Design of a Reconfigurable Reflectarray Unit Cell for Wide Angle Beam-Steering Radar Applications Francesca Venneri, Sandra Costanzo, and Giuseppe Di Massa DIMES – University of Calabria 87036 Rende (CS), Italy
[email protected]
Abstract. A reconfigurable aperture-coupled reflectarray element is proposed for the realization of beam steering antennas, suitable for radar applications. Each reflectarray element is coupled to a microstrip line, which is loaded by a single varactor diode acting as a phase shifter element, thus providing a continuously variable reflection phase. A reduced size reflectarray unit cell is properly designed in order to extend the antenna beam scanning capabilities within a wide angular region, but avoiding the occurrence of undesired grating lobes. The radiating structure is properly optimized to obtain a full phase tuning range at the frequency of 11.5 GHz, thus assuring a good agility and accuracy in the reconfiguration of the reflectarray radiation pattern. Keywords: Reflectarrays, phased arrays, radar antennas.
1
Introduction
Phased array antennas are usually adopted in the design of modern radar systems, offering many advantages such as agile beams, low profile and scalability. Unlike mechanically scanned antennas, phased arrays integrate the actual radiating structures with phase shifter components and/or more complex T/R modules [1] which control the input signal of each radiating element, thus allowing the radiated main beam to be electronically steered. By electronic scanning, the radar beams can be positioned almost instantaneously, without time delays and vibration of mechanical systems. Furthermore, electronically scanned antennas offer increased data rates, instantaneous positioning of the radar beam anywhere, avoiding also mechanical errors and failures associated with mechanically scanned antennas. A very attractive alternative to traditional phased array antennas is offered by the fairly new reflectarray concept [2]. As a matter of fact, this antenna type may be specifically designed for those applications requiring beam scanning capabilities or pattern reconfigurability. Furthermore, reconfigurable reflectarrays may offer many advantages over conventional phased arrays, such as reduced costs and volume, a simpler architecture due to the absence of complicated beam-forming networks, and increased efficiencies thanks to the use of spatial feeding. Á. Rocha et al. (Eds.): Advances in Information Systems and Technologies, AISC 206, pp. 1007–1013. DOI: 10.1007/978-3-642-36981-0_95 © Springer-Verlag Berlin Heidelberg 2013
1008
F. Venneri, S. Costanzo, and G.D. Massa
They consist of a printed array illuminated by a feed antenna. Each radiating element is properly designed to give a phase response able to create a total reradiated field with some desired features, such as prescribed beam directions and/or shapes. Many different reflectarray configurations have been proposed in literature and, recently, many efforts have been spent in the design of reconfigurable reflectarray elements, which are usually based on the use of microstrip patches integrated with one or more electronically controllable components, such as MEMs and varactor diodes [3-5]. Recently, the authors have proposed a novel reconfigurable reflectarray element based on the use of an aperture-coupled patch electronically driven by a single varactor diode [6-9]. The radiating patch is coupled to a microstrip line printed onto a different substrate and loaded by a varactor (Fig. 1). By changing the bias voltage across the diode, a variable phase shift is added to the reradiated field, thus providing a dynamic control of the element backscattering features. A detailed description of the proposed phase tuning mechanism is reported in [9], where the procedure giving a proper phasing line optimization is also illustrated.
Fig. 1. Single element geometry: (a) top view; (b) side view
The phase tuning capabilities of this configuration have been already demonstrated in [6], while in [7-9] the proposed element has been successful adopted for the design of a reconfigurable reflectarray prototype composed by 3×15 elements and characterized by a unit cell with size equal to Δx×Δy=0.7λ0×0.7λ0 at 11.5 GHz. The reconfigurability of the proposed antenna has been tested in [7-9], through several measurements of its radiated pattern, for different configurations of the controlling varactors voltages, which are properly computed through the implementation of the synthesis algorithm described in [10]. In [7-9] the antenna beam steering capabilities have been demonstrated within an angular region, which extends from -25° to 25°, in the principal radiation plane containing the largest number of array elements. This limited scanning range is principally imposed by the unit cell size.
Design of a Reconfigurable Reflectarray Unit Cell
1009
In this work, the reflectarray element proposed in [6-9] is redesigned in order to offer the possibility to obtain a reflectarray with an increased scanning region. Pointing out that a large scan angle requires a close element spacing, less than or equal to half wavelength at the operating frequency [11], a reflectarray unit cell with a reduced size equal to 0.46λ0×0.46λ0 (f0=11.5 GHz) is proposed. In order to accommodate the phasing circuitries inside the reduced available area embedded in the unit cell, the antenna stratification layers are properly modified. According to the considerations reported in [9], the varactor loaded line is accurately resized in order to maximize the phase agility of the antenna. A phase tuning range of about 330° is numerically demonstrated, by varying the capacitance of the varactor diode within the values ranging from 0.2 pF up to 2pF.
