Horizon Distance

[Athene V30]

I understand that with 5 foot height of eye I have a visual horizon of about 2.6 miles.

Height of eye calculator

If I mount an AIS antenna on the stern rail - guess 4 feet and a ship has its antenna at 60 feet do I have a horizon of just over 9 miles or does AIS follow the earth's (water's) curveture and have a greater range? If so what sort of range and if I swap my masthead (VHF) antenna into the AIS would I greatly increase the range?

My brain hurts! /forums/images/graemlins/confused.gif

 

[alan17]

Yes you would increase the range but do you need to. I once had an AIS message from a vessel over 100 miles away. Hardly relevant to me really with a time to closest point of over 4 hours. I reckon that the 9 miles you mention is more than enough to enable you to take any necessary avoiding action.

 

[CPD]

Peter, I had a 1.5m aerial on the pushpit and was picking up vessels at over 30nm. I never did look at the theoretical range.

 

[srm]

Under normal conditions of VHF propogation radio range will be slightly more than the optical line of sight between the two antenna. However, under certain atmospheric conditions, such as summer high pressure, the lower atmosphere creates a wave duct giving ranges well in excess of usual.

 

[NickiCrutchfield]

I'll second that. Coming out of Lerwick in 2006 I was listening to Dover coastguard. Somewhat depressing when you are headed for Lowestoft with a F5 southerly guaranteed for the next 6 days. Lerwick to Dover is over 600 miles.

 

[jesterchallenger]

I have my antenna mounted on the pushpit and the set (NASA) works fine. Frankly anything over 8 miles away is academic. I normally set my guard zone at 4 miles which gives plenty of time for decision making, even with the high speed ferries. I believe there is an issue with a masthead mounted antenna if you are transmitting with an adjacent VHF antenna - I think I read somewhere that AIS and VHF antennae need to be 1 metre apart. If you mount it on the pushpit, it saves having to drag another wire through the mast, as well. I would add that I consider my AIS to be one of the more useful bits of kit, excellent value for money.

The only disadvantage is that once you've set the guard zone, anything inside it sets off the alarm. So if a ship enters the zone but is going to pass clear, you have to wait until it's out of the zone until you can reset the alarm (ready for other targets) and resume your catnapping. Not a problem once you're off soundings, but sailing alonside Channel shipping lanes requires more vigilance.

Hope that helps.

 

[Noddy]

I have my AIS ariel mounted aft at deck level.

I'm pretty sure that the cabin obstructs signals from forward leading to a relative blind spot ahead.

When I get all the other jobs done I will probably mount it higher.

P

 

[helixkimara]

I have my AIS antenna mounted on the top of a small mast fixed to the pushpit rails. The bottom of the antenna is about 1.5 m above deck level. I often picked up ships over 100 miles away. (frightening near TSZ's, if I zoom out as my gps screen filled up with a swarm of triangular wasps)
I seem to remember that another reason for mounting the antenna over 1 m above head height was that it gave off "brain frying waves". This is why Noddy still has his pirates beard and talks with an OOH AAARRRR JIM LAD !

 

[lenseman]

[ QUOTE ]
I have my AIS antenna mounted on the top of a small mast fixed to the pushpit rails. The bottom of the antenna is about 1.5 m above deck level. . . . . I seem to remember that another reason for mounting the antenna over 1 m above head height was that it gave off "brain frying waves".

[/ QUOTE ]
Unless you are running an AIS transponder (Class A or B), then there will be no RF signals emanating from your kit. The same goes for GPS as both these are receive only and do not transmit.

Even if there were, the transmission is only a 62mSeconds burst one a minite or so and if you compare that to a mobile (cell) phone 1" (2cms) from your brain and continuously transmitting at up to 2 watts of power (variable power levels).

Just out of interest, AIS Transponders transmit their two signals at ~160MHz whereas your mobile phone frequency of 950, 1800, 2200MHz transmits its signal much nearer to the dangerous frequency of 2450MHz which is the absorption frequency of water and hence your brain, all be it, non-ionising radiation even so! /forums/images/graemlins/shocked.gif

Two centimetres from mobile phone to your brain, this very short distance in RF terms is known as 'being in the Reactive Field' [Near Field, Far Field and Reactive Field]. During the late 80's and early 1990's I conducted research at the H M Government research laboratories to accurately measure the field-strength of various mobile telephones when in close proximity to an 'artificial head'. The findings of 98volts per metre were not uncommon at this proximity.

This is why I have the 'one-minute' bleeper turned on on my cell phone to remind me to change ears if the conversation is going to be a lengthy one!

It isn't called the Reactive Field for fun!! /forums/images/graemlins/shocked.gif

 

[helixkimara]

Yeh thanks for that Lenseman. It is an AIS transponder.

No time for a long chat cos I'm bussy building a trailer to tow 6m behind me with a 2 m mast to put my mobile phone on top of /forums/images/graemlins/confused.gif Well at least I know its not "old age" now /forums/images/graemlins/ooo.gif