Electronically Reconfigurable Aperture (ERA): a New Approach for Beam-Steering Technology Vladimir A. Manasson, Lev Sadovnik, Vladimir Litvinov, Robert Mino, Irina Gordion, Aramais Avakian, Michael Felman, Dexin Jia, Mark Aretskin, Viktor Khodos, Alexander Brailovskiy. Sierra Nevada Corporation, CNS/ATM, Irvine, CA
[email protected]
Abstract — In order to satisfy the demanding SWaP requirements of modern microwave/MMW instruments, designers are looking for novel integrated solutions. In particular, phased array packaging, especially at Ka and higher frequencies, is extremely challenging as halfwavelength spacing is prohibitively small. The cost of implementing high density phased array packaging is another impediment to wider use. This communication presents a new electronically beam-steering technology that is compatible with highly integrated antenna design and dramatically simplifies packaging. It is based on the coherent scattering of the evanescent field associated with waves propagating through a dielectric waveguide. The antenna scattering elements are controlled electronically and constitute a dynamically reconfigurable hologram. The switching time from one hologram pattern (one beam position) to another is on the order of tens of nanoseconds. Because of the hologram nature of this approach, the beamforming capabilities of the electronically reconfigurable aperture (ERA) approach are comparable to those of phased arrays: 1D and 2D beam-forming and beamsteering, multiple simultaneous individually controlled beams, steerable nulls and variable beam width(s).
constraints. In this paper we present some results of the development of electronically controlled beam-formers based on this approach.
2. ERA OPERATING PRINCIPLE ERA feed An important departure from traditional phased arrays is the antenna feed. Instead of branched waveguide/transmission line networks we use continuous dielectric waveguides. In addition to simplification of the feed design, the dielectric waveguide provides two distinct features: 1) the fields supported by a dielectric waveguide are not restricted to the physical volume occupied by the waveguide, they penetrate the boundaries of the waveguide body; 2) the field modes supported by a dielectric waveguide are slow, i.e. their propagation velocity v is slower than in free space c. In contrast to metal waveguides or metal-based transmission lines, the spatial localization of the propagating modes in a dielectric waveguide is due to the total internal reflection occurring at the boundaries between the waveguide’s physical body and the surrounding space. The total internal reflection creates an evanescent field that propagates outside the waveguide’s body and can be perturbed by objects placed in the waveguide’s vicinity. Such objects break the total internal reflection and scatter energy from the waveguide. The phases of the scattered waves depend on the positions of the scatters. Consequently, a set of properly distributed (phased) scatterers works as a hologram.
1. INTRODUCTION Passive Phased Array Antennas suffer from severe losses in the phase-shifters, circulators, and branched feeding network. Active arrays have been developed to compensate losses by use of amplifiers located immediately near the antenna radiating elements. To prevent the appearance of secondary lobes in the PAA beam patterns, the spacing between the antenna a half-wavelength. This requirement is increasingly challenging as the element spacing becomes prohibitively small at short MMW waves. This is the stimulus for the search for alternative beam-forming technologies.
The simplest hologram is a diffraction grating with equidistant distribution (a single spatial period) of scatterers. A more complex hologram can be represented as the composition of overlapping diffraction gratings with different periods. We restrict our discussion to a single diffraction grating.
Phased Array Antennas can be considered to be quasioptical devices that form wave-fronts by manipulating the antenna element’s phases without changing the position (distribution) of the antenna elements. An alternative method is to use a set of scatterers properly distributed in space, similar to the approach employed in optical holograms. The reconfiguration of scatterer patterns produces a desired beam shape and direction of propagation. The scatterer size can be much smaller than a halfwavelength, thus the device may easily satisfy the specified 978-1-4244-5128-9/10/$26.00 ©2010 IEEE
An optical hologram is usually fed (illuminated) from free space. A diffraction grating fed from free space can create a number of diffraction beams associated with different diffraction orders and governed by the following equation : (1)
673
where D and E are the angles of the incident and diffracted beams, correspondingly, O is wavelength, / is the grating period (spacing between the scatterers), and p = 0 ±1,±2,… is the diffraction order. By varying the grating period, it is possible to steer the secondary beams in desired directions. For most antenna applications, however, it is necessary to have only one controlled beam (main lobe). For p = 0, equation (1) always has a solution, and this solution does not depend on /. Thus, when the diffraction grating is illuminated from free space the zero-order diffraction beam is always present and negatively affects antenna performance.
linear arrays oriented parallel to the direction of the feeding wave propagation. For the case when the direction of the feeding wave propagation and scatterer lines are vertical, the beam direction is determined by two equations:
(6) where T and M are the elevation and azimuth angles of the radiating beam, S is the spacing between the linear arrays, n is the delay factor in the waveguide, / is the grating period in the vertical plane, and s is the incremental shift of the grating hologram pattern from one hologram line to another.
