ENABLING TECHNOLOGIES FOR ULTRA WIDEBAND PHASED ARRAY FOR RADAR AND EW APPLICATIONS WITH FOCUS ON SMALL AND MEDIUM PLATFORMS

A. Ouacha, R. Gunnarsson and A. Eneroth(*)

FOI, Dept. of Sensor Technology, P.O. Box 1165, SE-581 11 Linköping, Sweden E-mail: aziz.ouacha@foi.se

(*) FOI, Dept. of Electronic Warfare Systems, P.O. Box 1165, SE-581 11 Linköping, Sweden

Abstract — Systems that can operate over wide frequency ranges with agile beam scanning and frequency hopping are receiving increased attention due to recent trends within communication, radar and EW applications. In addition to requiring wideband apertures, T/R modules, beamforming and beam shaping, a wideband multifunction antenna system must also be able to instantaneously reconfigure the array to the required function(s) and other parameters such as effective radiated power (ERP) and beam shape. Our findings on the feasibility of realizing crucial components for wideband phased arrays and in particular array antennas, ultra wideband beamformer and front-end for small and medium platforms such as UAV’s is reported in this paper. A compact and lightweight phased array concept for 360° coverage is presented.

I. INTRODUCTION Recent trends within communication, radar and EW (electronic warfare) applications calls for systems that can operate over wide frequency ranges with agile beam scanning. Wideband active array antennas are highly interesting alternatives to conventional antennas in applications including wireless communications, wideband radar, EW, radioastronomy, and multifunction RF systems [1-4]. Their potential advantages; multiple target tracking and jamming, directive communication and jamming, graceful degradation, flexibility, weight, size, etc. offer significant improvements provided that they can be shown to be technically and economically feasible. Further, conventional antenna array systems are intended for only one application, i.e. separate systems are used for functions such as radar, electronic warfare or communication, commonly with limited bandwidth. This imposes a multitude of separate and separately optimised antennas on a platform which might have limited payload capacity due to size, weight and signature restrictions. Here a wideband multifunction antenna system might be an attractive solution. Such a system does not only require wideband apertures, T/R modules, beamforming and beam scanning, but also modular and reconfigurable architectures in order to, instantaneously, reconfigure

the array to the required function(s) and other parameters such as effective radiated power (ERP) and beam shape. In his paper we report on our findings on the feasibility of realising crucial components for wideband phased arrays including the antenna and reconfigurable beamforming components.

II. PHASED ARRAY DESIGN To be able to perform the desired functions for a multifunction phased array for small and medium platforms, e.g. EW, ESM (electronic support measures) and communication, and to reduce the cost, the array antenna should be wideband, easily manufactured, have low weight and have wide azimuth coverage. A popular antenna candidate for broadband, wide-angle scanning, antenna arrays is the tapered slot antenna (TSA) element, see Fig.1a. Different forms of TSA antennas have been used both as single antenna elements and in array antennas [5-7] and can theoretically be designed for multi octave bandwidths. These antenna elements are, however, relatively heavy, making them unsuitable for small and medium platforms, which often have limited payload capacity. Further, they also have a considerable extension in the direction of radiation, making them difficult to use if platform integration is desired. Stacked aperture-coupled microstrip arrays are conceivable candidates due to their potentially large bandwidth, low profile and low weight, see Fig.1b. However, broadband designs are complex and the manufacturing cost relatively high. We have studied circular and faceted antenna arrays consisting of bowtie elements, see Fig.2, [8-9]. The arrays were designed and evaluated using the commercially available software Ansoft HFSS and an in-house developed FDTD-program [10]. Apart from being wideband and having a low profile, phased arrays consisting of bowtie elements are also easy to manufacture using PCB (printed circuit board) technology. The realized gain for the faceted array in Fig.2 is presented in Fig.3.

