Class B AIS - radiation hazard?

Playtime

Playtime

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I'm considering fitting a Class B AIS transponder but am unsure where to site the antenna.

The obvious place is on the pushpit rail or maybe on a T bar above the pushpit. However, some transponder installation instructions warn about radio frequency exposure. For example, this is an extract from the SIMRAD AI50 installation manual -

"To meet FCC (Federal
Communications Commission) rules on Radio Frequency
Exposure, it is recommended that the VHF antenna
is mounted at least 3m (10ft) away from any area
accessible to any personnel on board. If this distance is
achieved by vertical separation, the antenna must be at
least 5m (16.5ft) above deck. This guideline applies only
to antennas not exceeding 3dBi gain.

Failure to observe these recommendations may
expose those within the MPE (Maximum Permitted
Exposure) radius of 1m (3ft) to RF absorption levels
that exceed the FCC safe limits."

Is this OTT for a transmission power of only 2W?

Where have others mounted the antenna?
 
Vertical whip type antennas radiate in a flattened doughnut pattern. So, if you're directly above or below it, there is little or no radiaton. If on the other hand you're directly along side it, you'll get the full radiation right through you.

Now does that matter with AIS? Class B radiation is 2W, many handheld VHF radios exceed that & the antenna is righht next to your brain. You can argue that AIS will be transmitting every few minutes continually or more often while a handheld is a few seconds very infrequently. But, the AIS transmission is very short.

Mount it on a pole above head height.
 
It's running on similar frequencies and powers to your ship's VHF - well clear of any frequencies that have ever been implicated in any health risk. You want the antenna as high as is reasonably possible for range, but you could sit holding it all day without any risk to your health.
 
maby said:
It's running on similar frequencies and powers to your ship's VHF - well clear of any frequencies that have ever been implicated in any health risk. You want the antenna as high as is reasonably possible for range, but you could sit holding it all day without any risk to your health.
Click to expand...

So the FCC are talking b - - - - - ks and their recommendation can be ignored? :confused::confused:
 
Playtime said:
I'm considering fitting a Class B AIS transponder but am unsure where to site the antenna.


"To meet FCC (Federal
Communications Commission) rules on Radio Frequency
Exposure, it is RECOMMENDED that the VHF antenna
is mounted at least 3m (10ft) away from any area
accessible to any personnel on board. If this distance is
achieved by vertical separation, the antenna must be at
least 5m (16.5ft) above deck. This guideline applies only
to antennas not exceeding 3dBi gain.

Failure to observe these recommendations MAY
expose those within the MPE (Maximum Permitted
Exposure) radius of 1m (3ft) to RF absorption levels
that exceed the FCC safe limits."
Click to expand...


note the terms recommended and may !

A definate maybe then ;)
 
Radiation Hazard

The actual hazard of radiation is a function of power that is absorbed into the body the frequency and the time that radiation is absorbed.
The power that is absorbed is a function of distance from the antenna the radiation pattern of the antenna and the power given for AIS as 2 watts. The pattern of the antenna has been given correctly as max at right angles to the whip and equal all round.
The time radiation is absorbed is a function of the on time of the transmitter and frequency of transmissions and of course the amount of time you spend on the boat and near the antenna.
The radiation standards are designed to cope with worse case scenarios. Like a full time crew person living or working near the antenna 24/7 most of the year.
I would imagine that most of us amateur sailors would not receive any harmful level of radiation over a life time or even a few days with a pushpit mounted antenna. Nevertheless to mount it up even a metre above head height would reduce radiation dramatically. Both being then out of the main radiation pattern of the antenna and a greater distance.
Regarding frequency. Our marine bands are something like 1/3 of the frequency of mobile phones. It seems to me that harm rises with frequency so microwave radiation radars etc are more harmful than VHF. (don't stick your head in the microwave oven). Mobile phones are a lot lower power but then are also held close to the head and the experts still can't decide if they cause harm when used for long periods. Radiation diminishes at the rate of cube of the distance roughly. So 1 metre away from 2 watts is far better than 10cms away from a few milliwatts. However it is all speculative.
I suggest OP does what is doable and ignores the warnings. But don't tell em I said so. olewill
 
William_H said:
...Radiation diminishes at the rate of cube of the distance roughly...
Click to expand...

I know in your case that was certainly a typo olewill but for the sake of good order I'll clarify that it diminishes at the rate of the square of the distance.

That is it diminishes at the rate of the increase of the area of the assumed spherical wave front, not as the volume of the sphere propagated through.

For practical antennas this gives a field strength greater than actual in the near field and so from a safety point of view is conservative for that case.

I am most familiar with FCC requirements rather than EU ones but even without calculation I join the views already stated that a 2W AIS is no problem at all even considering the likely greater exposure in the time domain when compared to VHF telephony. I suspect the warning in the AIS documentation is along the mollycoddling lines of those often seen in workplace lunchrooms on the western side of the Atlantic warning that microwave ovens are in use :eek:.
 
Playtime said:
I'm considering fitting a Class B AIS transponder but am unsure where to site the antenna.