2
Performance Limitations of Beam Steering Arrays
The angular displacement of an electronically scanned radar beam is practically limited by two principal factors, namely the element pattern and the array elements spacing. As a matter of fact, the radiation pattern of an array of identical radiators is given by the product of the array factor and the element pattern. If the single array radiator is isotropic, i.e. the array elements radiate an electric field quite uniform along those directions belonging to the scanning plane, only the array factor will affect the total radiation pattern. However, practical array element patterns are not omnidirectional, showing an amplitude that decays moving away from the broadside direction. In these cases, the single element will significantly reduce the amplitude of the scanned beam, except in the zone where it is nearly isotropic [11]. The second limitation, namely the array elements spacing, is more relevant. As a matter of fact, it is well-known that a large scan angle requires close element spacing, in order to avoid grating lobes appearance. The maximum scan angle, that a linear phased array can achieve, may be derived from the well-known relation [11]:
λ − 1 d
θ s max = sin −1
(1)
where θsmax is the maximum scan angle from broadside direction, d is the spacing between two adjacent elements and λ is the operating wavelength. Equation (1) is derived from the array factor expression of a linear array placed along the x or y-axis and its validity can be extended to the principal cuts of a planar array placed in the x-y plane [11]. If the array scan angle exceeds the value imposed by (1), grating lobes will appear along other directions. Equation (1) also states that half wavelength spaced arrays will have a complete theoretical scan range of ±90°. On the contrary, when the spacing between elements increases beyond a half wavelength, the scan range of the array is significantly reduced, due to the appearance of grating lobes having the same amplitude of the main beam.
1010
F. Venneri, S. Costanzo, and G.D. Massa
The maximum scan angle achievable by a phased array is also a function of the array length and the desired half-power beam-width [11], however the condition imposed by (1) is necessary for the design of an array with prescribed beam-steering capabilities.
3
Design of a Reconfigurable Reflectarray Element Embedded into a Reduced Size Unit Cell
In order to improve the scanning capabilities of the reconfigurable reflectarray configuration proposed in [6-9], the single reflectarray element is properly redesigned by reducing the unit cell size. In fact, as discussed in the previous paragraph, a closer array elements spacing assures a larger scanning region. The unit cell dimension is fixed to a value less than a half-wavelength at the operating frequency f0 =11.5 GHz. In particular, the array grid size Δx×Δy is set to a value equal to 0.46λ0×0.46λ0. Furthermore, as demonstrate in [12-14], a reduced unit cell size allows to improve the bandwidth performances of reflectarray antennas. This last aspect is not considered in the present paper, however it could be analyzed in a future work. In order to allow the accommodation of the tuning circuitries in the smaller area embedded inside the unit cell, the phasing line substrate adopted in [6] is properly substituted with a layer of Arlon AR600, with εr=6 and thickness h=0.762mm (see table 1). The use of a substrate with a higher permittivity allows to reduce the wavelength inside the printed lines, thus providing the possibility to design a shorter phase tuning line. As reported in Table 1, the other layers composing the antenna stratification are equal to those adopted in [6]. Table 1. Element stratification
Layer
Element designed in [6]
Element designed in this work
Δx×Δy = 0.7λ0×0.7λ0
Δx×Δy = 0.46λ0×0.46λ0
Material
Thickness
Patch
Material
Thickness
Copper
Antenna substrate
Diclad870 (εr1= 2.33)
35 μm
Copper
35 μm
t= 0.762 mm
Diclad870 (εr1= 2.33)
t= 0.762 mm
Air
d= 0.762 mm
Air
d= 0.762 mm
Ground plane with slot
Copper
35 μm
Copper
35 μm
Phasing line substrate
Diclad870 (εr2= 2.33)
h= 0.762 mm
AR600 (εr2=6)
h= 0.762 mm
Phasing line
Copper
35 μm
Copper
35 μm
-
Air
s= 3.7 mm
Air
s= 3.7 mm
Ground plane reducing back radiation
Copper
35 μm
Copper
35 μm
Design of a Reconfigurable Reflectarray Unit Cell
1011
The following dimensions are obtained for the different parts composing the antenna: W= 7.75 mm, L= 7.75 mm, Wa= 0.5 mm, La= 5.7 mm, Ws= 1.6 mm (Fig. 1(a)). The assumed line width Ws corresponds to a characteristic impedance of value equal to 37 Ω. A varactor diode, with a tunable capacitance ranging from 0.2pF to 2pF, is integrated to the microstrip line in order to obtain the required reconfiguration capabilities. As described in [9], the two line sections Lv and Ls (see Fig. 1) are optimized in order to maximize the phase agility of the element for the assigned varactor capacitance range. At this purpose, the length Lv is set to a value of 4.2 mm, while the stub length Ls is varied from 3.5 mm to 5.4 mm. Fig. 2 shows the reflection phase curves versus varactor capacitance computed for each considered value of the stub length. It can be observed that by increasing Ls a higher phase tuning range of about 330° is obtained. As accurately demonstrated in [9], this last result is due to the introduction of a proper inductive effect, which is directly related to the stub length.
