Posted by Doug Miller on 4/29/2011 to Products & Technology

A customer calls and says they just read a post on a forum that VHF antenna splitters are bad and shouldn't be used. What's the deal?

Yes, I have read the same stories and it has been a frustration for us for several years. All I can tell you is we have sold thousands of antenna splitters over the last 6 years and overall these products are some of our most successful product lines. We have had less than 5 returns for any reason and only two returned for failure – both due to electrically damage from mis-wiring or lightning. They are also probably one of the products that I get fewest support calls on. In fact, I can’t remember the last time we had a support call on a splitter.

I have had a Comar AST100 splitter installed on my sailboat for years and have never had an issue. I also find the performance with the splitter using the single MV158 antenna on the top of my mast far exceeds the performance I have found with using the same antenna as a dedicated AIS antenna on the stern rail (antenna height and positioning clear of other metal interference objects is everything). I typically get 40-50 mile AIS receive range with my setup and have never had any issues with range or performance with the VHF radio, although theoretically I would expect to see about a 10% reduction in receive range by sharing the antenna.

But won't I damage my radio if my AIS transponder is transmitting on the same antenna as my VHF radio?

While splitters obviously allow you to share a single antenna with two devices, the best feature of a transponder-compatible splitter (such as the Vesper Marine AIS VHF Antenna Splitter or the Comar AST200) is that it protects both the VHF radio, the AIS device and the optional FM radio at all times while any of the devices are transmitting. That said, if you use one of the AIS receiver compatible splitters such as the Comar ASR100 or the Smart Radio Splitter then yes, you might damage the VHF radio while an AIS transponder is transmitting since these splitters are not designed to be used with two transmitting devices. These receive-only splitters work great with AIS receivers but not with AIS transponders. Also note that all splitters must be powered whenever any radio or AIS device is powered. Failure to do that can cause damage since the splitter is not protecting your other devices.

So if you are going with an AIS transponder (any brand) and want to use a splitter, choose either the Comar AST200 or the Vesper Marine Splitter. Note that the Vesper product also has the advantage that it includes an antenna amplifier so you may find range is extended for reception for both the radio and the transponder. The amp only works for receiving signals and not for transmitting since that is technically not legal.

Given all that, having a dedicated VHF antenna for AIS use is also a great way to go if you have the room and laying new cable is not an issue. Always keep in mind though that any VHF antenna should be placed as far away as possible from other VHF antennas or any vertical metal objects - especially ones on the same plane. Most manufacturers recommend at least 6 feet of separation. About the worse thing you can do is install two VHF antennas at the top of a sailboat mast right beside each other. Better to either use a splitter or install the second antenna on the stern rail.

But this is just our experience. We'd love to hear from you about your experiences with antenna splitters.