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Old 30-11-2008, 08:45   #1
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AIS/VHS antenna systems

Rather than running a second antenna for the AIS system, (because my boat's starting to look kinda porcupine'ish)...I plan on using the current VHS antenna for the AIS system. I plan on doing this by splicing them together in some kind of junction box. Behold! Of course they make a special unit for this...sold by West Marine for about $87.00!!! (which further asserts my belief that anything is available for a price!)...anyway...My understanding of the system is that it has two inputs for VHF radios (VHF 1 and VHF 2) that when you key the mike on either, the other is blocked and drops off. What I'm concerned about is that after you transmit on say VHF 1, it should never return the AIS signal coming into VHF 2 because the AIS isn't transmitting...thus there's no reason for the automatic switch to go to VHF 2.

Anyone have any experience with this product or can recommend something better or different? As usual all help appreciated.

Ray and Sandy
aboard S/V Pure Joy
Olympia, Wa

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Old 30-11-2008, 08:51   #2
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VHF is an important piece of safety equipment and you can not afford to have the antenna fail when you need it the most. Saving a buck in some areas is OK but not when it comes to safety. This is not an area for you to get cheap in. Milltechmarine makes an excellent unit at

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Old 30-11-2008, 09:03   #3
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Thanks Chuck

Wow! Looks like a great unit...and I agree safety is paramount! Will check it out.

Ray and Sandy
S/V Pure Joy
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Old 30-11-2008, 09:16   #4
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Other forums have had lengthy discussions on this subject. Consider:

A separate AIS antenna can be tuned for the two specific (adjacent) frequencies used for AIS, while a general VHF antenna is designed for the lower middle of the VHF spectrum.

With a switcher, you miss what's going over the other system when you transmit. Some AIS information is sent only once every 6 (?) minutes.

If you have two antennas and lose one, the other can be plugged in to use in an emergency.

If a switcher fails to isolate the other radio when you transmit, it is fried.

If thirty minutes warning of an impending collsion is enough for you, your AIS antenna can be mounted lower.

When you go to an AIS B transceiver, you will want separate antennas to maximize your investment, and you can use the coax you pull today.
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Old 30-11-2008, 09:25   #5
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I would keep things simple and reliable by adding an antenna dedicated to AIS only. You don't have to install a long 8 foot whip antenna to do this. The shorter (lower dB) VHF antennas will work.

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Old 30-11-2008, 11:15   #6
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The $87 unit does more than splice the wires. If you do that with your VHF radio and the AIS receiver you will fry the AIS the first time you transmit from the VHF. RF energy will go directly into the AIS, the box does a signal block during the high power transmitting allowing RF only to the antenna.


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ais, antenna

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