Figure 1 Wideband phased array antennas developed at FOI. (a) 6-18 GHz TSA array [7], (b) 6-15 GHz aperture-coupled microstrip array [11], and (c) 8-18 GHz bowtie element array [8].

Figure 2 Faceted array with 2×22 bowtie elements.

Figure 3 Realized gain as a function of frequency and φ when the faceted array in Fig.2 is scanned at φ=180°. Maximum gain method used for antenna diagram synthesis.

The active reflection coefficient for the faceted array is better than -10 dB over, at least, 6 – 17 GHz for the element in the scanning direction (element 1), see Fig.4.

Figure 4 Active reflection coefficient as a function of frequency when the faceted array is scanned at broadside for the facet containing element 1. A planar array (facet) consisting of 4×4 bowtie elements has been manufactured and experimentally evaluated, see Fig.1c. The antenna elements are excited by 32 coaxial cables. (One pair of coaxial cables is needed for each of the antenna elements since the two halves of the element should be excited with a phase shift of 180°. The scattering matrix of the array was measured using a network analyser, and the resulting couplings were used to calculate the active reflection coefficient. The active reflection coefficient for a centrally situated element is presented in Fig.5 when the array is scanned at broadside and to 30° in the H-plane (azimuth scan). Comparisons are made with the simulation results for an infinite planar array.

Figure 5 Active reflection coefficients for element 10 in the experimentally evaluated 2×22 element array (see Fig.1) and an element in an infinite array. The arrays are scanned at broadside and to 30° in the H-plane (azimuth scan).

III. RECONFIGURABLE BEAMFORMING Reconfiguration of the antenna array to different frequency bands and/or to share it to a number of sub-arrays can be achieved at either the beamformer level or at the aperture level. At the aperture level, several approaches can be used to reconfigure the aperture. However, in this work we have focused on the reconfiguration of the beamformer and its associated MMIC’s, reconfigurable bidirectional power splitter/ combiners (PS/PC) and TTD’s (true time delays). A reconfigurable feed network provides the ability to switch branches and antenna elements on and off. This feature allows, among other things, the beamwidth to be kept constant at different frequencies by reconfiguring the array to the appropriate size [12]. Further, if tunable low-pass filters are used it is possible to obtain constant beamwidth for signals covering the entire bandwidth. Fig.6 shows an architecture of a wideband one-dimensional active phased array. The beamformer in our concept was developed based on the following requirements:

(a) Active instead of passive power distribution will allow very good control of the system power gain and linearity budgets.

(b) The architecture of the power splitters should be broadband, reconfigurable and allow variable gain.

(c) The system should be able to handle wideband signals.

The requirements (a) and (b) are satisfied using our reconfigurable concept. In order for the system to be able to handle a broadband signal, i.e. to fulfil (c), TTD’s must be used as control devices instead of conventional phase shifters which delivers a constant phase shift with frequency. The use of TTD’s eliminates the “beam-squinting” and “pulse-stretching” effects resulting from using conventional phase shifters.

Figure 6 Schematic of a broadband beamforming network based on TTD’s and tunable low-pass filters.

The proposed active beamforming network in Fig.6 provides reconfigurability, flexibility and beam shape management. Key components including TTD’s (true time delays) and reconfigurable bi-directional power splitters and power combiners (PS/PC) have been designed and experimentally evaluated [13]. A more affordable and less advanced beamformer solution, especially suitable for small and medium platforms such as UAV’s, has been designed using Rotman lenses, see Fig.7.

Figure 7 Schematic of a broadband beamforming network based on a Rotman lens. The beamforming network in Fig.7 includes both transmitter and receivers and can therefore be used in multifunction systems for EW, signal intelligence and communication. Rotman lenses have several favourable properties making them attractive alternatives for use in beamforming networks.

Figure 8 Photograph of a Rotman lens designed for a 4×8 element phased array. The lens consists of 4 input ports (one for each beam) and 8 output ports (one for each column of antenna elements).

