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Old 05-09-2011, 17:19   #16
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Whew , glad I didntconfuse myself reading this before I bought a VHF /ais radio. I bought a standard horizon gx2150. Plugged it in , to the old VHF antenna that was already mounted on the mast top , for a 30 yr old radio. Wired it to my garmin chart plotter according to the instructions (instructions actually say special ais antenna not recommended ) and the stuff all works great , receive only. And it wax only $279.00. Simple is good.
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Old 05-09-2011, 17:27   #17
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Re: AIS and VHF Performance

Stay tuned!
Eric is, of course right that VHF marine band employs FM.
As to range being limited by LOS or Rx sensitivity , well it depends on the design / installation / components of the antenna SYSTEM
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Old 05-09-2011, 17:44   #18
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Re: AIS and VHF Performance

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Originally Posted by fairbank56 View Post
Sorry, but this is TOTAL BS. From the "VHF is AM" on...
Aviation radios use amplitude modulation on their VHF bands, but the VHF Marine Band is frequency modulated (FM).

It is frequently observed that VHF Marine Band radio installations with completely useless antennas can receive quite well but cannot transmit 100-meters. This is because their antennas have a huge loss, perhaps -40-dB compared to a proper antenna. They continue to work quite satisfactorily on receive due to all the reserve gain available in the receiver, but on transmit, where there is a limit to the gain at 25-watts, these systems cannot be heard around the corner. This is a good example of how a receiver makes up for deficiency in the antenna with its extra gain, while a transmitter cannot due to its fixed power output. Again, a 3-dB loss in the receiver antenna is not a giant problem for most radio circuits; only those circuits at the extreme limit of the receiver's sensitivity will be affected.

Of course, a 3-dB loss on transmit is significant, but as (carefully and frequently) explained in this discussion, the transmitter path through the typical splitter does not undergo a 3-dB loss.
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Old 05-09-2011, 17:56   #19
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Re: AIS and VHF Performance

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Aviation radios use amplitude modulation on their VHF bands, but the VHF Marine Band is frequency modulated (FM).

It is frequently observed that VHF Marine Band radio installations with completely useless antennas can receive quite well but cannot transmit 100-meters. This is because their antennas have a huge loss, perhaps -40-dB compared to a proper antenna. They continue to work quite satisfactorily on receive due to all the reserve gain available in the receiver, but on transmit, where there is a limit to the gain at 25-watts, these systems cannot be heard around the corner. This is a good example of how a receiver makes up for deficiency in the antenna with its extra gain, while a transmitter cannot due to its fixed power output. Again, a 3-dB loss in the receiver antenna is not a giant problem for most radio circuits; only those circuits at the extreme limit of the receiver's sensitivity will be affected.

Of course, a 3-dB loss on transmit is significant, but as (carefully and frequently) explained in this discussion, the transmitter path through the typical splitter does not undergo a 3-dB loss.
I guess it had to happen - Thread Drift - and I am the guilty one!
FWIW, the aviation band uses AM on voice between 118 & 136 MHz and on LOC (b/w 108 & 112) but uses both AM & FM combined for VOR (b/w 108 & 118).

Now back to the interesting debate on AIS, antennas, splitters and marine VHF performance.
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Old 05-09-2011, 18:11   #20
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Re: AIS and VHF Performance

There is no AGC in a marine VHF radio. The gain of the RF/IF amplifiers is fixed. In a receiver that has AGC, the AGC control voltage is derived from the level of the input signal and is applied to the RF/IF amplifiers to control their gain. This is not the case in marine VHF radio's. I service these radio's. I have service manuals for them. I'm a marine electronics service tech for 36 yrs. They DO NOT have AGC.

Eric
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Old 05-09-2011, 18:30   #21
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Re: AIS and VHF Performance

This thread seems like a good opportunity for me to get an answer to a very puzzling AIS behaviour.

I have an AIS receiver with an antenna splitter connected to a standard masthead vhf antenna at around 65ft from the water. Typically I only see AIS targets appearing at a range of 10-12 miles although my VHF works regularly at 20 miles +

However, on just one occasion, I suddenly gots loads of AIS targets at around 20 miles range. This coincided with a very powerful VHF transmission from a South African Coast Radio station and continued all the time the station was transmitting a weather forecast and then navigation warnings. As soon as the transmission ended, the distant AIS targets dropped off and I was left with just my normal 10-12 miles range.

Can anyone explain this phenomenon?
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Old 05-09-2011, 19:23   #22
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Re: AIS and VHF Performance

i'll avoid the limiter vs agc argument as much as i love potato/potahtoe debates about engineering nomenclature.

