# Field Strength

Discussion in 'Electronic Design' started by Paul Burridge, Dec 12, 2003.

1. ### Paul BurridgeGuest

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?

p.

2. ### Rene TschaggelarGuest

Paul,
antenna type ? loop type or dipole type ?

Rene

3. ### Scott StephensGuest

I'll SWAG & hope I'll be corrected if I tell ya wrong. Assuming a 1/2
wave antenna on the xmit & rcv, take the volts/meter you're applying on
the antenna, divide by 4 PI / (distance)^2, with distance being in
wavelengths.

Then there is the famous propogation equations which involves 32,
recieve and transmit antenna gains, and the log of the distance and
frequency. You can then go from power to voltage according to the
antenna Z. I'll look up the equation for you if you don't get a better

--
Scott

**********************************

DIY Piezo-Gyro, PCB Drill Bot & More Soon!

http://home.comcast.net/~scottxs/

**********************************

4. ### Active8Guest

E = sqrt(30PG)/d Volts/meter - Field Strength

P = Tx power
G = Tx antenna gain - and don't forget the gain of the meter's
dipole.

S = PG / (4.pi.R^2) Watts/meter^2 - Power density

R = distance

S = E^2/377 (E field^2)/(Z of free space in the far field)

Where the far-field starts is antenna dependant, but it is
acceptaed the the boundary is where the inverse-square-law for
power density above becomes invalid, that is, as you approach the
antenna, the rate of change of S decreases and S is no longer
inverse-square-law dependant.

For large aperature antennas (dishes, dipoles, etc.) this seems to
work out to

R = 2L^2/lambda

lambda is wavelength and L is length of antenna

HTH,
Mike

5. ### Michael A. TerrellGuest

Your web site is very hard to read with the dark blue background and
black text. A lot of people have vision problems, and can not read this
color combination.
--
13 days!

Michael A. Terrell
Central Florida

6. ### GPGGuest

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

7. ### John CrightonGuest

Hello Paul,
have a look here,

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

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
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

at one end of the room, and field strength meter at the
other side of the room. Note the meter reading. Now
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
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

8. ### Active8Guest

On 13 Dec 2003 03:50:41 -0800, said...
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

9. ### Paul BurridgeGuest

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?)
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?

10. ### J M NoedingGuest

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

11. ### Active8Guest

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.
clues. so you're still on that project.
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 fucking 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

12. ### Mike AndrewsGuest

On my system (FreeBSD) with the Mozilla FireBird browser, the text
and images are in light-colored windows inside the dark background,
and it's not at all difficult to read. The Netscape 4.6 browser on
the same system _does_ put the black text directly on the dark-blue
background, and it is decidedly unpleasant.

A newer browser might be nice, but it also would be good if web page
designers built pages with older code in mind.

It _definitely_ has some cool stuff. Thanks, Scott!

13. ### Roy LewallenGuest

It's too bad it isn't that simple.

A 1V/m field doesn't result in one volt at the feedpoint of a perfectly
matched one meter dipole or monopole, and the value it does induce
depends on the quality of the impedance match as well as the fraction of
a wavelength the one meter antenna length represents. And, if one volt
does appear at the feedpoint, it's very unlikely that a simple circuit
will measure it as one volt.

Probably best to stick with your \$1.5 kilobuck meter if you really want
to measure field strength.

Roy Lewallen, W7EL

14. ### Active8Guest

you mean antenna to free space, right?
amplify, very please. por favor. Refresh my ram.

| E(uV/m) |
V (dBmV) = 20log | --------- / 1000 |
| 0.021f(MHz) |

plus correction for distance (regulations for limits are for
specific measuring distances), etc. I'll mull the above eq over.
Gotta figure out where the .021 came from, but not now. My eyes are
getting fatigued from this 'puter.
It would have to be calibrated to compensate for the circuit. Maybe
that's why it's called a "relative" field strength meter. Relative
to another signal or no signal
I don't. He does At least not tonight. But my SLM *will* measure
field strenth using a cheap ass dipole cut to the frequency of
interest with or without an external preamp and do it to the
satisfaction of the FCC, assuming it's calibrated. I even have a
near-field probe, not so cheap. It beats guess work.

I wouldn't expect his sniffer to be real accurate but he did ask
for guesstimates. Started off as "around 4 feet" for a half watter
now we're at 100mW - prob his reference Tx.

BRs,
Mike

15. ### Michael A. TerrellGuest

I have several other browsers, but I prefer using Netscape 4.79. Some
websites are a royal pain. I recently ran into an electronics
distributor who put their entire website in "Flash". There is no way I
will wait five minutes or more per page to download and run stupid
animation when I am looking for parts. I have seen a bunch of sites with
a white background and a very pale yellow text.

My website isn't perfect, but I but a lot of work into making it easy
to use and I asked for, and used, feedback from members of a couple
newsgroups. You can see it at:
http://home.earthlink.net/~mike.terrell/ I still have a lot of work to
do to the site, but a website is never really finished, is it?
--
11 days!

Michael A. Terrell
Central Florida

16. ### Paul BurridgeGuest

Well, it's *related* to "that" project, yes.
Yes! I'm only interested in *relative* field strength. That's why I
asked for a "ballpark figure" to be plucked from the air. I think you
may be thinking of some fancy type of instrument like some guy
mentioned he had that cost 1500 bux. These ham-type jobs I'm
interested in cost just pennies to make as they don't need any
absolute standard of accuracy; only a relative indication. You tweak
the meter's sensitivity control to show antenna A of the TX giving
rise to say S5 on the meter. You then change to antenna B and see if
the reading is any higher or lower. It's really as simple as that.
Noted, thanks.

17. ### Active8Guest

'twas me talking to someone else.
Yup. gave you the eqs to figure out your ballpark figure. I figured
a ballpark figure might work if you gould figure out how much
reference transmitter i might have suggested checking your RFSM
reading at one location with the ref and another closer location
with the test Tx, identical readings indicating sucess or close to
it.

18. ### Paul BurridgeGuest

Just curious, but would carrying out comparative measurements at say
only 6 to 10 feet give rise to invalid readings??

19. ### Active8Guest

Lots of things can foul you up.

I think I estimated the near field/far-field at 3.7m so I wouldn't
do it. IIRC the change in field strengh with respect to distance in
one of the zones of the near field varies inversely with the cube
of the distance, as opposed to the square as it does in the far-
field.

BTW, I've seen other far-field eqs where the antenna diameter (not
aperature size) is used and I haven't bothered looking into the
origin, but I'm just trying to impress upon you the fact that
nothing's written in stone.

The 6m I gave as an example might not be all that great. These
short distances you mention have me thinking you're locked in a
dungeon somewhere so I tried to keep it reasonable. A sniffer might
be ok at close range and an active RFSM would be good at greater
distances.

Personally, since you've got a reference Tx, I'd take the whole
deal outside (they do let you out, don't they) and see what happens
with greater distances. Even a gym or auditorium would work if
you're concerned with how well it works indoors.

I remember the 40MHz part, but I'm not sure what yer up to. Just
curious.

-mike

20. ### Paul BurridgeGuest

You once referred to me as the "battlebots guy" - although I post on
many other aspects of electronic design as well, so that's not
entirely accurate. I trust this jogs your memory.