What you are descrbing is a "signal sniffer", not a signal strength meter.

On Fri, 12 Dec 2003 16:41:06 +0000, Paul Burridge
wrote:

Hi all,

I wanted to build an RF relative field strength meter, so set about
searching on the Web for any existing designs. Those I turned up
weren't particularly impressive, so I decided to start from scratch
and design my own. I've just completed that this afternoon. I've
allowed for 0.25mV input to give rise to FSD on the microammeter.
Question being, however, is that going to be sensitive enough? Does
anyone have any idea what the field strength in microvolts or
millivolts is from a half Watt transmitter at about 6 feet away? I
guess I should have posed this question *before* designing it, but who
among us can honestly say they haven't designed something without
knowing what the spec is? :-)
Anyway, ballpark figures gentlemen, please.

p.
--

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


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

Here is a crystal set circuit.
http://circuitos.tripod.cl/schem/r85.gif
Convert it to a field strength meter by
Replacing the headphones with a large sensitive meter,
something big enough that you can still see the pointer,
when viewed from across the room. Bigger the better.
Use a germanium diode.
Use a metal box.
Use a short telescopic aerial.
Coil and variable capacitor to cover, 40Mhz,. 35Mhz and 27Mhz,
I am guessing those are the frequencies of interest, use
a switch if necessary to add/remove some turns or
add/remove a capacitor.

I know you have a grid dip oscillator so fiddling the
coil and capacitor values to get the frequency ranges
will be a snack for you.

Using your field strength meter only six feet away is too
close. Keep it as far away as possible from your radio
control transmitter but still being able to see it, that is
the reason for the big meter movement. I am thinking
of big cheap plastic 6 inch square types. Anything will
do so long as you can see it from across the room and
the movement is microamps full scale and not milliamps.

I am sure I have explained this to you yonks ago, well,
if I have, never mind. maybe you forgot :-)

Set up your known good commercial radio control set
at one end of the room, and field strength meter at the
other side of the room. Note the meter reading. Now
compare readings with your experimental transmitter.
Is it more or less? Make adjustments to your experimental
transmitter. This is the fun part.

I found that sitting my field strength meter (even though it
had rubber feet) on my wife's metal serving tray reduced
hand capacitance and made it nicer to adjust. So experiment
with and without a sheet metal base. Maybe the first circuit
without a parallel tuned circuit would be less fuss to use.
Maybe you can knock up both types and tell us which
was better. Heh heh heh....

Regards,
John Crighton
Sydney

On Fri, 12 Dec 2003 16:41:06 +0000, Paul Burridge
wrote:

Hi all,

I wanted to build an RF relative field strength meter, so set about
searching on the Web for any existing designs. Those I turned up
weren't particularly impressive, so I decided to start from scratch
and design my own. I've just completed that this afternoon. I've
allowed for 0.25mV input to give rise to FSD on the microammeter.
Question being, however, is that going to be sensitive enough? Does
anyone have any idea what the field strength in microvolts or
millivolts is from a half Watt transmitter at about 6 feet away? I
guess I should have posed this question *before* designing it, but who
among us can honestly say they haven't designed something without
knowing what the spec is? :-)
Anyway, ballpark figures gentlemen, please.

p.
--

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


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

Here is a crystal set circuit.
http://circuitos.tripod.cl/schem/r85.gif
Convert it to a field strength meter by
Replacing the headphones with a large sensitive meter,
something big enough that you can still see the pointer,
when viewed from across the room. Bigger the better.
Use a germanium diode.
Use a metal box.
Use a short telescopic aerial.
Coil and variable capacitor to cover, 40Mhz,. 35Mhz and 27Mhz,
I am guessing those are the frequencies of interest, use
a switch if necessary to add/remove some turns or
add/remove a capacitor.

I know you have a grid dip oscillator so fiddling the
coil and capacitor values to get the frequency ranges
will be a snack for you.

Using your field strength meter only six feet away is too
close. Keep it as far away as possible from your radio
control transmitter but still being able to see it, that is
the reason for the big meter movement. I am thinking
of big cheap plastic 6 inch square types. Anything will
do so long as you can see it from across the room and
the movement is microamps full scale and not milliamps.

I am sure I have explained this to you yonks ago, well,
if I have, never mind. maybe you forgot :-)

Set up your known good commercial radio control set
at one end of the room, and field strength meter at the
other side of the room. Note the meter reading. Now
compare readings with your experimental transmitter.
Is it more or less? Make adjustments to your experimental
transmitter. This is the fun part.

I found that sitting my field strength meter (even though it
had rubber feet) on my wife's metal serving tray reduced
hand capacitance and made it nicer to adjust. So experiment
with and without a sheet metal base. Maybe the first circuit
without a parallel tuned circuit would be less fuss to use.
Maybe you can knock up both types and tell us which
was better. Heh heh heh....

Regards,
John Crighton
Sydney

On 13 Dec 2003 03:50:41 -0800, said...
What you are descrbing is a "signal sniffer", not a signal strength meter.

Who are you replying to? Paul did not say Signal Stength Meter, but
the guy you replied to kinda hints at it when he mentions power
level.

I would have to say that my 1GHz Signal Level Meter, which cost
$1500 would be the better than a relative field strength meter, but
if he measures say, 1V with his Tx off and 2V with it on, then
that's 1V and if his Rx antenna is a 1m dipole, that's 1V/m.

Mike

On 13 Dec 2003 03:50:41 -0800, said...
What you are descrbing is a "signal sniffer", not a signal strength meter.

Who are you replying to? Paul did not say Signal Stength Meter, but
the guy you replied to kinda hints at it when he mentions power
level.

I would have to say that my 1GHz Signal Level Meter, which cost
$1500 would be the better than a relative field strength meter, but
if he measures say, 1V with his Tx off and 2V with it on, then
that's 1V and if his Rx antenna is a 1m dipole, that's 1V/m.

Mike

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

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

On Fri, 12 Dec 2003 16:41:06 +0000, Paul Burridge
wrote:

Hi all,

I wanted to build an RF relative field strength meter, so set about
searching on the Web for any existing designs. Those I turned up
weren't particularly impressive, so I decided to start from scratch
and design my own.
PA0SE made a good one for 136kHz and higher, believe you'll find it
described on http://www.qsl.net/on7yd/ and many other places

73
Jan-Martin, LA8AK
http://home.online.no/~la8ak/c.htm
--
remove ,xnd to reply (Spam precaution!)

On Fri, 12 Dec 2003 16:41:06 +0000, Paul Burridge
wrote:

Hi all,

I wanted to build an RF relative field strength meter, so set about
searching on the Web for any existing designs. Those I turned up
weren't particularly impressive, so I decided to start from scratch
and design my own.
PA0SE made a good one for 136kHz and higher, believe you'll find it
described on http://www.qsl.net/on7yd/ and many other places

73
Jan-Martin, LA8AK
http://home.online.no/~la8ak/c.htm
--
remove ,xnd to reply (Spam precaution!)

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?