Against the backdrop of electromagnetic space integration, the radio system of equipment platforms, such as next-generation aircraft, must possess multifunctional integration and electromagnetic stealth performance. Meanwhile, the equipment platforms need to evolve towards flat structures. These requirements pose significant technical challenges for antenna system design. The antenna must possess ultra-wideband to facilitate multi-function integration through the use of continuous radio frequency synthetic aperture. In order to ensure good aerodynamics of the flat airborne platform, it is required to implement conformal design, while the ultra-low profile is the greatest challenge in conformal design. Against this background, this work proposes a novel airborne tightly coupled antenna with ultra-low profile, ultra-wideband, and vertical-polarized omnidirectional radiation. The antenna unit utilizes a long slot structure and implements circular conformal design, where the resistive frequency selection surface is used to expand the operating bandwidth. This antenna has a profile height of only 0.047 times the low-frequency wavelength. Simulation and measurement results show that it achieves an impedance bandwidth of nearly 12∶1 with omnidirectional beam coverage, which meets the requirements of multifunctional future airborne antennas.