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Old 06-29-2009, 01:04 AM   #1
Join Date: Oct 2004
Posts: 3,067

SAAB's R4 AIS Class A Transponder System's 2 page pdf brochure describes the system in


"Ships equipped with an AIS transponder can receive transmitted messages from other ships within VHF range"

No discussion on what is VHF range The extreme could be that enjoyed by 2 Supertankers, say

50nm apart.

What about the average size sail boat, take two 37ft sail-boats, what would the maximum range could they transmit and receive on at 25 watts, with masthead antennas :- 20nm in good conditions ?? 10nm in poor ?

MMNETSEA is offline   Reply With Quote
Old 06-29-2009, 03:11 AM   #2
Join Date: Mar 2007
Posts: 129

Vhf radio range has many factors such as height of the antenna, orientation of the antenna and power delivered to the antenna.

Radio output power is a nominal 25watts on high power and 5 watts on low power. Antenna design and the db rating of the antenna describe the "focusing" of the broadcast signal. Typically sailboat antennas have low db ratings so that the signal can propragate even when the sailboat is heeled over are pitching in the seas. The higher the db the more focused in the horizontal plane perpendicular to the vertical axis of the antenna element. A high db antenna on a sailboat heeled over would be broadcasting its main power down towards the ocean on one side and up into space on the other side.

The the really big limitation is the line loss and swr of the cable between the radio and the antenna. On average the fixed cable length of the average sailboat antenna cable ends up with half or less of the 25 watts getting to the antenna. Add in the seepage of rain water into the cable and the design of the cable and the output wattage can drop to 5 watts when broadcasting on high power.

I use a special "zero loss" cable LMR600 with the length calculated for the minimum swr losses. This gives my main radio the ability to deliver 25 watts to the antenna element. However, this cable is US$5 per foot but I get 25 to 50 miles of range in flat seas.

Since the AIS that most private pleasure sailboats use is only a passive receiver the range of vessel we receive AIS from is only limited by the power, height, and db ratings of the broadcasting cruise ship/freighter or whatever is broadcasting. The AIS receiver is only a passive receiver and not limited by its receiving antenna (well, almost - there is still some line loss on the received signal on it way to the radio and other minor losses). But you should be able to receive AIS info from ships up to 25-50 nm.

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Old 06-29-2009, 04:13 PM   #3
Join Date: Jun 2009
Posts: 19

That is very good money spent @osirissailing! If you cannot get LMR600 you can use RG-213 cable - has a very small loss. Make sure you get the best compression termination connectors you can afford.

VHF is line of sight, so for a 40' boat with standard average mast heights, your maximum range should be in the order of 40-50nm. You can work this out if you have the height of the aerial above the sea surface and what dB value your aerial is (6 or 9d.

Basic safe ranges:

Sailboat to shore: 35-50nm (higher shore mast)

Sailboat to sailboat: 15-20nm

As they say, the best liferaft is the one on top of the mast.
mygoggie is offline   Reply With Quote


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