Design of a wideband wearable antenna for head microwave sensing applications

In recent years microwave technology is gaining attention in several fields thanks to its important benefits. The employment of microwave sensing in medical applications relies on the fact that microwave signals are sensitive to the electrical properties of tissues, such as permittivity and conductivity, different in healthy and pathological cases. This sensitivity enables the differentiation between healthy and diseased tissues by irradiating them via electromagnetic fields and analysing the different back-scattered signals. Hence, this technology can aid in the detection and characterization of abnormal tissues, such as tumours and strokes and can differentiate healthy and unhealthy cerebrospinal fluid that can be an evidence of the Alzheimer’s disease onset. These are pathological situations where there is dielectric contrast between the healthy tissues and the pathology at microwaves, and in which the variations of the field are footprinted in both the reflection and transmission scattering parameters. An early detection of the mentioned pathological conditions is of paramount importance in healthcare as it plays a crucial role in improving outcomes, as prognosis and survival rates by allowing the management of chronic conditions and the prevention from complications by providing a baseline for long-term diseases progression monitoring. Microwave technology is, in this sense, a promising solution. In fact, against other common detection technologies, it is low-cost, portable, almost unlimitedly accessible and affordable. Besides, it is non-invasive and allows real-time monitoring. Among the Microwave technology instrumentation, a crucial role is played by the antennas which are the sensors that transmit and receive signals to and from the target that is a body district in case of biomedical applications. In this Thesis, the target body district is the head and so the required antennas need to be quite flexible, wearable and compact since the application area is small and more than one sensor need to be applied on. Moreover, to be more versatile among different head possible diseases, the device is asked to show good performance in a wide frequency band. Thus, the most demanding challenge that has to be faced has been the trade-off between the need of compact size and the obtainment of a wide operational frequency range that is not trivial when the sensor works near the head tissues. In the proposed design, the antenna structure is basically made of two components that are the radiating element, the metallic component responsible for the transmission and the reception of the signals, and the substrate, a dielectric layer that acts as antenna physical support. For the on-body application a matching medium is then needed to improve the matching at the air-skin interface because of the non-flat body, or head, surface. Since all of the three components influence antenna performances, working on the design of an antenna means to find the best shape of the radiating element, the best substrate and the best matching layer in terms of thickness and dielectric properties. The optimization of the latter parameters is, hence, carried out via several parametric simulations. The final optimum design of this Thesis, numerically validated, is a compact wearable Z-shaped ultrawideband monopole antenna printed on the substrate Rogers Duroid RT5880® characterized by a -10 dB bandwidth from 1.3 GHz to 4.7 GHz.