[ARC5] Some info on the S-38

[Fuqua, Bill L]

  If you have an antenna sufficient to hear atmospheric noise on HF that is the best you can  do other than improving the effective noise bandwidth of the receiver.
That is why the application of low noise figure devices are not common or necessary in HF receivers. The atmospheric noise becomes the limiting factor. 
Microwave systems using frequency division multiplex systems used many pole filters not due to noise issues but for isolation between adjacent voice channel rejection.
You can still find very good mechanical filters used in these systems from time to time. It is just that they are at odd frequencies for amateur use and are relatively wide
in bandwidth. 
73
Bill wa4lav