AIS Problems with Antenna

[MarkJ]

1.2 is excellent!!!!!!! Well done!


Like you on your masthead antenna I used to get less than 1:1 i.e. better than perfect. but after a few months that came back to 1:1.
But is good to be better than perfect

[ka4wja]

Chuck,
This is a good plan....especially with the specs that Dan posted...

Originally Posted by AnchorageGuy

Thanks, which is why I'm removing it and adding another antenna just for the AIS, and the VHF will be on it's own. Chuck

Dan, thanks for finding/posting this!!
This is one reason why "splitters" got a bad reputation....(as well as introducing another failure point into your primary VHF system, which is a primary safety tool aboard!)

Originally Posted by transmitterdan

Insertion loss, AIS Receive path Typical 4dB
Insertion loss, VHF Radio Receive path Typical 4dB
Insertion loss, AIS Transmit path Typical 3.5dB
Insertion loss, VHF Radio Transmit path Typical 1.5dB

These are among the worst specifications of any AIS splitter I have seen. There are a lot out there better than this. IMO these specifications are really not very good.

Fair winds...

John
s/v Annie Laurie

[ka4wja]

roetter,
No worries....we've all done odd things when we are tired!!!

Originally Posted by roetter

Sorry about the error in my upate post. I guess I should not write any replies when I am dead tired.

The ohm meter was showing 0 conductivity (infinite ohms) between core and shield.

And as I wrote earlier, I don't have the factory specs for the Shakespeare 4400....but the paper work that came with the antenna should tell you whether it is a "DC Grounded" antenna, or not....(I suspect not, just like the 5215, etc.)
If it is NOT a dc grounded antenna, then infinite ohms would be the correct reading between the center-conductor (core) and shield of the coax....
(versus a dc grounded antenna, which would show a direct short / zero ohms, or VERY close to zero, when measuring between the center conductor and shield...)







As Bill correctly pointed out, most modern transmitters will reduce power (or shut-down completely) when subjected to high VSWR's (typically starting at VSWR's of 2:1 or higher....), and the added cable losses associated with higher VSWR's, etc., both play significant roles in reduced transmit performance...
So, making sure your radio or AIS transponder sees a VSWR of 2:1 or less IS important....

Originally Posted by roetter

New information:
I exchanged the antenna for a new one today. Result: Much better but not great.
Thanks all you guys that recommended to check for any metal object running parallel to the antennay nearby. I had completely missed the man-over-board-pole. Once I removed the pole near the antenna the VSWR was way better.

Picture below shows the antenna and GPS on the starboard side of the bimini. Behind the antenna in the white and red is a MOB-pole only a few inches away.

Screen shot below shows same conficuration but MOB-pole removed.

Now that is much better. Maybe some people can learn from my mistake. For some reason I thought the MOB was pastil, but it is metal.

I guess I will still get the Vesper antenna "splitter", as I still can not see my boat on Marinetraffic.com, which I can when the AIS is conncted to the mast-head VHF. The VHF on the bimini will remain there as a back up.

And, since my MOB pole is yellow fiberglass (GRP), I wouldn't have thought of telling you to move your MOB pole....
So, good thought you had checking it and seeing it was alum!!!
(yes, anything metallic that close to the antenna / running parallel to the antenna, WILL have a significant effect on not just the VSWR but also on antenna pattern / performance...)



As, for using the Vesper "splitter"....
In the past, I was like Dave and Bill and eschewed them.....

But, after seeing, using, and testing the Vesper SP-160, I was convinced that it was a different sort of animal (as Dan pointed out, by supplying Chuck with the abysmal specs of the Ameco "splitter")....its low loss and complete "fail safe" design, allows for a good deal of piece of mind...
So, keep the bimini-mounted antenna as a back-up, and install the Vesper SP-160....and you'll be in good shape...

One note of caution...
Make sure the coaxial cables / connections you use are GOOD....as even slight problems here can add up to big issues at VHF....
(and except for a short 2' - 3' length, do NOT use RG-58 coax, as it is too lossy...)
See the bottom half of my posting here for some details about cable...
SSCA Forum • View topic - Vesper AIS SP-160 "relay/splitter" test results



Fair winds..

John
s/v Annie Laurie


P.S. Just an FYI...
I have my AIS transponder (and Vesper SP-160) powered via a separate breaker, than my M-602 VHF....and when just using my VHF (such as at the dock) and not the AIS, the SP-160 is of course not supplied power and is in "bypass" / fail-safe....and has been working this way just fine for > a year now....
The SP-160 is powered on when the AIS trasnponder's breaker is switched on....

[Paul L]

Originally Posted by roetter

....

I guess I will still get the Vesper antenna "splitter", as I still can not see my boat on Marinetraffic.com, which I can when the AIS is conncted to the mast-head VHF. The VHF on the bimini will remain there as a back up.

The splitter might be an overkill. Class B AIS is specicifically designed for short distance. There is little advantage to trying to get your signal out past 5 or 8 miles. For long distance AIS there's class A equipment.

[Jaxxiam]

Great knowledge base, I'm grateful to know help is a question away.
I'm sure I will have a few.

[roetter]

Now that I discovered the problem with the MOB pole close to the AIS antenna, I am wondering what 2 VHF antennas (one AIS, one radio) will do to each other. Would I get a bad VSWR as well?