The situation is quite different when one illuminates a diffraction grating from a medium that supports slow waves. This is the case when a diffraction grating is placed in the evanescent field of a dielectric waveguide. For this case and a linear dielectric waveguide, equation (1) is modified to:
To launch a wave with a planar wave-front propagating parallel to one of the principal scanning planes, one can use corporate feeds, planar optics such as lenses, reflectors or linear leaky waveguides coupled to the planar waveguide with or without the help of a diffraction grating (see Figure 1).
(2) where c and v are the light velocity in free space and in the dielectric waveguide respectively, c > v. Equation (2) has a single solution (only one controlled diffraction beam) when
and (3) where n = c/v is the delay factor, or the medium’s index of refraction. This beam can be steered within a wide anglular range: (4) by varying the grating period / according to the equation: (5) Beam steering antennas based on leaky-waves diffracted by a grating placed in the evanescent field of a dielectric waveguide have been reported in a number of publications (see references [1-2]). Beam control can be implemented by using a spinning drum or a spinning disk carrying a diffraction grating with a continuously varying period, a back-and-forth (seesaw) moving plate or tape, spring coils, flat springs, etc.
Figure 1 – Different Launchers Form a Planar Wave-Front in a Planar Dielectric Waveguide Using a corporate feed or planar optics, it is possible to launch a feeding wave perpendicular to the edge of the planar waveguide and have it propagate along one of the principal scanning planes.
For ERA scanning in two orthogonal planes the feeding is done by a planar dielectric waveguide. A planar waveguide can support modes with differently shaped wave-fronts (planar, cylindrical, etc.); however, the simplest and most convenient case is when the feeding mode has a planar (or quasi-planar) wave-front and scatterers are organized in
Hologram carriers and electronically reconfigurable patterns The scatterer positions in a 2-D array are controlled electronically, making it equivalent to a dynamic hologram. 674
The simplest case of electronic control is when the scatterers are driven by binary switches: they scatter when they are in the ON state and do not scatter when in the OFF state (Figure 2). Interacting with the evanescent field of the previously described feed waveguide, this dynamic hologram generates a free-space propagating field according to the pattern of the ON/OFF states of the switches. With a number of densely distributed switchable scatterers, it is possible to create diffraction gratings and more complex dynamic holograms.
Figure 4 – Simulated ERA Beams Corresponding to the Hologram Patterns Shown in Figure 3. Beam Spacing is 0.5 deg, Aperture Length is 20 Wavelengths, 3dB Beamwidth is 3.4 deg, Scatterer Density is 8 per Wavelength, Amplitude Taper Function is cosine Figure 2 – Dielectric Waveguide Loaded with an Array of Switchable Scatterers The ERA described above provides discrete beam steering. The smallest angular step between the two closest main lobe positions depends on the density of scatterers, their distribution, and the total number of the scatterers in the aperture. To steer a beam in a desired direction, one can use equation (5), where the grating parameter / should be taken as an average period in the hologram pattern. There can be a number of hologram patterns that have the same average grating period. All will form beams with main lobes pointing in the same direction. Additional degrees of freedom, such as scatterer distribution, ON/OFF patterns, and coupling to the feed waveguide, give the antenna designer the ability to control both the grating and quantization lobes. While ON/OFF patterns can be changed electronically, scatterer distribution and coupling to the feed waveguide are fixed and should be optimized, taking into account all desirable beam positions.
Figure 5 – Segment of Hologram Pattern (top) and the Corresponding Simulated ERA Beam (bottom). No tapering was applied.
Some examples of 1D and 2D hologram patterns and simulated ERA beam patterns are presented in Figures 3 through 8.
Figure 3 – ON/OFF Hologram Patterns (Segments) for Three Different Beams with Pointing Angles 0.5 deg Apart
Figure 6 – Segment of Hologram Pattern (top) and the Corresponding ERA Beam (bottom) with Cosine Taper 675
The approach is flexible enough to scan/steer a beam with small angular steps, control side lobes, instantly create and control more than one beam, and potentially to perform other antenna functions not discussed here (nulls, focusing, specific beam shaping, etc.) Scatterer Control To achieve maximum flexibility of beam pattern control, each scatterer should be controlled individually. If the required switching time from one beam pattern to another is of prime concern, the scatterers may be controlled in parallel, by using multiple output devices, such as FPGA controllers (see Figure 10). Using an FPGA controller we have demonstrated switching times as short as ~20 ns. In many cases a signal processing part of the radar that follows the ERA antenna cannot process information at this rate. For this case of longer beam switching time (~10 Ps) ERA serial control can be used. Commonly available shift registers used to control LED arrays are perfectly suitable for use in the ERA (Figure 9).
