  In this paper, we review and compare HPM sources operating without a magnetic
field to guide the electron beam that are capable of producing high-power
microwave (HPM) pulses with a duration of about 100 ns. The proposed analysis
summarizes multi-year research carried with three types of HPM sources: a
split-cavity oscillator (SCO); an axial vircator; and a virtual cathode
oscillator in Reflex Triode geometry. These options were simulated for electron
beam energy $\sim$400 keV and for pulsers with demonstrated capability to
provide high power microwave pulses with the required pulse duration. Designed
sources were experimentally tested, and their advantages and weaknesses are
discussed with respect to high output power, long pulse duration, and good
operating stability.
