A constant-gain printed circular monopole antenna (PCMA) is presented that operates from 4 to 12 GHz. The gain of the PCMA is enhanced with the use of surface-wave excited, proximity-coupled (SW-PC), conductor-backed, twin-slot radiators. A significant improvement in 2 and 3 dB peak gain bandwidth in the XY plane (θ = 90°) is achieved, and the peak measured gain of the antenna is nearly constant (5.5 dBi ± 1.5 dB) from 3 to 10 GHz. It has a very broad radiation pattern in the azimuth plane and a typical figure of eight pattern in the elevation plane. The antenna is also compact, robust, and easy to fabricate.