Miniaturized Dual-band Circularly Polarized Full-duplex Antenna in VHF Band with Improved Radiation Efficiency and Isolation

This paper proposes a super-compact full-duplex antenna with dual-frequency operation in the VHF band to integrate transmitting and receiving functions and opposite circular polarization characteristics to enhance anti-interference capability. The transmitting and receiving antennas are in a basic form of quadruple inverted-F antennas (QIFA) with operating frequencies of 236.5 MHz and 281.5 MHz, respectively. The coupled shorted strip (CSS) is introduced in the receiving antenna to reduce the footprint and improve radiation efficiency by enhancing the co-directional currents and weakening the reverse ones. Meanwhile, the radiating arm of the transmitting antenna is folded horizontally to counteract the coupling current for isolation enhancement. The measured results demonstrate that the transmitting antenna can achieve a gain of -4.9 dBic with a left-hand circular polarization (LHCP) while the receiving antenna can realize a gain of -6.5 dBic with a right-hand circular polarization (RHCP). The isolations between the transmitting and receiving ports are better than 25 and 27 dB, respectively, in the two operating bands by introducing a super compact π-shaped low-pass circuit. The total size of the proposed antenna is only 120 × 120 × 20.1 mm³ (0.095λ × 0.095λ × 0.095λ for 236.5 MHz), resulting in a remarkable gain-to-volume ratio of 2458.