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Old January 1st 07, 11:26 PM posted to rec.radio.shortwave
Stephan Grossklass Stephan Grossklass is offline
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First recorded activity by RadioBanter: Jul 2006
Posts: 42
Default What does it mean when you get an "image" on shortwave?

dead of night schrieb:

Thanks for the help, Stephan, very imformative. So if it's an image, on
SSB the vocals will go up as I tune up?


That should happen, though I don't think the vocals are a good guidance
when the image comes from an AM station (as they're always present in
both upper and lower sideband) - I'd rather rely on the carrier. If it's
an SSB station causing it, you'd also observe that the image is on the
other sideband. (So if you find a utility station below 10 MHz in LSB,
check whether it's an image.)
BTW, depending on how well-aligned your rx is, a slight shift of the
carrier can be enough to discern an image from an AM station in SSB.
Many stations are very accurately on freq, so if you've set SSB fine
tuning to give zero beating on a real ("known good") station, you may
find an offset of a few 10 or 100 Hz (i.e. hum/whine) with an image if
the 2nd IF is not exactly 455 kHz. And chances are that this is the
case, since it is neither possible to align a receiver like yours so
precisely nor would it make much sense (frequencies tend to be affected
by crystal aging and temperature changes anyway, the IF filter isn't
super narrow either, and lastly readout is to 1 kHz only).

BTW, to receive anything real at all on 120m I think it takes (in that
order) a very quiet location, a good (typically large) antenna in the
right direction, a "real" rx not affected by intermod down there, and
last but not least some patience. (Finding some hams on 160 might be
more doable, at least I have heard some in the past, even with the 7600G
and some weird antenna contraption that somehow kept the intermod away
yet provided more signal. The antenna tuning of the whip tends to be
lousy down there, so you can pretty much forget about using that.) These
low ranges are quite special, and I guess the best kind of indoor
antenna would be a large tunable active AM loop adapted for about 50%
higher frequencies. Then the only thing missing is the quiet location,
due to the selective antenna the 7600GR would be entirely sufficient
receiver wise, though I'd prefer something with tone controls like my
trusty Redsun. (This assumes no kind of heavy interference requiring any
advanced bells and whistles, of course.)

Hmm, maybe I'll be dragging out the '7030 now...

Stephan
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