This paper describes the thermal vacuum (TVAC) through conductance test and model correlation results performed on the protoflight Mobile Satellite (MSAT®) L-Band Transmit (Tx) feed assembly. Very good correlation was obtained between the model and the measured results. Specifically, the correlated average temperatures for the feed array components, were all within +/- 5°C of the measured test results for all three (3) plateaus. Similar results were also obtained at 10 feed panel locations for each component. In addition, the differences in temperature gradients through the feed assembly from the cup walls to the Beam Forming Network (BFN), comparing the modelled and measured results, were less than 3°C for all three plateaus.The correlation results for the Tx feed assembly were subsequently used to update the spacecraft (S/C) L-Band antenna model. The updated S/C model temperature results for both the Tx and receive (Rx) feed assemblies were found to be 2°C to 3° C more benign that the original S/C model results. Therefore, the through conductance testing and correlation results demonstrate the validity of using the mathematical model developed for the S/C L-Band antenna thermal analysis.