AIS and VHF Performance

[Paul Elliott]

Originally Posted by fairbank56

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

Stop, you're both wrong! (and both right)

The front-end noise figure is a real issue, and no amount of gain in the following stages can recover a signal that has fallen below the front-end noise floor due to attenuation between the antenna and the preamp. Fortunately in most cases the signals are strong enough that a fair amount of loss can be accommodated. At VHF, the external noise sources are fairly low, so a low-loss, low-noise front end will make a difference with very weak signals. In contrast, at HF frequencies the atmospheric noise will overwhelm the noise floor of all but the poorest receivers, so coax loss is less of an issue for reception.

The traditional FM receiver does not have AGC. The limiter amplifier, which is between the mixer and the demodulator, is designed to cleanly saturate, removing any amplitude variation. The limiter is a fixed-gain section. It often has a logarithmic level detector which can be used for signal-strength indication. Here is a link to a spec sheet of one of these FM IF system asics: http://www.nxp.com/documents/data_sheet/SA676.pdf

I did find one FM chip that provides AGC of a sort (http://www.atmel.com/dyn/resources/p...ts/doc4838.pdf), where the AGC is fed to a front-end PIN attenuator. I suspect that this serves the same function as the "Local / DX" switch, which is used to control an input attenuator. This is done to reduce the chance of preamp or mixer overload when in the presence of strong signals (this is not the same thing as traditional AGC.) Preamp overload can cause intermodulation distortion (IMD) where strong adjacent, or out-of-band, signals can generate interference on the same frequency as the desired signal.

By the way, a proper AIS splitter works reasonably well, and may be a good compromise.

[goboatingnow]

it matter not that the receiver has AGC or not, continouswave is right re the sensitivity range and the fact that a 3db loss on transmission does not have the same effect at reception.

Most splitters also seem to have transmit losses around 1-1.5 db.

The fact is that such splitters, leaving aside the bit about connectors and wraping the cable etc ( which equally could apply to ordinary installations) are reliable and in reality have little effect on the installed performance.

Your masthead antenna spends most of its time doing nothing , now at least its getting some use.

Talk about using an AIS antenna as a spare etc, miss the point, you cant re-determine the failure modes that might render one or all your antenna faulty or just missing. The proper place for a reserve antenna is down below in a drawer

Dave

[boatman61]

[QUOTE=twistedtree;767304]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.

QUOTE]

Ahhah... could explain the disappearing fishing boats in the Biscay and Atlantic coast... you can see them... but they ar'nt there...

[fairbank56]

Originally Posted by goboatingnow

it matter not that the receiver has AGC or not, continouswave is right

His whole argument was based on his notion that a marine VHF radio receiver has AGC, it doesn't, and could compensate for transmission line signal loss, it can't.

Eric

[continuouswave]

Originally Posted by mrm

With respect to your level of training as a radio *operator*

The FCC Commercial Radiotelephone License I have permits me to make repairs and adjustments to almost any sort of FCC licensed transmitter, including ones operating at 100,000-watt levels, so I wouldn't just blow it off as a dumb-bell operator license. I only mentioned it because others claimed to have special knowledge because they were employed as a radio repairman.

Radio receivers operate quite similarly at their radio frequencies, and the difference between AM and FM occurs mainly in their demodulators. Signal detection, amplification, and noise influences affect all receivers.

This nonsenses about AGC has nothing to do with the discussion. I don't know why it keeps coming up. Forget about AGC.

Lossy feedlines and poor antennas work quite well with receivers. A very good example is a car radio. Most car radio AM band receivers are extremely lossy and very short. They have no gain. Yet I can receive AM broadcast stations on my car radio antenna from thousands of miles away. Of course, if I tried to transmit back to those stations on my car radio antenna, it would be impossible. This is why broadcast stations construct very large and very efficient antennas to transmit, and why car receivers are able to use very small and very inefficient antennas to receive.

Let me say it again, a 3-dB loss from a splitter reduces the receiver's ultimate sensitivity, but only when the receiver must be at its ultimate sensitivity to hear a signal. Otherwise the receiver has plenty of extra gain and easily makes up for that 3-dB.

I will give you an anecdote from my own recent experience. We were heading by boat to visit a friend. He owns an island and has a cottage there. We called him on the radio a few times, but got no reply. Finally , when we were about a mile away, we established contact. His signal was weak. He told us he had been hearing us call him for an hour or so, but apparently we could not hear his reply. When we finally got to his place, I saw the reason. The antenna he was using was on a tower, and the actual radiator element had been blow away in a storm. His antenna was really just a stub of the coax sticking up in the air. He has been using this mainly for receiving for months, and for very short range transmissions, and he was not aware that his transmit range was so short. He could receive stations from miles away. This is an example of how a lousy antenna works fine for receiving but will not cut it on transmit.

