VHF and AIS antennas

Boat Systems
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I have an ICOM VHF with AIS receiver - most of you will be familiar with this radio.

I also have a full AIS transponder independent of the radio.

The objective was to get some AIS receive redundancy should I experience a transponder failure …esp. offshore. After nearly 10 years I have not had a device failure. Pretty happy with that reliability.

But

I can not say as much for the mast top whip antenna which is shared by the VHF and AIS transponder via an antenna splitter. I have had to replace a broken antenna three times over that same 9 years. One of those antenna failures occurred about half way back from Hawaii. In that case I had to rig my emergency antenna to a long RG 58 cable (part of my emergency kit) which went up on the flag halyard and used that for 1000 miles. It was not ideal. I want better backup capability.

On original install, I wired low loss LMR 400 coax to the masthead antenna from the splitter located close to the chart table - about 65 feet of cable overall with three Barrel joints along the way.
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The cable looks good inside the boat as far as the RF connector at the mast base. The cable does not appear to be shorted, so I suspect an issue with the masthead connector or some other issue with cable condition aloft.

Meanwhile I am a lot less enthusiastic about climbing the mast than I have been in the past. I have never been comfortable with this climb and when I get to the mast top, I find it extremely difficult to do useful work. Hiring someone to climb and troubleshoot this is an expensive exercise as is pulling the mast and repairing on the ground myself.

So… In order to comply with offshore race requirements I must carry a radar reflector at least 13 feet above the water’s surface. I found this really difficult to accomplish reliably with the free standing mast. My solution was to install a 15 foot carbon windsurfer mast on my transom. I have built a reliable mount and basically skewered a classic aluminum radar reflector with the pole and got the reflector to ride about 15’ above the water. Sweet! But there is a tantalizing empty masthead on my new “mast”. So I used that real estate for my independent AIS GPS receive antenna. Works like a charm.

So, years after installing the radar “mast”, I run into the VHF and AIS issue described earlier.

I really did not like not having a good AIS signal. I needed a temp fix and that temp fix could become my emergency backup when I finally repair the masthead issue, So I installed an AIS rcv/xmit stub antenna right below the AIS GPS receiver at the top of the radar pole. While range is somewhat reduced due to reduced height, the gain is improved somewhat by elimination of the splitter and reduction in cable length, Overall it is close to a wash sensitivity wise. Range will be reduced by the lower height of the antenna. I am getting about 15 NM in open water. I can live with that.

But it has worked so well (and so beautifully accessible if a problem arises) that I am now thinking of installing a hi gain VHF antenna on the same pole Just below the radar reflector which is below the AIS antenna which is below the AIS GPS receiver antenna. Yea, it’s getting crowded but everything is fairly light.

I had originally thought ti install a 36 inch whip similar to a masthead antenna but I then thought, why not just go with one of those fiberglass pole antennas one might see on a power boat? I would align this with the radar mast, stand it off a few inches and we now have a complete backup system. I think,

I wonder if any of you RF experts could shed any light on the performance impact of installing (for example) a six foot fiberglass antenna standing off a carbon fiber pole (2.5" diameter) . The standoff would be about 2 inches or so. The base of the new antenna would wind up about 8 feet above the water.
My biggest concern would be creating an RF shadow behind the 2.5" carbon mast. I really can not allow the new antenna to flop around in the breeze, it needs to be integrated to the radar mast and, as a result, behind, on the side of, or in front of the radar mast pole and vertically aligned with it.

Eventually I will have the mast out to deal with whatever problems I find but in the meantime I have an operating system and am left with a good emergency backup for both AIS and VHF when the issue with the primary antenna is resolved.

Could any RF experts comment on this? esp the RF shadow issue. Also any general comments, criticism/alternative approaches are welcome.

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Can I see some pictures of your carbon windsurfer mast?

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Will do. I am out of town for two days. As soon as i get home.

2 Likes
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Zephyr, here are two photos of the radar mast. Better late than never.

Above the reflector is the stub AIS antenna and on very top is the AIS GPS receive antenna.

My thought is to install a long hi gain VHF antenna aft of the Pole and aligned with it. I would be arttached to the pole with stand offs about two or three inches long. The antenna base would then be about four feet above the pushpit. This would place the antenna base about 6-8 feet above surface of water.

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Holy smokes, that thing is huge! How long is the pole? Where did you get it? How much flex does it have?

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Yea, I get some ribbing from my cohorts in the SSS. I sail a Yawl according to them.

The mizzen is 12 feet long. it is 2/3 of a carbon windsurf mast and nicely matches the mainmast🤣. Offshore rules require the reflector to be 14 feet from the water surface. This serves. I used to windsurf so I just repurposed a two part mast. I permanatized the joint with laminate.

It is bendy but not too much. It is quite effective as a radar reflector according to passing ships I’ve chatted with. It has sailed 5000 NM to Hawaii and back, twice. So it’s pretty durable too. About 5 years ago I added a small carbon spar from the base to the ladder fitting on the transom to prevent slight lateral movement of the base In heavy air/sea. Now it’s rock solid.The whole thing is very light but getting heavier as I keep adding stuff.

What do you think re the original question?

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In general placing an antenna alongside a conductor, within some distance (~1 wavelength as a yardstick) the current in the antenna induces current in the conductor, changing the radiation pattern and probably reducing the efficiency (not designed for it, like a Yagi-Uda). You could use fiberglass or wood (even just alongside the antenna), or increase it’s standoff, or put it above.

VHF marine frequencies are right around 1.9m - Ham radio operators and forums have lots of discussions about side mounting antennas close to things (2m is a Ham band). Even though carbon fiber is significantly less conductive than metal, it is likely conductive enough to have an impact.

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Swo,

Thanks for this info. In the meantime I have spent a little time at the Shakespeare site looking at their very basic antenna guidelines. In particular I became aware VHF propagation is impacted by antenna gain, the higher the gain the flatter the pattern. According to them our rocky/rolley boats are better served with the lower gain antenna, say 3dB at the masthead. I was already aware antenna height matters. I currently have a stub antenna about 13 feet off the water and have decided to share this with my VHF radio. I realize the stub and the splitter will reduce performance substantially but I should be a little better off than I am using my low power handheld. If I can eke out range of 5 to 10 miles (probably dooable assuming receive station/antenna is at a decent height) I will be happy with that until I bring the mast down at some point and deal with whatever the issue is at the masthead or on the way to the masthead.

It is easy for me to reconfigure my gear to do the above. I will test AIS post mod to ensure I have not degraded my receive/xmit too much, same with the radio. Worst case I go back to the original config and mount a new temp antenna on the pushpit away from the CF pole.

Interestingly, during prior testing, I got reasonably good AIS performance while using the antenna at the masthead WITHOUT the splitter in line. Just AIS direct to the masthead and VHF offline. So that might be another avenue for temporary ops.: AIS to masthead, VHF to stub.

Thanks also for the link. Interesting info.

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VHF ariels need at least 1 m horizonal gap , larger vessels use at least 2 m Would use the vhf on mast head for distance
Ais transceiver on stub will give at least 10nm transmittion at 14 ft for merchant ships
I have ais vhf stub specific to ais channel on an a frame about 12 ft from waterline picks up boats up to 12 mile away . remove all splitters
Ais stub can be used as vhf in emergency
Other option is a tee bar on your post