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A surface acoustic wave (SAW) hydrogen sulfide (H S ) gas detector has been designed, fabricated an3 tested. The detector consists of twin SAW delay lines fabricated on a single LiNbO substrate. Each delay line i s confi uredas an i s coated with a sputfered f i l m of activated tungsten trioxide (WO$ while the other i s uncoated and used as a stable reference. Phase dela changes due to the mass loadin stress and elecrric field effects induced by the 82s gas sorbed on the WO3 film result in corre ponding frequency shifts which are directly proportional to the as concentration. Gas detector properties time and shape of the response curve gignature). The detector was found t o be quite stable and s stematic responses to concentrations less than 18 parts per billion were observed.oscillator. The propa ation path o 9 one delay line stu 2 led include selectivity, sensitivit , response

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The interaction of an electromagnetic wave with a time varying medium is governed by the property of conservation of the wavenumber. This property can be utilized to construct a frequency transformer. The time variation in the relative permittivity of the medium is easily produced in a magnetoplasma medium by altering the ionization level. The change of the ionization level can be produced by the dynamic control of the source of ionization. The switching-on action of the source produces a build-up plasma, whereas a switching-off action produces a decaying plasma. Simple mathematical models, however, for the two cases are different. In the case of building-up plasma, the model has to ensure the continuity of the current density and in the case of decaying plasma, the average drift velocity of the surviving electrons has to be continuous. Using these two appropriate models, the effect of the time variation of the plasma frequency as well as the background magnetic field of the magnetoplasma medium are studied. Switching off the external magnetic field for the wave in the magnetoplasma medium converts a whistler wave to a wiggler magnetic field. Similar results are obtained with an intensification of the source wave as well as frequency upshift of the whistler wave for the collapsing plasma medium. By controlling the static magnetic field parameters and the level of ionization, the frequency shift ratio can be changed and the frequency transformer becomes tunable. The principle of frequency up-shifting using plasma permits the generation of signals from easily obtainable frequency bands and upshifts them into frequency bands not easily obtainable by other methods.