Using a VHF Antenna splitter for AIS

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This splitter does look like it's fit for purpose with proper failsafe qualities.
The big problem with endorsing splitters as a general concept is that people might buy those passive coax splitters for around £10. They are bad news in this application for the reasons stated elsewhere.
The other issue with using an active splitter is, as others have mentioned, losses of up to 3.5db, which halves the signal power. This particular splitter may have much better performance but, when powered down due to a fault, it may degrade the signal significantly.
With a very good antenna system such a loss may not matter hugely but should be a consideration.
Remember that sharing your VHF antenna means you can't be receiving AIS data whilst talking to ships to discuss avoidance, but that might be of little significance.

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I use an EasyAIS splitter and have not experienced any degradation of VHF transciever performance. I get about 34nm range with large vessels on AIS (even over the top of the Isle of Wight) instead of the average 28nm I can get at home with the same receiver without a splitter and similar antenna hieght. I also get better FM performance than from any other marine antenna installation I have tried. I understand the insertion loss of the EasyAIS unit to be at most 0.7dB for VHF transmission, trivial if you have a 25W transmitter for line of sight communication. I have not found the <3.8db reception loss to make any noticeable difference, CROSS Joburg still interrupts the peace(?) in the Solent frequently.

Furthermore, for the persistant doom and gloom merchants, it takes less than a minute to disconnect the splitter from the VHF and connect the antenna cable directly, if for any reason one may wish to do so. I do also carry an emergency VHF antenna for when the mast comes down and wonder if the second antenna on the pushpit brigade also carry one, incase a falling mast demolishes there pushpit?
 
Given that the splitters seems to cost around the same price as a VHF antenna, I fail to understand why anyone would use them given that they will defintly induce an amount of loss, and there is a chance (maybe small) of failure.

I always recomend two antennas, your not really achieveing much by going downt he splitter route.

Anthony
 
I tend to agree oldhand. HOWEVER as I am not an electronics expert (and wonder how many of the doom and gloom exponents are either) I have a theory.
I think people are looking at something called "Coupling Factors" and saying with great authority Ah there you are that's a loss of signal strength. I think these are switching levels at which the splitter switches IN the antenna to the appropriate ports. I think therefore that the majority of posts on here are feeding off the same false assumption just as they were originally when they said "It will blow your chartplotter to bits"
But what do I know? I am only a humble mechanical engineer. So I have again sent an e-mail to Germany asking them what the figures mean. I will also ask them on the stand on Friday when I go to the boat show. We will see if the pundits are right then won't we? Good night all.....
 
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Given that the splitters seems to cost around the same price as a VHF antenna, I fail to understand why anyone would use them given that they will defintly induce an amount of loss, and there is a chance (maybe small) of failure.

I always recomend two antennas, your not really achieveing much by going downt he splitter route.

Anthony

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If you mount a 2nd aerial (AIS) at the mast-head, close to the VHF aerial is there not a risk of overloading the input circuit of the AIS when you transmit at 25W ?

I always thought that you need a minimum horizontal separation between transmit and receive aerials for this reason?

John
 
Each to his own. It's not a case to doom & gloom. It's just a case of not seeing any advantage other than fitting one antenna.

Having spent >20 years as an electronic design engineer including 6 years designing radios, I tend to look at these things from a technical pros & cons view.

OK if the stick came down & demolished the pushpit, I'd be a bit stuck other than having a handheld on board. It's not a case of risk removal it's just risk reduction. That's why we fit fuses. I assume everyone here does have fuses on board.
 
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But what do I know? I am only a humble mechanical engineer. So I have again sent an e-mail to Germany asking them what the figures mean. I will also ask them on the stand on Friday when I go to the boat show. We will see if the pundits are right then won't we? Good night all.....


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Difficult to guess how that conversation will go..

Is it safe to fit one of these switch thingies, I know nothing about RF

Salesbloke - (rubbing hands) oh yes sir, look at the pretty case we have built.

But will it degrade my signal

Salesbloke - Not noticeably (if you are talking to the marina office)

I have heard that it might fail and stop me being able to transmit

Salesbloke - it is CE marked so it must be ok to use, did I mention the pretty case

But, what if it fails

Salesbloke - All our products come with a 12 month guarantee

Phew, that's ok then, hey, nice case...

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Whilst a signal degradation can be measured in dB, Tx and Rx powers can't. Therefore your statement doesn't make sense.

lso coupling factors are nothing to do with switching.
 
I have deleted the previous post Simon as I am no longer sure of my facts. I am NOT an electronic engineer and couplings to me are things that join shafts. However I was fed the information I posted by someone who SHOULD know what he was talking about but it is possible I have misunderstood. So to avoid confusing others I am backtracking somewhat and am now inclined to believe the figures mean what you say they do but cannot come to terms with claims that this will halve the signal strength. Perhaps you would look at the spec again and tell me what you think it means in practical terms as a percentage signal reduction. Those that have them fitted (including some on here) are seemingly happy with them and don't seem to suffer significant signal loss. Can you explain why? For clarity, what I have in the technical description is this. (FM port ignored)

Coupling factors (standby, VHF transmitter off)

Coupling factorAntenna-AIS Port: >3.8dB@145.....165MHz
Coupling factor Antenna-VHF Rx >3.8dB@145.....165MHz
Decoupling VHF-Transmitter Rx -AIS Port >20dB@145.....165MHz

Coupling factors (active,VHF Transmitter on)

Coupling factor VHF Radio Tx-Antenna < 0.8dB
AIS Port switched off (decoupling to transmitter>30dB)

I need to understand this properly without emotional bias as I have not installed it yet and as I already have 2 antenna (one on the rear arch feeding a VHF already and a new one which is going up the mast) how and where I fit it (if at all) is important. I may let the new masthead antenna be dedicated to VHF and the lower one used for AIS and keep the old radio on there for emergency use. Wouldn't hurt then to have the FM connected too... If it all then goes to rats I don't degrade the new DSC radio or have any possibility of failure. I am still very curious about the practical effects of a reduction of 3.8dB though. Surely it cant reduce the antenna efficiency by 50%?
 
