Wireless Microwave Biosensors: The Effect of Sample Size and Sample Location on Sensitivity

In this paper, a wireless microwave biosensor is used to investigate the effect of changing sensing samples permittivity, size and location on the sensitivity of the sensor. The sensor is based on a horn antenna designed to operate as a wireless sensor for biomedical applications in the X-band (8 GHz-12 GHz). The sensor characterizes a true remote wireless sensing system where the sample is located at a distance from the microwave sensor. The permittivity of the sensing sample is first changed, for a fixed sample size and location, to investigate the change in the reflection coefficient (S11), as a sensing parameter, versus the change in the permittivity of the sample. Next, the location of the sensing sample is changed, while the size kept fixed, and the S11 magnitude is recorded. It is found that the sensitivity of the sensor decreases with the increase in the distance between horn antenna and sample location. However, it is shown that the sensitivity has non-linear behavior at locations close to the sensor; whereas it becomes approximately linear at the far locations $(> 4\lambda)$ . The highest sensitivity achieved is 181 $\mathrm{x}\ 10^{-3}\text{dB}/\mathrm{F}/\mathrm{m}$ achieved at $\mathrm{D}=\lambda$ from the sensor for a sample size of 1.5 mm.