Advantages include straightforward manufacturing, low cost and low weight as well as simultaneous availability to several lobes. Since it is a TTD component, Rotman lenses give frequency independent beam steering and can therefore be used in wideband systems. A Rotman lens designed for a phased array consisting of 4×8 element bowtie elements is shown in Fig.8. Measured couplings (S-parameters) for the Rotman lens combined with ideal element diagrams produce the normalised directivity shown in Fig.9.

Figure 9 Normalised directivity calculated from measured S-parameters for the Rotman lens in Fig.9 and assuming ideal element diagrams.

IV. ARRAY SIMULATION AND BEAM SHAPING

For signals with small instantaneous bandwidth, beamshaping can be performed by switching on or off the signal to the antenna elements [13]. In the case of signals with wide instantaneous bandwidth, apart from switching elements on/off it is also necessary to perform a frequency taper. One way is to use a taper where for a desired beamwidth θ3dB and scanning angle θ, the antenna element n is excited only with the signal whose frequency band is [fmin , f(n)] where fmin < f(n) < fmax and f(n) is given (in one dimension) by the expression:

)cos(212)(

3 θθ ⋅⋅⋅=

dBnAnf (1)

Here A = 0.886 and n = -N/2,…, N/2, where N×N is the number of elements in the array. In the studied case, the frequency band 6-18 GHz has been chosen to demonstrate the frequency dependent taper concept. Fig.6 shows the different RF blocks involved in the beamformer. An array of 16×16 accessible elements was selected. The reconfigurable feed network is formed using the active reconfigurable PS/PC for antenna reconfiguration. The TTD’s are used for beam steering, the tunable low pass-filter is used for

frequency band selection, i.e. for the antenna element (n) the allowed frequency band is [fmin , f(n)] where f(n) is given by the above equation. Here fmin and fmax are equal to 6 and 18 GHz. The time equalizer is used to adjust for the delay introduced by the low-pass filter. Fig.10 illustrates the resulting radiation pattern when the beamformer is configured to give a frequency independent beamwidth.

Figure 10 The normalized radiation pattern in one plane, when the array is excited from 6 to 18 GHz. TTD’s and filters are used to obtain a frequency independent direction and beam width [13]. Fig.11 illustrates a small UAV equipped with a faceted phased array. The screenshot shows the GUI (graphical user interface) of an in-house developed tool for visualisation of conformal antennas (VISCA) [14]. Using measured or theoretical element patterns VISCA can be used to study scanning, coverage, cross polarization and pattern synthesis for phased arrays. It is possible to include PO (physical optics) contributions from the platform on which the array is mounted. A small UAV equipped with a faceted phased array is illustrated in Fig.12.

Figure 11 Concept for a small UAV equipped with a faceted multifunction phased array for ECM, ESM and direction finding. (The antenna diagram shown is not for the proposed faceted array.)

Figure 12 Illustration of a small UAV equipped with a faceted phased array with bowtie elements. The UAV in Fig.12 was used as a generic platform in a study where the performance of a small UAV was investigated. In the studied scenario the UAV, which was equipped with a wideband multifunction phased array, was used to protect a target against a hostile radar. To assess the best tactical behaviour of this small UAV different scenarios have been studied. It was found that by placing the UAV in relatively close proximity to the radar and performing mainlobe jamming it is possible to protect large targets against powerful radars even when the ERP for the UAV is very low. It is therefore conceivable that small, and thereby relatively inexpensive, UAV’s will play an increasingly important role in the future battle field scenario. Fig.13 shows the JSR (jamming-to-signal ratio) as a function of distance between the jammer and the radar for targets with varying RCS (radar cross section).

Figure 13 Jamming-to-signal ratio (JSR) as a function of jammer-to-radar distance for targets with varying RCS. ERPjammer = 600 W, ERPradar = 50 MW, and frequency = 10 GHz.

