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900MHz yagi safe?

Discussion in 'Electronic Design' started by Mark Powers, Jan 24, 2006.

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  1. Mark Powers

    Mark Powers Guest

    One of my employees is questioning the safety of our new 900MHz
    broadband antenna (horizontal 10 segment yagi) located on the roof
    above his table.

    Our ISP recently replaced the 2.4GHz system with these.

    Apparently, the output is now 1W instead of the previous 100mW and it
    is not as directional or prone to shielding by metal surfaces.

    Can anyone with experience in this area suggest what might be the
    maximum dBi at 12 feet from the antenna through a corrugated steel
    roof?

    And would this generally be considered safe for continuous exposure?

    Many thanks,

    Mark
     
  2. Mac

    Mac Guest

    Are you the same person who asked a very similar question a while back?
    Was that with the 2.4 GHz system which has been replaced?

    --Mac
     
  3. Ken Smith

    Ken Smith Guest

    How far above the roof in the yagi? It can make quite a difference to the
    answer.

    In this sort of situation, you are best off measuring what in there in the
    real world.

    Get lots of expensive looking gear and someone in a white lab coat to say
    "that's less than the background, no problem here".
     
  4. Jim Thompson

    Jim Thompson Guest

    Makes a more accurate homing beacon for the black helicopters ;-)

    ...Jim Thompson
     
  5. Leon

    Leon Guest

    A friend of mine who used to live in a village outside Cambridge always
    had USAF A-10 aircraft targeting his house, for some reason. Perhaps it
    looked like a tank from the air. :cool:

    Leon
     
  6. Guest

    Does the roof leak?

    I think a large part of the answer would depend on how well the roof
    forms a continous conductive plane.

    Then you'd look at any way substantial power could be leaking off the
    feedline, or around the edges of the roof. Got that yagi pointed at an
    inside corner of an adjacent metal building that would make a nice
    corner reflector to send it back in through a window at eye level?

    It would seem like most of these paths are going to have large losses
    compared to the input power - but if you need to be sure, the only way
    to do so is to have someone qualified take measurements.
     
  7. Jim Thompson

    Jim Thompson Guest

    A Nike base in NH, for practice, used to target commercial aircraft
    flying over the other Cambridge on their approach to Logan Airport ;-)

    ...Jim Thompson
     
  8. Rich Grise

    Rich Grise Guest

    You have a 10-element Yagi, with one watt of feed power, on top of
    a steel roof?

    I'd say the risk is neglibile. You probably won't even pick it up
    on a FSM, at least from inside the building.

    Cheers!
    Rich
     
  9. Rich Grise

    Rich Grise Guest

    Let's put this in perspective. When I was in the USAF, I worked with
    radar jamming transmitters. One model was a "pod", which mounts to an
    airplane wing much like a bomb or external fuel tank. It had its own
    antenna, and when we fixed one, we ran it in the shop, transmitting,
    and the techs would check to see if it was radiating by feeling the
    radiation with their hand. It feels kind of warm. That was significantly
    more than a watt, and no one seems to have had any ill effects. Admittedly,
    it was only for a few seconds, but, as has been mentioned, with 1
    watt, to a Yagi, over the top of a steel roof, your risk will be below
    background.

    Cheers!
    Rich
     
  10. Ken Taylor

    Ken Taylor Guest

    There is a distinct difference between 'occupational' exposure (which tends
    to be short-term by people who know what they're doing and will take
    precautions if there's 'something wrong here') and 'environmental' exposure,
    such as the OP is talking about. That's why the various national standards
    set out two differing acceptable levels of exposure. Having said that, it
    sounds like there's not going to be a problem in the OP's case, as you say.

    But the OP sounds like he's asking as an employer, checking up on what to do
    about a concerned employee. To the OP - get a qualified fucking contractor
    to check it! Why ask for freebie advice on the internet, for Christ's sake??

    A/ You'll get advice worth what you pay for in either case; and
    B/ If he gets sick and sues, how can you know it *wasn't* dangerous if you
    haven't had it checked?

    And as a follow-up to B, if it is above the levels proscribed in the
    standards (I forget the US standard off-hand), do something about it. It's a
    pretty easy equation.

    Ken
     
  11. Ian Stirling

    Ian Stirling Guest

    Point out that 900Mhz is a frequency in common use in cellphones.

    These all have about a watt or so max output, and are often used clamped
    to the head.
     
  12. Since you seem to be fine, I guess it's ok! ;-)

    Frank
     
  13. Mark

    Mark Guest

    1 Watt out of an isotropic antenna at 12 feet = a field strength of
    about 1.5 Volts/meter or power density of 0.0005 mW/cm^2

    if the yagi is aimed away, it will be less than isotropic.
    the metal roof will also reduce the field strength

    The legal limit in the US is listed as SAR and MPE, see:

    http://www.fcc.gov/oet/rfsafety/rf-faqs.html
    and

    http://www.fcc.gov/Bureaus/Engineering_Technology/Documents/bulletins/oet56/oet56e4.pdf

    For 900 MHz MPE is about 0.6 mW / cm^2.

    So your case is at least 1000 times below the legal limit

    Someone check my figures please.

    Mark
     
  14. Chris Jones

    Chris Jones Guest

    None will pass through the roof, but some tiny amount might go around and
    get in through the windows. To demonstrate how little, get him an analogue
    meter movement (with a rating of 1mA or less full scale) with a fast
    Schottky diode (e.g. HP5082-2835 etc.) connected across the meter movement
    with wires about 5cm long. This will detect RF and show the relative
    strength, (though it will not be very flat vs. frequency).

    Now let him hold this thing near a working mobile phone (cellphone) and near
    a working microwave oven, and in his normal working position.

    This might help him to put the exposure at his desk into perspective.

    Chris
     
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