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Marine VHF/AIS Range

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What sort of typical range should I expect from the VHF/AIS. 

I get there are some variables in this, but today for example I was only intermittently picking up the ferry on AIS when they were 2.8NM away with some terrain in the way.... see picture.
AIS and VHF share an antenna with the appropriate splitter. (XB8000 and a Standard Horizon VHF - Furuno antenna (looks old))


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I’ve frequently seen ships at 30 to 40nm however class B to class B will be only 7 to 10nm depending on the mast heights and sea conditions 

Transmission intervals vary according to your speed and theirs also so if when they transmitted if you had a house or land in the way you wouldn’t receive their transmission 

VHF will bend around corners but not reliably so that’s why sometimes you would pick them up.

Hope that helps

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If the antenna (and coax) is 10yrs + is will have degraded. It's often the Class B AIS that shows this 1st. Change the cable and co-ax - they are not expensive.

Class B should Always give you 7-10 miles across water. Ships with class A should be 20 miles if your antenna and theirs are line of sight. I have seen over 100 Miles once, but that's weird atmospherics.  I one had a ship call me from 46miles and ask my intentions - I didn't even know he was there!

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Because AIS channels are at the very top end of the Marine VHF band, dedicated AIS antennas are usually cut for the the top  two or three frequencies in the band resulting in a considerably shorter antenna with a centre frequency of around 162 mhz where a normal VHF antenna will be centered around 159 mhz. On our commercial boats the AIS antennas  are  about 30cm shorter than the normal VHF communication antenna's. Two separate dedicated antenna's at similar height will always  work better than one single antenna but on small boats compromises have to be made. Step 1 would be  to disconnect the antenna couplings any sign of corrosion can be cleaned up with a small wire brush  and sandpaper. If this does not fix the problemhave a look at the solder or crimp joints for signs of moisture. Next would be slice the outer insulation off the coax and have a look at the braid, any sign of green ditch the antenna and coax and buy new.

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