900MHz yagi safe?

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

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".

--
--
kensmith@rahul.net   forging knowledge

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

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
     It\'s what you learn, after you know it all, that counts.

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. 8-)

Leon

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.

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

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

| 1962 | It's what you learn, after you know it all, that counts.

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

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

Admittedly,

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 f****ng 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

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.

Since you seem to be fine, I guess it's ok! ;-)

Frank

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:

and

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

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