[Boatanchors] Why 455?

[Bob Macklin]

Not all IFs are 455. But 455 was the most common.

There were some that were 465. These were not just retuned 455s. They were
manufactured as 465.

There were also 265KC IFs used in auto radios.

But I believe that the choice of 455KC was because Miller produced them in
very high quantities for a very low price. Up to the late 70's you could
still by the Miller transformers over the counter at most Radio/Electronics
supply dealers.

Also the BCB variable capacitors were made in large quantity to use with a
455KC IF.

Bob Macklin
K5MYJ
Seattle, Wa,
"Real Radios Glow in the Dark"


----- Original Message -----
From: "Al Klase" <al at ar88.net>
To: "Boatanchors" <boatanchors at theporch.com>; "Boatanchors List"
<boatanchors at mailman.qth.net>; "New Jersey Antique Radio Club"
<njarc at mailman.qth.net>; "Delaware Valley Historic Radio Club"
<dvhrc at mailman.qth.net>
Sent: Tuesday, January 29, 2008 10:44 AM
Subject: [Boatanchors] Why 455?


> Every so often the question comes up: Why are all the IF’s 455 KHz? I’d
> like to get an article together that solves this riddle while the people
> who know are still with us. I know parts of the story, but I need help
> with a couple of issues.
>
> There are two major consideration is the choice of the intermediate
> frequency used in a superheterodyne receiver. The lower the frequency,
> the easier it is to attain high selectivity. Also, in the early days,
> before tetrode and pentode tubes, it was easier to achieve a high degree
> of amplification at lower frequencies. Conversely, a higher IF frequency
> results in better image rejection.
>
> Early superhets had the IF at 100KHz or lower in order to get adequate
> gain from the available triode tubes. They suffer severely from
> “two-spot tuning” (images). By the early 1930’s, broadcast set had
> settled in at 175KHz, and automobile receivers would later adopt 262KHz
> as a standard.
>
> The advent of the short-wave craze, and multi-band broadcast receivers
> dictated a higher IF frequency to achieve adequate image suppression on
> the short-wave bands. The broadcast band occupied 550-1500KHz at this
> time, and the designer encounters sever problems if his radio tunes
> across it’s own IF. Some shortwave sets used 1600-1700KHz for better
> image rejection, but one couldn’t go higher if the 160-meter ham band
> (1800-2000KHZ) was to be covered. Most multi-band receiver settled in
> near 450KHz, a comfortable distance from the first broadcast channel at
> 550KHz.
>
> Questions:
>
> Odd multiples of 5KHz, 455, 465, etc., were usually chosen so that the
> image of the carrier of a broadcast-band station could be zero-beat with
> the carrier of the station being tuned to achieve minimal interference.
> (This assumes 10KHz channel spacing. Did the Europeans (9KHz) do
> something else?)
>
> The Radiotron Designers Handbook, Third Edition, p. 159, states “A
> frequency of 455 Kc/s is receiving universal acceptance as a standard
> frequency, and efforts are being made to maintain this frequency free
> from radio interference.”
>
> (1) Do FCC and international frequency allocations reflect this?
>
> (2) I’ve heard the term “Clear-Channel IF.” Can anyone cite references?
>
> (3) At lease one news group posting claims that broadcast frequencies in
> a particular market are assigned to prevent strong inter-modulation
> products from falling near 455KHz. Is this factual? Need reference.”
>
> (4) Was this (3) at least part of the reason for “Radio Moving Day” in
> 1941? See:
http://www.dcmemories.com/RadioMovingDay/032341WINXFreqChange.jpg
>
> (5) Many National Radio sets used a 456KHz IF’s and I think I remember a
> 437 somewhere. Why? Are there different considerations for short-wave CW
> operation?
>
> Further input, corrections, and elaborations are greatly appreciated.
> Scolarly reference will be looked upon with great favor.
>
> Regards,
> Al
>
> --
> Al Klase - N3FRQ
> Flemington, NJ
> http://www.skywaves.ar88.net/
>
> _______________________________________________
>