Fig. 2. Phase curve versus diode capacitance for different stub length
The element pattern of the designed unit cell is reported under Fig. 3. The depicted diagrams refers to the reflectarray element with a phasing line having the following dimensions: Lv= 4.2 mm and Ls=5.2 mm. The radiation patterns computed in the two principal planes show a nearly isotropic behavior within the range from -45° to 45°, as in the case of a typical cos(θ) source. In conclusion, the proposed unit cell could be suitable for the design of reflectarray antennas with improved beam steering capabilities. As a matter of fact the main beam could be scanned within an angular region greater than about 40° with respect to the one demonstrated in [7-9], without occurring in the grating lobes phenomena.
1012
F. Venneri, S. Costanzo, and G.D. Massa
—o— x-z plane; —+— y-z plane Fig. 3. Element pattern of designed reflectarray element
4
Conclusions
The reflectarray concept has been applied in this work to the design of beam steering antennas suitable for radar applications. A reflectarray unit cell based on the use of a single varactor diode has been proposed and specifically optimized to provide wide angle reconfigurability features. At this purpose, the antenna has been properly designed by reducing the unit cell size, in order to achieve a large angular scanning. As a specific numerical example, a varactor loaded reflectarray element, embedded into a 0.46λ0×0.46λ0 cell at f0=11.5 GHz, has been synthesized, obtaining a full phase tuning range of about 330°.
References 1. Fenn, A.J., Temme, D.H., Delaney, W.P., Courtney, W.E.: The Development of PhasedArray Radar Technology. Lincoln Laboratory Journal 12, 321–340 (2000) 2. Huang, J., Encinar, J.: Reflectarray Antennas. Wiley-IEEE Press (2008) 3. Sorrentino, R., Vincenti Gatti, R., Marcaccioli, L., Mencagli, B.: Electronic steerable MEMS antennas. In: 1st EuCAP, France (2006) 4. Hum, S.V., Okoniewski, M., Davies, R.J.: Realizing an electronically tunable reflectarray using varactor diode-tuned elements. IEEE Microw Wirel. Compon. Lett. 15, 422–424 (2005) 5. Riel, M., Laurin, J.J.: Design of an electronically beam scanning reflectarray using aperture-coupled elements. IEEE Trans. Antennas Propag. 55, 1260–1266 (2007) 6. Venneri, F., Costanzo, S., Di Massa, G.: Reconfigurable Aperture-Coupled Reflectarray Element Tuned by a Single Varactor Diode. Electronics Letters 48, 68–69 (2012)
Design of a Reconfigurable Reflectarray Unit Cell
1013
7. Venneri, F., Costanzo, S., Di Massa, G., Borgia, A., Corsonello, P., Salzano, M.: Design of a Reconfigurable Reflectarray Based on a Varactor Tuned Element, EuCAP 2012, Prague (2012) 8. Venneri, F., Costanzo, S., Di Massa, G., et al.: Beam-Scanning Reflectarray Based on a Single Varactor-Tuned Element. Int. Journal of Antennas and Propagation 2012, Article ID 290285, 5 pages (2012), doi:10.1155/2012/290285 9. Venneri, F., Costanzo, S., Di Massa, G.: Design and validation of a reconfigurable single varactor-tuned reflectarray. IEEE Transactions on Antennas and Propagation (2013), doi: 10.1109/TAP.2012.2226229 10. Venneri, F., Costanzo, S., Di Massa, G., Angiulli, G.: An improved synthesis algorithm for reflectarrays design. IEEE Antennas Wirel. Propag. Lett. 4, 258–261 (2005) 11. Balanis, C.A.: Antenna Theory: Analysis and Design, 2nd edn. John Wiley and Sons (1997) 12. Costanzo, S., Venneri, F., Di Massa, G.: Bandwidth enhancement of aperture-coupled reflectarrays. IEE Electronics Letters 42, 1320–1321 (2006) 13. Venneri, F., Costanzo, S., Di Massa, G., Amendola, G.: Aperture-coupled reflectarrays with enhanced bandwidth features. Journal of Electromagnetic Waves and Applications 22, 1527–1537 (2008) 14. Venneri, F., Costanzo, S., Di Massa, G.: Bandwidth Behavior of Closely Spaced ApertureCoupled Reflectarrays. International Journal of Antennas and Propagation 2012, Article ID 846017, 11 pages (2012), doi:10.1155/2012/846017
