[Hallicrafters] Re: Which Models had the 50 KC IF?

[GARDGORE at aol.com]

455 KC is a compromise frequency chosen to give the best combination of 
selectivity and image rejection capability in the IF strip for single 
conversion receivers. It was also favored because it was below the tuning 
range of most receivers. The engineers wanted a frequency that would be low 
enough to yield fairly good selectivity but yet high enough to still provide 
decent image rejection on the higher frequencies. Usually if band conditions 
were not too bad you could get by with this scheme up to about 14 megacycles. 
The 455 kc IF worked out great for standard broadcast radios but its 
limitations were soon apparent for shortwave work in the crowded amateur 
bands and in trying to tune in to frequencies above about 14 megacycles. For 
reasons the engineers call "percentage of frequency difference" a lower 
frequency of say 50 kc is even better for selectivity but is a poor choice 
for image rejection for the same reason. This is why the engineers invented 
the dual conversion scheme. Dual conversion was first introduced briefly in 
the early Thirties by the Sargent Radio Company of Oakland, CA but wasn't 
really popular until around 1950 when Hallicrafters offered it in their 
SX-71. It works by converting the incoming signal to a lower frequency (50 kc 
in the SX-88) to get good selectivity and converting again to a much higher 
frequency of around 2075 kc (also in the SX-88) to get good image rejection. 
The SX-88 uses 1550 instead of 2075 as a second conversion frequency for band 
two because 2075 falls within its coverage range on that band. The S-76, 
SX-88, SX-96, SX-100, SX-101, SX-111, SX-115, SX-117 & SX-122 are several 
that come to mind that use the 50 kc IF. The S-76 was the first to use 50 kc 
in a dual conversion receiver in 1951. Interestingly enough though, the 
single conversion S-53 pioneered the use of 2075 IF in 1948 probably as a 
pilot model to get some experience with that frequency. All of this is why 
many inexpensive low and mid-priced receivers seem to be so hot, especially 
on the first two bands. You're really hearing lots of interfering signals all 
at once in a crowded space. It's not what you hear that counts, any receiver 
with modern high gain tubes can do that, it's what you don't hear that 
matters the most. Reminds me of a nice old National FB-7 I have around here. 
It doesn't really matter which coils you plug in. There are so many images 
and harmonics of images that you can find the same signals on any of the 
coils if you patiently hunt around the dial. Not to throw down on the FB-7 
because it marks an important place in history in the beginning of the 
communications receiver industry when hams began to buy their receivers 
rather than build them. I suppose you could say the 1932 Hammarlund Comet 
really started it all though.

Greg Gore
Charlotte, NC