A novel synthesis design approach to multimode resonant, azimuthal null frequency scanning antennas (ANFSAs) coordinated by null-depth is presented. The antenna is composed of a principal radiator and an auxiliary one fed by different ports. A synthesis chart gauged by different null-depths is developed to determine the respective exciting amplitude and phase conditions for the principal and auxiliary radiators. As is theoretically demonstrated and numerically verified, the ANFSA can exhibit an adjustable null-depth varying from −12∼−30 dB. A three-port unequal-split power divider with the simplest configuration is properly designed, utilised and integrated to meet the oriented power and phase conditions. A prototype with in-band null-depth of −30 dB is designed, fabricated and validated in such a way that its available bandwidth can reach up to 32.7%, and the null can continuously scan from φ = 160°–275° in azimuth plane.