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Author Topic:   Safe distances between VHF and GPS aerials
Novice John posted 03-12-2010 05:00 PM ET (US)   Profile for Novice John   Send Email to Novice John  
My boat has a Raymarine GPS antenna and a 2.4-meter VHF antenna mounted three-feet apart on an arch above the flybridge. I wish to install an AIS receiver with its own one-meter VHF antenna and a separate Garmin GPS, as a backup. Can I site the two VHF aerials within 15-cm of one another on the arch? Similarly, can I mount the two GPS aerials within 15-cm of one another on the arch?
The only other place to mount the new GPS and VHF aerials would be in front of the instrument console on the fly bridge, which would invoke the wrath of my wife and a fate worse than death!
I should greatly appreciate some advice, as Garmin and Raymarine installation instructions appear to contradict one another.
SJUAE posted 03-13-2010 01:28 AM ET (US)     Profile for SJUAE  Send Email to SJUAE     

Yes confusing, No problem with the GPS aerials providing they are not insight of any radar

It seems after looking at the following installation manual extracts that vertical separation between normal comunications VHF and AIS VHF is prefered especially if it's both TX and RX

As your only putting a reciever on why not just use your existing VHF aerial and a splitter as I presume this AIS is not a compulsory requirement for your boat.

I would also guess that as it's only a reciever firstly the 1-meter aerial is 1.4-meter below your existing VHF and any likely interferance would only be when you are TX on the VHF

If you already have the aerial why not jury rig it to see before any executions by the Mrs :)

VHF antenna for AIS use NOTE:

If you are installing a Class B AIS transponder on a vessel because you are required to do so, you must use a separate VHF antenna and a separate GPS antenna dedicated to the Class B device in order to be compliant with the regulations.

Use the antennas supplied with this kit. The VHF antenna supplied for use with the Nauticast™-B is omni-directional, vertically polarized with unity gain (0 dB) and a band width sufficient to maintain VSWR <1.5 over the frequency range 156–163 MHz. As a minimum the 3dB bandwidth will cover the two AIS channels and the DCS channel. The antenna is also suitable for marine shipboard applications (index of protection, ruggedness, means of mounting, etc.).

If you elect to not use the antenna supplied, the alternate VHF antenna employed for AIS use: Must be a dedicated antenna, i.e. not shared with any other VHF transmitter/receiver. Should be mounted with at least a two meter vertical separation distance from any other VHF antenna used for speech or DCS communication. Also see Appendix for warnings such as “Radio Frequency Exposure Warning

The receiver requires its own marine VHF antenna and cannot be shared with a transceiver antenna. It should be mounted as high as possible to maximize range but should be spaced not less than 1 metre from a transmitting antenna. The antenna cable should be at least 3 metres long
and the antenna should be sited at least 2 metres from the AIS receiver. php?object_id=navidoc~49549225&f=/images/TIBE/device/navico/library/ navidoc/navidoc/navidoc/j2009/m02/t04/0000305_2.pdf& fc=988-0168-003-Simrad-AI50-Manual.pdf&type=download

The most important factor in the performance of any
AIS transceiver will be the quality and positioning of the
As the range of VHF signals are governed by line of sight,
the antenna should be placed as high as possible, while
remaining clear of any metallic objects.
Long whip antennae are generally recommended for
larger boats, although the most popular antennae for
marine use is 1m (3ft 3in) long. On sailboats these are usually mounted on the masthead, where the length of
the antenna keeps it clear from the navigation lights and
wind vanes. This type of antenna can also be mounted on
the cockpit roof or powerboat garages.

For maximum range, it is recommended that a VHF
antenna specifically tuned for use with an AIS is used,
and mounted away from the standard VHF antenna.
Vertical separation is preferred, but where this is
not practical, at least 5 metre horizontal spacing is


jimh posted 03-13-2010 09:38 AM ET (US)     Profile for jimh  Send Email to jimh     
The antennas for the GPS receivers can be located close together. However, both should be kept distant from the VHF Marine Band radio transmitter antenna.

The AIS receiver antenna can be located near to the GPS receiver antennas. The AIS receiver antenna should be kept distant from the VHF Marine Band radio transmitter antenna.

To help separate the antennas, use vertical separation. Raise the height of the VHF Marine Band transmitting antenna using a two-foot extension mast.

SJUAE posted 03-13-2010 02:35 PM ET (US)     Profile for SJUAE  Send Email to SJUAE     
Good point Jim if he has any problems use the extender to raise the existing VHF antenna


Novice John posted 03-14-2010 03:41 PM ET (US)     Profile for Novice John  Send Email to Novice John     
Very many thanks to you all for taking so much time to help me. I learnt a lot and have enough info to explain to my wife my reasons for siting both aerials at the front of the flybridge, rather than up on the radome/VHF/GPS arch at the rear of the flybridge.
I'll let you all know how it worked out during the coming season. Thanks again and have a good season.

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