RFI measurements at UHF on a pulsed plasma thruster

Abstract

E Pulsed Plasma Thrusters (PPT) were flown aboard the Lincoln Experimental Satellite LES-6 to provide the thrust necessary for a stationkeeping experiment. The thrusters were manufactured by FairchildHiller, Republic Division, and provided an approximate 6 jidb-sec impulse from each discharge of 1.85 joules of electrical energy. A Radio-frequency interference (RFI) specification was drawn up along with a measurement procedure. The RFI specification, measurement procedure, and measured results are the subjects of this Note. The thruster nozzle and teflon propellent subsystem are described in Ref. 1. Thrust from the PPT was provided by the acceleration of gaseous ionized teflon through a nozzle (horn). The igniter plug initiated a conductive discharge between the anode and cathode which vaporized and ionized some of the teflon fuel element. The ionized particles were accelerated by two mechanisms, one being the gas pressure difference between the ionized particles and freespace and the other due to magnetic forces set up by the conduction current. The process is inherently noisy and thus arises the concern over RFI effects on the rf transponder aboard. The rf transponder operated at an assigned uplink frequency in the neighborhood of 300 MHz with a 100 kHz bandwidth. The receiver contained a bandpass limiting IF amplifier that had approximately 60 db of dynamic range and limited on front end input noise. Receive signals equal in power to the total threshold noise power would capture approximately one-half of the downlink transmit power.