Radar reflectors

[Paul Elliott]

I was having this discussion with someone about radar reflector heights, and why higher may not be better (it may have been here...) Anyway, there are nulls in the reflected signal as seen by the radar, a result of the destructive interference (cancellation) of the direct reflection and the reflection that is bounced off the ocean.

The paper that Adaero gave us a link for contains this "probablilty of detection" plot:


Notice the deep nulls at four miles, and again at two miles (and many more as the range decreases). This is with the radar antenna at 30 meter elevation, and the reflector at 4 meters (these are fairly typical, I guess). Also notice that these nulls are fairly wide.

If you raise your reflector higher than 4 meters, that first big null moves out, and I don't think you get any stronger return at the ranges where it is important. The motion of the boats in the swells will move the location of the null somewhat, and the sea-state will effect the depth of the null.

There was an article that recently ran in one of the sailing magazines that went into this in some detail. I've tried to find it again, but so far no luck. Anyway, the bottom line is that there is a "optimum" reflector height that places these nulls where they will have the least impact on your boat been seen when it needs to be seen.

AIS -- Here's another vote for AIS. Yes, ships run their transponders continuously, and AIS is a great tool for collision avoidance (standard disclaimer about still using eyes and ears).

[David M]

Originally Posted by Paul Elliott

I was having this discussion with someone about radar reflector heights, and why higher may not be better (it may have been here...) Anyway, there are nulls in the reflected signal as seen by the radar, a result of the destructive interference (cancellation) of the direct reflection and the reflection that is bounced off the ocean.

The paper that Adaero gave us a link for contains this "probablilty of detection" plot:


Notice the deep nulls at four miles, and again at two miles (and many more as the range decreases). This is with the radar antenna at 30 meter elevation, and the reflector at 4 meters (these are fairly typical, I guess). Also notice that these nulls are fairly wide.

If you raise your reflector higher than 4 meters, that first big null moves out, and I don't think you get any stronger return at the ranges where it is important. The motion of the boats in the swells will move the location of the null somewhat, and the sea-state will effect the depth of the null.

There was an article that recently ran in one of the sailing magazines that went into this in some detail. I've tried to find it again, but so far no luck. Anyway, the bottom line is that there is a "optimum" reflector height that places these nulls where they will have the least impact on your boat been seen when it needs to be seen.

AIS -- Here's another vote for AIS. Yes, ships run their transponders continuously, and AIS is a great tool for collision avoidance (standard disclaimer about still using eyes and ears).

That's interesting Paul...partial to full wave cancellation caused by two different distances between the reflected and the direct radar signal. Who woulda thunk? I bet by placing two different reflectors at two different heights this problem is eliminated...but then, your boats mast starts looking like a Christmas tree.

[Zach]

Very cool thread!

Learned a lot in this one.

Say, aren't most Radar units directional, in that they in a pretty straight line? (Remembering reading about heights of objects to be detected at 48miles...) Wonder if a radar reflected at the top of the mast (boo to sailing performance negatives...) would be more in line with big ships radar antennas?

Also... what about putting aluminum foil or something reflective inside a fiberglass hull?

[Paul Elliott]

Originally Posted by David M

That's interesting Paul...partial to full wave cancellation caused by two different distances between the reflected and the direct radar signal. Who woulda thunk? I bet by placing two different reflectors at two different heights this problem is eliminated...but then, your boats mast starts looking like a Christmas tree.

I would love to do some experimentation with this. I'm going to crunch some numbers tonight to see the effect of ship radar height (which ought to behave the same as reflector height). I suspect that the actual real-world performance is so chaotic that the height effect may be washed out in the general noise of all the other uncontrollable factors -- but this is just a suspicion.

It would be nice if there were a set of guidelines that, if followed, would at least keep you from ruining whatever chances you might have. I wish I could find that "Optimum Reflector Height" article, and see what assumptions the author was making. I do remember thinking (as I was reading it) "Hmmm.... This does sound plausible..."

[Wotname]

Originally Posted by Zach

Say, aren't most Radar units directional, in that they in a pretty straight line? (Remembering reading about heights of objects to be detected at 48miles...)

The radar RF wave is directional, the antenna that transmits it is rotating through 360 degrees (for a standard ships radar).

Here is a very simplistic analogy: Its a very dark night and you have a big powerful searchlight that you scan the horizon with (that's the radar transmitter), whatever you see is because the light from your searchlight was reflected back into your eyes (the radar receiver). A dark object is a poor reflector of light so you can't see it very well while a white object is good reflector of light and is easier to see. With radar, metals reflect well, wood and fibreglass don't.

