Resonance frequency and Ohmic Q factor in klystron cavities

Including a coaxial circular insert separated by a gap spacing from the cavity end, reentrant cavities with dimensions conveniently small can operate at large wavelengths, thereby finding direct application in klystron cavities for low frequencies. It is illustrated that a reentrant cavity 8.0 cm in diameter, loaded with a coaxial insert of radius 2.0 cm and a 2.0 mm gap spacing, resonates at a 30.0 cm wavelength (1.0 GHz).