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Old 04-09-2011, 15:52   #1
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AIS and VHF Performance

I am looking at AIS receivers. Some units have an internal AIS / VHF signal splitter that obviates the need for a seperate AIS antenna or external splitter. I have heard that that VHF performance is compromised with use of the AIS internal signal splitter. Fact or fiction? Any comments?
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Old 04-09-2011, 16:14   #2
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Re: AIS and VHF performance

Fact.

A few things happen.

- The splitter introduces some signal loss.

- Only one device can be transmitting at a time, and I believe all give priority to the VHF. That means your AIS transmissions can get cut off. It's not the end of the world, but the more you are on your VHF the less your AIS will be updating your location.

- The antenna specs are different for AIS than for VHF. AIS calls for a max of 3db gain on the antenna which typically translates into a 3' or 4' antenna. With your VHF you typically want an 8' antenna with 6db of gain, sometimes more.

My first AIS was a Raymarine with built-in splitter. The antenna was small for the boat, but the AIS still constantly complained that the SWR was out of spec, I think because the antenna was too high gain.

When I did the electronics on my current boat I made a point of separating them. The AIS gets an antenna specifically designed for AIS, and the VHF get the best VHF antenna it can have. Both are happier as a result. It's obviously more work to install, but I think worth it. There's also a little added redundancy which never hurts.

One place where the shared antenna might make sense is on a sail boat with only a 3db antenna on top of the mast. If a 3bd antenna is all you are going to have for your VHF (and all you need because you can get a lot of height from the mast), sharing that antenna might make a lot of sense and not pose the compromise that other situations would.
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Old 04-09-2011, 16:28   #3
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Re: AIS and VHF performance

Most internal VHF/AIS units don't incorporate an internal splitter. They time share on the frequencies as required for TX. For RX separate receive channels are mostly used. A splitter is only required when you use separate AIS/VHF units and a single VHF antenna.
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Old 04-09-2011, 17:12   #4
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Re: AIS and VHF performance

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Most internal VHF/AIS units don't incorporate an internal splitter. They time share on the frequencies as required for TX. For RX separate receive channels are mostly used. A splitter is only required when you use separate AIS/VHF units and a single VHF antenna.
Yes, but I think he's asking about the latter.
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Old 05-09-2011, 08:38   #5
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Re: AIS and VHF Performance

Splitting an antenna for simultaneous use by two receivers is typically done in a manner that results in at least a 3-dB loss of signal to the receiver from the antenna. However, in most cases this 3-dB loss is not particularly tragic and will not affect reception. I explain:

A receiver generally has a great deal of reserve gain, so if there is attenuation of 3-dB between the antenna and the receiver, the receiver just increases its gain to make up. The only time this 3-dB loss can be important or could affect reception is when the received signal is very weak and the receiver is operating at its maximum gain. Also, loss between antenna and receiver affects the noise figure of the receiving system, and in very weak signal work an increase in the noise figure of 3-dB (from a splitter) would be unacceptable.

In most VHF Marine Band radio installations on boats, there are other factors affecting the ultimate receiver sensitivity. Locally generated noise is often a factor. Noise from other electronics or from spark ignition may create a higher noise floor, rendering the 3-dB loss in the antenna as insignificant. Of course, there could be some instance at some time when reception would be be better without the 3-dB loss in an antenna splitter. This assumes, or course, that the separate antennas used in place of a splitter would each be as favorably located as the shared antenna used with a splitter.

In regard to transmitting through an antenna splitter, there is no universal design for how this is to be done. I would assume that most of these devices must employ some sort of very fast acting detector which senses the transmitter power applied to one port of the splitter and disconnects the receiver on the other port. More elaborate splitters might be able to share an antenna between two transmitters, but there could never be simultaneous transmission.

As for loss through the splitter device for the transmitter, there may be some. I suspect that the loss will be low. The splitter should be designed so that the transmitter is coupled directly to the antenna, and not through the receiver portion of the splitter. That is, the transmitter should not be attenuated by 3-dB. However, some loss can occur in the transmitter path from the presence of relays or other devices. In a well-made splitter of this type I would expect the transmit loss to be minimal, less than 0.5-dB, which would for the most part be undetectable in any useful communication circuit. Again, there might be some instance at some time where a 0.5-dB loss could affect communication with another station, but I don't think it will occur too often.
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Old 05-09-2011, 09:35   #6
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Re: AIS and VHF Performance

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A receiver generally has a great deal of reserve gain, so if there is attenuation of 3-dB between the antenna and the receiver, the receiver just increases its gain to make up.
While there is automatic gain control circuitry in an HF SSB radio, they do not exist in a marine VHF-FM radio. The receiver cannot make up for a loss of signal in the antenna system.