Don't take this analogy to far, it is only a very simple look at radar.

[David M]

Originally Posted by Zach

Very cool thread!

Learned a lot in this one.

Say, aren't most Radar units directional, in that they in a pretty straight line? (Remembering reading about heights of objects to be detected at 48miles...) Wonder if a radar reflected at the top of the mast (boo to sailing performance negatives...) would be more in line with big ships radar antennas?

Also... what about putting aluminum foil or something reflective inside a fiberglass hull?

A good open array antenna meant for a larger yacht will have a horizontal beam width of less than 2 degrees. Large ship antennas have even narrower beam widths because they have some very wide antennas. For reasons relating to physics, the wider the antenna, the narrower that the horizontal beam width can be made and the better the radars resolution becomes. If you were to place a typical yacht antenna higher, such as up on a ships mast, you would get more distance to the horizon but not better resolution like a ships radar antenna. On the other hand, the small dome antennas you see on smaller yachts have a wider horizontal beam width giving them less resolution. It all works out to price vs resolution.

A 48 mile radar is kind of overkill for a yacht unless you are trying to pick out mountain tops over the horizon. The distance to the horizon for 100 feet above the water is 11.7 nm.

Distance to the Horizon Calculator

[Paul Elliott]

Originally Posted by Zach

Say, aren't most Radar units directional, in that they in a pretty straight line? (Remembering reading about heights of objects to be detected at 48miles...) Wonder if a radar reflected at the top of the mast (boo to sailing performance negatives...) would be more in line with big ships radar antennas?

Also... what about putting aluminum foil or something reflective inside a fiberglass hull?

Radars are directional, but mostly in the horizontal plane. For example, the Furuno 6-foot open array has a horizontal beamwidth of 1.4 degrees, but a vertical beamwidth of 22 degrees (this isn't a ship's radar, but the performance is probably close and I happen to have the specs handy). A 22-deg beam paints a very large vertical strip. We use our radar (a smaller unit to be sure) to look at both squalls and other vessels.

So, I don't think that a top-of-the mast reflector will be painted any better than the typical spreader-height reflector. Also, I'm much more concerned about being detected at 10 miles, and don't care so much about 48 (or 20) miles.

As for the foil, it certainly couldn't hurt but I have no idea if it would help significantly. I suppose I can safely say "It Depends".

By the way, I'm no radar expert, and I welcome any better-informed comments.

[senormechanico]

Originally Posted by Zach

Very cool thread!

Learned a lot in this one.

Say, aren't most Radar units directional, in that they in a pretty straight line? (Remembering reading about heights of objects to be detected at 48miles...) Wonder if a radar reflected at the top of the mast (boo to sailing performance negatives...) would be more in line with big ships radar antennas?

Also... what about putting aluminum foil or something reflective inside a fiberglass hull?

Why not just require all the crew to wear tin foil hats?

Steve B.

[Paul Elliott]

Originally Posted by senormechanico

Why not just require all the crew to wear tin foil hats?

Your regulation anti-alien tinfoil hat is actually an excellent stealth shape. Many people think these hats are for shielding, but actually we wear them to avoid being detected. They can't abduct you if they can't find you! I've had a 100% success rate with my particular hat configuration.

[Maren]

Originally Posted by Paul Elliott

Your regulation anti-alien tinfoil hat is actually an excellent stealth shape. Many people think these hats are for shielding, but actually we wear them to avoid being detected. They can't abduct you if they can't find you! I've had a 100% success rate with my particular hat configuration.

That's utterly silly.

Aluminum foil hats aren't for avoiding detection and abduction. They are for the orbital mind control laser. You are just being a pawn of the man. It's already been shown to be a collaboration between MUFON (which is controlled by the Vatican), PETA, Lockheed Martin and the porn industry.

You just gotta read man, it's all there!

