VHF antenna and radar reflector distance

[Ken O]

I will be installing a new VHF antenna (Morad 156 hotrod) and using the old VHF antenna for AIS (as an experiment--if I don't like the results, I will switch to a 162 antenna for AIS). Anyway, I need to fit 2 antennas into the very limited C-22 cabin roof real estate. With solar panels and a dinghy, I'm left with the corners. Starboard forward has the existing Shakespeare mount, starboard aft has pretty much no room, port aft could work, but is a longer run and is my favorite route forward, and port forward would work, but my radar reflector is there. Port forward would be best, but the radar reflector would be less than a foot away, and in-plane with the bottom of the antenna. What to do?

[Ken O]

Oops! Wrong forum. Can I move this post?

[jkidd]

Temporarily put them were you want them and check your swr's. They are suppose to be 3 feet apart. If the antenna is going try to couple to the radar reflector it will ruin your swr's and you won't be able to do it. You could get some reflection and that will mess with your distance. Our little 22's don't give many options for multiple antennas. You could put the reflector on a mast at the back of the boat or raise it above the antennas.

[Ken O]

Jody, I knew I could count on you for a good idea. Disappointed I didn't think of that myself. Thanks s bunch.

[thataway]

Are you transmitting AIS--if so what protocol? ( 5 watt B + SOTDMA vs 2 watt B, with less data?) I do not believe that the radar reflector in the rain catch position, should interfere with VHF transmissions, and certainly not reception.

There are many more factors involved. (Antenna height, antenna swr, etc.) You want to avoid parallel wires close by the antenna. ). In today's world I would go with the B+. and the Morad 162 mhz antenna. Your 'basic Vhf' antenna may or may not have ideal swr (or resonant frequencies) with the random VHF (made to be at best close to 156 to 157 mhz where the voice channels on Marine VHF are located.

We ran all sorts of frequencies (Marine SSB, Ham SSB, VHF and UHF) on our ocean crossing boats--I did a number of experiments to see what would affect the transmission and reception. We had long wires, resonant antennas, half wave, 5/8 wave, dipoles, etc--also the reflecting effect of the rig of the sailboat and its mast. We had a radar reflector on the mast and also one on the 9' off the deck radar arch near the transom.

[Ken O]

Thanks, Bob. I got the idea for giving the old 3db Shakespeare a try as the AIS antenna from Rob at Morad (thanks for suggesting Morad, I am very impressed with them). He said to test it before buying another antenna. I always take the advice of a company rep who is not in a hurry to sell their product! Lots of advice, including your advice, to avoid a splitter for AIS. Thanks

[jkidd]

You could also mount the reflector on the top of the boat and mount the antennas so their base is above the reflector. Mount the antennas on the front and back of the pilot house on diagonal corners.

[Ken O]

I forgot to answer Bob's question: the new AIS will be a 5 watt SOTDMA.

[jkidd]

The old antenna is probably tuned for channel 16. You should be able to tune it to channel 70 for the best swr's. Even if you left it alone it should work just fine it just depends if you want the best distance. My guess would be if you can be seen for a couple of miles your not going to get run over.

[thataway]

Good choice of B+ protocol and power. This is more important when at higher speeds. I also agree with a separate antenna.

I believe that the below is a photo of Ken's boat. Has the radar reflector been moved or changed since this photo?



No question that the normal VHF antenna will work with AIS. As for ease of changing the lowest SWR frequency--easy with a base loaded whip--not so easy in some of the fiberglass encased antennas.

Morad is one of a very few antenna manufacturers who check the SWR on each antenna before it is shipped. I have taken my antenna analyzers to West Marie in the past, to check the SWR on each antenna before I purchased one--and some were significantly off..

My experience is that in a small boat there is a significant chance of being run down by larger vessels. I have had to take evasive reactions about a dozen times in over 200,000 miles of voyaging. I have a number of friends who perished this way. Some even in "coastal" waters. Some larger vessels turn off or reduce gain on their AIS (often for nefarious reasons). AIS is just another tool--like radar, chart plotters, your eyes & ears to prevent collisions. Situational awareness on the boat is always paramount.

[DayBreak]

We are using a Raymarine AIS700 AIS Transceiver with Vesper SP160 antenna splitter and a Shakespeare 5225-XT Galaxy VHF Antenna. Bob's comment on "situational awareness on the boat is always paramount" is so true! If anyone is thinking of putting an AIS transmitter on your boat to transmit your vessels information while receiving broadcast from other vessels, I encourage them to do so. It is really nice and a safety factor to anticipate a larger commercial vessel coming around the corner, their size, speed and direction even before you see it. Gary

[ssobol]

The boats you are most likely to get run down by are unlikely to have AIS receiving and display capability.

[Ken O]

Yes, Bob, that's our boat. The radar reflector is still in the same location. We did add a solar panel in the area behind it, so only front or back locations are available on that side. I see that Morad offers a 2 ft. stanchion, which would get the antenna above the radar reflector.

...and, thanks everyone for the reminders about keeping a good lookout.

If you want to read some scary reports, try some of these:

https://www.bst.gc.ca/eng/rapports-reports/marine/index.html

I'm sure vessels do no better in American waters. I read the Canadian reports because they are clearer and more accessible.

Be careful out there...
Ken

[DayBreak]