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Old 26-08-2014, 02:05   #1
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AIS Problems with Antenna

I am installing an EM-trak B100 class B transponder AIS. I installed a Shakespeare 4200 on bimini about 2m above the water. Not perfect but easily doable. On receive I can see 50+ AIS signals in our area, with the furthest boat about 30nm away, ATON up to 50nm. I am in a marina inside a deep bay with shallow land around.

I cannot see my boat on MarineTraffic, but other boats closel by. A friend on the same docks can see me on his AIS receiver.

Antenna Shakepeare 4200 with 15' of RG58 cable and PL259 connector. Originally I had extended this with 15' RG8 with PL259 ends and a joiner. Initially I thought that my extension may be bad. Today I moved the B100 temporarily to the aft cabin and hooked it up directly to the Shakespeare pig tail. Same or worse results (number to follow further down). I redid (soledered the PL259 twice with same reults. I checked with multi meter that I have no connection between shield and core.


Testing so far.
When I hook up to the mast head antenna from the VHF I can see my boat on MarineTraffic.

The ProAIS software has a diagnostics page with VSWR. Here are some screen shots.

Picture 1: SWR 0.9:1 with mast head antenna
Click image for larger version

Name:	SWR <a title=bimini.png Views: 201 Size: 120.7 KB ID: 87136" style="margin: 2px" />

Picture 2: SWR 1.9:1 with 4200 on bimini and RG extension
Click image for larger version

Name:	SWR mast head.png
Views:	184
Size:	111.1 KB
ID:	87135

Picture 3: SWR 3.1:1 without extension on supplied 15' pigtail RG58, connector twice redone - same readings for SWR
Click image for larger version

Name:	SWR in aft <a title=cabin.png Views: 176 Size: 109.9 KB ID: 87137" style="margin: 2px" />

Is the antenna bad?
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Old 26-08-2014, 02:44   #2
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Re: AIS problems with antenna

Roetter,

I have an even worse problem. My AIS antenna is mounted at the rear of the port hull, and I have a huge blind area to the front and to the right. I am installing a splitter to use the VHF antenna at the top of the mast for the AIS as well. That should resolve it.

Oliver
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Old 26-08-2014, 03:07   #3
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Re: AIS problems with antenna

Quote:
Originally Posted by roetter View Post
Is the antenna bad?
It might be bad. VSWR going higher with less cable is a clue. Less cable means less power loss so the ratio of forward to reflected goes up and thus VSWR goes up. So that tells us the problem is toward the antenna and not toward the AIS unit. I presume the antenna doesn't have a connector. If it did you might try a different cable.

Check the installation instructions for the antenna. Make sure you installed it per their recommendations. No big metal parts next to it and stuff like that. If you are sure it's installed right take it back and get another one.
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Old 26-08-2014, 08:27   #4
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Re: AIS problems with antenna

Quote:
PL259 connector
Interesting problem!
Nice to see you soldered the internal wire into the connector few do.
One error that I kept on making was losing just 1 filiment of the outside wire and it would get itself caught up and make a short.
If its not that, and you have checked connections a zillion, trillion times then it must be the antenna. Hopefully the shop you bought it from is close and you can swap it and test it..

The ProAIS2 is terrific with the VSWR meter. how the hell would we know if we didnt have it?
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Old 26-08-2014, 10:12   #5
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Re: AIS problems with antenna

I had a chance to look up the antenna. It has a connector on the base. Try bringing the antenna inside and connect it with the shorter cable you have. I now suspect the cable or connector at the antenna end. That is more probable than a bad antenna. The VSWR should not be bad even with the antenna inside the boat.
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Old 26-08-2014, 11:02   #6
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Re: AIS Problems with Antenna

roetter...there are varied opinions as to coax, just my preference but RGX8 seems to have worked best for my VHF/AIS..
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Old 26-08-2014, 11:04   #7
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Re: AIS Problems with Antenna

I agree with Markj on soldering the quality PL259. No worries of connection nor corrosion.
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Old 26-08-2014, 11:07   #8
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Re: AIS Problems with Antenna

In the cases of your high VSWR numbers, the number of samples taken (TX events) is very low. So there is probably a fair amount of error in the displayed values. Not sure where on the bimini you mounted your antenna, but you might want to try temporarily moving away from potential metal interference and see if the VSWRs change. And measure with a lot more TX events over time.
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Old 26-08-2014, 11:12   #9
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Re: AIS Problems with Antenna

Beg, borrow, or buy an antenna analyzer; it's clearly in the cable or term's. A quick sweep will confirm the problem. Or, just replace the cable with a professionally crimped or soldered one...
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Old 26-08-2014, 13:14   #10
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Re: AIS Problems with Antenna

Quote:
Originally Posted by Paul L View Post
In the cases of your high VSWR numbers, the number of samples taken (TX events) is very low. So there is probably a fair amount of error in the displayed values. Not sure where on the bimini you mounted your antenna, but you might want to try temporarily moving away from potential metal interference and see if the VSWRs change. And measure with a lot more TX events over time.
Thanks for all the good replies.

