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Old 14-07-2013, 21:43   #16
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Re: AIS transponder and VHF antennas on the masthead ?

I've argued there is no navigational value in seeing AIS targets >10 or 15nm away. Hence, herculean efforts to increase AIS range at the detriment of VHF comms seems silly.

Two separate antennas is the cleanest and most simple choice. If you feel the need to see or be seen at greater range, do like MarkJ and swap coax between the radios.

FWIW, recently with my rail mounted Class B AIS, I was >10nm offshore and the USCG sector hailed me by boat name to assist in relay comms. Obviously, they have tall land-based towers.
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Old 15-07-2013, 04:28   #17
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We have our VHF antenna mounted at the masthead within a foot of the AIS VHF antenna. Both have worked flawlessly with no discernible interference for the last 3 years.
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Old 15-07-2013, 04:56   #18
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Quote:
Originally Posted by DotDun View Post
I've argued there is no navigational value in seeing AIS targets >10 or 15nm away. Hence, herculean efforts to increase AIS range at the detriment of VHF comms seems silly.

Two separate antennas is the cleanest and most simple choice. If you feel the need to see or be seen at greater range, do like MarkJ and swap coax between the radios.

FWIW, recently with my rail mounted Class B AIS, I was >10nm offshore and the USCG sector hailed me by boat name to assist in relay comms. Obviously, they have tall land-based towers.
This is a AiS receiver mentality. With a transponder every effort should be made to ensure your weak class B signal reaches its destination over as wide an area as possible. Antenna height is the biggest factor in this regard

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Old 15-07-2013, 05:57   #19
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Re: AIS transponder and VHF antennas on the masthead ?

Quote:
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I've argued there is no navigational value in seeing AIS targets >10 or 15nm away. Hence, herculean efforts to increase AIS range at the detriment of VHF comms seems silly.
Many ships are required by their companies to keep a CPA of 1 nms, or often 2nms. They will divert at 10nms.

So you really do need as good as transmitting range as possible.

The ship below diverts at 10nms





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Old 15-07-2013, 06:06   #20
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I went the splitter route when I first installed the AIS receiver and now I have upgraded to a transponder and have kept the splitter.

For coastal the range probably does not matter but if you are in the middle of an ocean, the greater the warning for you and the other vessel seems to me to be a great thing.
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Old 15-07-2013, 06:29   #21
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Re: AIS transponder and VHF antennas on the masthead ?

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Originally Posted by MarkJ View Post
Many ships are required by their companies to keep a CPA of 1 nms, or often 2nms. They will divert at 10nms.

So you really do need as good as transmitting range as possible.

The ship below diverts at 10nms





Mark
And you are sure that course change was due to you?

I have evidence that I can be seen by a high antenna @10nm and are contacted often by general pleasure vessels @6nm. These distances are obtained with a rail-mounted AIS antenna.

I have no issues with masthead mounted AIS, just can't see degrading VHF comms to accomplish it. All considered, I don't find it necessary. My experience is that simple is better than complication.

IMO, of course!
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Old 15-07-2013, 07:02   #22
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Re: AIS transponder and VHF antennas on the masthead ?

I have the Garmin 600 AIS, which comes with a built in splitter. I have been using the splitter with no problems so far. Good transmit/receive on both AIS and VHF.
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Old 15-07-2013, 12:11   #23
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All of the large vessels I have encountered offshore have altered course to a CPA of at least 2 miles were possible. I always call them via VHF and thank them. Of course I alter course to help. My AIS transponder set up seems to have a Tx range of about 10-15 miles as reported. I see large vessels at a much longer distance and look at CPA at around 20 miles. Inshore ie coastal then a CPA of 2 miles is not always possible and careful navigation is always prudent
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Old 15-07-2013, 13:09   #24
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Re: AIS transponder and VHF antennas on the masthead ?

Quote:
Originally Posted by MarkJ View Post
Many ships are required by their companies to keep a CPA of 1 nms, or often 2nms. They will divert at 10nms.

So you really do need as good as transmitting range as possible.

The ship below diverts at 10nms





Mark
They did not divert to steer clear of you because of the AIS!!! They were too stoned to read the AIS screen or hold a straight course

HNLMS Amsterdam....

As it was a replenishment ship, they probably had the best weed in the Dutch navy
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Old 15-07-2013, 13:30   #25
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I went through all these same questions last year before installing my ais transponder antenna. A lot of opinions, reasonable science, and real experience on CF. My final call was to Shakespeare since they make the antennas. Their advice was to have both at the masthead with a preferred 3+ feet separation. Mine are there now with 3' 9" apart. My VHF and AIS seem to work perfectly fine although admittedly it's hard to know for sure. How do you know what you're not getting? But I'm satisfied with any range I've had on both.

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Old 15-07-2013, 17:52   #26
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How did you get them 3 ft 9 apart without making a wonderful bird perch for a dozen seagulls?
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Old 15-07-2013, 18:45   #27
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Re: AIS transponder and VHF antennas on the masthead ?



This picture was taken while the mast was being raised when I was buying the boat in '11 and before I redid a lot of things. You can see my masthead has a lot going on. The two whip antennas you see are the VHF and now AIS (originally it was a unique DSC antenna before DSC was integrated into VHF). The VHF sticks out way aft on the rod holding the windex. The solid antenna you see was a cell antenna but I have replaced it with a Wifi antenna (also solid). Note the forward wind sensor is not yet installed.

I got plenty of advice not to have the Wifi antenna near the VHF/AIS but to me the use cases are completely different so it doesn't matter. I only actively use VHF/AIS while moving, and I only use Wifi when in port.

Also there's a good quiz here for the observant: what's wrong with this picture?
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Old 15-07-2013, 18:55   #28
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Re: AIS transponder and VHF antennas on the masthead ?

To address some other concerns mentioned above I'll also add that I have a spare VHF antenna all wired up inside my lazerette so if something goes wrong on the mast I can pull out the backup and put it on the aft rail. There is a switch inside my nav station cabinet that selects which antenna my VHF radio uses. I suppose at some point I should do the same for AIS.
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Old 15-07-2013, 20:10   #29
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Re: AIS transponder and VHF antennas on the masthead ?

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Originally Posted by jr_spyder View Post


Also there's a good quiz here for the observant: what's wrong with this picture?
The tricolor is rotated 90 degrees away from the correct orientation.

I would rather have the AIS antenna somewhere other than on the mast. AIS does not need a lot of height and having it somewhere else gives redundancy in case of a dismasting.
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Old 15-07-2013, 20:42   #30
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Re: AIS transponder and VHF antennas on the masthead ?

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They did not divert to steer clear of you because of the AIS!!! They were too stoned to read the AIS screen or hold a straight course

HNLMS Amsterdam....

As it was a replenishment ship, they probably had the best weed in the Dutch navy

Thats what I thought!
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