I have a question.
I really don't understand well, the workings of the tuner section of a radio, so I'm not sure my question is even at all important.
But anyways here goes :
What is the significance of the changes to intermediate frequencies during the 30s?
My research reveals it started out at 460kHz, then went to 470 kHz in 1937, . . . stayed there for a few years. Then it dropped to 455kHz in 1940, where it stayed for the remainder of AM broadcast history.
Started out down below 100khz went to around 175khz early on.
There is a lot to it, like images and stuff (station at multiple locations on the dial). For this one you are probably going to need a book.
Ok...I have one here....guess it's time to crack it. Thanks.
The early 30's Philco superhets used 175 and 260 kc. Philco also use several tuned radio frequency (trf) circuits too, these don't require any IF circuitry. Most of these are from the late 20's and early 30's.
I read somewhere the lower frequency transfer energy better, though I doubt 455 vs 470 is going to be noticable.
Most all car radios used 262.5Kc.
Theory says that the lower frequencies allow for tighter selectivity. I think for that reason some communications receivers have double conversion - a high first IF frequency to improve image rejection, then a low IF where bandwidth can be well controlled.
In early sets, low frequencies probably made it easier to derive lots of IF gain with reasonable stability.
Another aspect to the choice of frequency is the potential for interference caused by the local oscillator. If the IF is 460 kHz, then a set tuned to 1000 kHz will have the oscillator running on the 1460 kHz channel. The move to 455 kHz put the LO from a correctly tuned and aligned radio inbetween stations - at least in the USA where channels are 10 kHz apart. In UK and europe, the channels are separated by 9 kHz. I'd have to do some arithmetic to see how that worked out.
.. and then there were the RCA "catacomb" sets. Those used a 40 KHz IF for the early superhet designs, and it made tuning in any given station a true challenge. Receiving the same strong station on multiple dial positions wasn't/isn't unusual... it's a fact of life. And those two dials didn't help, either... one for the antenna, one for the LO.