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Old 18-06-2009, 13:31   #1
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Radar Reflector Height

I was thinking of taking a trip up the mast today to inspect some stuff and was thinking of repositioning our radar reflector, it is currently just above the first spreaders which is maybe 20 ft off the deck and 25 from sea level. Is there a calculation as to how high you want your radar reflector or is it just as high as you can get it?
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Old 18-06-2009, 14:24   #2
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Higher is better.
See also ➥ Distance to Horizon & Collision Avoidance
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Old 18-06-2009, 18:07   #3
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Thanks Gord, I was vaguely aware of that formula but will have to crush it into my mind until it can't sneak out again. But does this pertain to a radar reflector? I thought that was the point of radar: it isn't line of sight, right?
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Old 19-06-2009, 03:03   #4
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Higher the better for radar detection, lower the better for weight aloft aspects and windage.
I have always stuck it in the middle (around spreader height) - assuming single spreaders.
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Old 19-06-2009, 06:01   #5
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Radar is very nearly line-of-sight.

The distance to the visible horizon can be calculated (visible dip):

D (Visible) = 0.97 x √Hf -or-D = 1.76 x √Hm
Where:
D = Distance in Nautical Miles
√Hf = Square root of Height of eye in Feet
√Hm = Square root of Height of Eye in meters.

Because a radio signal is slightly refracted, the Radio/Radar horizon (range - or radio dip) is calculated:

D (Radar)= 1.22 x √Hf -or- D = 2.21 x √m


Note: The above radio range calculation also applies to Radar, which explains why Radome mounting height is more important than Power (Watts) in determining maximum Radar range.
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Old 19-06-2009, 06:26   #6
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25 feet up is pretty good. Remember the radar transmitter on a ship is going to be way up there -- 80 feet? 100 feet? So according to the formula [the one in the article Distance to Horizon & Collision Avoidance -- is it right?], you've got line of sight between a ship's radar and your reflector up to something like 6 miles plus 10 miles -- 16 miles. Even a radar on top of a fishing trawler should give you something like 10 to 12 miles visibility of your reflector.

Look at it another way: If you put your reflector way up at the masthead, say at 60 feet off the water, what would you gain? A little more than 3 miles.

So if you move it up only by 10 or 15 feet, it's hardly going to make much difference, and you'll be moving weight aloft, and don't forget windage, both of which are very harmful to your boat's sailing qualities.

I wouldn't bother, personally.

And don't forget that there are numerous reported cases of ships not monitoring their radars and not keeping any decent watch at all, and running down yachts. So plus or minus 5 minutes of radar warning is pretty irrelevant in the grand scheme. In any case you've got to keep a good watch, and make sure your own radar alarms work.

The height of your own radar antenna may be more important than the height of your reflector -- assuming you get a better radar watch than ships do, as well you should considering how small and fragile you are compared to a ship. Also don't forget an AIS transponder.
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Old 19-06-2009, 10:08   #7
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That's funny, I could have used that advice before I had my 110 pound girlfriend hoist my 180 pound butt up the mast to move that last night. : )

Gord I am starting to get confused. In your link you say that the distance to the horizon is 1.17 times the square root of height on deck but then in this thread you say it is 0.97. Can You clarify?
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Old 19-06-2009, 10:38   #8
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Quote:
Originally Posted by unbusted67 View Post

Gord I am starting to get confused. In your link you say that the distance to the horizon is 1.17 times the square root of height on deck but then in this thread you say it is 0.97. Can You clarify?
The different coefficients in the formulas Gordan gave are caused by the fact that visible light and radar waves have a different refractive index because they are two very different frequencies. The lower the frequency, the more electromagnetic waves will bend over the horizon. The visible light band is a much much higher frequency than the band used by commercial or yacht civilian marine radars.

Merchant ships have radars with both 3cm and a 10cm wavelengths. The 10cm wavelength allows them to see further over the horizon, but with slightly less resolution. Yacht radars are typically single wavelength 3cm radars. Ten centimeter radar antennas are typically larger and therefore not suitable for smaller boats.

Refractive index
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