Radar Height and Range

[RDW]

I admit I am being lazy but I will appreciate your information.
Is the height of the radar mount a big factor? How much difference in the radar range would there be in a radar 35 feet off the water versus 15 feet?
Thanks
RDW

[a64pilot]

There is a mathematical formula that will tell you that, based I believe on the Earth's curvature.
I was going to buy a 4G Radar and wondered about the exact same thing, I called Simrad asking that as I wanted to mount it on a Radar pole, but wouldn't if it reduced the range. Their answer is that there is no practical difference in performance based on numbers very close to what your giving, and if a pole mount was available, they preferred it largely based on any future maintenance that may be required.

Now of course for larger, much more powerful Radars, I feel that may not be the case

[Hydra]

The distance to the radar horizon is roughly proportional to the square root of the antenna height above sea level. Then, raising the antenna from 15 to 35 feet increases the range by 52%.

In metric units, distance to horizon in NM = 2.2*sqrt (height in m)

Alain

[Dsanduril]

Raymarine says essentially the same thing:

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 target's radar horizon as well. Knowing that, we add the distance to the target's 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.

[Jim Cate]

Another factor is that with low mounts large seas can obscure the normal horizon a significant part of the time, and that high mounts can "see over" low lying land features better than low ones. Finally, I believe that with a low mount sea clutter is a bigger factor, but I'm not sure about this. Whether either of these matter to you depends a lot on where you propose to use the radar.

Jim

[fryewe]

Water vapor of the atmosphere varies with height and the radar beam is refracted downward by the normal density distribution of the atmosphere.

Radar horizon for this condition is about 4/3 x the geometric horizon. And that range, in NM, is 1.43 times the sq root of HOE (in feet).

Radar horizon at 15 feet HOE is about 5.5 NM, vs a horizon at 35 feet of about 8.5 NM.

Of course, targets have height as well, and will extend above the horizon (above the shadow zone mentioned above) into the radar's energy field, which extends the effective range above the horizon...to a range dependent on the radar's power, the reflective characteristics of the target, and the receive sensitivity of the system.

[Paul L]

The need for distance is overrated for Radars on a cruising boat. Resolution is far more important. A 24"dome will have much better angular resolution that a 19" - no matter what the power and distance spec'd by the seller is. We are most interested in good resolution and closer in operation for traffic avoidance and navigation. We use long distance ranges to look for storm cells. In this case it doesn't matter what your radar height is as the storm height is way up there. We also use long distance ranges to verify that our charts match up with reality, again dome height is not important in this case. The lower dome height might have some issues with big seas, getting blocked at the bottom. In this case the dome mounted at 45 feet off the water is going to be swinging wildly about as the mast gets bounced around, so the radar single is not going to be that good anyway.
You can play with dome height and target heights here to get max distances:
Radar Horizon Calculator
Example that might apply to a freighter in the distance
Radar height 15ft, target height 60 ft = range 14miles
Radar height 35ft, target height 60 ft = range 17miles

[jongleur]

And will it be mounted above your
lower shroud attachment or above it? If
you lose the top portion of your
mast, you might still have the
radar if mounted under.

[Adelie]

Don Dodds in his book "Modern Seamanship" has a chapter on radar. He make good arguments for mounting in the 16'-25' range, too low and you had radiation issues with crew on deck as well as sea clutter and too high you started eating into your short range returns, shorelines at about 1/8 nm.

You could put the radar at the top of your mast and only gain a couple/five miles max range while also severely cutting into your near field returns. To me I would rather see ships at 15nm instead of 18nm in exchange for being able to tell if that low rock is 100m vs 300m away.




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[wolfesmy]

There have been a number of formulas bandied about the actual formula is 1.17 x (sqrt) height of eye.

Bottom line higher is better based on you radars range. if it is a 6 mile radar not much advantage of putting it at the top of the mast where it becomes unnecessary weight aloft. Your target has elevation as well.
Hope that helps

[Adelie]

Originally Posted by wolfesmy

There have been a number of formulas bandied about the actual formula is 1.17 x (sqrt) height of eye.

Bottom line higher is better based on you radars range. if it is a 6 mile radar not much advantage of putting it at the top of the mast where it becomes unnecessary weight aloft. Your target has elevation as well.
Hope that helps

That's the formula for visual range, radar will refract around the curve of the earth slightly giving significantly more range hence the different formula.




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[Cadence]

Do what is most practical for you. The distance at those heights is academic. Good arguments have been made for both. As long as you're high enough not to nuke someone's brain.

[Capt Phil]

Certainly a good idea to mount the radar at the highest practical point on your boat. However, these so called Mariners that have been sold a 48 or 72 mile open array radar by some slick sales person deserve to be parted with their hard earned or inherited $!
Must important when buying a new radar in my opinion is the signal strength first and ease of operating second. For coastal cruising where you want at least a 16 mile with a strong return or if you spend time out a few miles a 24 mile with a strong return would be fine. The only reason to go to a 36 mile would be if you occasionally close the coast and want a warning of how much lee shore you might have.
It might be fun to pick up Catalina as you leave San Diego but a 72 mile radar is overkill IMO. Many of these new radars have dropped in price significantly and are married up with chart plotters, autopilots and sounders/fish finders. Simrad which is the power boater version of the old standby, Brooks and Gatehouse, that sailors have used for years is one of these. Garmin have marinized their line of land based equipment with many cool options. Coastwise, prices have dropped to a small fraction of what you would pay even 3-5 years ago and competition has kept prices very attractive.
That's my two cent opinion.... cheers, Phil

[Cadence]

Originally Posted by Capt Phil

Certainly a good idea to mount the radar at the highest practical point on your boat. However, these so called Mariners that have been sold a 48 or 72 mile open array radar by some slick sales person deserve to be parted with their hard earned or inherited $!
Must important when buying a new radar in my opinion is the signal strength first and ease of operating second. For coastal cruising where you want at least a 16 mile with a strong return or if you spend time out a few miles a 24 mile with a strong return would be fine. The only reason to go to a 36 mile would be if you occasionally close the coast and want a warning of how much lee shore you might have.
It might be fun to pick up Catalina as you leave San Diego but a 72 mile radar is overkill IMO. Many of these new radars have dropped in price significantly and are married up with chart plotters, autopilots and sounders/fish finders. Simrad which is the power boater version of the old standby, Brooks and Gatehouse, that sailors have used for years is one of these. Garmin have marinized their line of land based equipment with many cool options. Coastwise, prices have dropped to a small fraction of what you would pay even 3-5 years ago and competition has kept prices very attractive.
That's my two cent opinion.... cheers, Phil

I love the truth about a slick salesman but it's good bragging rights. "I have a 48mi. radar." but the horizon plus a little is 15mi. at your mounting height.

[CGirvan]

I think the answer to radar height has changed with the introduction of the new broadband radar. Now with no radiation issues to deal with and a much improved close in resolution I feel that having the radome lower has significant benefits. The broadband radars don't have quite the long distance advantages as the pulse radars anyway and as previously mentioned apart from stormcells or headlands what do you need to see more than 6 miles out.
It's the close in stuff that's important.