wrote:

hi,

I'm experimenting with my superhet. My IF is 8mhz (cause I have 8 mhz
crystals for IF filter). I first experimented on 40m. VFO was 15mhz. At
design time I'm using 3 meters long wire as an "antenna" (and I'm
sitting in my basement). I heard different european stations (EA5FKX,
RW6XE, UR4GI etc) which are several thousand km-s away. Pretty amazing
for such "antenna"!

So I decided to go for 20m band. I had an option to set my VFO either
to 6mhz or 22mhz. I first went with 6mhz. But for my disappointment I
heard nothing but the band noise.

So I'm confused now - is it because the short "antenna" or am I wrong
thinking that VFO 6mhz + IF 8mhz should give me 14mhz?

any ideas?

What everyone else said, plus:

Yes, a mixer should work on both sum and difference frequencies, so an
LO that's lower than the IF is just fine for receiving a good strong
signal.

You leave yourself open to more spurious reception problems. When folks
are talking about 'spurs' and 'mixer charts' they mean you need to watch
out for funky combinations of harmonics of LO and RF signals mixing down
to your IF -- having the LO frequency higher than the RF frequency tends
to reduce this problem. You can do this by hand by assuming that you'll
be mixing 6, 12, 18, 24, etc., with 14, 28, 42, etc., and seeing if any
of the combinations boils down to an IF of 8. If they do you could have
spurious mixing problems, particularly if they're not high harmonics.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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"Applied Control Theory for Embedded Systems" came out in April.
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