Microfabricated field emitter arrays are being used in an ongoing DARPA/NRL program as a means for gating or prebunching electrons in a microwave amplifier tube. The goals of the program are to demonstrate 10 dB gain at 50 W and 10 GHz in a gated Klystrode amplifier tube with 50% efficiency. The proposed cathode specifications call for 160 mA peak emission and 10 GHz emission modulation from an annular emitter array having a 600 mu m outer diameter and an inner diameter to be determined by transconductance and capacitance requirements. Experimental results have shown an average array capacitance of 6 nF/cm(2), and that a transconductance of 1 mu S/tip can be achieved at emitter-tip loadings of 10 mu A/tip. Calculations based on these results show that emitter arrays having 0.4 mu m diam gate apertures on 1 mu m centers, a 600 mu m outer diameter, and a 560 mu m inner diameter should meet the tube specifications. Such arrays have been fabricated and shown to have essentially the same characteristics (Fowler/Nordheim coefficients) as the test cathodes used to develop the design parameters. These microwave cathodes have been successfully modulated at 10 GHz rates in an experimental Klystrode amplifier tube at CPI (formerly the Varian Associates Microwave Power Tube Division), and microwave output power has been achieved. Ongoing trials are showing steady pro, toward the program goals.