AIS transmission without VHF splitter

[Gitane]

I am considering installing a AIS transponder.

With respect to aerials, there appear to be two obvious options; either use a VHF signal splitter and feed both the AIS and VHF signals to the same aerial, or use separate aerials, one for the AIS and one for the VHF.

For some reason, marine VHF signal splitters are quite pricey, so I am considering the second option.

Would using separate aerials work, or will the signals from the VHF and AIS interfere with each other?

 

[PaulRainbow]

I am considering installing a AIS transponder.

With respect to aerials, there appear to be two obvious options; either use a VHF signal splitter and feed both the AIS and VHF signals to the same aerial, or use separate aerials, one for the AIS and one for the VHF.

For some reason, marine VHF signal splitters are quite pricey, so I am considering the second option.

Would using separate aerials work, or will the signals from the VHF and AIS interfere with each other?

They work OK, but i'd fit the AIS antenna on the pushpit, not at the masthead.

• Separation at the masthead is obviously not ideal.
• The second antenna at the pushpit gives a backup VHF antenna in the event of a dismasting or any other antenna failing.
• Range with a pushpit mounted antenna is perfectly adequate, ask anyone with a small mobo.

 

[pvb]

Yes, an antenna on the pushpit would be fine and, as PaulRainbow suggested, would provide redundancy.

 

[Norman_E]

One on the pushpit is fine, and you can even buy one specifically made for AIS. If you have a bimini you could always clamp a vertical pole with a screw thread on the end to take the aerial onto the frame and raise the aerial well above your head height. I don't know if its true or not but I have been told that you should not place yourself close to and in line with any transmitter aerial for any length of time and one on the pushpit will be transmitting quite close to you at head height. For that reason I have my active radar reflector on such a pole.

 

[pvb]

I don't know if its true or not but I have been told that you should not place yourself close to and in line with any transmitter aerial for any length of time and one on the pushpit will be transmitting quite close to you at head height. For that reason I have my active radar reflector on such a pole.

AIS transceivers transmit at very low power output (2W) and each transmission lasts only milliseconds, so there's no danger.

 

[rszemeti]

AIS transceivers transmit at very low power output (2W) and each transmission lasts only milliseconds, so there's no danger.

It is worth remembering that compared to a mobile phone, AIS is very, very safe. The average power is about 1/10th that of a mobile phone (GSM average power is around 500mW, due to the high repetition rate, AIS average power is around 50mW, due to the 2W peaks but very low repetition rates), and the separation distance reduces the actual power delivered to the body by a factor of 100 compared to a mobile phone (remember that in the near field, the power reduction is cube law, not the square law found in the far field, and mobile phones clamped to the head are definitely near field) .. .so overall, the power delivered to the body is probably down by a factor of 1000 compared to a mobile phone, so no perceivable risk and any residual risk is way lower than almost any of the other risks present in sailing.

 

[rszemeti]

They work OK, but i'd fit the AIS antenna on the pushpit, not at the masthead.

• Separation at the masthead is obviously not ideal.
• The second antenna at the pushpit gives a backup VHF antenna in the event of a dismasting or any other antenna failing.
• Range with a pushpit mounted antenna is perfectly adequate, ask anyone with a small mobo.

That's what I did ... a second half-wave on the pushpit, with easily acessible bulkhead connectors and flying leads in the nav position. Should the main aerial fail (dis-masting, bad coax, meteor strike etc) I can switch over to the pushpit aerial quite easily.

 

[Mistroma]

It is worth remembering that compared to a mobile phone, AIS is very, very safe. The average power is about 1/10th that of a mobile phone (GSM average power is around 500mW, due to the high repetition rate, AIS average power is around 50mW, due to the 2W peaks but very low repetition rates), and the separation distance reduces the actual power delivered to the body by a factor of 100 compared to a mobile phone (remember that in the near field, the power reduction is cube law, not the square law found in the far field, and mobile phones clamped to the head are definitely near field) .. .so overall, the power delivered to the body is probably down by a factor of 1000 compared to a mobile phone, so no perceivable risk and any residual risk is way lower than almost any of the other risks present in sailing.

COMAR obviously play it safe as my CSB200 manual states.

"To meet the requirements for Radio Frequency Exposure it is necessary to install the VHF antenna correctly and operate the AIS equipment according to the instructions.

The VHF antenna must be mounted at a minimum distance (vertical separation) of 3 metres from the head of any person standing on deck in order to meet international safety directives on Maximum Permissible Exposure (MPE) / Specific Absorption Rate (SAR).

Where no suitable structure exists to achieve a 3 metre vertical separation then the antenna base must be mounted at least 1 metre above the head of any person within range, all persons should stay outside the 3-metre safety radius and if practical a grounded RF shield should be interposed between people and the antenna.

Failure to adhere to these limits could expose persons within the 3 metre radius to RF radiation in excess of the MPE / SAR limits.

I guess they are covering themselves in case someone in USA gets a headache and sues.

I fitted mine on a pole, offset from wind-gen and almost 3m above head height. That was more to do with range and the fact I already had the pole in place anyway.

 

[Gitane]

Thanks for all the replies.

 

[vyv_cox]

I have just installed an AIS transponder from Digital Yacht with a short antenna above the radar scanner on my stern arch. Seems to be working fine.