AIS / VHF Splitter

[boogiediver]

Hi All,

I have both an AIS receiver and a VHF radio connected to a single antenna via a 2-way adapter with a manual switch. AIS has been connected for over a year, but the built-in VHF is new for me - installed last week.

I can't remember which way the switch was set, but strangely both VHF and AIS were receiving just fine simultaneously. However, I noticed the AIS signal dropped a few times over the weekend (my chartplotter beeped and showed a warning message). I will have to check again next weekend, but in retrospect, it might be that AIS dropped on the few occasions that I did a test transmission on the new VHF, which I can understand*would make sense.

My local retailer tells me I should use an EasySplit which (a) allows AIS and VHF signals to be received simultaneously, and (b) quickly disconnects the AIS when VHF transmits.

I can't imagine that will fully help my situation as it will still cause interruption to AIS receiving, thus triggering the same AIS warnings on my chartplotter.

I might be able to live with all this as it is, but what frightens me is what my retailer has said (in very broked English) about the likelihood my VHF radio will "burn out" if I forget the switch in the "AIS" position (ie the VHF radio has no antenna connection). Does that sound right? A VHF radio will self destruct if transmitting without an antenna attached?

Will be pleased to hear what you know about this danger and the best solution (in your experience) for connecting an AIS (RX only) and VHF radio to a single antenna for simultaneous use.

Fyg, I have a Navman 8084 chartplotter (now Nortstar M84), a Nasa AIS Engine, and a new Ray55 VHF radio with RayMic at flybridge helm.

Cheers,
Mark

 

[RickB]

Both radios are in the same band and share the same antenna so you can't transmit on one and still receive on the other as you have already discovered.

It seems a bit silly to me to spend money on a device that isolates the AIS receiver while transmitting just so you don't have to spend a lot less money to install another antenna dedicated to the AIS.

Just make sure you install the AIS antenna a respectful distance from your VHF antenna to minimize the interference when transmitting. Even then, unless the antennae are so close together that you overload the "front end" of the AIS receiver and burn it out, or you have teenagers who spend hours chatting non stop on the VHF, who cares if you lose two or 3 seconds of AIS reception a few times an hour or day.

 

[boogiediver]

My main reasons for trying the single antenna approach are (1) I didn't know you can't operate AIS and VHF simultaneously, and (2) I will only very rarely be transmitting on VHF... basically only for emergency/urgent communications. Much less important, I hate to drill more holes and fit more bits on my boat if not really necessary for my purpose. And it turns out an imported antenna costs more than a switch here in Hong Kong.

I agree, I don't care about loosing a few seconds of AIS here or there, but unfortunately the chartplotter's "no AIS signal" alarm beeps until one acknowledges it, which would not be ideal when I'm operating the VHF at a different station... but how often does that happen? Practically never in my case I suspect. So basically a non-issue for me.

Except I'm still left wondering if there is any truth to my retailer's advice that a VHF radio will burn out if transmitting without an antenna attached. Any thoughts?

 

[sunchaser]

I just finsihed install of a Class B AIS. I elected to go with a separate antenna. A single/dual use*works on paper but the installer ( a very good bunch) said not all dual use systems behave well. Plus, I like the redundancy aspect of having a spare antenna. I might add, the first trip out the AIS (hooked into laptop running Nobeltec) gave me great data, crossing point times &*distances for those pesky*21 knot freighters. A blinking red course path appears when crossing points become too close. My wife sure likes it better than Marpa for big ship dodging. For those that don't have it, AIS when hooked into a laptop or a fast read plotter*is a tremendous asset. The puny displays that come with AIS units are a joke though. However, they are*an excellent*data transfer device when in the NMEA 2000 mode..

 

[Penta]

In answer to the first question, yes, a transmitter can self destruct if operated without an antenna or test load connected. This is a result of the mis-match of the output circuits causing the power transistors to draw excessive current thereby causing them to fail. This is especially a concern in the lower price range marine type transmitters where in order to reduce costs the manufacturer has not included protective circuitry that reduces the current draw of the output transistors to a safe level when no load is attached.
As to adjacent antennas allowing damage when transmitting on one antenna while an unprotected receiver is attached to the adjacent antenna, this is caused by the tuned circuits in the initial stages of the receiver absorbing the radiated power and causing an overload to the somewhat delicate receiver transistors. Once again the higher end equipment generally has protective devices in the unit to reduce this problem.
Antenna switches, whether automatic or manual can also cause these problems unless they are provided with protective circuitry for the "disconnected" unit.
The safest way to operate AIS and two-way VHF radios on a vessel is with separate antennas spaced as far apart horizontally as physically possible or spaced vertically, directly under one another and at least 6 feet apart (tip to base) which negates this method for most of our vessels.
While many installations will work apparently fine for some time, eventually the units will fail due to the repeated overload of the fragile devices.
Sorry for the length of this post but I don't know how else to explain it.
Have a great day
John
Penta

 

[Marin]

When we bought our boat it had a selector switch to connect either the flying bridge radio or the lower helm station radio to the single antenna.* In my opinion this was an accident waiting to happen.* We knew the day would come when the switch would get left in the wrong position, we'd key the wrong radio, and most likely damage it.* So we* added a second antenna within a few months to eliminate the potential for a problem.

While it's a personal thing, I sure wouldn't worry about drilling holes in the boat to mount a second antenna.* That's what boats are for--- to mount stuff on. If the installations are done neatly and in intelligent locations there's nothing detrimental about having however many antennas one needs on a boat. We have two VHF antannas, one of them 24 feet high, a Loran-C antenna (Loran-C still works in the PNW although it is supposed to be shut off this fall), and two GPS antennas.

Anyway, if you don't drill holes to mount the necessary antennas, the next owner will, so you might as well do it yourself and get the benefit while you own the boat.



-- Edited by Marin on Wednesday 28th of April 2010 11:26:43 AM

 

[RickB]

boogiediver wrote:
*I didn't know you can't operate AIS and VHF simultaneously,

*That isn't necessarily true either. It is true if you are trying to use the same antenna but not true if you have separate and separated antennas.

The band is the same but the frequencies are different so if your VHF transmitting antenna is far enough away from the AIS receiving antenna*so it*does not overload the front end then you will not have any problems.

The reason your alarm goes off is probably because the switch you are using puts the AIS antenna to ground in order to protect it from the RF signal just millimeters away from the switch contact and the plotter detects a loss of signal. If you have a separate receive antenna, even if the receiver*is swamped*temporarily and you lose incoming data, the radio still has a signal and doesn't cry wolf.

 

[boogiediver]

Thanks very much everyone. Gives me a few things to think about... including how I would squeeze another cable into the already packed ducting from flybridge down to lower helm. Aaaarrgh!

 

[Keith]

You can always squeeze one more in!

 

[RickB]

Tight spots require lube, lots of lube. You might even have some water based stuff close to hand, so to speak.

If not then try this: http://ideal.datacomtools.com/idealcatalog/wire-lubricant.htm

This stuff makes wire pulling a piece of cake and eliminates almost all friction and the risk of damaging the conductors or outer covering and it evaporates without a trace. It's non staining and makes a great ultrasonic couplant too, if you're into that sort of thing.

-- Edited by RickB on Friday 30th of April 2010 05:19:48 AM

 

[Baker]

Keith wrote:

You can always squeeze one more in!

That's funny Keith....I thought the same thing when I read his post!!!