in the horizontal planeXZ, at frequency of 5.8 GHz.

Figure 10: Simulated radiation patterns with twoports fed, XZ Plane

4

4. Prototype and experimental resultsThe prototype that was built in this work has acylindrical shape, consisting of two modules: an in-ner module and an outer module, which includesthe elements of the structure. The inner moduleis constituted by a cylinder of greater height andcan be removed and it is made from a polystyreneblock and wrapped with an aluminum foil film. Theouter module is a hollow cylinder, also made ofpolystyrene, wherein the outer face is filled by mi-crostrip Kapton with a thickness of 0.05 mm.

Figure 11: Prototype

In this section we analyze the experimental re-sults obtained in the laboratory in order to studythe performance of the built antenna. Measure-ments were performed for the reflection coefficientsas well as for the Sij parameters using a vectoranalyzer VNA. Although simulation and measure-ment Sii results have shown some discrepancy, pos-sibly due to the insufficient number of mesh cellsused, the antenna proved to be fairly tuned, with|S11| ≤ −10dB for the desired frequency, 5.8 GHz.Simulated and experimental results for the radia-tion patterns have also been compared. Experimen-tal patterns obtained in an anechoic test chamberwhen only one port is excited are shown in figure12. These patterns behave as a sectorized antenna.

Figure 12: Antenna in the anechoic chamber

The blue color represents the simulated patternsusing CST and the red color represents the resultingexperimental patterns.

(a) XZ Plane (b) YZ Plane

(c) XY Plane

Figure 13: Radiation patterns: Simulations (inblue) and experiments (in red) at 5.8 GHz

It is possible to conclude that the results aremostly similar, with only a few differences in themagnitude of the side lobes.

An experimental study was performed with twosimultaneously and in-phase excited ports. Theradiation patterns obtained, in the XZ plane, areshown in figure 14, in red, along with the simulatedones in blue.

Figure 14: Radiation pattern: Simulated (in blue)and experimental (in red) at 5.8 GHz with two portsfed (XZ Plane).

Again, simulated and experimental results aresimilar. It is inferred that by using consecutive radi-ation elements and combining port signals the goalof having 360◦ can be achieved.

5. ConclusionsThe work presented here has, as main objectives,the study and project of a cylindrical conformal ar-ray, to operate in the ISM band of 5.8 GHz. One

5

possible application of this kind of antennas is thelocalization of people or objects in indoor environ-ments. The design of the structure was made usingthe CST Microwave Studio software, where werealso carried out all the simulations of this work. Inorder to study the proposed structure, a number ofsimulations in free space was carried out, aiming atthe characterisation of the antenna performance. Aprototype was built and measured in the RF labora-tory of the Institute of Telecommunications, Lisbon.It was concluded that the antenna can be a candi-date to be used in indoor localisation applicationswhere the angle of arrival is needed.

Possible future work is listed below:

• Field tests in indoor environmental and deploy-ment of location algorithms.

• Better characterization of dielectric materialsfor other frequency bands of interest.

• More calculation capacity to enable simula-tions with a more number of mesh cells.

• Study the impact of deformations on the arrayperformance.

• Implementation of other antenna designs.

References[1] L. Bras, N. B. Carvalho, P. Pinho, L. Kulas,

and K. Nyka. A review of antennas for indoorpositioning systems. International Journal ofAntennas and Propagation, 2012:1–14, Novem-ber 2012.

[2] P. Bahl and V.N. Padmanabhan. RADAR: anin-building RF-based user location and track-ing system. In INFOCOM 2000. NineteenthAnnual Joint Conference of the IEEE Com-puter and Communications Societies. Proceed-ings. IEEE, volume 2, pages 775–784 vol.2,2000.

[3] L.M. Ni, Yunhao Liu, Yiu Cho Lau, and A.P.Patil. LANDMARC: indoor location sensingusing active RFID. In Pervasive Computingand Communications, 2003. (PerCom 2003).Proceedings of the First IEEE InternationalConference on, pages 407–415, March 2003.

[4] Jeffrey Hightower, Roy Want, and GaetanoBorriello. SpotON: An indoor 3D loca-tion sensing technology based on RF signalstrength. UW CSE 00-02-02, University ofWashington, Department of Computer Scienceand Engineering, Seattle, WA, 1, 2000.

[5] Ubisense [Online]. Available:.http://ubisense.net/en/about.

[6] Tadlys Ltd Topaz [Online]. Available:.http://tadlys.co.il/pages/Product content.asp.

[7] X. H. Wu and M. Y. A. Abdalla. A cylin-drical antenna array for MIMO radar applica-tions. Antennas and Propagation Society Inter-national Symposium (APSURSI), IEEE, pages484–485, July 2014.

[8] S. Raut and A. Petosa. Wideband PrintedBowtie Array for Spectrum Monitoring. An-tennas and Propagation Society InternationalSymposium (APSURSI), IEEE, pages 235–236, July 2014.

[9] R. Guinvarc’h, F. Boust, M. Serhir, andN. Ribiere-Tharaud. A Wideband Omnidi-rectional Conformal Array for Passive Radar.Antennas and Propagation Society Interna-tional Symposium (APSURSI), IEEE, pages237–238, July 2014.

[10] Maciej Klemm and Gerhard Troster. Charac-terization of small planar antennas for UWBmobile terminals. Wireless Communicationsand Mobile Computing, 5(5):525–536, 2005.

[11] Madhuri K Kulkarni and Veeresh GKasabegoudar. A CPW-fed triangularmonopole antenna with staircase ground forUWB applications. Int. J. Wireless Communi-cations and Mobile Computing, 1(4):129–135,2013.

[12] Yingsong Li, Wenxing Li, Qiubo Ye, and RajMittra. A Survey of Planar Ultra-WidebandAntenna Designs and Their Applications. InForum for Electromagnetic Research Meth-ods and Application Technologies (FERMAT),2012.

[13] Wen-Chung Liu and Ping-Chi Kao. CPW-fed triangular monopole antenna for ultra-wideband operation. Microwave and opticaltechnology letters, 47(6):580–582, 2005.

[14] Joao Vicente Faria. Flexible antennas designand test for human body applications scenar-ios. Master of science thesis, Instituto SuperiorTecnico, January 2015.

6