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Old 25-06-2008, 23:47   #1
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ais vhf antenna mounting

i was thinking of mounting my ais vhf antenna on the forward face of the mast. there is currently at sat radio antenna at this location (about 30ft/10m above deck) and i don't require it.

would being near, and immediately in front of, the mast degrade the ais performance significantly? the ais is a class b transponder (i.e. not just a receiver)

thanks
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Old 26-06-2008, 04:14   #2
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Quote:
Originally Posted by bobsadler View Post
i was thinking of mounting my ais vhf antenna on the forward face of the mast. there is currently at sat radio antenna at this location (about 30ft/10m above deck) and i don't require it.
would being near, and immediately in front of, the mast degrade the ais performance significantly? the ais is a class b transponder (i.e. not just a receiver)
thanks
Yes the mast will degrade the SWR & Radiation pattern (shade).

Because the AIS is a VHF, the antennas should be located according to the same principles:
As high as possible [1], as free as possible from obstructions [2], and as far as possible away [3] from other antennas and strong sources of RF.

Obviously, this will lead to some sort of compromise between the VHF Radio and AIS antenna placement (they can’t both occupy the ideal mast head location).

I’ll be interested in hearing from experienced AIS users (I’m not).

1. Antenna height
AIS uses VHF radio frequencies, whose propagation is close to line of sight, therefore, the higher the antenna location the longer the range.

2. Clear view of the horizon
The AIS VHF antenna should be mounted with a relative clear view of the horizon.
Large obstructions ( especially parallel metals - ie: mast) might shade the AIS radio communications.

3. Antenna separation
AIS transponders use frequencies on the high side of the marine mobile band (Channel
1 is 161.975 MHz, Channel 2 is 162.025MHz). The frequencies are close to the duplex
channels used for marine communication.
The AIS VHF antenna should be separated as much as possible from the voice VHF installations used for main communication to avoid unnecessary interference.
Best separation is achieved by installing the antennas over each other or on separate sides of the mast. The VHF antenna should be mounted at least 3 meters away from, and out of, the transmitting beam of high-power transmitters or other VHF antenna installations.
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Old 26-06-2008, 04:37   #3
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My AIS antenna is mounted on 6' pole attached to the pushpit making it about 12' (base) above the water. I receive AIS target from at least 8-10 miles and often even over land.. defying the line of site notion. I find this more than adequate to identify CAP targets well in advance.

I don't use it much as I am sailing in local protected waters and there is little "use" for AIS.

I don't think high up the mast will make much difference for collision avoidance.
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Old 26-06-2008, 05:36   #4
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My AIS antenna is mounted on 6' pole attached to the pushpit making it about 12' (base) above the water. I receive AIS target from at least 8-10 miles and often even over land.. defying the line of site notion. I find this more than adequate to identify CAP targets well in advance ...
... I don't think high up the mast will make much difference for collision avoidance.
Your experience confirms the line of sight science.

Line of Sight distance (nautical miles) = [1.17 x square root of your eye-height (in Ft)] + [1.17 x square root of target height (in Ft)]

Hence:
A 12 Ft high antenna would be expected see a 30' high object at a Line of Sight distance of (4 + 6.4 = ) about 10.4 nm - just as you say.
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Old 26-06-2008, 09:28   #5
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I have a ketch, so it's a bit easier, but you can do what I did on my mizzen in order to mount a 2m (144 MHz) ham antenna:

A 6' flat bar is mounted horizontally fore/aft on the masthead. The AIS antenna is mounted on the forward end, and the 2m antenna aft. The new Air Breeze wind generator is mounted between the two. With 6' of separation, interaction is minimal - especially since the AIS frequencies are up around 162 MHz...
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Old 28-06-2008, 14:10   #6
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With the VHF aerial on top of the mast, I've opted to mount the AIS aerial on the second spreaders about 50 cm out from the mast, hoping to avoid too much mast "shadow". On a one-masted yacht I guess it will always be a trade-off between max. reach for either the VHF or the AIS.
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Old 30-06-2008, 02:16   #7
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multihullsailor6: with the antenna on the spreader do you feel you're getting good ais system performance?
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Old 30-06-2008, 07:38   #8
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Gord,

But I am seeing targets which are blocked by land - hills taller than either antenna.
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Old 30-06-2008, 08:38   #9
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Gord,But I am seeing targets which are blocked by land - hills taller than either antenna.
Blink quickly 3 times, then close and rub your eyes.
Targets still there?

I'm no expert in Radio Signal Propagation, so perhaps Bill T. or one of our other Hams, might chime in with better explanations.

FWIW:
This may be caused by Tropospheric Ducting or enhancement, due to signal refraction off a temperature inversion.
A warm/dry airmass, on top of a cooler/humid airmass usually produces the best ducting conditions
Sea breeze ducts form where the cooler sea breeze meets a warmer off land breeze, at the top of an escarpment.
Several other mechanisms can also cause ducts to form, such as along fronts.
Look for significant weather changes, such as a cold front moving in, or a major storm system. Also early evening as the earth cools and early mornings as the earth warms can cause these conditions.

Some of the other conditions that can affect VHF propagation are ionospheric irregularities, including meteor ionization in the E region (sporadic E) and the F regions, and the northern lights (aurora borealis).
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Old 30-06-2008, 10:11   #10
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Are you talking about a transponder or reciever?

If only a reciever - Smart Radio makes an antenna splitter that allows using your masthead antenna to function as the AIS antenna. It automatically disconects the AIS when you transmit on VHF radio. Seatech sells the unit. This will not work on a transmitting transponder.
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Old 03-07-2008, 01:14   #11
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transponder
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Old 03-07-2008, 13:53   #12
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Quote:
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multihullsailor6: with the antenna on the spreader do you feel you're getting good ais system performance?
Bob, have to defer an answer for the moment as it's a new installation (transponder) and I haven't yet been able to test it.

Roger
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Old 04-07-2008, 22:42   #13
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speciald@ocens: this may be what you're looking for - a splitter that allows transmit and recieve by ais on a shared antenna

AIS equipment - New offers
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Old 05-07-2008, 08:48   #14
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Eight foot fiberglass whip on pushpit, 6 feet above water. From reading other forums I distrust sharing antennas via a switch. I know there are AIS repeaters in Europe, particularly around Gibraltar, and suspect that there are repeaters on the Chesapeake because I can receive signals well beyond line of sight, even considering that shipboard antennas can be a couple hundred feet above the water. Anyone know for sure?

Anyway, I don't want much more range, the screen would be too cluttered. For AIS B, at it's lower power, it questionable that great height would serve much purpose.
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Old 05-07-2008, 10:37   #15
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I use a Raymarine AIS receiver. The dedicated AIS VHF antenna sits on the port side of my stern-mounted "antenna farm" which holds the GPS, Sirius weather satellite and, on the starboard side of the array, about four feet away, the 6dB VHF comms antenna. The AIS VHF antenna also doubles as an FM radio antenna, shunting off at the unit to my stereo. In the nav center, I have a selector switch to choose whether I want the 3dB masthead VHF comms antenna or the 6dB, above deck antenna. I have a multihull with a 15 degree (generally) maximum heel, so I can get away with the 6dB.
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