Directional wide band printed monopole antenna for use in microwave breast cancer imaging

Abstract

Breast cancer is the most common cancer in women. Detection of small breast lesions by mammography screening facilitates the cancer treatment by noninvasive techniques. Recently, new therapies than traditional surgery have been explored to satisfy these demands. The physical basis for breast cancer detection with microwave imaging is the difference in dielectric properties of normal and malignant breast tissues. Microwave imaging involves illuminating the breast with an ultra-wideband pulse from a number of antenna locations, then synthetically focusing reflections from the breast. The detection of malignant tumors is achieved by the coherent addition of returns from these strongly scattering objects.Radar-based microwave imaging techniques have been proposed for early stage breast cancer detection. Radar-based microwave breast imaging approaches involve illuminating the breast with an ultra-wideband pulse of microwaves and detecting reflections. The reflections are then processed to create images that indicate the presence and location of tumors in the breast. A key component of these systems is the antenna that is used to radiate and receive the ultra-wideband pulses. So the antenna design requirements for use in near field near surface measurement applications, such as radar-based microwave breast cancer imaging are as follows: radiation of ultra-wideband signal to transmit short pulses, size of the antenna on the order of a few centimeters to selectively illuminate and permit scanning, an optimum half power near-field beam width( HPBW) to avoid smearing of the scatterers that occurs if the field of view of each antenna is too broad, and finally a good impedance matching across the entire band, This ensures that most of the energy is transmitted. In order to decrease the HPBW of an antenna we have to increase the directivity of the antenna in a desired direction. Nevertheless, most of the wide band and UWB antennas like planar monopoles, which are in use, have almost Omni-Directional radiation pattern.Directivity can be achieved if the antenna is large in a desired direction, such as Horn or Vivaldi antennas. Printed disc monopole antennas with an L-shaped or parabolic-shaped ground plane are introduced as another type of directional antennas. In these antennas it has been shown how partial ground optimization influences the antenna?s performance, in maximizing the directivity and gain of the antenna. These kinds of directional antennas are similar to the UWB type Omni-Directional monopole antennas, where it is shown the effect of ground plane on obtaining the desired directional characteristics of the antenna.This Thesis presents a new design of directional wide band monopole antenna with parabolic-shaped ground plane. Ground plane of the antenna consists of a symmetrical parabolic curve, which its axis extended along the direction of the substrate?s diagonal. In order to accomplish high gain and directivity, axis of parabola in the ground plane is extended throughout the direction of square substrate?s diagonal that maximizes the capability of symmetrical ground plane as a reflector. The directivity of the antenna is further improved by inserting parabolic-shaped slots at the corners of ground plane. The second edge of the ground plane which is created by inserting the slots, behaves as an additional reflector which cause to increase in the gain and directivity.Then, the presented planar antenna is composed of a disc-monopole fed by a 50? microstrip line printed on a FR4 substrate. Simulation and measurements show that the proposed antenna has stable directional radiation pattern and higher gain compared to the previous directional monopole antennas. Impedance bandwidth of the antenna covers the frequency range of 4-9 GHz. Measured HPBW is among the degrees 54-22 in the same range of frequencies. In comparison with conventional antennas with a similar structure, gain of the antenna is improved between 1.1 and 3.1 dBi among 4-9 GHz. HPBW of the antenna is also between 5 and 15 degrees through the bandwidth .Results confirm the good characteristics for use in radar and microwave Breast cancer imaging applications where high resolution is required. For example, at 8.5 GHz, measured HPBW of the antenna is decreased from 38 degrees to 23 degrees (mentioned in the result section), which confirms a 40 % decrease in HPBW of the antenna (simulated HPBW is 26 = 33 % improvement). That is very important in order to increase the resolution of a radar system.As an additional attempt, another novel compact directional monopole antenna in microstrip technology is also presented. Dimensions of this antenna are considerably miniaturized in comparison with conventional directional antennas. The main effort is to convert an Omni-directional radiation pattern of a compact monopole antenna to the desired directional radiation pattern, by using a novel ground plane, and a parasitic element. The ground plane and parasitic element are accurately designed in a way that make the surface currents of radiating elements to move toward the desired direction, which increase the radiation density in the preferred direction and also decrease the radiation intensity in the opposite sides. Simulations confirm a good directional characteristic of the antenna at the frequencies between 5 and 9 GHz. Gain of the antenna is increased over 5 dBi at the desired frequencies. Reflection coefficient bandwidth of the antenna covers the frequencies among 5-9 GHz. Miniaturized size and an acceptable directional characteristic of the antenna make it possible to use it in the microwave imaging systems and radar applications.