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Old 15-09-2014, 14:51   #16
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Re: AIS Antenna Not Working

It is true that some (many?) VHF marine antennas show a DC short...not measurable impedence.

You can't assume that because you are getting a short at the radio end of the coax that something is wrong UNLESS you have disconnected both ends.

Also, the manufacturers are getting sloppy. I'm holding in my hand right now a PL-259 UHF connector which came off a brand new Shakespeare VHF antenna. It is a solder type, not a crimp type, connector but guess what?

There is absolutely NO SOLDER! not in the center pin and not in any of the four holes. Someone at the factory simply slipped the PL-259 over the coax and forgot to solder it, never mind test it.

Bill
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Old 15-09-2014, 19:14   #17
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Re: AIS Antenna Not Working

The Shakespeare AIS antenna is an end fed half wave design that doesn't require a tuning coil to produce a 50 ohm load for the transmitter. It shouldn't read short at DC, but any of the models that are 5/8 wavelength(Λ) or colinear with a 5/8Λ element would read shorted. They'll typically have a loading coil that is shorted at some point to the radiator. If you aren't sure, disconnect the antenna if possible.

Or, get the crimper and get it right almost every time. Another thing I like to do, if you have a thru deck connection near the base of the mast, is to use N type connectors to join the cables. N type connectors are better made and much more durable than the typical PL259. You can use the same crimp tool too. None are waterproof or very weatherproof - seal any exposed connectors.
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Old 07-10-2014, 14:18   #18
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Re: AIS Antenna Not Working

The continuing saga.. Sent it back to the manufacturer they said they tested it on their antenna in the shop and it worked ok. Contacted Shakespeare and they sent me a new AIS antenna no charge (awesome service guys) unfortunately that did not fix the problem either. I've used three crimp on connectors now and I even chopped the last one off a few inches from the end and checked for shorts as well as continuity between the two conductors and it tested fine so my connector that the AIS reported as bad seems to have been just fine.

Also.. upon further testing I cannot get this ting to work with ANY antenna I have. It sometimes works for one or two transmissions then goes dead.


One other random observation is that the AIS diagnostic tool they provide tells me that it sees a SWVR of 4.2:1 consistently with NO antenna connected. Seems suspicious to me consider what I read on the net tells me that with no antenna connected the SWVR should be infinate meaning that the device should probably read an error. Out of curiosity I plugged in a new connector with no cable or antenna to the end of the AIS and my SWVR went up to 6.1:1. Seemed quite odd that it would change at all due to a few MM of connector that is not connecting to anything. I think I am going to push to send it back again and force them to just send me a new one, maybe they got lucky on their bench test or they are lying or the antenna they said they tested with was not actually connected!
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Old 07-10-2014, 18:16   #19
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Re: AIS Antenna Not Working

The Emtrak B100 is particularly sensitive to high SWR. I have one, and had a problem, too. Mine would transmit sometimes, but when it rained the SWR went up enough so the B100 would not transmit.

I solved the problem by using my antenna analyzer (MFJ 259B) to measure the SWR as seen by the B100. It was pretty high, right out of the box. That means the antenna was not trimmed properly and/or was being affected by other nearby antennas and structures.

The fix was easy enough: the resonant frequency was too LOW, so I just progressively trimmed a bit off the s/s whip until I brought the SWR down to an acceptable range....about 1.2:1, if I remember correctly.

Anyway, no problems at all since. You can see mine transmitting on marinetraffic.com or vesselfinder.com. My MMSI is 367119560.

Bill
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Old 07-10-2014, 19:05   #20
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Re: AIS Antenna Not Working

Quote:
Originally Posted by btrayfors View Post
The Emtrak B100 is particularly sensitive to high SWR. I have one, and had a problem, too. Mine would transmit sometimes, but when it rained the SWR went up enough so the B100 would not transmit.

I solved the problem by using my antenna analyzer (MFJ 259B) to measure the SWR as seen by the B100. It was pretty high, right out of the box. That means the antenna was not trimmed properly and/or was being affected by other nearby antennas and structures.

The fix was easy enough: the resonant frequency was too LOW, so I just progressively trimmed a bit off the s/s whip until I brought the SWR down to an acceptable range....about 1.2:1, if I remember correctly.

Anyway, no problems at all since. You can see mine transmitting on marinetraffic.com or vesselfinder.com. My MMSI is 367119560.

Bill
An excellent point!

I was using an 8ft, +6dB marine VHF whip antenna with an AIS receiver, and decided to measure the SWR. Wow! It was quite good on channel 16 (156.8 MHz), but at the high end of the band where the AIS channels are (161.975 and 162.025 MHz) the SWR was around 5:1! A shorter "sailboat-style" antenna is probably not as sharply tuned as my long collinear antenna was, but this is one reason why using a splitter to share a VHF antenna may not work as well as it should.

Since I couldn't tune this particular antenna I ended up building another from copper pipe. It's on top of my garage...

And for what it's worth, on one boat I do have a single antenna and a splitter for the AIS transponder. I haven't measured the SWR on the AIS channels, but it does seem to work well.
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Old 07-10-2014, 20:30   #21
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Re: AIS Antenna Not Working

We have a Vesper splitter sharing our Digital Antenna 3' loaded coil whip with the VHF. The SWR is 2.0 at the AIS frequencies.

Not perfect, but good enough, and probably better overall performance than mounting a better-matched antenna 40' lower on a rail.

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Old 15-12-2015, 07:17   #22
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Re: AIS Antenna Not Working

I got reminded of this thread from another thread, and need to add information to my post above.

While at the time, we were getting SWR=2.0 on that antenna, over time it got worse and variable. We would see 4:1 for a while, then 1:1 for a while, and variable between until it finally just went high and stayed there.

I removed the antenna and opened up the loading coil to find that the solder connection to the coil was a cold joint and was not electrically attached - just touching. This was a new antenna at the time.

The replacement antenna gives a 1:1 VSWR, and has continued to do so consistently.

I also tried several other brands of antenna from both our spares and from borrowing others. These all gave VSWR <1.5.

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