Wideband and dual-polarized Twin Hilbert curve antennas for capsule endoscopy

In this study, we design a pair of third-order conformal Hilbert curve antennas for use in capsule endoscopy at three different bands (Industrial, Scientific, and Medical [ISM]), especially at 433 MHz, 915 MHz, and 2.45 GHz. The antennas are placed orthogonally with respect to each other, offering dual linear polarization characteristics, which have been highlighted in literature as the most robust communication link between a roaming transmitting capsule within the GI tract and the receiver antenna. The robustness of the link depends on (i) invariance of linear polarization with the position and orientation of the roaming transmitter, as well as (ii) tolerance of linearly polarized fields to depolarization effects when penetrated through the body. The antennas were manufactured on polyimide substrate, which was wrapped around a 3D-printed capsule volume. Both antennas were measured inside bovine muscle tissue and the obtained results have been compared with the simulated results to validate the performance of the propose solution. The presented results showed that, the return loss characteristics and the measured bandwidth, for both antennas inside bovine muscle tissue, have matched with the simulated results. To the best of our knowledge, these demonstrated antennas are the first designed capsule antennas to address three ISM bands with dual linear polarization. The designed antennas can be adapted for clinical use with further development.