The typical loss on these splitters is 3-4db on the receive side and 1db on transmit. The best one Iv'e seen is the Vesper Marine SP160 which claims 1.5db loss on receive and it employs a 12db amplifier on the AIS receive side.

Eric
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Old 05-09-2011, 09:58   #7
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Re: AIS and VHF Performance

I have a marine VHF antenna on top of the mast (it lis a J-pole antenna which is a 1/2 antenna fed through a 1/4 wave matching stub) so probably 6dB gain. I feed it with a rather expensive lowloss thick coax, to be honest the VSWR worsened slightly to a 1.3-1.7:1 all over the marine VHF band when installing this coax instead of the thinnish RG-58 coax that was in place as it usually is - a 20m of this RG58 coax gives considerable loss on 156 Mhz! The "lossy" coax dampened the SWR! Expect 3db loss... so I really improved my system, at times I can contact Dover Coast Guard from Zeebrugge (Belgium) which is about 65 NM away, with good signal report.

However, this spring I installed a Raymarine 250 AIS and wanted to use the splitter it has onboard (1 VHF antenna splitting to marine VHF, AIS RX and VHF broadcast).
SWR is OK behind the splitter, so I thought to make good use of the low-loss atenna system on top of the mast. The AIS splitter is however causing interference on some VHF channels (eg. ch27 used often for weather forecast or securité messages) and is causing loss on reception.

So I stopped using the splitter, installed a dedicated AIS reception antenna on the pushpit, yes a 3dB antenna but its base is only about 1.8 m above sea level...but AIS is working OK. Before I did receive AIS info from ships > 30NM away but what the heck...15-20NM is still enough.

And the AIS RX antenna is giving me a spare VHF antenna just in case the one on the mast fails or even worse....

Morale: install a seperate dedicated AIS reception antenna.

Jan
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Old 05-09-2011, 10:02   #8
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Re: AIS and VHF Performance

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Originally Posted by fairbank56 View Post
While there is automatic gain control circuitry in an HF SSB radio, they do not exist in a marine VHF-FM radio. The receiver cannot make up for a loss of signal in the antenna system.

The typical loss on these splitters is 3-4db on the receive side and 1db on transmit. The best one Iv'e seen is the Vesper Marine SP160 which claims 1.5db loss on receive and it employs a 12db amplifier on the AIS receive side.

Eric
Eric--I am not sure where you got your view of FM receivers. An FM receiver typically employs a limiter stage which acts to automatically control receiver gain. See

Limiter - Wikipedia, the free encyclopedia

for a simple explanation of limiters in FM receivers. The gain control is entirely automatic. An FM receiver typically runs at maximum gain all the time until the limiter acts to shut down the gain based on the signal it is receiving. Because of the frequency modulated system, the detector works best if there are no amplitude variations. Automatic gain adjustments in the receiver maintain the signal level at the limiter threshold or higher.

Again, there is no threat to receiver sensitivity from a 3-dB loss unless the signal you want is at the margin of the receiver's ultimate sensitivity. In terms of range, 3-dB of signal usually means about an 18-percent change in range. So with 3-dB loss on receiver, your ultimate range of reception might degrade 20-perent in terms o range. That might mean you'd pick up ships at 40-miles instead of 50-miles.

Who is to say that if you use a separate antenna that its location, height, gain, feedline loss, and other influences, will all be the equal of the main ship antenna that could be shared?

As for ultimate sensitivity, a simple test can be made as follows: unsquelch the FM receiver and set the noise to a moderate level. Compare the noise when the antenna is connected and when antenna is disconnected. As long as the receiver noise increases when the antenna is connected, you have enough sensitivity to hear weak signals that are above the receiver noise floor. If the receiver noise does not increase, you have too much attenuation in your antenna feed line, or you have an exceptionally quiet location.

As for gain in an FM receiver, one can easily calculate this. A typical receiver has a sensitivity -107-dBM and can produce at least 1-watt of audio output, or +30-dBm. The receiver thus has a gain of +137-dB. This amount of gain is not needed all the time, and can be reduced when stronger signals are received. Also, the audio stage has its own gain control, which permits the loudspeaker level to set as needed.
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Old 05-09-2011, 10:39   #9
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Re: AIS and VHF Performance

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Eric--I am not sure where you got your view of FM receivers. An FM receiver typically employs a limiter stage which acts to automatically control receiver gain.
I'm not talking about FM receivers in general. Yes, a marine VHF uses a limiter. This effectively acts as an AGC but not as you explain. The limiter is a high gain IF amplifier that is intentionally driven into saturation so that the output is a constant amplitude over the wide range of antenna input signal levels. It does not vary the IF gain like AGC does.

As far as general FM receivers go, some use AGC instead of limiters, some use only limiters and some use both. I service marine VHF radio's, they do not use AGC circuits.

