A Feasibility Study of RF Time-Domain Reflectometry as a Railgun Armature Tracking Technique

Since past investigations have shown that radio frequency (RF) signal transmission in the bore of a railgun can be practical, the feasibility of using time domain reflectometry and well-established transmission line theory is investigated as a means to determine armature position during launch. This report describes the transmission line model of the HEMCL and initial measurements suggesting that such an approach might be used to provide dynamic armature position profiles. However, subsequent continuous wave analyses of RF transmission line measurements shows that the electrical transmission line characteristics of the HEMCL railgun make it unfeasible to use RF time-domain reflectometry to characterize its dynamic behavior. The laminated containment structure enclosing copper rails of the HEMCL form a very lossy, poorly matched transmission line making it impractical to couple the RF signal into it; high RF attenuation diminishes that portion of the signal that it is coupled. Measurements showed that signals below 900 MHz are completely reflected at the 50-ohm/HEMCL-muzzle interface. A fraction of RF signals at higher frequencies (900–2400 MHz) can be coupled, but will attenuate by 20 dB or more after propagating only a few tens of centimeters.