The obvious place is on the pushpit rail or maybe on a T bar above the pushpit. However, some transponder installation instructions warn about radio frequency exposure. For example, this is an extract from the SIMRAD AI50 installation manual -

"To meet FCC (Federal
Communications Commission) rules on Radio Frequency
Exposure, it is recommended that the VHF antenna
is mounted at least 3m (10ft) away from any area
accessible to any personnel on board. If this distance is
achieved by vertical separation, the antenna must be at
least 5m (16.5ft) above deck. This guideline applies only
to antennas not exceeding 3dBi gain.

Failure to observe these recommendations may
expose those within the MPE (Maximum Permitted
Exposure) radius of 1m (3ft) to RF absorption levels
that exceed the FCC safe limits."

Is this OTT for a transmission power of only 2W?

Where have others mounted the antenna?
Click to expand...

you need to do a cost/ benefit analysis
 
Thanks guys - some sensible replies above.

I think we are agreed that SIMRAD are OTT quoting the FCC recommendation when applied to a 2W AIS transponder.

Nevertheless, as suggested, I will mount the antenna on a pole above head height at the pushpit (when I have completed the cost/benefit analysis ;) ).

Not only will this minimise the radiation hazard but also give some elevation (and hence range) to the signal.
 
Seajet said:
I sometimes wonder about RadHaz when I see mobo's with a radar just behind the heads of the people on the flybridge, but I wouldn't worry about a transponder !
Click to expand...

I think it's safe to say that the greatest risk we all run is from our mobile phones. They run at surprisingly high power outputs and we hold them pinned to the side of the head - often for an hour or more per day. In comparison, almost every other RF exposure we suffer is tiny.
 
So do mobile phones

wooslehunter said:
Vertical whip type antennas radiate in a flattened doughnut pattern. So, if you're directly above or below it, there is little or no radiaton. If on the other hand you're directly along side it, you'll get the full radiation right through you.

Now does that matter with AIS? Class B radiation is 2W, many handheld VHF radios exceed that & the antenna is righht next to your brain. You can argue that AIS will be transmitting every few minutes continually or more often while a handheld is a few seconds very infrequently. But, the AIS transmission is very short.

Mount it on a pole above head height.
Click to expand...

Mobile phones run up to 2 Watts as well (yes I know on a different frequency), but they are attached to your brain for hours ...............
 
jfkal said:
Mobile phones run up to 2 Watts as well (yes I know on a different frequency), but they are attached to your brain for hours ...............
Click to expand...

The frequency difference is the critical factor - mobile phones run on frequencies that are pretty close to those that very successfuly cook your dinner! AIS, in comparison, runs at very low frequencies - which have never been implicated in any health hazard.
 
maby said:
The frequency difference is the critical factor - mobile phones run on frequencies that are pretty close to those that very successfuly cook your dinner! AIS, in comparison, runs at very low frequencies - which have never been implicated in any health hazard.
Click to expand...

Au contraire. VHF can do damage at sufficient power.
In general though, the biggest danger with vhf is poking yourself in the eye with the antenna.

The heffalump in this particular room has to be radar, how many yachts have a kilowatt or more of radar beam sweeping the decks?
 
lw395 said:
Au contraire. VHF can do damage at sufficient power.
In general though, the biggest danger with vhf is poking yourself in the eye with the antenna.

The heffalump in this particular room has to be radar, how many yachts have a kilowatt or more of radar beam sweeping the decks?
Click to expand...

A kilowatt ERP, possibly, but at the pulse level and in a fairly narrow beam! The total exposure to someone standing in the beam sweep several feet away will be a lot less - the power drain off the 12v supply is measured in tens of watts. Even so, I agree that the health risks are poorly understood. Planning on putting Simrad Broadband radar on ours - that's about the lowest power output on the market at the moment.
 
maby said:
The frequency difference is the critical factor - mobile phones run on frequencies that are pretty close to those that very successfuly cook your dinner! AIS, in comparison, runs at very low frequencies - which have never been implicated in any health hazard.
Click to expand...

Exactly :-)
 
Just use a good splitter like the OCB and share the VHF antenna. Class B AIS transmits at 3/4 Watts, fitting the antenna on the pulpit reduces the range from which you can be received dramatically. I have received range reports averaging around 8nm with the mast antenna. I would expect the range to be halved if the antenna is near the water level.

Danger from radiation of a Class B AIS is negligible compared to what you get every day from your mobile phone, mobile phone masts near you, your WiFi router, your WiFi laptop, even those components that do not have a transmitter and yet radiate plenty of RF on several frequencies like your computer network card and ADSL modem/router.
 
Last edited:
haydude said:
Class B AIS transmits at 3/4 Watts, fitting the antenna on the pulpit reduces the range from which you can be received dramatically. I have received range reports averaging around 8nm with the mast antenna. I would expect the range to be halved if the antenna is near the water level.
Click to expand...

My present receive only antenna is mounted at the pushpit, maybe 7' above the water (it's on a T Bar) and receives signals from 12-15 miles away, plenty for all practical purposes.

Why introduce a splitter in the safety critical main VHF antenna when a separate AIS antenna at the pushpit is less than half the price?
 
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