[Hallicrafters] SX-28, Collins R-390A, and Ten Tec RX-340

[GARDGORE at aol.com]

Practically any communications receiver from the Thirties up will hear well 
provided you have connected it to a fairly good antenna. Sensitivity refers 
to the weakest signal which can be satisfactorily received by the receiver. A 
very weak signal needs a lot of amplification to make it audible. There are 
limits to the amplification process because of noise. Noise, the hissing or 
rushing sound when no station is selected, can be the noise generated by the 
electronics used to amplify the weak signal or it can be the noise picked up 
by the antenna attached to the receiver.
In a receiver design, the engineer can always lower the internal noise of the 
receiver to such a low level that the lowest signal that can be tuned in and 
detected is limited by the external noise. One simple way to check if a radio 
receiver amplification is good enough is to unplug the antenna. Listen to the 
audio output. When the antenna is reattached, there should be a rise in the 
noise output of the receiver if no station is received. If there is no 
increase in output, you can assume that the receiver is not sensitive enough. 
There is a misconception about the importance of sensitivity, however. It's 
not what you hear that's important but rather what you don't hear that makes 
all the difference. For example take the case of a typical low cost receiver 
such as the rather inexpensive and simple little Hallicrafters S-38. 
Practically anyone who has ever used one that is working properly is usually 
pretty impressed with the little receivers hot performance especially on the 
lower bands. The truth of the matter is it seems so hot because the passband 
is so wide and the shape factor so poor that many times you are really 
hearing three signals at the same time. This problem exists in all low priced 
sets and to a certain extent in many manufacturers mid priced models too. The 
quality of a good receiver that should rank higher in importance is 
selectivity. Selectivity means the ability of the receiver to pick out a 
required signal out of a jumble of many others. On the shortwave band, it is 
a common experience to tune to a busy radio band and find many stations all 
seemingly crowded together at the same frequency. Only a receiver with high 
selectivity will enable one to pick out one signal above the others. Another 
important quality is stability, especially for CW and SSB reception. I would 
say that the degree of selectivity and stability have a greater impact on a 
receivers price than anything else. 

Regards, Greg Gore