By the way, I have been involved in radio, building my own stuff, including lots of antennas, for 50-years, and used to be very active in Amateur Radio--but I gave it up to go boating.

73 de K8SS

[continuouswave]

Originally Posted by fairbank56

His whole argument was based on his notion that a marine VHF radio receiver has AGC, it doesn't, and could compensate for transmission line signal loss, it can't.

Eric

My argument has nothing to do with AGC, and I never mentioned it. Eric brought it up.

A receiver has surplus gain. That is all I have said.

Please let me make my own argument. Eric should make his argument, and not tell people what my argument is. I will let Eric make his argument, but, as I noted, the last time he did he proved my point perfectly for me. And, Eric, thank you for that.

[continuouswave]

Originally Posted by Paul Elliott

...At VHF, the external noise sources are fairly low, so a low-loss, low-noise front end will make a difference with very weak signals....

Yes, I completely agree, except when you start up your spark-ignition engine which probably has poor RFI suppression. It will generate a lot of radio noise.

Also your SONAR. It's main pulse might be a source of wideband radio noise, too.

And then there are all the processors running at various frequencies in your navigation electronics.

If you are on a sailboat, no engines, no SONAR, no electronics, in a region without atmospheric noises (thunderstorms, lightning), then you will have a very quiet receiver enviroment. Otherwise, maybe not so quiet.

[fairbank56]

Originally Posted by continuouswave

My argument has nothing to do with AGC, and I never mentioned it.

"the receiver just increases its gain to make up"

"An FM receiver typically employs a limiter stage which acts to automatically control receiver gain"

"Automatic gain adjustments in the receiver maintain the signal level at the limiter threshold or higher"

"This amount of gain is not needed all the time, and can be reduced when stronger signals are received"

These were your statements about how the gain is automatically adjusted in our VHF-FM radio's. There is no auto gain in these radio's. I agree with you in that if you are receiving a signal that is full quieting and then if reduced by 3db but we are still at full quieting then it is of no consequence. There was still plenty of gain. If however, the signal level is down below a microvolt and there is some noise on the signal and the signal is reduced by 3db, you will hear a difference and it may be significant depending how close we are to the sensitivity threshold of the receiver. I apoligize for my remark about you not knowing what you are talking about, but I still disagree about your belief that the gain of the receiver is changing. Maybe it's just a matter of misinterpreting what you are tying to get across.

Eric

[continuouswave]

Originally Posted by fairbank56

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

The LOCAL-DISTANT switch is intended to protect the initial RF amplifier stage from overload. The initial RF amplifier stage operates without knowledge of what follows it. It just amplifies signals. If there are very strong signals, it overloads, causing problems. The LOCAL-DISTANT switch inserts some loss between the antenna and the first amplifier to protect if from overload. This circuit device works exactly the same for all radios, whether or not they have AGC or not. AGC has nothing to do with LOCAL-DISTANT switches.

However, thanks again for demonstrating that a receiver will still work quite well when we operate the LOCAL-DISTANT switch and insert some attentuation between antenna and first RF amplifier.

On the notion that readers of this thread must have access to the schematic diagram of current VHF Marine Radios in order to understand the points being made by Eric, this is going to be a problem. Most manufacturers do not publish their schematic diagrams and you cannot just order up a copy. A lot of manufacturers these days consider their circuitry to be a trade secret and they don't just publish schematic diagrams of it. In my work we have a lot of smart people and we often repair rather sophisticated electronic devices--we use a very wide variety of equipment in our plant. But in the modern era manufacturers are getting more and more reluctant to disclose their circuity. We've had some suppliers refuse to send us a schematic diagram.

Most all receivers operate like this:

ANTENNA-->RF Amplifier-->Mixer--->IF Amplifier and Selectivity-->Demodulator--->AF Amplifier---Loudspeaker

What we are discussing is the influence of a 3-dB loss between antenna and RF Amplifier stage. If there is a 3-dB loss, the receiver has plenty of gain in its various stages, RF, IF, and AF, to compensate.

The only time we take a hit on the 3-dB loss is when we need all the gain available to receive, amplify, and detect the signal. If we run out of gain, we suffer from the 3-dB.

