Frequency and

A compact passive hydrogen maser with many unique features, including a significant reduction in size over previous hydrogen masers, is des-cribed. It uses the passive mode of operation, thereby permitting use of a small microwave TE cavity which is dielectrically loaded by a low-loss alumina (Alz03). The cavity is 1 4. 6 cm O.D. and 1 3. 7 cm high, weighing only 4. 4 kg. The unloaded cavity Q factor is about 6000. With a conventionai source, hexapole state selector, and 4 magnetic shields the volume of the er?tire H-maser resonator package is only about 20 11. 011 The teflon coated quartz bulb which is commo~~ in other masers has been replaced by a teflon coating on the inside wall of the cavity. This has yielded a simpler design and more rugged H-maser package. The technique for the application of liquid emulsion FEP 120 is discussed. The cavity and attached endcaps comprise the vacuum envelope, thus allowing use of a single vacuum system. The dimensional Stability of the ceramic cavity under barometric changes is suffi-ciently within the range of the electronic cavity servo that a second vacuum system is not needed. For temperature control, a single oven is located in the magnetic shield nest. The electronics for this small passive hydro-gen maser is very similar to that previously deve-loped at NBS. Preliminary measurements on a proto-type small maser system yield a frequency stability of approximately q ( t) = 6. 6 x X- ’ to at least one day, with a measurement bandwidth of 1 kHz