that, once developed, the coupler circuit could be integrated with the FETs and other passive circuitry into a workable MMIC. Couplers were fabricated on cleaned and etched semi-insulating GaAs (100) subttrates by using the evapFration and lift-off technique. 300 A of titanium and lo00 A of gold were deposited by evaporation. Finally, gold electroplating was done to increase the gold thickness to 3 p n , the required gold thickness for MMICs.
EXPERIMENTAL RESULTSMeasurement on the 6-, lo-, and 15-dB couplers, illustrated in figure 1 have been taken using a Wiltron network analyzer. Figures 2,3, and 4 show the coupling, insertion loss, isolation, and input return loss of the 6-, lo-, and 15-dB couplers, respectively. The performance of the couplers introducing tapered sections are shown by the dotted curves.
DISCUSSIONExperimental results show that the coupling remains almost same around the center frequency irrespective of whether the tapered sections are there or not. Comparison of the figures reveal that the isolation of the coupler increases considerably if the tapered sections are introduced at all the four ports. Around the center frequency, the input return loss is greater than 15 dB.
CONCLUSIONThis paper demonstrates the realization and performance predictability of directional couplers, with and without tapered sections, for monolithic microwave integrated circuits on GaAs. Fabrication of these couplers is compatible with the present MMIC technologies. Measured results also show that the isolation characteristics of the couplers improve if tapered sections are introduced at all the four ports of the couplers.
ACKNOWLEDGMENTThanks are due to D.T.S.R., New Delhi, for financing the work. the probe (e.g., liquids, fine pow&rs, etc.).
REFERENCES K. Wilson, "Monolithic Microwave Integrated Circuits," GEC.
ABSTRAfl
In this paper, the method of measurement of dielectric constant by wing open-ended coaxial lines with a hemispherical center conductor extension is examined in great detail. Calculation results show that this method can considerably improve the sensitivity of measurement while retaining the advantages of a simple mechanical construction. Such a probe is conveniently used with any material that can intimately embrace the hemispherical tip of