V. CONCLUSIONS Our findings on the feasibility of realising crucial components for wideband phased arrays and in particular array antennas, ultra wideband beamformer and front-end for small and medium platforms such as UAV’s has been reported. A compact and lightweight

wideband faceted phased array with 360° coverage, which is particularly suited for small platforms, has been designed. Two broadband beamforming networks were proposed and described. Measurements of key components, including Rotman lens and power splitters and combiners, have shown good results. A technique for realising wideband phased arrays with frequency independent radiation characteristics was described. Finally we have shown that even with a low input power, the ECM capability of a UAV equipped with the proposed phased array system is powerful enough to make the useful assets in the future battle field scenario.

VI. ACKNOWLEDGEMENT The authors would like to thank the Swedish Armed Forces (FM) and the Defence Material Administration (FMV) for supporting this work.

VII. REFERENCES [1] S. Hong et al, A Multiband, Compact Full-Duplex Beam Scanning Antenna Transceiver System Operating From 10 to 35 GHz, IEEE Trans. Ant. Prop. Vol. 54, 2006, pp 359-366. [2] P.K. Hughes, J.Y. Choe, Overview of Advanced Multifunction RF Systems (AMRFS), IEEE Int. Conf. Phased Array Syst. Tech., Dana Point, CA, May 20-26, 2000, pp. 21-24. [3] P. Lacomme, New Trends in Airborne Phased Array Radars, IEEE Int. Symp. Phased Array Syst. Tech., Boston, Oct.14-17 2003, p17-22. [4] D. Schaubert et al, The Square Kilometer Array (SKA) Antenna, IEEE Int. Symp. Phased Array Syst. Tech., Boston, Oct. 14-17, 2003, pp 351-358. [5] B Carlegrim, et al, Characteristics of a Wideband Antenna Array, JINA, Nice, 12-14 Nov. 2002. [6] B. Carlegrim, R. Gunnarsson, L. Pettersson. R. Erickson, Properties of a 2-6 GHz tapered slot antenna array, FOA Report No. FOA-R--00-01742-616--SE, Dec. 2000. [7] R. Gunnarsson, R. Erickson, L. Pettersson, B. Carlegrim, and S. Leijon, Radiation characteristics of a broadband tapered slot antenna array demonstrator, FOI Report FOI-R--0687--SE, 2002. [8] R. Gunnarsson, T. Martin and A. Ouacha, Wide-Band Circular Antenna Arrays Consisting of Bicone, Semi Bicone or Bowtie elements, APMC2006, Yokohama, Japan, 12-15 December 2006. [9] R. Gunnarsson, and T. Martin, A wide-band conformal antenna array consisting of bi-conical antenna elements, Antenn 06, Linköping, Sverige, 30 May – 1 June, 2006. [10] Torleif Martin, Broadband Electromagnetic Scattering and Shielding Analysis using the Finite Difference Time Domain Method, Ph.D. Thesis No. 669, Linköpings Universitet, 2001, Linköping, Sweden. [11] R. Gunnarsson, L.-G. Huss, P. Andersson, S. Leijon, R. Erickson, Broadband arrays with dual-polarized, aperturecoupled, microstrip antenna elements, Antenn 06, Linköping, Sweden. [12] R. Gunnarsson, A. Ouacha, B. Andersson, A. Nelander, R. Erickson, Comparison of wideband phased array concepts for achieving frequency independent radiation characteristics, 2008 IEEE Radar Conference, Rome, Italy, 26-30 May, 2008. [13] A. Ouacha, R. Erickson, R. Gunnarsson, B. Carlegrim, C. Samuelsson, S. Leijon, Wideband Antenna Arrays with Reconfigurable Beamforming and Beamshaping, IMS2007, Honolulu, Hawaii, 3-8 June 2007. [14] T. Martin, A visualization tool for conformal antenna arrays on platforms, 4th European Workshop on Conformal Antennas, Stockholm, Sweden, May 23-24, 2005.