From a completely different viewpoint, external splitters have some issues
heretofore unmentioned.

1. With a splitter one has 4 terminations between each receiver/transmitter
and the antenna as well as an active device (the splitter itself and any associated power connections). A dual antenna has only two terminations (the unit and the ant) and no active device. This is not only a difference in potential signal loss but to me more importantly 50% less reliable (ok, cut me some slack, lets not argue the math of reliablility and availability and the magnitudes of unit/antenna failures in the calculations. Let me treat those as second order unless someone wants to debate it offline in email). Twice the connections (which are trouble points) and
an additional device in the path results in a less reliable system whose primary purpose is safety. Sounds bad put that way doesn't it?

2. When installing a dual antenna system, there is a connection, a length of coax
through the boat, and an antenna. When using a splitter, there are two jumper cables and the splitter, usually coiled up in the back of an electrical panel in too tight quarters around lots of noisy electrical lines, then a length of coax to the ant.

Not only is this less reliable with tight connections oft crammed in a cabinet, but wire wound in coils and wrapped around
in electrical supply cables and connected to a receiver generate all kinds of interesting noise and inductive pickup especially if your panel has AC and DC in the same cabinet (as most sailboats do unless you are on a modern goldplater with lots of room or the builder happened to have an actual engineer on staff instead of using his brother-in-law who used to wire monster trucks).

Hey, maybe you like listening to the switching noise from your fridge instead of eavesdropping on the gossip in the anchorage.

IMHO, less loss, less noise, and higher reliability with dual antennas.
costs you an extra run of coax, an extra antenna mount, and some extra work.
sounds like a no brainer to me.

gello
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Old 06-09-2011, 09:08   #23
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Re: AIS and VHF Performance

@Jeannius: indeed a strange phenomenon...! Sounds like some super-imposed signals? But AIS is on 161Mhz - then again marine shore stations usually broadcast on duplex channels where also 160mHz is used for the duplexchannels....

I suggest you try out with only the AIS receiver hooked into the masttop antenna and leaving the marine VHF tranceiver out?

I got away from the AIS splitter operation using only the masttop antenna.
AIS signal now comes from a dedicated 3dB gain vhf antenna on the pulpit, its base is only about 1.80m above sealevel but I receive AIS data from 20NM away.

Jan
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Old 06-09-2011, 10:26   #24
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Re: AIS and VHF Performance

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There is no AGC in a marine VHF radio. The gain of the RF/IF amplifiers is fixed. In a receiver that has AGC, the AGC control voltage is derived from the level of the input signal and is applied to the RF/IF amplifiers to control their gain. This is not the case in marine VHF radio's. I service these radio's. I have service manuals for them. I'm a marine electronics service tech for 36 yrs. They DO NOT have AGC.

Eric
Whether or not Eric thinks there is any sort of gain control circuit in a typical VHF Marine Band FM receiver is not of any importance to this discussion. I will no longer reply to that sidebar topic.

What is important to the topic, use of an antenna splitter to operate two receivers simultaneously, is the notion that all receivers have plenty of excess gain. Receivers typically have more gain than they can ever use, which is established by the fact that they have so much gain they amplify the background noise they receive to a very high level.

If you reduce the signal to any receiver by 3-dB, as occurs when you insert a typical antenna splitter, the receiver just makes up for the loss with its gain. This process is in effect until the receiver is running at its maximum gain and runs out of gain. Now we experience the effect of the 3-dB loss. Until that point, we never noticed the 3-dB loss.

Receiver systems are generally limited in their sensitivity by noise. Noise comes from two places: noise is generated by the receiver's circuits themselves, and noise is part of the input from the antenna. Both of these noise levels act to determine the ultimate sensitivity of a receiver. Modern VHF Marine Band radio receivers are well designed and their internal noise is very low, so that noise is not a factor. Noise from the antenna input is more likely to be a limiting factor. There is a reasonably good chance that in the typical ship installation there may be noise sources, both on the boat or in the surrounding environment, which will tend to limit the ultimate sensitivity of the receiver. If there is a desired signal which is below this noise level, you cannot receive it very well, and no amount of gain helps; the noise is amplified along with the signal.

If you were in a remote location, lets say far at sea, and there were no local noise sources, no local environmental noise sources, and no noise propagating in, then you could use the ultimate sensitivity of your receiver. In that situation if you reduced the signal from the antenna by 3-dB you would be throwing away some useful sensitivity.

