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Old 12-04-2012, 09:08   #16
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Re: AIS antenna

I found the Shakespear 5215- AIS found here:

Shakespeare Antenna Specifications: Shakespeare Squatty Bodyģ 5215-AIS VHF Marine Band

to be the same price as a regular VHF antenna about $50 so I figured since I was buying a dedicated AIS antenna I might as well get the one optimized for AIS. Probably not a tremendous difference in performance but if youre going to get a dedicated antenna why not? If your going to go the splitter route then sure why bother.
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Old 12-04-2012, 13:29   #17
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Re: AIS antenna

We have had some fun discussing AIS vs. Marine VHF antennas, but perhaps have not solved the OP's problem. His poor reception might be improved by an AIS antenna, but maybe not. Before doing anything I would recommend testing at the back of the radio for VSWR, and then at the base of the antenna if possible. And use quality coax to interconnect. Personally I used a VHF mobile in place of the AIS box (which was receiving poorly) and found that the brand new, name brand marine VHF 3dB antenna had a VSWR of 5:1! Changing to a new AIS 4.5dB antenna made a huge difference; a functioning marine VHF antenna would also have made a huge difference, so it is hard to tell the extra benefit of a purpose-built antenna. The point I would make is that it is not certain that a new antenna is performing to spec, so check first.
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Old 12-04-2012, 14:04   #18
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Re: AIS antenna

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Originally Posted by jeremiason View Post
The problem I have found is when in heavy weather the swaying mast causes me to loose targets... I don't know the technical reason why, just that when the mast is moving back and forth I will loose targets over a couple miles away.
A low gain antenna typically works over a broader range of angles than a high gain. That's why you'll see power boats with 8db (or more) antennas, but on a heeling sailboat the norm is 3db. It might be worth checking that you do have 3db antennas up there.
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Old 12-04-2012, 14:21   #19
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Re: AIS antenna

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Originally Posted by David M View Post
Actually it is not a ploy if you have a transceiver. For my Furuno FA-150 I tried a regular VHF antenna and it did not work. I called Furuno tech support and they said I need an AIS transceiver antenna, I changed out my regular antenna for this AIS antenna and it then worked...but this is just for a transceiver and not the OP's situation. A regular VHF antenna will work for a receiver.

Shakespeare Antenna Specifications: Shakespeare 396-1-AIS Broadband VHF
This is correct as far as the FA-150 goes. Just over a year ago we had our Product Detail - GPA017S/S antenna destroyed with no spare on board, we did however have a spare antenna for the VHF radios which we connected up until we received a replacement for the damaged one.
As David M states it basiclly didn't work, we did receive some sporadic signals from vessels within 10nm that just dropped in and out, the same with our outgoing signal (and this reported from sister vessels fitted with the exact same equipment) When the new antenna arrived and was fitted all was back to normal and the equipment was functioning as it should....and we now keep a spare onboard....

Obviously this doesn't help the OP with his/her question, i just posted to back David M's statement re the FA-150.....
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Old 12-04-2012, 16:40   #20
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Re: AIS antenna

I don't get it, you replaced your damaged GPS antenna with a VHF antenna?
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Old 12-04-2012, 16:42   #21
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Re: AIS antenna

Quote:
Originally Posted by IslandHopper View Post
This is correct as far as the FA-150 goes. Just over a year ago we had our Product Detail - GPA017S/S antenna destroyed with no spare on board, we did however have a spare antenna for the VHF radios which we connected up until we received a replacement for the damaged one.
As David M states it basiclly didn't work, we did receive some sporadic signals from vessels within 10nm that just dropped in and out, the same with our outgoing signal (and this reported from sister vessels fitted with the exact same equipment) When the new antenna arrived and was fitted all was back to normal and the equipment was functioning as it should....and we now keep a spare onboard....

