I think you need to check your AIS. If you are showing vessels 50-100nm away, these are most likely ghost images or images where you have not received a transmission for quite some time. The should not be on the screen. The AIS works on a VHF frequency. This is a direct line of sight transmission. Meaning, that both the transmitting antenna and the receiving antenna need to “see” each other. Due to the curvature of the earth this limits the transmission range. The maximum range of a VHF transmission is 25 nm.
The range in nautical miles - that is, how far your radio can 'see' to the horizon is equal to 1.23 times the square root of the height of your antenna in feet.
So if, for instance your antenna height is 60 feet, the distance it can transmit before it runs into the horizon is ~ 9.5 nautical miles. ( the square root of 60 = 7.74. Multiply that times 1.23 to come up with approximately 9.5.)
Now, keep in mind, the vessel to which you are transmitting also has his antenna above the water. So, let's assume, for example, his antenna is 18 feet above the water. He can transmit about 5.2 miles. You can add your range to his to come up with 14.7 nautical miles.
A land based station is usually much higher. An antenna on a hill on a tower might be very high, but for argument's sake, let's say it's 400 feet. So theoretically, he can transmit 24.6 miles. Add that to your 9.5 miles and you have about 34.1 nautical miles.
This seems to exceed the roughly 25 mile maximum range limit. Well, many other factors come in to play here. The first of which is the rapidly diminishing strength of the transmitted signal with distance. The power density of an electromagnetic wave is proportional to the inverse of the square of the distance from the source. So, for every 4 miles your signal travels, its strength is reduced by a factor of 16. It won't take too long for that 25 watt signal to reduce itself to next to nothing.
From: amelyachtowners@... [mailto:amelyachtowners@...] On Behalf Of markghayden@...
Sent: Thursday, March 20, 2014 9:56 AM
Subject: Re: [Amel Yacht Owners] ais antenna
Several years ago we installed AIS on Northfork. We initially used the existing AM/FM antenna and a year later had a VHF antenna that was designed for use with AIS professionally installed on the top of the main mast with a new run of coax to our nav station.
The difference was dramatic. The range increased from 10nm (with AM/FM antenna) to 50-100nm (with VHF/AIS antenna).
On Thu, Mar 20, 2014 at 12:31 PM, <sailw32@...> wrote:
When I bought a Garmin AIS 600 ,I was told by the salesmen at the GPS store as well as a Garmin rep. that I should install a separate antenna . Now that I am ready to install it on top of the mizzen mast and thinking of the difficulty of the install, I am questioning if its worth the effort and the cost of the antenna. Putting a splitter into the vfh would be much easier. My question is for those that have a splitter , do you have a good AIS signal and is there any reason to have a separate antenna ?
; Thanks, Pat SM #123
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