As for the notion of having a dedicated antenna for an AIS receiver, and that antenna being useful as a back-up antenna for the main VHF radio, I think that is a wonderful idea. In fact, I would say that is the best plan. The AIS receive antenna will be a great back-up.

But, on the notion that a splitter is going to create a 3-dB loss for my main radio receiver and my AIS receiver, I would not abandon all hope for every hearing an AIS transponder or communicating with other vessels on the radio. The 3-dB loss will only come into play in the very marginal signals.

If reception of the very marginal signal is your goal, of course you don't use a splitter. You'd be foolish to use one in that situation.
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Old 06-09-2011, 11:04   #25
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Re: AIS and VHF Performance

Your post is so rife with misinformation, that I won't even bother wasting my time trying to correct it. It's obvious to me, as a professional, that you don't have a clue as to what your talking about. Based on your theory, I'm just going to rip out that big honkin low loss coax of mine and run a piece of cheap lightweight rg174 cause the radio is going to automagically compensate for the loss ...Oh, wait a minute, that only works on receive...damn

Eric
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Old 06-09-2011, 11:13   #26
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Re: AIS and VHF Performance

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Your post is so rife with misinformation, that I won't even bother wasting my time trying to correct it. It's obvious to me, as a professional, that you don't have a clue as to what your talking about. Based on your theory, I'm just going to rip out that big honkin low loss coax of mine and run a piece of cheap lightweight rg174 cause the radio is going to automagically compensate for the loss ...Oh, wait a minute, that only works on receive...damn

Eric
Good try to poison the well by attaching me. However, your own reply undoes your argument. If it were impossible for a receiver to operate with loss between the antenna and the receiver, then the very thing you propose, having loss in the coaxial transmission line, would cause all receivers to stop working properly. Of course, they don't. They just make up for the loss between antenna and receiver, either in the coax, the connector, or a splitter, buy using more gain. I think you have made my point.

At no time has anyone, particularly I, proposed that we ought to tolerate a 3-dB loss in the transmitter path to the antenna. Why? Because the transmitter gain (its power output) is fixed by regulation and cannot be increased to make up for the loss. This is the exact opposite of the case for a receiver.

By the way, I have held the highest grade FCC commercial radiotelephone license since 1976, and have been employed professionally using that license since then. However, the principles of radio receiver design are the same for all, licensed or unlicensed, radio repairman or radio user.
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Old 06-09-2011, 11:50   #27
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Re: AIS and VHF Performance

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Originally Posted by Jeannius View Post
This thread seems like a good opportunity for me to get an answer to a very puzzling AIS behaviour.
I can't explain the phenomenon, but I'd start with removing the splitter and seeing if a dedicated straight through connection to the antenna increases AIS range.
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Old 06-09-2011, 12:01   #28
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Re: AIS and VHF Performance

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Originally Posted by continuouswave View Post
By the way, I have held the highest grade FCC commercial radiotelephone license since 1976, and have been employed professionally using that license since then. However, the principles of radio receiver design are the same for all, licensed or unlicensed, radio repairman or radio user.
With respect to your level of training as a radio *operator* I am getting impression that you are somewhat mixing principles from AM (amplitude modulation) field with FM (frequency modulation). Automatic gain control is of more importance in AM systems because the useful signal is carried in the amplitude and detectors have a limit on signal levels they accept. In FM land the useful signal is a difference in frequency and amplitude is of (almost) no importance to detector (mixer) circuits. This is of course oversimplified (crosstalk, mirror images, intermediate frequencies, etc.), but it is the general idea.

BTW, in continuous wave (CW) systems it is the bare *presence* of a radio frequency signal that carries information..

Returning to main topic, on our boat we frequently receive AIS information from over a hundred miles across but that is easy to explain. We are near a repeater station :-)
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Old 06-09-2011, 13:08   #29
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Re: AIS and VHF Performance

Ever notice the LOC/DX setting on a marine VHF radio? Switching to LOC (local) reduces the sensitivity of the receiver by switching in a small attenuator circuit between the antenna jack and the 1st RF amp. If the radio had AGC, this wouldn't work. Marine VHF radio's do not have AGC. Show me a block diagram or schematic from one that proves otherwise. I can show plenty that show they do not have AGC. Only for those who can actually read a schematic though, but if you can and have already looked at an actual service manual from a marine VHF radio, then you already know that they don't have AGC.

Eric
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Old 06-09-2011, 13:20   #30
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Re: AIS and VHF Performance

I have a bimini mounted Shakespeare dedicated AIS antenna and have picked up targets consistently up to and over 100 miles,in calm seas.
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