AIS Problems with Antenna

[transmitterdan]

These antennas don't need a counterpoise like HF SSB systems. They can be affected by nearby metal changing the inductance of the coil in the base. But the high VSWR (3:1) reported suggests a cable or connector is bad. It might be the antenna itself but the more likely cause is a cabling problem.

[colemj]

Originally Posted by Auspicious

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.

http://www.simrad-yachting.com/Root/...0379-001_w.pdf

Antenna Splitter for Class B AIS Transponders

Mark

[DotDun]

Originally Posted by Auspicious

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

I believe ProAIS(2) ships with every SRT manufactured unit, which I also believe Raymarine OEMs.

[bill good]

Works on mast aerial OK system is OK. AIS aerial or cable. Disconnect the cable from the AIS unit. Unscrew PL259 from base of aerial, check DC resistance on open plug it will be open circuit. Put short beween centre pin & outer part of connector. Now reading should be close to zero ohms. Cable tested - all that is left is the aerial or the mounting plate thickness is preventing the PL259 connector making a proper connection at the aerial base.

Regards

[smac999]

unless you know the location of the nearest person sharing data to the ais websites (normally peoples houses) I wouldn't put much trust into that. it might be 40nm away. maybe your class b is going 35. and the other boats are using class A and going 50nm and reaching the guys house who is collecting and sharing the data to the net. the class A's transmit much further with more power.

best to find boats far away and see if they can see you.

1.9 with ant and ext cable sounds normal to me. 1.1 is a dream never seen in real life

does the boat show up on marine traffic with the mast ant? (you have to leave it on for a little while before it'll show up)

[roetter]

Originally Posted by bill good

Works on mast aerial OK system is OK. AIS aerial or cable. Disconnect the cable from the AIS unit. Unscrew PL259 from base of aerial, check DC resistance on open plug it will be open circuit. Put short beween centre pin & outer part of connector. Now reading should be close to zero ohms. Cable tested - all that is left is the aerial or the mounting plate thickness is preventing the PL259 connector making a proper connection at the aerial base.

Regards

The "pig-tail" cable is firmly attched to the antenna and cannot be disconnected without destruction. Since the antenna is a whip I guess I can measure the connection from the core pin at the end of the cable to the metal whip.

I checked 0 Ohm core to shield. Tomorrow I will do more tests.

[bill good]

Originally Posted by roetter

The "pig-tail" cable is firmly attched to the antenna and cannot be disconnected without destruction. Since the antenna is a whip I guess I can measure the connection from the core pin at the end of the cable to the metal whip.

I checked 0 Ohm core to shield. Tomorrow I will do more tests.

It is likely that 0 is normal. If you test the extention in the the way indicated I would be betting on the aerial. Can you connect this aerial to your VHF radio for a recieve only test?

Regards

[transmitterdan]

Originally Posted by roetter

The "pig-tail" cable is firmly attched to the antenna and cannot be disconnected without destruction. Since the antenna is a whip I guess I can measure the connection from the core pin at the end of the cable to the metal whip.

I checked 0 Ohm core to shield. Tomorrow I will do more tests.

0 ohms inner to outer is correct. But that doesn't prove it is ok either. Make sure there is no metal within about a foot horizontally from the antenna. Metal below the antenna is ok. If you have a picture of the antenna mounting that might trigger some further advice.

Check the coax run for kinks or knots. Sometimes that will cause VSWR problems.

[bill good]

Originally Posted by roetter

The "pig-tail" cable is firmly attched to the antenna and cannot be disconnected without destruction. Since the antenna is a whip I guess I can measure the connection from the core pin at the end of the cable to the metal whip.

I checked 0 Ohm core to shield. Tomorrow I will do more tests.

The 4200 is terminated with a PL259 socket and not supplied with a lead. There is no flylead from the information on the net. Are you sure you have a suitable VHF aerial? What you have maybe a different aerial from what you expect.

Regards

[btrayfors]

I had a similar problem last year while installing the same AIS unit, i.e., high SWR was limiting transmitted AIS signal range.

After checking everything, I determined that problem was the antenna itself. It's mounted on a 1' extension on the pushpit rail, between a couple of other antennas. Problem was likely interaction.

Unfortunately, it would not have been easy to move it or find another spot. So, I just used an antenna analyzer to check it out. Found that the resonant frequency was LOWER than desired, i.e., the whip antenna was too long. I trimmed it, little by little, until I achieved a 1.2 to 1.0 SWR.

Voila! No more problems. It's worked perfectly for almost a year, and range is as good as can be expected from an antenna mounted just 8' above the water.

