A quasioptical image terminating diplexer may be placed before a double-sideband receiver to reduce the SSB system noise temperature. We present a detailed analysis that includes losses in the diplexer which indicates that the improvement is not as great as previously assumed. To achieve any worthwhile improvement the losses in the diplexer have to be kept very small (less than 10%) and the receiver noise temperature also has to be low. For a receiver DSB noise temperature of ~5hv/k the sensitivity improvement may be only ~10% for moderate losses and atmospheric opacity. The same improvement can be achieved for a DSB receiver by reducing the noise temperature from 5hv/k to 4.2hv/k. A Dual-polarization DSB receiver is found to be a very competitive option.
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