[NJARC] Why 455?

[John Ruccolo]

Hi Jim,

Your theory is very interesting, but that has nothing
to do with it. The same engineer who developed the 455
KC IF also developed the 455 cu. in. V8 for
Oldsmobile. He had a fixation on the number 455.

I thought you knew that....

JR


--- Jim Whartenby <antqradio at sbcglobal.net> wrote:

> Visit our web site - See http://www.njarc.org
> _______________________________________________
> Al
> Just playing with the calculator and using
> approximate frequencies for
> the old AM band limits such as 500kcs and 1500kcs
> (600 to 200 meters)
> you get a geometric mean of 866kcs.  Divide this by
> 2 gives 433kcs.  So
> depending on what the frequencies actually were for
> the old AM band,
> you end up quite close to 455kcs.
> Just a thought,
> Jim
> 
> --- Al Klase <al at ar88.net> wrote:
> 
> > Visit our web site - See http://www.njarc.org
> > _______________________________________________
> > 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/
> > 
> > _______________________________________________
> > NJARC mailing list
> > NJARC at mailman.qth.net
> > http://mailman.qth.net/mailman/listinfo/njarc
> > 
> 
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