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Author Topic: AIS Receiver - Need help  (Read 9978 times)

Posts: 1

« on: March 03, 2014, 11:24:53 AM »


I have been building a network of AIS receivers along the Mississippi River. The latest one, my 10th install, has me questioning my knowledge on all aspects of this.

A typical receiver kit is a 4' 3db gain omni antenna to an n-type connector, 30-75' of Davis RF Bury-Flex with a PL-259 on the lower end, to a Comar AIS-3R receiver.

I've installed on a privately-owned telephone pole, building walls, 30-50' towers (Rohn 25/45 series), etc. Always above-ground, no buried or conduit work.

This one's a bit different in that it is on a custom 60' tower that is triangular, 36" sides, made entirely from 2" hot-dipped galvanized steel. It has a pair of 4" conduits at its base that lead inside the adjacent building. The conduits are PVC outside, but come up inside as electroplated galvanized steel.

All connectors are Amphenol nominal 50 ohm. I've been terminating my own cables since the first install 2 years ago. I use dial calipers to measure the cut-backs or making terminations and use Kester 44 solder.

Typical range is 20-30 nautical miles. I was expecting this one to be 40-50nm. I'm seeing 3-4nm!

I've replaced receivers twice, replaced the receiver->PC USB cable, replaced the processing PC, replaced connectors on both ends, and replaced the antenna.

I have never used lightning protectors/ground blocks, but I installed one on the new receiver as a troubleshooting step.

Also tried adding ferrite beads to power cables and USB cable as a final shot in the dark.

The antenna lead tests open circuit when not connected to the antenna. Short circuit(0.3-0.6 ohm) when shorted.

Any ideas? Is the conduit or buried section affecting something?

About me: I'm fairly experienced with AC and DC electronics and PC's/servers both operating and building. Very experienced with soldering. I know relatively little about RF theory or finer points. Almost entirely self-taught, so I'm missing the typical classroom/textbook concepts.

Thanks for any help!

PS - This is supposed to be Slash2cpu. Not sure why the forum shows as CarsonZ06???

Posts: 14710

« Reply #1 on: March 03, 2014, 01:24:16 PM »

Placing the coax under ground or in conduit shouldn't have any affect. Is the total length of the coax run considerably longer than your other installations? All coax cable has loss so if you double the length of the run you double the loss. Make sure you haven't damaged the coax while pulling it through the conduit. If you scrape off the outer insulation then moisture can enter the coax in a relatively short time and cause the attenuation to go way up. Sometimes underground conduits collect water in them from condensation or leaks. That won't hurt direct burial cable UNLESS you damage the outer insulation during the pull. 

Does the antenna have a clear line of sight out over the water? If there is anything blocking that (tower legs, other metal structures, etc) then the signal will be attenuated.

Do you have a portable receiver that you can monitor the frequency with? You might want to listen for noise or interference from other devices that might be operating in the same building or somewhere nearby. If you have interference or a high noise level then the receiver will only hear signals that are strong enough (i.e. close enough) to overcome that noise or interference. Noise can be generated by non-radio electronic devices like computers, power systems, etc.

« Last Edit: March 03, 2014, 01:28:19 PM by AA4PB » Logged

Bob  AA4PB
Garrisonville, VA

Posts: 1

« Reply #2 on: March 05, 2014, 02:40:54 PM »

Thanks for the troubleshooting ideas.

I pulled the cable out and checked the jacket. A few scratches, but no nicks or scrapes.

It's not a significantly longer run than 2 of the other stations.  I have one that is almost identical and one that is about 10' longer.

It's a clear line of sight for at least 15 miles. I can visually see ships while on the tower, and the receiver isn't tracking them.

Antenna  is at the top of the tower. There is a 3" diameter aluminum mast about 3' behind the antenna, but it's on the land-side. I wouldn't expect that to matter?

That leaves interference. I was hoping there was some easy fix that I was missing.

I have a Motorola Saber VHF handheld that I can program to the 2 AIS channels. I think I'm just listening for static, hum, etc to get louder on those channels?

Posts: 14710

« Reply #3 on: March 05, 2014, 04:40:49 PM »

Probably a "buzz" or something like that. Since the receivers are FM they won't be as sensitive to the noise as would an AM receiver. You might also try connecting the receiver to the coax and antenna to see if some interference farther away is being picked up by the antenna. I expect that with the antenna connected you will hear the burst transmissions from the ships. You might get yourself a "reference" by connecting the receiver to the antenna at another working site and compare what you hear.

The really bad news is that if you do have interference you'll probably have to track it down and eliminate it at the source rather than using some kind of filter at your receiver to get rid of it.

Bob  AA4PB
Garrisonville, VA
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