New approach for design and verification of a wideband Archimedean spiral antenna for radiometric measurement in biomedical applications

A wideband, body contact, directive beam, circularly polarized, single arm, Archimedean spiral antenna for radiometric temperature measurement inside a human body in S-band and an SAR (specific absorption rate) measurement method using a thermal camera are described in this paper. A new approach of designing the spiral antenna based on using the estimated effective permittivity of a multilayered microstrip line to derive the dimensions of the spiral antenna is proposed. The purpose of using the effective permittivity is to set the position of the active region at the operating frequency (3.5 GHz), which achieves the highest radiometric efficiency. Two simulation models are developed to verify the reflection coefficient (S₁₁) and SAR distribution. The reflection coefficient (S₁₁) of the fabricated antenna is less than -15 dB in the frequency range of 1-4 GHz. A new multilayered, low cost phantom is purposed to be used in SAR measurement. Four layers of sponge soaked with (6 g/ℓ) saline solution are used as a load for measuring SAR distribution of the fabricated antenna. The radiometric efficiency has been estimated from simulations and verified by SAR measurements. By assuming a target of 1 cm diameter, placed 1 cm away from the antenna within a human tissue compatible environment, the highest efficiency is 1.4% at the operating frequency.