The experimental and theoretical results of coaxial diode and the theoretical results of coaxial vircator are presented in this paper. The cathode is a cold, field- emitting graphite ring and needle-shaped copper applied to a grounded cylinder. The anode is a semi-transparent cylinder located inside of, and concentric to the cathode cylinder. The anode cylinder is pulsed positive. The coaxial vircator generates microwave by injecting a radial electron beam into cylinder such that the space-charge limited current is exceeded. A virtual cathode forms and oscillates in radial position and amplitude, generating microwaves which are extracted by an attached waveguide with a circular cross- section. Analytic and PIC simulations were used to study coaxial diode and vircator, with aid of the two dimensional PIC code, KARAT. The comparisons between the theoretical and the experimental results for a coaxial diode are presented.