On Sat, 13 Dec 2003 20:50:47 +0000,
said...
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.

Because you didn't provide enough info, dude. That much could've
been inferred from the eqs I gave you. And you asked for "field
strength" in the wrong units. it's V/m or mV/m or uV/m. Hell I'd
give it to you in kV/m if you wanted. You'll never know what it is
if you can't determine the voltage present at the meter antenna's
terminals and it should be a dipole.

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

clues. so you're still on that project.

tweak the sensitivity and get a peak at S9 on the CB radio type signal
meter I'm using for this purpose.

! Adjust the meter to get the answer you want. Ok.

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?)

lessee 40MHz is 7.5m lambda so the far field starts at around 3.75
meter per the eq I gave you for that.

I think that 1st eq I gave was for a vertical and it's ****ing with
me.

I'll use a distance of 6m.

since you seem to want to know power try the path loss eq

32.45 + 20log(f) + 20log(d) = 20db

f in MHz, d in km

10log(Pr/Pt) = -20dB

but EIRP = Pt.G

G is antenna gain and you didn't give that info.

so I'll use G = 1

Pr = 1mW

Pr.G = E^2/Z voltage at input to Rx

G is Rx antenna gain, I'll use 1

E = 224 mV rms

Let me know if I f'd up anything. Lots of distractions and I'm
trying to hurry and do other work.

Mike

Buy 'em books, send 'em to school, and all they want to do is eat
the teachers




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?