Novel wideband multi-layer Archimedean spiral antenna with integrated coupler for radiometric measurement in medical applications

A wide bandwidth, body contact, directive beam, circularly polarized, and single arm Archimedean spiral antenna with an integrated coupler is proposed for radiometric temperature measurement inside a human body at S-band. An innovative integration of two functions in one configuration is done in a straightforward design. The coupler consists of 50 Ω grounded coplanar waveguide placed under the active region of the spiral antenna. After determination of the effective dielectric constant of the multilayered structure, the spiral structure is designed by adjusting the position of the active region at the operation frequency of 3.5 GHz. Two different simulation models are built using 6 g/ℓ of saline water solution to verify simulated results of S-parameters and the radiometric efficiency with measured ones. The prototype of this configuration shows very low return loss of -20 dB, a flat coupling factor of 21 dB and high isolation factor of 27 dB in the frequency range of 3 to 4 GHz. By using a low cost multilayered phantom of human tissues, the radiometric efficiency of the prototype is estimated from the simulations and verified by SAR measurements. An observed object of 6 mm diameter assumed inside the multilayered phantom in a depth of 1 cm is used to verify the radiometric efficiency. The estimated efficiency is around 1.2% at the operation frequency.