In this paper, a novel ultra-wideband (UWB) antenna with a planar single-layer structure is proposed. The antenna consists of a main circular patch that is capacitively coupled to six circular patches of very small size relative to the main patch. The coupling is achieved through narrow gaps of semicircular shape which are uniformly distributed on the circumference of the main patch. A coplanar waveguide (CPW) is used for feeding the antenna to get the complete antenna structure with the feeding line printed on one face of a flexible dielectric substrate. The antenna is fabricated and subjected to experimental assessment of its performance regarding the bandwidth, gain, and radiation efficiency. The measurements show good agreement with the simulation results. It is shown that the proposed antenna operates efficiently over the frequency band of 3.1-10.6 GHz. The antenna has a radiation efficiency that ranges from 99% to 100% over the entire band. This high efficiency is attributed to the planar single-layer structure of the antenna and the use of a thin low-loss substrate. The antenna maximum gain ranges from 2 dBi to 5 dBi over the entire frequency band. The substrate material is Rogers RO3003 TM which is flexible and can be conformal to planar and curved surfaces. The total substrate dimensions are 35 × 39.4 × 0.5 mm.