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Tom · December 21st 06, 04:46 AM posted to rec.radio.shortwave

wrote:
Btw, I was wrong about Kaito 1103 tuning to AM 1040 when dialing 140 -
it is, as I understand, really 140, i.e. Long Wave channel of the same
station.

No, it's not the LW channel - there are no LW broadcasters in North
America. Rather it is an unwanted effect of the superheterodyne
receiver: a signal can be shifted into the intermediate frequency stage
at more than one setting of the tuning control. The DE1103 uses a 2nd
IF of 450kHz. When the radio is tuned to 1040 kHz, the local oscillator
is effectively operating at 1490 kHz to produce a mixer difference of
450 kHz. However, a LO frequency of 590 kHz also will produce the IF -
that corresponds to a tuning of 140 kHz, known as the image frequency
of 1040 kHz. Better superhet radios have sufficient selectivity ahead
of the mixer that the 1040 kHz energy and consequent image signal are
very weak and almost undetectable.

Tom