900MHz yagi safe?

[Mark Powers]

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

 

[Mac]

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

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

 

[Ken Smith]

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.

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".

 

[Jim Thompson]

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

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

...Jim Thompson

 

[Leon]

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.

Leon

 

[[email protected]]

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?

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.

 

[Jim Thompson]

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.

Leon

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

 

[Rich Grise]

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

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

 

[Rich Grise]

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

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

 

[Ken Taylor]

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

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

 

[Ian Stirling]

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?

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.

 

[Frank Raffaeli]

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

Frank

 

[Mark]

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

 

[Chris Jones]

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.

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