On Sat, 13 Dec 2003 11:58:39 GMT, (John Crighton)
wrote:

Hello Paul,
have a look here,
http://www.alphalink.com.au/~parkerp/noapr97.htm

Hi John,

Always nice to have your input. I did actually come across the circuit
you point to above during my search of the Web, but rejected it as
probably not being sensitive enough. I thought I could maybe do a
little better by having a stab at it myself - with the assistance of
LTSpice of course!
Many thanks to the other respondents to this thread, but no one seems
to have been able to SWAG the actual likely signal level in mV or uV.
So I thought what the hell and built my original design from
yesterday. I've just finished it and am surprised and pleased to
report that it worked fine first time! The sensitivity is a little on
the low side, but *remarkably* close to what I'd set out to achieve.
With a 100mW transmitter some 4 feet away, I can tune for 40Mhz and
tweak the sensitivity and get a peak at S9 on the CB radio type signal
meter I'm using for this purpose. That would have done me just fine
had you not suggested making the measurements from some greater
distance! So I either live with it as it is and use is at say 6 feet
away or stick in an extra voltage amplification stage for 'far field'
testing (do I *really* need this for my purposes?)
Incidentally, you were dead right about the tea-tray idea. It turns
out the grounded areas of the PCB really need to be earthed to a
decent, proper ground/earth rod via the mains supply. It makes a
*huge* difference to the sensitivity and drastically reduces the
annoying effects of hand capacitance when tuning and tweaking.
I'll post the schematic I arrived at later under another thread and
maybe someone can suggest a few mods that might up the sens. without a
complete redesign. Is it really that important to make the
measurements from 10 feet or more away?
--

"I expect history will be kind to me, since I intend to write it."
- Winston Churchill