A patch antenna is presented which has been designed to radiate frequencies in the range 4–9.5 GHz into human breast tissue. The antenna is shown by means of previously unpublished simulation and practical measurements to possess a wide input bandwidth, radiation patterns that remain largely consistent over the band of interest and a good front-to-back ratio. Consideration is also given to the antenna’s ability to radiate a pulse, and in this respect it is also found to be suitable for the proposed application.
Breast cancer is the most common cancer in women. X-ray mammography is currently the most widely-used detection technique . However the X-ray contrast between a tumour and the surrounding tissue is of the order of a few percent and as a result it suffers from relatively high misseddetection and false-detection rates. X-rays are also ionising and, hence, not generally suited to frequent screening. They also require uncomfortable compression of the breast. Microwave detection of breast tumours is a nonionising and indeed potentially low-cost alternative. The high contrast between the dielectric properties of a malignant tumour and the normal breast should manifest itself in terms of lower numbers of missed-detections and falsepositives. This potential has led to the exploration of detection techniques based on microwave-radar by a number of groups around the world [2, 3]. Research at Bristol employs a post-reception synthetically-focussed detection method originally developed for landmine detection [4, 5]. All elements of an antenna array transmit a broadband signal in turn, the elements sharing a field of view with the current transmit element, then record the received signal. By predicting the path delay between the transmit and receive antennas via any desired point in the breast, it is then possible to extract and time-align all the signals from that point. Repeated for all points in the breast, this yields a 3D image in which the distinct dielectric properties of malignant tissue are potentially visible. This process depends on: † achieving high resolution † overcoming the high attenuation in human tissue, to permit the detection of relatively deep-seated tumours † preventing reflections from skin, bones and other anatomical features (clutter) obscuring the signals from tumours. Achieving high resolutions and good anticlutter performance requires wide bandwidth operation, and an operating bandwidth of 4–9.5 GHz is the objective herein. Over this bandwidth the antenna design employed must exhibit good performance, both in terms of input match and radiation pattern (low sidelobes, a beam that is wide enough to encompass a reasonable portion of the breast, good front-to-back ratio and consistent patterns). Furthermore, a compact, low-profile antenna design is additionally desirable in order to reduce the complexities of the physical array structure and to achieve a degree of conformality with the body. The wideband bowtie antenna employed at Bristol in the past for landmine-detection research is therefore not ideal . Various different types of antennas are being considered by research groups involved in tissue-sensing applications using pulsed radar techniques. Typical examples of such antennas include the resistivelyloaded bowtie , slotline bowtie , ridged pyramidalhorn , resistively loaded dipole  and microstrip Archimedean spiral . This paper presents a low-profile stacked-patch antenna design that can operate over the necessary wide bandwidth for this application. While stacked-patch antennas are well known to have good operating bandwidths, the bandwidths achieved are usually of the order of 20% . The stackedpatch antenna presented here has been designed to radiate directly into a medium  which has similar dielectric properties to breast tissues, and furthermore achieves a bandwidth of approximately 77%. As described in subsequent sections, initial antenna design and optimisation were carried out using FDTD techniques. The paper discusses in detail for the first time both the FDTD modelling and the subsequent practical measurements conducted in contact with a biological equivalent medium.
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