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A Modulating Retroreflector as a Passive Radar Transponder
Abstract - A novel retro-reflecting transponder is described. The structure is a modulated antenna array which reflects an interrogating signal back toward its source. It may be realised as a printed circuit and has a low power requirement as an RF transmitter is not employed, unlike conventional transponders.
Introduction: Simple retro-reflectors comprising corner structures are sometimes employed on small craft to increase their radar cross section (RCS). These are passive devices, designed along geometric optics principles. If a target?s RCS can be modulated by some means, the reflected signal carries this modulation and the target is more easily distinguished from the background clutter. The concept of the modulated retro-reflector combines the twin aims of retro-reflectivity (high RCS) with modulated reflectivity (clutter rejection) so as to realise a passive transponder which is envisaged to find wide applicability.
Design Philosophy : To achieve the above aims, the retro-reflective antenna array  has been adopted as the basic structure. This comprises an antenna array where antenna pairs are joined by equal-length transmission lines to achieve phase conjugation. This structure lends itself to RCS modulation by employment of switches in the transmission lines, as exemplified in figure 1 below, where radiating elements 1-6 are joined in pairs via switches.
Figure 1. Example of 1-Dimensional Modulated Retro-Array.
Since where G is the gain of the array and G =N g(q,f)
where N is the number of elements and g(q,f) is the gain pattern of an element,we see that the choice of array element is of fundamental importance in defining the performance of the retro-reflector in its angular coverage, as well as the magnitude of its reflectivity, and polarisation response.It is also interesting to note that if the array is illuminated in a multi-path environment, retro-reflection occurs for each path to recombine the original waveform at the transmitter.
Modulated Retro-array Prototype:The retro-array was first conceived by Van Atta - Refs and  reported on the performance of a 2-dimensional array at 2.85 GHz, consisting of a 4 x 4 rectangular matrix of half-wavelength dipoles, horizontally aligned a quarter wavelength above a ground plane and spaced at intervals of 0.61 wavelengths. As a convenient starting point, this design was replicated but scaled to an operating frequency of 2.5 GHz. The ground plane measured 32 cm along the axis parallel to the dipoles and 28 cm along the orthogonal axis. RCS modulation was achieved by utilising purpose built PIN diode microstrip switches in series with the coaxial lines behind the ground plane. Double-pole matched switches were initially employed, to achieve amplitude modulation by switching between the conductive state (retro-reflection) and matched state (low reflection). These were later replaced by pi-phase shifters, which produced phase modulation by switching between different microstrip line lengths. In each case, a square wave modulation signal was applied to the switching network at frequencies up to 250 kHz, while the array was illuminated with RF at close to 2.5 GHz. The nature of the reflected signal was examined on a spectrum analyser, as exemplified in figure 4 below where the modulation frequency is 25 kHz.
a) Amplitude modulation. Illumination at 0ø b) Phase modulation. Illumination at 35ø
Fig 2. Typical modulation products for the reflected RF.
The modulation products were studied as the orientation between the array and the incident RF was varied by rotating the array in azimuth (H-plane). The power returned in the first upper sideband was recorded as a function of rotation angle, and seen to vary slowly. The angular response is a function of the antenna element radiation pattern, as stated above.
Printed Circuits for Modulated Retro-Arrays:To develop the modulated array toward a more marketable structure, a planar array utilising patch antennas was built. This involved the fabrication of two circuits photo-etched onto Rogers RO3000 microwave laminate (er = 3.0, h = 0.76 mm ) which employed aperture coupling to obviate hard-wired connections between the antennas and feed/switching circuit. Overall dimensions were restricted to 296 mm x 300 mm for practical reasons. The radiating elements were linearly polarised 34.09 mm square patches. The planar array was functionally equivalent to the dipole array, ie. a 4 x 4 array operating at 2.5 GHz, with pi-phase shifters. The power in the first upper sideband was measured for the H-plane and E-plane responses and compared to the theoretical patterns based on the square of the radiation patterns from a patch radiator  as shown in figure 3.
Fig 3. Reflected sideband power (solid) = measured (dotted) =- theoretical. (Plots are normalised)
The technique may be scaled to other frequencies.X-band transponders are under development and a planar prototype, also utilising a 4 x 4 matrix of aperture coupled patch antennas and very simple single diode amplitude switches has been demonstrated at 9.4 GHz.
Conclusions:A novel passive microwave transponder has been developed. The basic concept is a retro-array whose radar cross section may be amplitude or phase modulated by switches in the transmission lines which join antenna pairs. The technique combines enhancement of RCS with modulation to impose information onto the reflected signal. The modulation products in the reflected waveform are visible over a broad range of angles - the angular response being a function of the antenna radiation pattern. Transponders have been realised as robust printed circuits which lend themselves to economic production. Prototypes have been demonstrated at 2.5 GHz and 9.4 GHz.
J.Thornton and D.J.Edwards (Department of Engineering Science, University of Oxford, Parks Road, Oxford, OX1 3PJ, United Kingdom)
 Antennas, John D.Kraus (second ed.) McGraw-Hill 1988 pp. 496 - 497.
 ?Van Atta Reflector Array?, Sharp and Diab, IRE Transactions On Antennas And Propagation VOL AP-8 July 1960.
 Radar Cross Section Handbook Vol 2, Ruck, Plenum Press 1970. pp 599 - 602
 ?Microstrip Antenna Technology? K.R.Carver and J.W.Mink, IEEE Transactions on Antennas and Propagation Vol AP-29 no.1 Jan 1981.
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