Low Metal Content Anti-Personnel Mines are very difficult to detect but can still be devastating in their effects. One such mine type is the R2M2 which is virtually invisible to conventional (metal) detectors. Usually such mines are buried up to 20cm beneath the surface.

With funding from the European Union, the DEMINE consortium is developing a new, hand-held detector using Surface Penetrating Radar technique to be able to localise and discriminate such targets. The approach taken is to employ a Pseudo Random Noise radar system. A known digital noise stream is transmitted and the scattered radiation passed to a correlator. The output of the correlator indicates the presence of coherent returned signals.

The basic system operates with a clock frequency of 10GHz and the frequency range used for imaging and target ID is from 1GHz to 4GHz. This gives a range resolution of approximately 2.5cm (depending on soil permittivity). This should allow objects of the size of an anti-personnel mine to be resolved in range. To provide azimuthal resolution an array of antenna elements will be used. The practical limit to the width of the array for hand-held operation is around 50cm so the antenna elements must be fitted into this space. Note that the azimuthal resolution is independant of the soil permittivity and for 25cm operating height is around 7.5cm.

The system has been designed around a custom built integrated circuit which contains essential radar electronics. The chip generates the pseudo random pulse stream for transmission as well as providing the correlator to process the received signal. Fabrication has used Silicon-Germanium technology to allow clock operation up to 10GHz. Each antenna element will be connected to its own dedicated chip with the system linked together by a common clock, enabling fully coherent array processing. The chips will be mounted on the antenna array itself. The array is fixed to the end of a hand held probe.

The antenna elements are required to transmit with as flat as impedance as possible across the frequency range 1-4Ghz. The elements chosen are bow-ties since these are TEM structures hence satisfying the requirements for a flat impedance.

Practical bow-ties suffer from reflections at the outer edges which mars their good characteristics. By using a resistance profile which increases along the radius of the bow-tie it is possible to reduce the effect of this reflection considerably. The resistance profile is based on the work of Wu & King^[1] and Schlager, et al^[2]. A planar ink-jet technology is being used to manufacture the antenna elements.

The impedance of the antenna elements is determined by the angle at the vertex and the shielding environment. For connection to the radar chip this impedance must be in the range 50-200 Ohms preferably towards the lower end of the range. Here, the optimum size was found to be a 5cm radius with a 120 degree vertex angle. To avoid T/R switching, separate transmit/receive arrays are being used and hence the configuration of the whole array is two rows of six antennas. One row is used for transmission, each element firing in turn, while the second row is used for 6 parallel channels of reception.

The images formed from the data received can be contaminated by scattering from objects behind the antenna array. It is therefore important to shield the rear lobes of the array. This has been done by mounting the antennas on a block of absorbing material with a metal shielding plate on top of this. The rear-lobe suppression achieved has been found to be approximately 15dB across the band of interest. An additional advantage is that the effective impedance of the elements is reduced by approxiamtely 30 Ohms and crosstalk performance is improved.

Simulations using in-house Finite Difference Time Domain codes have been employed in the design work. To test the final design, simulations were carried out with the full antenna array 20cm above the targets in free space or above a block of "dry soil" (dielectric material with relative permittivity of 2.6) containing a mine (R2M2 ant-personnel) and a metal sphere of the same size. The simulation layout is shown above.

Coherent array processing from the 36 received channels is used to generate an image of the target. Time domain backpropagation accounting for dielectric interface effects is used to generate the focussed image. The array processing causes suppressionof unwanted surface clutter and yields shape and size information for false target rejection. [The picture to the right shows a series of processed images of the two targets in free space beneath the array of bow-tie elements, as the latter is scanned forwards over the target scene.] The R2M2 is clearly seen and has a characteristic shape and size which permits easy descimination against the sphere.