Although I am an electronics engineer and make my living dealing with satellite communications systems and use dBs (and dBm, dBW, dBV, dBm0, dBmi)evey day of my working life ............. I don't want to get into this discussion as I found the manufacturers specs less than clear.
However to answer your last question 3.8 dB is a ratio of 1:2.3988 (41.69%) ................. so a little more than half; and yes, if you have an antenna and split the receive output with a 3dB splitter then you will have 50% of the power in each output port of the splitter (neglecting VSWR mismatches e.t.c.).

Also without seeing the schematic for this "splitter" it is hard to say exactly how it functions ........ it could be a resisitive splitter, a coupler or even a coupler and circulator combination.!!

Hope that answers your last question.

Alan.

P.S. I use two VHF antennas with the AIS receive only antenna on the pushpit.
 
A coupler is alike ... Think of a house main sewer pipe and then the coupled dishwasher pipe, around 30mm feeding into what 150mm pipe. Run it backward, i.e. run pressurised water up the sewer pipe and the dishwasher pipe is a take off. Easy to understand that the take off will have much less power than the main pipe. It is possible to have a direct coupler, sewer pipe with a sewer pipe take off.

In RF it is similar, you can tap off some or all of the power with a coupler. What you have here is a 3.8 dbm tap in essence (wow my theory is rusty??).

If I am understanding this correctly, then the signal loss to the AIS and fm radio is half or over than if you connect directly to the antenna. Not to good, but by the by as this is not why I don't like them.

Lets look at the VHF only. 0.8dbm coupling factor, if this is correct, you are losing nearly 1/3 signal strength on rx, 1/3 will be best case, as you go through the band this may rise significantly, it may fall (ha ha). Think of it like changing the angle of attack of the dishwasher pipe, different take off angles will work better for different flow rates in the main pipe, no one take off angle will be perfect across any range of flow rate??

Decoupling is just a switch, this is where my cynicism arises, all switches are noisy and can fail, this is where this device changes from passive to active. This switch must work from 2W and less upward. A switch that was just switching 25W would be a lot easier to design without introducing noise etc.

My cynicism comes from replacing 130twenty RF switching type components from failed outright to increased noise over time.

Without emotion, as the VHF is mostly in standby, the splitter will mostly be passive, thus will mostly work with some attenuation to the signal, probably not enough to notice in the real world.

But..

If it fails, and things fail, it may stop TX. Now, you have stated, if it fails it fails in such a way that TX is allowed and the AIS will lose it's signal. My concern here (I am guessing as I have not seen your particular device), is that must mean the unit is not sat in a passive state all the time but active, it is keeping the switch held to rx until, rather than switch the switch when.. I would need to know more about it.

Emotionally.. I don't see the reason at all for taking the risk, the VHF is such a simple device, you are putting a pelican crossing on a motorway, it should be ok as not many people will use it. But if it fails it could be the worst money you have ever spent. As it gets older will you change it, like you change your coax?

I will expect to be flamed, I am sure some of my theory is painfully incorrect, but, would I have one... nope, I just don't see the point when you look at costs of antenna's. It is a product that may have had a place 40 years ago, but nowadays the other equipment is cheaper..
 
the 'magnetic' antenna conditioner looks good..... it said so in sales blurb
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And when you get tired of conditioning you can wear it to cure rhumatism or wrap it round your fuel lines to kill the bugs ..

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I need to understand this properly without emotional bias as I have not installed it yet and as I already have 2 antenna (one on the rear arch feeding a VHF already and a new one which is going up the mast) how and where I fit it (if at all) is important. I may let the new masthead antenna be dedicated to VHF and the lower one used for AIS and keep the old radio on there for emergency use. Wouldn't hurt then to have the FM connected too... If it all then goes to rats I don't degrade the new DSC radio or have any possibility of failure. I am still very curious about the practical effects of a reduction of 3.8dB though. Surely it cant reduce the antenna efficiency by 50%?

[/ QUOTE ]I think your idea of using the masthead antenna exclusively for your main VHF set is correct. Having a splitter on your main VHF will degrade its performance - not much in day-to-day use, but enough for it to be important in critical circumstances. Having the AIS running from the antenna on the rear arch will be fine, with performance probably as good as with a masthead antenna connected via a splitter. You could probably return the splitter for a refund. I reckon you've hit on the perfect solution.
 
Well here is another question then. The AIS splitter only cost me £40 so I am not worried if I don't use it. But if I retain my old non DSC radio as an emergency use set it will be switched off most of the time. In that case the only degradation of signal will be to the AIS which EasyAIS say is insignificant. Does the forum think that with a splitter in the circuit and the VHF connected to it but turned off the 3.8dB still applies? According to the "plumbing analogy" it shouldn't..... I can then connect my FM to it and listen to "the archers" as well /forums/images/graemlins/smile.gif
 
If the old non-DSC VHF is for emergency only, couldn't you just swap the antenna connection from the AIS receiver to the VHF if you ever needed to use it?
 
Frankly thelast thing you want to do in an emergency is to fart about with wires at the back of a radio set. Does anyone know the answer to the question I asked?
 
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