Bill

[AnchorageGuy]

Don't want to hijack the thread but I do have a question on a couple of comments. My antenna is mounted on the side of the flybridge and is withing inches of the aluminum frame for the hardtop. A few comments say stay away from metal. This is an 8 foot Shakespeare. Is the aluminum frame going to affect the antenna? It's my understanding that the fiberglass antenna is only using the top 19". If that is the case, that is well above the frame. In a regular installation like the OP talks about, his antenna is mounted on the bimini frame. Chuck

[continuouswave]

Based on the measured VSWR data you have posted, my inference is as follows:

The antenna you are using for your AIS transponder is not properly tuned for the AIS channels. The AIS channels are far removed from the usual frequencies used by a VHF Marine Band radio. Unless you purchased an antenna that was specifically tuned for AIS use, the antenna's VSWR will probably be above 3:1 at the AIS frequencies.

It is common that solid-state transmitters protect themselves against transmitting into a high VSWR load by reducing and limiting their power output. Because the AIS transponder is already rather low power, 2-Watts for a Class B, any reduction in transmitter power due to bad antenna VSWR is to be avoided.

Any measurement of transmission line VSWR in which the VSWR measurement is made at a distance away from the antenna will be affected by transmission line loss. At VHF or UHF, where transmission line loss is greater, it is generally considered that on any antenna installation with a long transmission line with attendant loss, if the VSWR is greater than 3:1 there is probably no antenna even connected on the far end.

Your measurement of VSWR 3:1 with a short (15-foot) transmission line indicates the antenna is connected but is badly tuned.

Originally Posted by btrayfors

I had a similar problem last year while installing the same AIS unit, i.e., high SWR was limiting transmitted AIS signal range.

The VSWR of the transmission line has no effect on the range of an antenna. It only tends to increase the line loss or to reduce the transmitter power output by a protection circuit. Once the radio-frequency energy reaches the antenna, it radiates just like any other energy that gets there; it cares not a whit for the VSWR on the transmission line that carried it there. If your range was reduced, it was due to less power delivered to the antenna, not because there were standing waves on the transmission line.

[btrayfors]

Originally Posted by continuouswave

Based on the measured VSWR data you have posted, my inference is as follows:

The antenna you are using for your AIS transponder is not properly tuned for the AIS channels. The AIS channels are far removed from the usual frequencies used by a VHF Marine Band radio. Unless you purchased an antenna that was specifically tuned for AIS use, the antenna's VSWR will probably be above 3:1 at the AIS frequencies.

It is common that solid-state transmitters protect themselves against transmitting into a high VSWR load by reducing and limiting their power output. Because the AIS transponder is already rather low power, 2-Watts for a Class B, any reduction in transmitter power due to bad antenna VSWR is to be avoided.

Any measurement of transmission line VSWR in which the VSWR measurement is made at a distance away from the antenna will be affected by transmission line loss. At VHF or UHF, where transmission line loss is greater, it is generally considered that on any antenna installation with a long transmission line with attendant loss, if the VSWR is greater than 3:1 there is probably no antenna even connected on the far end.

Your measurement of VSWR 3:1 with a short (15-foot) transmission line indicates the antenna is connected but is badly tuned.




The VSWR of the transmission line has no effect on the range of an antenna. It only tends to increase the line loss or to reduce the transmitter power output by a protection circuit. Once the radio-frequency energy reaches the antenna, it radiates just like any other energy that gets there; it cares not a whit for the VSWR on the transmission line that carried it there. If your range was reduced, it was due to less power delivered to the antenna, not because there were standing waves on the transmission line.

Well, that's only half right. Or less.

High SWR at the radio can and does result in reduced transmitter power (as you said yourself).

In the old days with tubes as finals, it didn't make much difference. Now, and for a couple of decades with transistorized emitters and amplifiers, it does. Most transmitters have a built-in protection circuit which will reduce power output when high SWR is detected in order to protect the finals.

Reduced power output of the transmitter can in fact affect transmission range, sometimes severely.

BTW, the AIS frequencies are at the top of the marine VHF band, very near the WX frequencies. The two frequencies used worldwide are 161.975 and 162.025 MHz (channels 87B and 88B, or AIS1 and AIS2).

The paragraph above concerning length of transmission line and VSWR is very misleading. Yes, transmission line length and type can affect VSWR at the transmitter, but it is perfectly possible to trim the antenna and/or ground system to bring VSWR to an acceptably low level so the transmitter won't reduce power output. That's what I've done with my AIS unit.

Bill
WA6CCA

[ka4wja]

EDIT:::
Oppss...I just saw the 2nd page of this thread....sorry if I repeated info that others have posted....


I hate that we are all still calling 'em "splitters", as they are really "splitters/relays/switches"....but, oh well....we call Marine HF radio "SSB" all the time....so, no worries...


