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  #1  
Old 02-12-2005
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Radome location

Adding radar on a J/35 and considering different locations. Favoring backstay, but not sure. Other options are mast and pole.
Some say leveling not really necessary. See through wet sails?
Suggestions?
Many thanks.
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Old 02-13-2005
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Radome location

We went through the same question last year for our new 41''. Settled on a stern pole, no leveling and are happy with the choice. Reasoning included most radar use is under 12 miles, we find often under 6 and the extra range from the height of a mast mount is meaningless. A mast mount beats up the headsail leech. A backstay mount was ruled out as we slack the backstay totally in light air and it would flop around. Leveling we have found unnecessary. The vertical arc of a typical radar is 25 dgrees or so, and unless you are on your ear there is no difference. As a practical matter we have seen no deterioration in range or definition from heeling (or from wet sails). We know two boats that had their leveling mechanism fail. Our philosophy is keep it light and simple whenever possible.
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Old 02-14-2005
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Radome location

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
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Old 02-14-2005
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Radome location

Wayne, your points are well taken. I had read the Raymarine website (we have a Raymarine setup), but also talked to a number of people with fixed mounts who had found in practice no ill effect, and our experience matches. So I was suggesting leveling benefits may be more theoretical than real, at least for most of us. If you need maximum performance at serious heel angles, get leveling. We find our radar use (and those we talk to) is almost always feeling through fog or at night with modest heel.

On height, a lower mount does handicap max range performance, but can enhance close in and that''s where the accurancy is appreciated, like feeling for buoys in fog. The stuff we look for at ranges over 6 miles tends to be either decent size boats (collision avoidance) or land (navigation), and these stick up enough to be seen. We turn the radar on the 24 mile range sometimes for kicks but serious work seems always close in. Our experience anyhow, others may be different. The question (as always) is how will you be using it. On the backstay mount, I''ve raced on a J-35 and they used to also slack the backstay to where a radar and mount would flop around, so I assumed the same. If you keep the backstay always tight enough, a backstay mount could be the right choice, but that reduces some of the adjustability of the J-35 rig.
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Old 02-15-2005
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Radome location

Marine radars use one of two frequency bands ó X-band (9000 MHz) or S-band (3000 Mhz). Because the shorter 3-cm (about one-inch) wavelength allows the use of a smaller and lighter antenna, most small-boat radars operate on the X-band. Both band systems will detect and display precipitation, but S-band units will do a better job of penetrating rain to spot targets on the other side. The X-bandís shorter wavelength will suffer greater attenuation when passing through precipitation, reducing the amount of reflected energy received from objects obscured by heavy precipitation.

The RADAR HORIZON (in nautical miles) is 1.22 times the square root of the scanner height (in feet), while the Visible Horizon in nautical miles is 1.169 (usually rounded to 1.17) times the square root of the height of eye in feet. The maximum radar range is determined by the height of the scanner, and the power output of the transceiver.
ie:
A radar antenna mounted at 12'' above the water will ďseeĒ about 4.23 nm. If the target was also 12'' high, the radar would see it at about 8.45 nm (4.2262 + 4.2262).
Range = 1.22 x sq. rt. 12 = 1.22 x 3.454 = 4.2262

The shape or angular dimensions of the beam also play an important performance role. The vertical angle of the beam emitted from the scanner is typically 15 to 30 degrees (equally divided by the horizontal axis of the mount). For a 25-degree beam width, 12.5 degrees of the beam will be below the horizontal axis, if the scanner is mounted level with the water. The horizontal beam width varies from 0.5 degrees for larger installations (largely dependant upon antenna length) to five degrees for smaller designs. Three to five degrees is typical for small-boat radars. In general, larger scanners have narrower beam angles, and more accurate bearing resolution. Radar is generally more accurate in determining target Range, than Bearing.

The horizontal beam width of a radar determines the bearing resolution (the radarís ability to discriminate between multiple targets that are close together). If the beam is wide enough to strike both objects at the same time, they will be displayed as a single target on the screen. Beam width also affects the bearing accuracy, by elongating a targetís displayed image as the wider beam sweeps across it. This must be taken into account when taking bearings with radar. A narrow-beam antenna does a better job of focusing the available power, and the result is more effective radiated power aimed at the target.

The antenna should be mounted at a height that places it at least:
two feet above,
and four to five feet from,
the head of anyone on board.

Mounting the antenna more than about 22 feet above the water will not make a worthwhile contribution to maximum range operation, and can degrade the radar''s ability to show important close-in targets.

Placing a radar on an insulated backstay used as an SSB antenna will likely produce unsatisfactory results. Most radar set scanners must be connected to the vessel''s ground bus. A ground on the radar scanner will therefore place a ground on the backstay, unless the mount were specially insulated from the stay. The insertion of a ground at this point will make tuning the antenna difficult, if not impossible, and reduce the effectiveness of the antenna during transmission. In addition, the relatively high-voltage RF field surrounding the backstay during transmission may damage components in the radar scanner.

Gimbaled mounts can cause problems with some radar scanner connecting cables. The constant motion of the scanner, relative to the surface on which it is mounted, can fatigue the cable, which is not designed to withstand this type of service.

It is likely that the best overall results will be achieved by mounting the radar on a short mast at the stern of the boat. It is unnecessary to mount the scanner at any great elevation; in fact, too high a mounting height can create problems when trying to see close-in targets. Remember, a higher antenna heigh will increase your maximum range - but will also exacerbate the ill effects of pitch and roll.

I would mount the radar on a stern mast so that it clears the deck by about eight to ten feet. This will likely place the radar antenna about 11 to 14 feet above the water. At this height the radar horizon will be at least four miles distant. Targets at this same height above the water will be visible at about eight miles, with higher targets visible at longer ranges.

A note on interference:
For GPS to work properly, it is recommended to be positioned 12 inches above the radar dome (or open array), and have an unimpeded look skyward at the satellites, It is also important that the VHF and cellular antennas are mounted 12 inches or more away from the radar and the GPS, so as not to have three or four bands of [electronic] waves interfering with one another.
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Old 02-15-2005
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Radome location

GordMay

In all of that you ignore at least one major radar manufacturer''s mounting recommendation and high school trigonometry. An antenna more than 12.5 degrees out of axis will see nothing but the sky. You and crew should keep particularly careful watch to the weather side when on the wind.

Wayne
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Old 02-15-2005
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Radome location

Jon,

I concur. If that is the way you use the radar and are aware of the plusses and minuses, then you have chosen the right installation for your use.

Wayne
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Old 02-17-2005
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Radome location

Thanks everyone for you thoughts!
Researching Raymarine and Furuno.
I''m impressed with the C series displays and like the MARPA.
Heading up to Nova Scotia, Sable Is. and Newfoundland late this summer so plan to put the radar onboard and practice well with it.
Thanks, again.
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Old 03-04-2005
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Radome location

Hi:

We are happy with the Raymarine radar and have found the MARPA easy to use. I was looking at the BT Global Challenge website and noticed they have their radars on a fixed mount at the stern, no leveling. That outfit knows as much as anyone about what works and what doesn''t. They must count on the radar. I suspect they have the found the theoretical gains from leveling not worth the risk of mount failure or cable fatigue. I also suspect the beam cutoff angle is not abrupt at 25 degrees. There will be degraded signal beyound that point but we still see things at normal heel. Newfoundland in the summer will be a good test!
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