Rolf

[transmitterdan]

Rolf,

If they are very close then maybe. But if they are 500mm apart them probably it will be ok. Better would be 1M apart.

[roetter]

Originally Posted by transmitterdan

Rolf,

If they are very close then maybe. But if they are 500mm apart them probably it will be ok. Better would be 1M apart.

Here is a picture that was taken to document broken spinnaker halyard fittings on the mast. It also shows the antennas with some jury-rigging at the bracket as was going bad on us. The antennas and brackets have since been replaced but would be in the same location. Looks less than 500mm, more like 250-300mm.

[transmitterdan]

About all you can do is try this, it might work ok. I know some people put their AIS antenna at the upper spreader (upside down) to avoid this problem. Another option is to put a bracket at the masthead that let's you space them further apart. If it were me that would be my preference. I have even seen some mount the AIS antenna upside down off a yardarm at the masthead.

[ka4wja]

Rolf,
As Dan wrote, 1m / 3' away, will have little, to no, effect on antenna VSWR, nor on antenna radiation patterns...

Originally Posted by roetter

Now that I discovered the problem with the MOB pole close to the AIS antenna, I am wondering what 2 VHF antennas (one AIS, one radio) will do to each other. Would I get a bad VSWR as well?

But, once you move it closer (say closer than 500mm) you will have some effect on antenna patterns, and once you get real close (< 200mm) there will be significant effects on VSWR, as well as antenna patterns....



But...
But, from this additional posting of yours, I'm wondering if you are referring to having BOTH your AIS antenna and VHF-marine antenna mounted near / next-to each other???

Originally Posted by roetter

The antennas and brackets have since been replaced but would be in the same location. Looks less than 500mm, more like 250-300mm.

If this is the case, then the answer is, NO....Do NOT do that!!!
Any distance less than 1' / 300mm and you'll probably damage the receivers in both units (certainly the AIS unit will be damaged, by the VHF's transmit energy!)

I recommend a separation of 6' - 10' (2m - 3m), which is fairly impractical / difficult to do, at the masthead...


A couple years ago, I addressed this (in layperson's terms) in this article...
AIS Transponder

If I wanted to use a mast-top antenna, I needed to move the antennas further apart horz and/or vertically, in order to keep any damaging radio energy from the main VHF transmitter and AIS transmitter out of the adjacent units' receivers…(this is something I've had a GREAT deal of experience with in VHF repeater design/installation over the past 30 years…and I determined that 6' - 10' of sep was required…but others have made do with 3' of separation…) This antenna isolation (separation) would entail building and installing a couple of stand-offs at the masthead, and adding extra cabling to the antennas and/or mounting one antenna on the top spreader and running all new cabling…(neither was a project that I looked forward to!!!) BUT… But, as I thought more about it I realized that many cruising boats have used a rail-mounted or bimini-mounted AIS antenna (about 10' off the water) to good success…so, I thought I'd give it a go, and see what results I got!!!

And, from 2.5 years ago....

My ultimate plan calls for a top spreader mounted AIS antenna (about 30" away from the mast) and moving the masthead-mounted primary VHF outboard on a 30" stand-off, placing it about 10' directly above the AIS antenna, giving me ~ 45db - 50db of isolation between them.....
BUT, this will require pulling the mast, etc. (I've already tried running the cables with the mast in place, and no joy!!!).....so, my ultimate plan will wait a few years!!!

I've decide that using an "antenna splitter" (although we know they're not really just splitters, that's become the accepted jargon for them) is not for me.....I came up with a "plan B"....

Plan B has me using a spare vhf antenna, mounted at the stern about 10' off the water, fed with ~ 30' of LMR400uf, for my AIS transponder antenna...

My current masthead has 2 "vhf" antennas, mounted about 12" apart
a) my primary VHF antenna...
b) a 144mhz/220mhz/440mhz mobile whip antenna (which I use with my current AIS receiver)
Although there is only ~ 20 db isolation (measured by me) between these 2 antenna on marine vhf freqs, over 6 years of use, I've not experienced any issues with my AIS receiver....it still works great!!!
BUT....
BUT, I'm not willing to place my primary VHF radio (Icom M-602) in jeopardy by placing a vhf transmit antenna in close proximity, without adequate isolation (30-40db)......I could accept some "desense" that will occur on the M-602, during the short AIS transmit burst, which will occur until isolation is raised to 40- 60 db or more (depending on channel selected and on the exact radio's design specs), but I cannot accept the chance that I'd damage the M-602's front-end, with inadequate isolation (>30db)....

Then last year, I made a few changes....
http://www.cruisersforum.com/forums/...ld-130803.html



Again, I'm not sure if you were asking about the viability of mounting BOTH the AIS transponder antenna and the VHF-marine antenna 250mm from each other....but, again, if this was the case....please do NOT do this....



I hope this helps...

Fair winds...

John
s/v Annie Laurie

[roetter]

Thanks John

The installation in in the picture is almost identical to the one I currently have on my catamaran. The initial problem in this thread was on my mono hull. The cat now has 2 whip antennas at the distance shown in the picture.

For the cat I had asked the company that does the additional electrical installations for Lagoon deliveries in Les Sables, France to install a VHF and a AIS antenna on my mast. This is what I got. It has been working for more than a year now. Neither the radio nor the AIS transponder have failed. I will definitely be looking at moving them further apart this fall or going to the Vesper "splitter".