We study field emission characteristics of an allmetal double-gate single nanotip emitter to explore the feasibility of such emitters for applications that require extremely high beam brightness and coherence. The single-tip device showed an excellent beam collimation characteristic including an order of magnitude reduction of the transverse velocity spread and an order of magnitude enhancement of beam intensity as reported with array devices previously. The evolution of the beam image with the increase of the collimation potential indicated the importance of subnanometer corrugation at the nanotip apex surface.