Transmitterda:
Sorry a typo in my post - was about midnight and I just finished reading hundreds of post on the "anchoring pictures" thread. It is the 4400 not the 4200. The 4400 has a pigtail attached. Would have liked the 4200 but was not sure it would fit my mount.

Three possible problems I can see:

1 Instructions said to be careful when screwing the antenna onto the mount to ensure that the cable can freely rotate. I should have done that correctly. I went as far as taking a drill and widening and straightening the path through the mount and pay attention that the cable was rotating and not being twisted off at the base as I screwed it on.

2 As transmitterdan mentioned check for metel nearby. I include a picture of the installation. Anything wrong with that.

3 The antenna is bad. I will take it back after checkin the potential install problem.

MarkJ - The little strand of shielding was my worry and I awlays check with the multimeter before comitting to the soldering and afterwards too. The only thing I have not checked is the conductivity of the core and the shielding from the connector to the antenna. Will do that today. Also will check with a different location away from the bimini.
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Old 26-08-2014, 14:35   #11
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Re: AIS Problems with Antenna

Quote:
Originally Posted by roetter View Post
I am installing an EM-trak B100 class B transponder AIS. I installed a Shakespeare 4200 on bimini about 2m above the water.

...

The ProAIS software has a diagnostics page with VSWR. Here are some screen shots.
Good on you for what you have done so far.

ProAIS and ProAIS2 are Raymarine software packages. I've never tried to use them with EmTrak hardware. Who suggested that to you?

0.9:1 VSWR shouldn't ever happen. 1:1 is perfect impedance match.

You are getting good reception. Without being there I suspect that Scott is on the right track and you need someone that can use and interpret the results of an antenna analyzer. If you find the right person I suspect you'll find a bad connector, a chunk of bad coax, or an antenna shielding problem. The fellow you find with an antenna analyzer should bring a known-good portable antenna.

Quote:
Originally Posted by Oliver L. View Post
I am installing a splitter to use the VHF antenna at the top of the mast for the AIS as well. That should resolve it.
Splitters induce significant attenuation of the signal to both your AIS and your VHF. Not recommended.
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Old 26-08-2014, 15:18   #12
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Re: AIS Problems with Antenna

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Originally Posted by Auspicious View Post
Splitters induce significant attenuation of the signal to both your AIS and your VHF. Not recommended.
"Significant" is an imprecise term. Navico's splitter has 0dB loss on receive and <1dB when transmitting for both AIS and VHF. Vesper's splitter has a 1dB loss for AIS and VHF transmit, a 1.5dB loss for VHF receive and a 12dB GAIN for AIS receive.

I suspect the gain from mounting the antenna on the masthead using one of these splitters exceeds the 1dB loss, and the performance of such will exceed an antenna mounted on a stern rail without a splitter.

However, one may think 1dB is significant while another doesn't.

Mark
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Old 26-08-2014, 16:25   #13
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Re: AIS Problems with Antenna

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"Significant" is an imprecise term.
Hi Mark. My experience with splitters has been pretty clear. Since you brought up a couple of specific splitters could you provide links to the tech data sheets? Noise figure and impedance are is as important as attenuation although attenuation is usually the biggest deal in the real world.

I'll be thrilled if someone has come up with a splitter that changes the game that I'm not yet aware of.
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Old 26-08-2014, 16:31   #14
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Re: AIS Problems with Antenna

Vesper's splitter is about as good as you can get these days. What little attenuation it has in the VHF radio transmit path is not "significant" for probably 99% of cruisers.
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Old 26-08-2014, 16:54   #15
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Re: AIS Problems with Antenna

One thing that comes to my mind is, how is the Shakespeare antenna mounted? You say on the Bimini but is the mount metal/plastic?

The thought I have is that while the antenna is connected, you have the radiating element, but what about a counterpoise or groundplane to balance it out. Maybe having a short run of wire from the outer conductor (shield) part of the PL-259 to, say, the lifelines, might give the antenna a larger groundplane for the transmitted signal.
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