Design, fabrication and testing of pressurized co-axial directional coupler for high RF power measurements for SST-1 ICRH system

Directional couplers are used to monitor forward and reflected high RF powers. At Mega Hertz range of frequencies, normally loop type directional couplers are used. The loop samples electric or magnetic fields. The penetration and orientation of the loops are adjusted in such a way that the magnitude of electric and magnetic current is same in the two arms of the loop. In this configuration one arm of the loop shows incident power while the other arm gives reflected power. Secondary arm is terminated into 50 ohm in order to avoid multiple reflections. The rectangular coupling loop used in the developed directional coupler is having the dimensions of 46 mm times 19.5 mm (W X H). It is made up of copper. The directional coupler developed can be pressurized up to 3.0 bar absolute pressure. A specific Teflon cup design is used to pressurize the directional coupler. The size of the directional coupler is 6 1/8" EIA standard. The inner and outer conductors are made from 99.5% ETP copper. Flanges are made from 60:40 electronic grade brass. Inner conductor joints are of brass with silver-plating. The return loss obtained is better than 30 dB. The coupling factor is variable from 40 dB to 57 dB depending upon probe penetration.