Obviously this doesn't help the OP with his/her question, i just posted to back David M's statement re the FA-150.....
Is the link correct, if so, I am not surprised it didn't work with the spare VHF antenna as the link details a GPS antenna.
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Old 12-04-2012, 16:55   #22
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Re: AIS antenna

OOPS! wrong link

Product Detail - 001-034-670

...to many beers in QANTAS club bar....
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Old 12-04-2012, 17:10   #23
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Re: AIS antenna

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Originally Posted by IslandHopper View Post
OOPS! wrong link

Product Detail - 001-034-670

...to many beers in QANTAS club bar....
Well, this seems to be a standard sort VHF antenna but I suspect it is tuned for the AIS frequency however note 3 on this link Product Detail - FA150
states "If an aftermarket antenna is used, it must be capable of transmitting at 156.025 - 162.025 mHz" which sort of contradicts note 1 "UAIS rated VHF Antenna is required option - use 000-113-498 or purchase locally"

Sorry to interrupt your beers!
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Old 12-04-2012, 17:23   #24
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Re: AIS antenna

Quote:
Originally Posted by Wotname View Post
Well, this seems to be a standard sort VHF antenna but I suspect it is tuned for the AIS frequency however note 3 on this link Product Detail - FA150
states "If an aftermarket antenna is used, it must be capable of transmitting at 156.025 - 162.025 mHz" which sort of contradicts note 1 "UAIS rated VHF Antenna is required option - use 000-113-498 or purchase locally"

Sorry to interrupt your beers!
I don't know the technical aspects of it all, i just know what worked and what didn't on the day.....

You didn't interrupt the beer drinking, i can type and drink at the same time.......after all i am skilled in multitasking....
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Old 12-04-2012, 17:56   #25
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Re: AIS antenna

It has been awhile since I looked at transmitter design, but I am guessing that the Furuno is shutting down as a result of a (somewhat) high VSWR. At a basic level the VSWR is a measure of the reflected power from an antenna that is not perfectly resonant at the output frequency. This reflected power is ultimately absorbed by the output transistor; the higher the VSWR the more energy is absorbed and turned into heat in the radio. The radio could shut down the output either because it sensed the temperature rise in the transistor directly, or because it measured a high VSWR and proactively shut down. Either way, it is done to protect the output transistor from overheating. It does seem that this radio is unusually sensitive to moderately high VSWR; whether that is the result of the transistor/heat sink choice or an overly sensitive protection circuit is unknowable by us. Good to know before buying/installing this unit.
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Old 12-04-2012, 20:27   #26
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Re: AIS antenna

Even at the maximum reporting rate of every 2 seconds, the output power transistor/module is never going to get overheated when the packet transmission time is 26 milliseconds! The typical reporting rate (class A) for a ship traveling up to 14 knots on a relatively straight course is 10 seconds so it would be transmitting .26% of the time.

I don't know what the issue is with the FA-150, if there even is one, but I installed the class B Furuno FA-50 model on the Naval Academy Navy 44's and they share the regular VHF antenna through a splitter and they are working just fine. There's probably only about 1 watt going out of the masthead antenna and I receive them down the bay to around 18-20 miles from my shop antenna.

Eric
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Old 13-04-2012, 00:41   #27
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Re: AIS antenna

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Originally Posted by CarinaPDX View Post
My AIS antenna (the Digital Antenna 4.5dB AIS above) is about 12ft high, on the radar mount on the transom. I regularly see USCG movements at the mouth of the Columbia River, about 100 miles away. YMMV
Not to sound like a smartass, but...

That's because you're picking up the Columbia River Pilot's AIS repeater network. It allows them (and anyone else receiving AIS) to see traffic on the entire river and calculate meeting points.

I see vessel movements in Portland when still down around Tillamook and 50nm offshore.
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Old 13-04-2012, 03:03   #28
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Re: AIS antenna

HeHe - that explains it! I admit I thought it was a little too good to be true, and I wondered about the possibility of a repeater, but never saw any indication of one. Thanks for the correction.

On the previous post, I admit I was thinking more of typical radios used for continuous data transmission. A 2W output with approx 1% duty cycle isn't a whole lot of power; it is hard to believe that anyone would use an output transistor that couldn't absorb the reflected power - at least not without some serious cutting of corners. Still, high VSWR is the only obvious difference between antennas in both of the cases described, so it is a puzzle.

0-2 for the day. Some days it's better to get rained out...
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