VHF/UHF Low Profile Vertically Polarized Airborne Antenna

To reduce the impact of airborne antennas on the aerodynamic characteristics of aircraft,it is necessary to conduct research on miniaturization with a low profile of airborne antennas,especially for VHF/UHF vertically polarized airborne antennas.In addition,traditional antenna forms that achieve vertical polarization are mostly electrically small antennas,which limit the electrical performance of airborne antennas.However,in the VHF frequency band,the aircraft platform has large-sized structures with good resonant characteristics,which can be fully utilized to improve the electrical performance of antennas.Against this background,this thesis proposes three airborne VHF/UHF antennas suitable for different scenarios.Firstly,a VHF/UHF low-profile vertical polarization airborne antenna based on cylindrical cavity theory is proposed.The TM₀₁ and TM₀₂modes of the cylindrical cavity were utilized to achieve vertically polarized omnidirectional radiation patterns in the horizontal plane.A tapered sleeve was introduced to improve the input impedance and effectively reduce the resonant frequency of the TM₀₁ and TM₀₂ modes,enabling the resonant frequencies of both modes to work in the target frequency range.Four-state broadband matching networks were designed to match the impedance of the antenna.The cylindrical cavity antenna was fabricated and tested,and the simulation results were consistent with the measured results.The measured results showed that the cylindrical cavity antenna achieved impedance bandwidths of 47.3%and 52.3%in the low and high-frequency bands,respectively,and achieved a stable vertically polarized omnidirectional radiation pattern in the horizontal plane.Next,a VHF tri-polarized multi-antenna system for airborne applications based on characteristic mode theory is proposed.Firstly,the characteristic mode of the vertical tail is excited using an L-shaped slot antenna to achieve vertically polarized lateral radiation patterns.Then,the characteristic modes of the aircraft platform are utilized to achieve dual-polarized bidirectional radiation patterns,with the main lobes covering the upper and lower areas of the aircraft platform simultaneously.Specifically,the target radiation pattern is achieved by linearly weighting the characteristic modes of the aircraft platform and optimizing the complex weights of each mode.The synthesized mode currents under the dual-polarization scenario are discussed and analyzed to determine the installation positions of the coupling excitation elements.The PIFA antenna is used to excite the mode currents of the aircraft platform,and the corresponding power divider network is designed to achieve dual-polarized bidirectional radiation patterns.Finally,a vertically polarized bidirectional radiating VHF aircraft antenna based on characteristic mode theory is proposed.The vertical polarization radiation pattern is achieved by utilizing the vertical current component in the aircraft platform’s lateral area,providing a feasible solution for realizing vertical polarization on aircraft platforms without vertical tail wings.Characteristic modes that contain the vertical current component in the aircraft platform’s lateral area were selected from the characteristic modes of aircraft platforms without the vertical tail.By linearly weighting them,a vertically polarized bidirectional radiation pattern is achieved,with the synthesized mode current mainly distributed in the aircraft platform’s lateral area.A PIFA antenna is selected as the coupling excitation element,and the feeding amplitude and phase for each antenna are obtained by linearly weighting and optimizing the active element pattern of each antenna.An unequal power divider network is designed to provide the required amplitude and phase for each antenna,achieving a vertically polarized bidirectional radiation pattern.