An improved gridded microwave tube is provided, the tubecontaining a cold cathode, an anode, and a grid located between theanode and cathode. In one embodiment, the cold cathode has arefractory metal substrate and carbon nanotube emitters, theemitters having a diameter of 1 to 300 nm and a length of 0.05 to100 µm. The grid-cathode spacing is 1 to 100 µm, the grid containsapertures having a maximum dimension of 0.5 to 100 µm, and thegrid thickness is 0.5 to 100 µm. Emission from the cathode directlyonto the grid material itself, which undesirably heats the grid, isreduced by either (a) the presence of a shadow mask between thegrid and the emitters or (b) selective formation of the emitters inlocations that correspond to the grid apertures. The microwave tubeoperates at a frequency of greater than 0.5 GHz, advantageouslygreater than 2 GHz.
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