Now the real question is what influence does this have on your range of receiving? The answer to that depends on what your path loss happens to be and what kind of terrain in between you and the other station. If you are in free space, you suffer one sort of path loss. If you are in rough urban land terrain, you suffer a different path loss. If you are at sea, you suffer a different path loss. It is hard to predict precisely how much distance in terms of receive range you will suffer. The greatest distance lost would be in the free space model. It is too complicate to explain here, but if you are interested, see my article on this topic at

continuousWave: Whaler: Reference: Estimating Path Loss on Marine Non-Line-of-Sight Paths

under the subheading "Calculating Influence of Antenna Gain on Range"

[continuouswave]

Originally Posted by fairbank56

"the receiver just increases its gain to make up"

"An FM receiver typically employs a limiter stage which acts to automatically control receiver gain"

"Automatic gain adjustments in the receiver maintain the signal level at the limiter threshold or higher"

"This amount of gain is not needed all the time, and can be reduced when stronger signals are received"

These were your statements about how the gain is automatically adjusted in our VHF-FM radio's. There is no auto gain in these radio's. I agree with you in that if you are receiving a signal that is full quieting and then if reduced by 3db but we are still at full quieting then it is of no consequence. There was still plenty of gain. If however, the signal level is down below a microvolt and there is some noise on the signal and the signal is reduced by 3db, you will hear a difference and it may be significant depending how close we are to the sensitivity threshold of the receiver. I apoligize for my remark about you not knowing what you are talking about, but I still disagree about your belief that the gain of the receiver is changing. Maybe it's just a matter of misinterpreting what you are tying to get across.

Eric

In an FM receiver the signal level to the detector wants to remain at a constant level no matter what the input signal was from the antenna. This is a result of the circuitry employed. If there is a 1-microvolt signal at the input, it become amplified to, say, 1-volt, and is applied to the detector. This is a voltage gain of 1,000,000. If the input signal happens to be 100-microvolts, the input of the detector will still be 1-volt. The receiver gain has been reduced to a voltage gain of 10,000. This happens without the operator making any manual adjustment. This change in overall gain was automatic.

I am familiar with other types of gain control systems in which a control voltage bus is used to vary gain of several stages in the receiver. In an FM receiver the gain change can occur in a a series of amplifiers called limiter amplifiers. As noted, in an FM receiver it is not necessary to maintain linearity in the amplifier stages, and it is simpler to just let some amplifiers go into a saturated gain situation. Although this is a somewhat crude method, it is an automatic gain control. A limiter is a very simple automatic gain control device.

OK--I broke my vow--I responded to the AGC gambit. Sorry. I will try to control myself better.

[IslandHopper]

Originally Posted by gello

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

IMHO, the only post giving the best advice....

The rest is just techi chest beating....

[fairbank56]

Originally Posted by continuouswave

The LOCAL-DISTANT switch is intended to protect the initial RF amplifier stage from overload.

That's not what it is for. It is not automatic. It is a user selected function whose purpose is to eliminate distant nuisance signals or strong signals causing interference.

Eric

[Svsilvergirl]

Damn , you two married or what ? My daughters don't argue this much LOL

[continuouswave]

Originally Posted by fairbank56

That's not what [the LOCAL-DISTANT SWITCH] is for. It is not automatic. It is a user selected function whose purpose is to eliminate distant nuisance signals or strong signals causing interference.

Eric

Of course it is not automatic. It is a switch. You have to reach down and throw it. What is your basis for argument that it has been suggested this manually thrown switch is an automatic device?

It controls--typically--an attenuator in the RF stage. I am sorry to disappoint you, but this attenuator has no knowledge of the distance of a signal it is attenuating. It attenuates all the signals the same amount, no matter what their distance away. You cannot possible attribute to this passive manual attenuator some ability to know what signals to let pass based on the distance away. That is just not even in the realm of the possible. But it is a good laugh.

The actual purpose is not to eliminate signals but to protect the front end from overload. I have seen many inexpensive VHF Marine Band radio receivers that will completely overload and produce no signal output if they're close to another station with a 25-watt signal.

Also, you can eliminate noisy and weak signals by turning up the receiver squelch. On a sophisticated receiver the squelch circuit will essentially let you select a minimum signal-to-noise ratio of the recovered audio that you want to listen to, and weaker, noisier signals will be suppressed. That is how you eliminate weaker signals.

I have to admire your creativity and persistence. But I am an old dog, if you want to keep gnawing on this bone, I will keep chewing it with you.

[Goudurix]

Hi quarreling boys Eric & Continuouswave!

I guess I have not even 10% of your electronis & transceiver knowledge...and even I am known in our marina as a techie.....

So please respect other people's limited knowledge on this subject- especially the starter of this thread Ibexx - and just come foreward with a good advice in layman's language... something that can really guide poor old Ibexx and others in similar situations.
As Islandhopper pointed out, IMHO Gello has done so.

KISS!

Because of interference caused by my Raymarine 250 AIS receiver & antenna splitter on several marine VHF channels, I was forced to set up a dedicated AIS RX antenna, reardless of eventual loss issues. It was only my old Icom M80 marine VHF who suffered from the interference. When using my Yaesu FT897 as receiver, no interference on those channel frequencies. So the problem is the combination of AIS250 and IcomM80.
My problem is solved. My marine VHF is again directly coupled via a lowloss coax to its masthead antenna. Only 2 PL259's in between...so I minimised possible losses.
I'm happy.

Make the other readers happy!

Jan
ON3ZTT