The project was developed after a requirement of Creo Medical Ltd. was identified to obtain accurate relative permittivity characteristics for biological samples in order for their device development to be more efficient, impedance mismatches to be minimised, with less prototyping required. A device was created which allows for materials with a relative permittivity of up to 80 with a minimal frequency range of 6.5GHz. Materials with a permittivity of up to 50 were accurately analysed with a frequency range of 0.5GHz to 20GHz. An algorithm was developed to extract the permittivity of the sample using the Nicholson Ross Weir model. The model is shown in this paper to be successful for simulated s-parameters for the device but not for measured s-parameters.
This device is used alongside a modified algorithm to evaluate the impedance of
dielectric materials. Currently, there is no design available to Creo Medical Ltd.
which can be manufactured quickly and cheaply, and so the method of choice for
many design engineers is to utilise software packages such as CST or HFSS to
simulate relying on existing material databases and estimations, such as IFAC[1].
Open-ended coaxial devices have been used extensively for this in previous research and whilst this design was considered for this project, there was concern about the accuracy of data measured using open-ended coaxial devices due to any fringing fields which can couple to surrounding objects. The closed arrangement utilised in this project with a cavity for a sample to be inserted removes the possibility of such inaccuracies.
This leads to the rationale behind this project and why there is a need for such
a device. The databases providing dielectric information do not always provide
specific enough information for all materials in all conditions, unhealthy or cancerous biological tissue for example, and so having a device which is able to provide this information for any sample is very useful, especially when this knowledge is required to create an accurate design to reduce impedance mismatches and losses in power transmission.