Figure 7 – Infrared Picture of a 2D-Hologram Created by PIN-Diode Controlled Scatterers. IR radiation from active PIN diodes were recorded with an IR camera and is seen as white spots. One can see irregularities and linear defects in the hologram pattern.
Figure 9 – Segment of ERA Controller Based on Shift Register ICs.
Figure 10 –Hologram Chip Integrated with ERA Controller Based on FPGA
3. EXAMPLES OF ERA IMPLEMENTATION 1D-ERA beam steering antenna operating at W-band Figure 8 – Samples of 2D Beam Created by Different Hologram Patterns Illustrating Scanning of a Single Beam and Two Beams Simultaneously (bottom right)
A major advantage of the proposed ERA technology is the possibility to use standard fabrication processes amenable to low cost mass manufacturing. An example is a 1D ERA 676
with hologram carrying boards made as a single Si chip IC. An array of shorted slots (scatterers) operates as a hologram carrier. Imbedded PIN diodes perform the switching function, controlling the scattering function of the slots. A quartz fiber is used as a cost effective and low-loss feeding waveguide.
Figure 14 –Measured Beam Patterns Generated by the ERA Shown in Figure 13 2D-ERA beam steering antenna operating at X-band
Figure 11 –1D ERA Assembly Operating at W-Band
A 2D ERA antenna operating at X-band is shown in Figure 13. The hologram carriers were built as a set of printed circuit boards interacting with the evanescent field of a planar dielectric waveguide. Hologram boards were aligned in the direction of propagation of the feeding wave and are half-wavelength spaced. GaAs PIN switches are used to provide very rapid beam steering. Biased GaAs PINs radiate at near IR. This allows us to visualize the hologram patterns seen in Figure 7. Measured ERA beam patterns are shown in Figure 14.
Samples of beam patterns generated by the 1D-ERA are shown in Figure 12.
ERA as a front-end sensor for imaging radar We have developed and demonstrated several ERAs operating as beam-steering antennas integrated with FMCW imaging radars. Figure 12 –Measured Beam Patterns Generated by the ERA Shown in Figure 11
Figure 15 W-Band ERA Used in Traffic Monitoring Radar
Figure 13 –2D ERA Assembly Operating at X-Band
677
Figure 17 – B-scope of an FMCW radar equipped with bistatic 2D ERA. The bright spot is a boat detected at 3.7 nm
Figure 15 –Single Frame Extracted from a Real-Time MMW Video Using the Fast Scanning ERA Shown in Figure 14. One such an antenna is shown in Figure 15. The imaging W-band radar was capable of collecting images at a 20 Hz frame rate. A snap shot of traffic monitoring data collection is shown in Figure 16.
Figure 17 – Same as above with a boat and a buoy detected at 7.4 nm
Another example is a 2D-ERA operating as part of an Xband bi-static surface search radar (Figure 16). Three samples of B-scope radar displays are shown in Figure 17.
Figure 17 Same as above with a land mass detected at 9.9 to 11.5 nm
4. CONCLUSIONS The major benefit provided by ERA technology, as compared to PAA technology, is that it does not require the use of phase shifters: all phased array-like capabilities are achieved by varying the distribution of the field amplitude formed by an array of scatterers . This is applicable for both 1D and 2D beam forming.
Figure 16 – One Aperture of a Bi-Static 2D-ERA Operating at X-band
Another salient feature is that a highly developed feeding network traditionally employed by PAAs is replaced by a simple dielectric waveguide. 678
Scattering amplitudes may be controlled via PIN diodes, or other binary switches, as well as by continuously adjusting impedance devices. Both, the switches and their controllers can be integrated with scatterers (hologram carriers) at the wafer level. SNC has demonstrated both 1-D and 2- D ERAs at frequencies ranging from X-band to W-band. ERA based antennas have been implemented using arrays of PCBs (at lower frequencies) and monolithically integrated siliconbased single-chip apertures (at higher frequencies).
ACKNOWLEDGEMENTS Authors thank their SNC colleagues who were instrumental in obtaining radar images and organizing the data collection: Csaba Mezei, Rodger Lakey, Dave Howard, Jan Dyer, Dean Rudy and Richard Schouten. We acknowledge the US Navy’s support of ERA development and helpful discussions with Douglas Marker, Gary Mason, Mark Tadder and Yavuz Dogrul in formulating ERA requirements.
REFERENCES [1] A. Oral Salman, The Millimeter Wave Radiation of a Dielectric Leaky-Wave Antenna Coupled with Diffraction Grating: Broad-Face Interaction,” J. Infrared Milli Terahz Waves 31, 196-213, 2010. [2] A.A. Kirilenko, S.A. Steshenko, “The Accurate TwoDimensional Model of the Effect of Surface Waves Transformation into Spatial Modes,” Radiofisika I Electronika 10, No 1, 30-38, 2005.
679
