Dear David, LF Group,
Having poked around inside quite a few radios over the years, I think
465/470kHz IFs were common in the days of discrete transistors and tuned IF
transformers, but are mostly ancient history now. Most of the more modern
broadcast radios use either 455kHz or 450kHz - the former being the default
centre frequency for ceramic filters, the latter especially for radios with
PLL synthesisers (it works out quite nicely with a 4.500MHz reference
crystal and 9kHz or 10kHz channel spacings). At least in theory, the more
modern cheap broadcast radios should have better IF rejection, since these
single-chip receivers have double balanced mixers, rather than the old
single-ended, self-oscillating, discrete designs.
The trouble with trying to avoid broadcast RX IF frequencies is that there
is now a wide range to be avoided. With f0 possibly of 450, 455, 465 or
470kHz, +/- tolerance, and a bandwidth typically around 9kHz, a big chunk of
frequencies from say 440 - 480kHz would have to be excluded on these
grounds. It would be a great waste of spectrum to say these frequencies must
be off-limits for all eternity for the sake of the inadequacies of cheap
receivers used by an ever-decreasing number of listeners. Anyway, these
frequencies always have been used by high power maritime telegraphy, NDBs,
etc.
Another consideration is that the proposed maritime "super-Navtex" that
might be centerd on 500kHz will inevitably have big powerful transmitters,
using OFDM modulation with several kHz bandwidth. Although the out-of band
emissions for this type of modulation is in principle very small, there will
inevitably be some "spectral re-growth" due to imperfections in transmitter
linearity. The high power levels compared to amateur signals, and wide
coverage areas will mean this would be a major problem for amateurs if we
have to operate on adjacent frequencies (not unlike the problems with
utilities on 136k). There would be no way of convincing the maritime users
they should take remedial action - they would just reply that their
transmitters were fully within spec, which they would be. So if the maritime
proposals go ahead, a large spacing between any amateur band and 500kHz
would be a good thing. I volunteer to help struggling experienced
non-appliance operators to wind a few more turns on their VFO coils if this
happens ;-)
Cheers, Jim Moritz
73 de M0BMU
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