"Count Floyd" CountFloyd@MonsterChillerHorrorTheater wrote in message
news:g40vCXBzNU8x-pn2-Q82zQcWLtrec@localhost...

Damn, you would think in 2006 that single-conversion radios were not
even being made! I have an old S-38 and an HE-10, both single
conversions, but they were made in the '40's and '50's! They still
pull in great DX on the BCB though and the tubes keep my room warm in
the winter!



Why shouldn't single conversion receivers still be made? They still work
well enough for people to buy them.

The only real problem with single conversion is image rejection, and older
higher quality single conversion radios obtained increased image rejection
with additional tuned RF circuits. Hallicrafter's top of the line radio for
1947, the $275.00 SX-42, was single coversion (tuned antenna and two tuned
RF stages) with an IF of 455 kc through 30 Mc. The second in line, the dual
conversion (on six meters) SX-43 (tuned antenna and one tuned RF stage),
cost $169.50. The S-38 (tuned antenna stage only) cost only $47.50. As a
comparision in complexity, the SX-42 used a 10 wafer bandswitch, as long as
the radio, to switch the oscillator and RF coils in and out. The S-38 used
a stubby 2 wafer bandswitch.

Here's an article about the restoration of a SX-42, with a particular
emphasis on the bandswitch:

http://antiqueradio.org/halli07.htm


For the same image rejection at high frequencies, a high IF converted to a
lower IF will be cheaper than that lower IF with multiple tuned RF stages.
However, conversion stages are noisier and potentially less linear than
amplifier stages. I suspect Hallicrafters went with the more expensive
single conversion scheme in order to sell an amateur radio with low noise,
low distortion high fidelity ability all the way from the AM broadcast band,
through SW, low VHF to the FM broadcast band. Hallicrafters soon replaced
the 42 with more specialized amateur receivers, and reskinned it as the
SX-62 (still expensive!) single conversion SWL radio. The SX-62 was around
'till 1965.

If anyone ever manages to design an inexpensive narrow bandwidth filter with
an excellent shape factor at 45 Mhz or so, we can pretty much say goodby to
the multiple conversion radio. Skip all those troublesome conversion
stages! Exept there might still be a few radios with extra conversion
stages. If so, there will surely be somebody saying "I can't believe
they're still using that old 20th century technology!!" Of course, the
manufacturer might be doing it to distribute gain at different frequencies
in order to avoid oscillation. But that's whole 'nother bit of radio design
trivia.

Frank Dresser