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Field Strength

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

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  1. 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.

  2. Paul,
    could you give us some more information, such as frequency and
    antenna type ? loop type or dipole type ?

  3. 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

    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



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

  4. Active8

    Active8 Guest

    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

    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

  5. 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. GPG

    GPG Guest

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

  7. Hello Paul,
    have a look here,

    Here is a crystal set circuit.
    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....

    John Crighton
  8. Active8

    Active8 Guest

    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

    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.

  9. 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?
  10. J M Noeding

    J M Noeding Guest

    PA0SE made a good one for 136kHz and higher, believe you'll find it
    described on and many other places

    Jan-Martin, LA8AK
  11. Active8

    Active8 Guest

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

    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.


    Buy 'em books, send 'em to school, and all they want to do is eat
    the teachers ;)
  12. Mike Andrews

    Mike Andrews Guest

    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 Lewallen

    Roy Lewallen Guest

    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. Active8

    Active8 Guest

    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.

  15. 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: 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. 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. Active8

    Active8 Guest

    '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
    voltage appeared at your RFSM antenna. had I known you had a
    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
  18. Just curious, but would carrying out comparative measurements at say
    only 6 to 10 feet give rise to invalid readings??
  19. Active8

    Active8 Guest

    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-

    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

    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

  20. 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. :)
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