Pattern-Reconfigurable Loop–Dipole Antenna for Electromagnetic Pleural Effusion Detection

Electromagnetic pleural effusion detection requires wideband and pattern-reconfigurable antennas to scan the chest area. Thus, a wideband and low-profile unidirectional loop-dipole antenna with planar structure that is capable of scanning in the azimuth plane is proposed. The antenna consists of a one-wavelength loop, a modified half-wavelength bow-tie dipole, and two parasitic directors to achieve wide operating bandwidth and high front-to-back ratio (FBR). The dipole antenna is directly fed using coaxial cable, whereas the loop is excited using offset proximity coupling. The unidirectional radiation pattern is achieved by quasi-Yagi mode at lower frequencies and a combination of loop and dipole modes at higher frequencies. The parasitic directors are utilized to alter the current distribution on the loop and, thus, enable beam switching in different directions. To verify the proposed concept, two p-i-n diodes are used to change the current nodes on the loop and, hence, switch the radiation pattern from 200 to 00 and +200 in the azimuth plane. The proposed antenna, which has the compact size of 0.32λ
₀
× 0.2λ
₀
× 0.004λ
₀
(where λ
₀
is wavelength at the lowest operating frequency), has a wide fractional bandwidth of 55% at 0.8-1.4 GHz, which is suitable for pleural effusion detection. The antenna has a peak gain and an FBR of 5 dBi and 14 dB, respectively.