Antenna Placement

[Easygoing]

I'm sorry to post this. I've read too much conflicting information on the web, and the more I read, the more confused things get.

Could someone please help me with my antenna placements?

I apologize if this is not the correct place to post this.

I have a 6' wide pilothouse, and would like to mount the following antennas:

1. DRS4DNXT radar (mounted in the center, elevated 8-12" above the roof)
2. VHF #1 antenna - was thinking of placing this antenna on the starboard side of the pilothouse roof. This will be an 8' Digital 529 antenna.
3. VHF #2 (backup...will be mainly monitoring channel 16) antenna- haven't bought this one yet. Was leaning towards mounting on the portside of the the pilothouse roof (6 foot distance from the other VHF antenna). Willing to go with either another 8' Digital 529 and mount a separate antenna for an AIS unit, or install a splitter and use a dual purpose AIS/VHF antenna, in either 8' or 4' height...whatever is recommended.
4. Furuno AIS FA50 antenna - would it be best to combine with my backup VHF and give 6' distance from the other VHF antenna, or get separate antennas and mount the two VHF antennas the farthest possible (6 foot) and buy a 4' AIS antenna that would be positioned between the radar and one of the VHF antennas...giving 1.5' distance from each.

I throw the idea of a 4' AIS antenna just to get some vertical separation.

Thanks in advance if anyone is able to help me.

 

[Taniwani]

Having experimented with this, my short answer would be not to worry too much, but let me explain what I found out.

Originally I had a very expensive combo antenna at the mast head, which fed several antennas into two coax cables down the mast of my sailboat, these ended in some boxes that split up into various connectors for VHF, FM-radio, SSB-receiver, UMTS-cell phone and DSC.

The interesting part here is the VHF and the DSC antenna. Physically the VHF antenna sticks out at the top about 3ft long with a matching coil for an optimum at CH16. The DSC antenna is a vertical dipole also rather short with a matching coils for the DSC frequency. The upper half of the DSC-dipole overlapped the VHF antenna and is horizontally about a foot away.

I used the DSC-antenna for my AIS and it worked just fine, until a few years ago corrosion destroyed the DSC dipole and part of it broke off.

Rather than buying a new combo antenna for more than $2000 I mounted an AIS matched short rod at the masthead, about a foot from the VHF antenna, vertically overlapping about 2/3 of the length and switched the second coax cable to this new separate antenna.

AIS reception was always ok, but it seemed even better with this new antenna. VHF worked as well as before. On AIS I can see that marine-traffic stations that are high enough, pick me up at 40-50 miles and on VHF I have been talking to boats some 35 miles away.

I also tried to transmit on a VHF channel very close to the AIS frequency and see wether I could still receive anything on the AIS. The result was that AIS signals from class-A AIS were still coming through from up to 4 miles away. Class-B in the same harbour where also still getting through.

Note that with a splitter, the AIS would be completely shut out while transmitting on the VHF. And both, AIS and VHF work better without a splitter.

So I'd say, that if it is easy for you to put some distance between the antennas, do it, but don't worry if it is too difficult.

With respect to the radar, I can say that I have a 10ft radar mast which used to hold a 4kW open radar scanner and among a few other things a spare VHF antenna. (In case the rigg comes down). I did test the VHF with the radar running and could not notice any interference on either. Must say though, that the VHF antenna is a bit higher and above the main beam. Even the main GPS which is also mounted on the radar pole is entirely unimpressed by the radar. With the lower power of the NXT radar, this should be even less of a concern.