High Power Combline Filter for Deep Space Applications

An S-band, compact, high power filter, for use in the Mars Orbiter Mission (MOM) of Indian Space Research Organization (ISRO), has been designed and tested for multipaction. The telemetry, tracking, and commanding (TT&C) transponder of MOM is required to handle continuous RF power of 200？W in the telemetry path besides simultaneously maintaining an isolation of greater than 145？dBc to its sensitive telecommand path. This is accomplished with the help of a complex diplexer, requiring high power, high rejection transmit path filter, and a low power receive path filter. To reduce the complexity in the multipaction-free design and testing, the transmit path filter of the diplexer is split into a low rejection filter integral to the diplexer and an external high rejection filter. This paper highlights the design and space qualification phases of this high rejection filter. Multipaction test results with 6？dB margin are also presented. Major concerns of this filter design are isolation, insertion loss, and multipaction. Mission performance of the on-board filter is normal. 1. Introduction Telemetry, tracking, and commanding (TT&C) transponder in a spacecraft meant for deep space mission requires a high power (100？s of watts) transmitter, a high sensitive (~135？dBm) receiver, and respective high gain antenna systems. Since both the transmitter and receiver require similar antenna systems, it will be highly taxing in terms of on-board weight and volume, if we use two antennas independently. It is economical to use a common antenna system for both uplink and downlink, with the help of a diplexer [1]. Diplexer is a passive component that connects the common antenna feed, simultaneously, to both transmitter and receiver with proper isolation. It consists of a high power transmit filter, a receive filter, and a combining network as shown in Figure 1. Figure 1: TT&C system configuration of a spacecraft. Multipactor [2] is an electron resonance phenomenon which occurs at radio frequencies in high power components like filters and resonators and transmission lines operating in vacuum. It represents a possible payload failure mechanism for communications satellites since it can destroy microwave components or transmission lines, or it can significantly raise noise levels [3]. Multipactor effect has been known for many years; it still presents a critical problem and more constraints in satellite communication system applications in terms of transmit power, number of carriers, and wider bandwidth. Multipactor occurs whenever the electrons are sufficiently energized by