[wwilson]

Jon,

Respectfully, I disagree with your recommendation. I have taken the liberty of cutting and pasting from a Raymarine FAQ page (below). Your conclusion that the VBW will compensate for heel is simply incorrect, it is however very popular with people who choose a non-leveling stern pole implementation. Also, if your philosophy is "to keep it light", you have picked the worst of the options available.

FAQ Response follows---------------------

When mounting the scanner on a sailboat, the primary "enemy" is the heel of the sailboat. All of our radar scanners (and most of our competitors as well) have a vertical beam width of 25 degrees. You can basically assume that it extends 12.5 degrees above or below the horizon, from the center of the scanner. Any target that is below this beam is invisible to the radar. On a sail boat, you can easily have 12.5 degrees or more of heel at any given time, which would create a large blind spot to the radar. A leveling mount eliminates this problem.

The other factor to contend with is blind spots caused by the masts and booms. If a scanner is mounted is close proximity to one (on a mast mount, for example), the radar will be blind in the direction blocked by the mast. On a typical mast mount platform, the radar will be subjected to a blind spot 1-15 degrees wide, depending on the thickness of the mast. Because this is directly astern, it is of relatively lower concern to the operator, as generally the higher risk area is in front and to the sides of the boat. On a backstay, with some distance between the scanner and the mast, there will be a small blind spot in the direction of the mast, however the increased separation tends to allow the radar beam to "fill in" the blind spot to some degree.

The height of the scanner above the water is directly related to the range the scanner can see. The equation to calculate the distance to the radar horizon is 1.17 times the square root of the height of the scanner. Therefore, a scanner mounted 20 feet above the water line will be able to see about 5.2 miles before the beam is completely blocked by the curvature of the earth. This may seem to run contrary to the advertised maximum ranges of the radar systems, at 24, 48, or 72 miles. We must also factor in to this equation the height of the radar target, and the fact that its height makes it stick up over-the-horizon. The same equation can be used to determine that targets radar horizon as well. Knowing that, we add the distance to the targets radar horizon, to our maximum radar horizon, and derive the maximum detection range for that target. For example, our 20 foot high scanner, radiating on a target that is 20 feet tall, can detect that target at a maximum range of 10.4 miles or so. Atmospheric conditions and weather will also play into this somewhat, and can either increase or decrease the range.

Taking all of this into consideration, it is my opinion that a scanner mounted on the backstay, utilizing a leveling mount, is the best choice for a sailing vessel. Although we sacrifice some range performance by mounting it slightly lower, we are also increasing our short range detection zone, and minimizing blind spots from both the boat''s super structure and motion.

---End FAQ paste --------------------------

mychaos

This seemed to me to be a pretty lucid and dispassionate assessment of the issues to consider in antenna location. I hope it will be of some help to you as well.

Wayne