A 3db loss is easily detected under marginal conditions and could be the difference between communicating and not.

The sensitivity of a marine VHF is typically around -118dbm or better for 12db sinad.

The noise you year on an unsquelced VHF is all receiver noise, not atmospheric noise.

Eric
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Old 05-09-2011, 10:41   #10
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Re: AIS and VHF Performance

I have separate 6dB VHF antennas for the AIS and VHF mounted on the antenna farm at the stern of my multihull (12 degree heeling angle). That way, in a pinch, I can switch over from a broken antenna to a functioning one. I'm also adding a 3dB whip at the masthead, when I pull the stick.
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Old 05-09-2011, 13:13   #11
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Re: AIS and VHF Performance

The notion that a 3-dB loss in the receiver antenna will always affect communication range is worth considering but only when the incoming signal is at the threshold of reception AND the noise floor of the system is sufficiently low for the receiver to detect a signal at that level. On many boats there will be a noise floor that is a limiting factor. A weak signal will be lost in the noise before the receiver runs out of gain and cannot make up for the missing 3-dB of antenna signal.

Although FM demodulation tends to minimize interference from amplitude modulated noise, those noises are still present and form a noise floor, below which it is impossible to detect a signal.

Give some thought to this: if the 3-dB loss in a splitter were so terrible as to render a receiver useless, why would so many be used in so many antenna distribution schemes? And, as implied earlier, any loss is easy to make up with a small pre-amplifier in the splitter, so the output to the receiver is actually higher than when directly connected to the antenna.

Now a 3-dB loss on transmit would be foolish to tolerate. On receive, I would not lose too much sleep over it.

As for the action of an FM limiter being a form of automatic gain, this is a debate for electronic designers. The result is the same, that is, the amplitude of the output signal stays in a constant range for a variety of input levels. If this is not an automatic gain, then call it something else if you like. There is nothing particularly different about an FM receiver compared to an AM receiver in terms of the receiver having plenty of reserve gain that can be employed to make up for a bit of signal loss at the antenna input. Having lots of gain is the measure of a sensitive receiver.
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Old 05-09-2011, 13:35   #12
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Re: AIS and VHF Performance

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Morale: install a seperate dedicated AIS reception antenna.
Jan

Exactly +1
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Old 05-09-2011, 13:59   #13
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Re: AIS and VHF Performance

Sorry to bust this up...VHF is AM modulation. There is no limiter, but there is an agc circuit. Range is typically limited by line of sight...not RX sensitivity. The typical sailboat cable run of 50 feet to the top of the mast will result in far greater loss (typically 10dB with connections) but the advantage is greater line of sight, thus extending range. Adding attenuation into the FRONT end (antenna and cable) adds noise figure to the system. This is a much more important factor as most modern receivers have excess gain. If you add 3dB in loss (like a splitter) you add 3dB to the noise figure. You also halve the transmit power (watts). 3dB is 1/2 the signal power. 10 dB is 1/10 the power. So a 25watt VHF radio is only transmitting 2.5 watts at the antenna on a typical sailboat...still plenty of power for line of sight.
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Old 05-09-2011, 14:00   #14
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Re: AIS and VHF Performance

We have a 76ft high VHF antenna, a Standard Horizon VHF, a Raymarine chart plotter and a West Marine AIS and splitter. The AIS integrates seamlessly into the chartplotter.

The AIS and VHF work out to 20+ miles. We found no practical change in VHF range after we added the AIS and splitter.

Certainly a dedicated VHF antenna for the AIS mounted much lower than the masthead would not improve the function of the AIS.

The AIS transceiver has greatly improved the safety of our interactions with big ships, it is of more practical value than the 48 mile radar!
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Old 05-09-2011, 17:06   #15
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Re: AIS and VHF Performance

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Sorry to bust this up...VHF is AM modulation. There is no limiter, but there is an agc circuit. Range is typically limited by line of sight...not RX sensitivity. The typical sailboat cable run of 50 feet to the top of the mast will result in far greater loss (typically 10dB with connections) but the advantage is greater line of sight, thus extending range. Adding attenuation into the FRONT end (antenna and cable) adds noise figure to the system. This is a much more important factor as most modern receivers have excess gain. If you add 3dB in loss (like a splitter) you add 3dB to the noise figure. You also halve the transmit power (watts). 3dB is 1/2 the signal power. 10 dB is 1/10 the power. So a 25watt VHF radio is only transmitting 2.5 watts at the antenna on a typical sailboat...still plenty of power for line of sight.
Sorry, but this is TOTAL BS. From the "VHF is AM" on. It would be real nice if the receiver could make up for losses in the antenna system, but it simply doesn't work that way.

Eric
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