Mark,
Except for the "0db" spec from navico, (and that if they really had "0db" loss, that would mean a permanent receive pre-amp and splitter is being used for both AIS rx and VHF rx, so that if 12vdc power is lost from this splitter or it fails for some reason, you lose your primary VHF radio's antenna....something that CANNOT happen with the Vesper SP-160 "splitter"!!), I agree with what you write here...

Originally Posted by colemj

"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.

And Dave, in addition to the Vesper specs, I have done my own tests, and I've posted the details and spectral scans on-line for all to see...

Originally Posted by Auspicious

... 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.

The Vesper unit has loss of < 1db on VHF tx and rx, and on AIS tx....and DOES provide an improvement in AIS reception with its pre-amp....it is 50ohms (of course) and has a low noise figure AIS rx pre-amp (as evidenced by my tests)....
Please have a look at my posting here...where you'll see
Vesper AIS SP-160 "relay/splitter" test results


As well as Vesper's site...
Antenna Splitter for Class B AIS Transponders
Antenna Splitter for Class B AIS Transponders


As for noise figure, I no longer have my HP8970, but nobody makes VHF pre-amps with a high noise figure anymore, I suspect 2db noise figure at most...
(heck, I've made some homebrew vhf pre-amps w/ 0.25db noise figure for about $5 in parts....)
But, as you know for terrestrial VHF FM comms, the "noise figure" of the receiving system is limited by the receive bandwidth....using a front end / first RF amp (or ext. pre-amp) of noise figures from 0.5db thru 2-3db, would have no effect on receive system noise figure....
So, for the sake of this discussion (terrestrial VHF FM signals and AIS/VHF antenna splitters/relays), "noise figure" is a fairly moot point...




Now as for VHF Marine antennas....never mind...
Well, seriously I don't want to start an argument, but if you can accept some things on faith....please accept that using a VHF antenna with coax attached is a bad idea for most cruising boats and please do not use RG-58 coax (unless the total length is just 2' - 3')!!!
Read some of the facts/info on, coax / coax loss / lack of connector loss / etc. in this posting as well...
Vesper AIS SP-160 "relay/splitter" test results

If, like me, you need/desire a stainless-steel whip-type VHF antenna....
I recommend the Shakespeare 5215 (or 5215-AIS)....or the 5442-A...
5215-AIS Squatty Body® | Shakespeare Electronic Products Group

Shakespeare 5215 Squatty Body VHF Antenna

Make sure you have the antenna mounted in the clear as best you can, and keep it 3', or more, away from other vertical metal structures / masts / antennas, etc....
And, please do not hinder yourself with 15' of RG-58 (which has 0.9db loss, just on its own).....and , no you do not need any counterpoise wires....

And, read this posting, and my earlier article on AIS...
SSCA Forum • View topic - Vesper AIS SP-160 "relay/splitter" test results




FYI, here are photos of my set-up that has worked very well for me offshore....

My masthead-mounted primary VHF / AIS antenna.....




My stern-mounted back-up VHF/AIS antenna (originally my back-up VHF and primary AIS antenna)...







I hope this helps...

Fair winds..

John
s/v Annie Laurie

[ka4wja]

Dan,
Are you SURE about this???

Originally Posted by transmitterdan

0 ohms inner to outer is correct.

The Shakespeare 5215's and 5241's, etc. are NOT DC Ground antennas....
5215-AIS Squatty Body® | Shakespeare Electronic Products Group
I don't have the factory specs for the 4400, but as a base-fed-halfwave-whip, I suspect it is like the other Shakespeare VHF whips, and therefore should NOT show "0" ohms (direct short), but rather "infinite" (open) resistance when measured with a ohm-meter / digital multimeter....

So, again I ask....
Are you SURE about the Shakespeare 4400 being a DC Ground antenna??

And, if as I suspect that the 4400 is not a dc grounded antenna, then we have found the proximate cause of his troubles....the coax and/or the coax/antenna connection...



FYI, anyone wishing to see a VSWR plot of my masthead Shakespeare VHF antenna (typical real world results)....have a look here...(it's NOT the "AIS version", which would have a slightly better VSWR at the high end...sorry I didn't measure my "AIS version", as it was starting to storm/lightning out, and I wanted to get my test gear ashore and safe!!)
It is this antenna....








For the spectral scans of the Vesper SP-160 splitter, please see the referenced thread (as there is detail there that explains things, it makes no sense to post 'em here without the details/explanation!!!)

Vesper AIS SP-160 "relay/splitter" test results



And, be sure to see the minute loss of 23 connectors/adapters/etc...(a total loss of 0.2db thru the whole 23 adapters/connectors and a short piece of coax...so next time someone wants to BS you about "connector loss", you'll know better..)






Fair winds...

John
s/v Annie Laurie