And yes, I'm as serious as a mime having a heart attack

[Alan Wheeler]

For reasons relating to physics, the wider the antenna, the narrower that the horizontal beam width can be made

That is because of the number of elements. No not heating elements. A scanner is a form of transducer. Transducer means Transmit/Receive. When you have one, the energy radiates away from the front of the transducer and dissipates at a rate of 6dB per doubling of distance. When you now place two elements side by side, the two work together and Sum the signal. The result is that in the middle of the two, the energy dissipates now at a rate of only 3dB/doubling of distance and the outside extremes of the two drop away at the 6dB rate. Now add 4 elements and now the centre of each combine and the drop off is now 1.5dB/doubling of distance and the outside still is 6dB. See where this is heading? So the centre of the beam becomes more directional and more focussed and has far more energy. The main advantage is when over a greater distance, that energy is not cluttering up the return with stray echoes. It is like using the Floodlight versus Spotlight on a theatre stage, shining from the back of the theatre. You want to see just what the spot is focused on. The floodlight illuminates a whole mess of unwanted scenery.
Plus remember that the scanner is doing two things. It is sending out the microwave burst and is also receiving the reflection almost instantly.

[Wotname]

Zach,
I mis-read your post (#33) thus giving you a far too simplistic answer - sorry.

David, Paul and Alan have given you far more eloquent answers that actually address what you were asking.

Initially I thought I mis-read you post 'cause I had only just woken up and not yet had the coffee; now I realize it was 'cause I wasn't wearing my aluminum foil hat - I will never take it aff again when I am on CF.

[panthablue]

I built a radar reflector mount some years ago (shown below). The wire you can see on the right is the backstay, the wires on the left are attached to the pushpit. The rope is an uphaul to raise the assembly in to position. A short rope above this is tied to the backstay using a rolling hitch with a couple of extra turns and it's tail end has an eye for the uphaul rope to pass through. The metal plate covering the backstay has some curved nylon blocks inside. The leverage holds the assembly in position. It works well but having now read this thread, I think I should polish it up a bit.

[Paul Elliott]

Warning: Stop reading this NOW unless you enjoy technical trivia. There may be some useful data here, but perhaps not.

This reflector height issue has been bugging the H*ll out of me, so i did some more research. My conclusion is that there is indeed a good reason to not elevate your radar reflector above the height it needs to be for detection at a reasonable range.

I sketched out the geometry of a radar, a reflector, over a perfectly-reflective flat earth, and arrived at the same numbers I've seen elsewhere. I also found a reference that gave the equations (which were the same as mine), as well as a nicely simplified equation for the approximate distance to the first null-zone. As the ship and the target are approaching, the signal will increase, but at the first null the signal will drop out, increase, drop out, increase, etc. I previously posted a chart showing this. Here is a chart I generated showing the same null position (radar @ 30 meters, Reflector @ 4 meters):

Note that the first null is around 4 naut. miles, and at closer range there are many more nulls. Between the range of first contact and the 4-mile null, the reflector return is reasonably stable. Please don't take the null depth shown on my chart to be accurate, they really just show the location. At short ranges, the nulls smear together on my chart, so use your imagination. Also, my charts do not show the signal attenuation with distance (path loss).

If you double the height of the reflector to 8 meters, the first null moves out to approx 8 miles (the distance depends on both radar and reflector height):


If I you put the reflector at the top of your 50-ft mast (16 meters), the null moves out to 16 miles:

Look at that mess of nulls, right where you want to be seen. Hardly likely to give a strong blip on the radar!

Here is what happens when the radar is at 16 meters (a fishing boat, perhaps), and your reflector is at 4 meters:


My last chart shows the first null range for varying radar and reflector height:


Caveats: In the real ocean, there are waves. These will effectively add to and subtract from the radar and reflector heights. You can use the chart above to determine the wave effect on the null position. Also, in the real ocean, the waves will cause multiple reflections to be received. These wave effects will cause the nulls to be not as deep, and their position to move around.

Also, the "flat-earth" math suggests that you might want to lower your reflector to the surface of the water. This is obviously not right: We live on a semi-spherical Earth, so distance-to-horizon is important. Wave height is also a factor in setting a reasonable reflector height. So there is a minimum reflector height needed to provide decent range.

My conclusion: Raising my reflector above the first spreader is not a great idea. And a ship's probably not going to see me anyway.

By the way, the equation (simplified) for range to first null is:

Range = (2 * RadarHeight * ReflectorHeight) / RadarWavelength

All units are in meters. The wavelength of a 9.410 GHz radar (X-Band) is approx 0.0319 meters.

[Wotname]

OK I like technical triva, especially electronic TT and this is good triva! As well as the varying sea state, factor in a pitching, rolling mast, the difficulty of mounting anything about deck height on a sailboat and I think the best mounting spot for the relector is wherever you can put the darn thing securely. Then cross your fingers, keep a good lookout, maybe fit an active reflector (radar transponder) and finally pray to one's own personal Deity. Oh and sleep lightly.

BTW, thanks to Paul for doing this work.