Radar Reflectors

st599 said:
A mast is a very poor radar reflector. It's curved, so the reflection back towards the emitter is limited.
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A curved surface tends to scatter relatively evenly in all directions perpendicular to the axis, so a vertical mast would scatter back towards the horizon.
A single flat plate will give a highly directional 'specular' reflection, much like shining a spotlight on a mirror. Unless the incoming wave is on axis, very little of the reflected energy will go back towards the source.
It's less directional than light though, because the dimensions of the mast are of the same order of magnitude as a wavelength, at least in the horizontal plane.

In general the transmit power of a radar is in kilowatts, the receive power is more like nanowatts, you don't need much of the energy to be returned.
 
lw395 said:
A curved surface tends to scatter relatively evenly in all directions perpendicular to the axis, so a vertical mast would scatter back towards the horizon.
A single flat plate will give a highly directional 'specular' reflection, much like shining a spotlight on a mirror. Unless the incoming wave is on axis, very little of the reflected energy will go back towards the source.
It's less directional than light though, because the dimensions of the mast are of the same order of magnitude as a wavelength, at least in the horizontal plane.

In general the transmit power of a radar is in kilowatts, the receive power is more like nanowatts, you don't need much of the energy to be returned.
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A further point is that the thickness of an (approximately) cylindrical emitter (e.g. a mast) increases bandwidth of signals for which it is an effective emitter; the thicker your mast, the wider the range of wavelengths for which it is an effective radiator. The reflection isn't a specular reflection from the surface of the mast as the wavelengths concerned are comparable to the diameter of the mast; it's a passive re-transmission from a signal induced in a long cylinder of metal. So, the curved surface of the mast is immaterial in terms of reflecting ability.

A mast will, however, tend to reflect better in the plane at right angles to the mast, so a heeled mast may not be as effective as a vertical one. However, chances are some of the rigging is vertical (more or less) even if the mast isn't.

Of course, all this analysis fails to take account of the intereference - both constructive and destructive - between the various metallic elements of a yacht's rigging. The overall result is pretty well unpredictable without serious modelling software. I'd imagine that a hedgehog would be a pretty good model of the 3D polar diagram of a yacht's rigging!
 
I agree.
A quick google will show some RCS plots for aircraft.
A medium size plane varies wildly from around 2 to over 20 sqm.

It also shows simple flat plates are good for stealth, right angle corners the opposite.

But the factor of 10 or more in the unpredictability of the RCS is small beer compared to the huge loss in power between transmit and receive, which is literally in the trillions or beyond.

The big issue with boats is that they operate in a random clutter environment, the sea and move too damn slowly for doppler to make life easy!
 
I have an Echomax up the main mast: called a ship off the west coast of Ireland in quite heavy weather when I saw them visually at 3nm; they had seen me on radar at 5nm but still hadn't seen me visually.

A radar reflector of some kind is a definite requirement: (1) SOLAS (yes, that does apply to small boats), (2) insurance: a friend on starboard collided with a boat on port tack that hadn't seen him due to not looking round the genoa; daylight, good visibility. His insurance claim was rejected because he didn't have a radar reflector up (the other boat took a photo of his boat specifically to prove no reflector).
 
"His insurance claim was rejected because he didn't have a radar reflector up (the other boat took a photo of his boat specifically to prove no reflector)."

IF that is true - which I doubt, there is an ombudsman for cases like that.

So lets see, broad daylight, other boat had time to take a picture but no time take avoiding action?

Its the blue M&M's again
 
It IS true.

He had time to take the photo because he stopped after the collision to do so.

Be not such a doubting Thomas: just expect the worst of everybody (including insurance companies) and you won't go far wrong.
 
Seajet said:
What gets me is that when radar was in its' infancy in WWII, it seems U-Boat periscopes were relatively easy to detect; we seem to have got worse not better !
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Except there are very good reasons why U Boat periscopes were easy to detect. They were bigger and they used to leave them up rather longer than the current boats do, and furthermore the Germans hadn't worked out how effective radar was. It skewed the odds in the radar's favour until they got wise to it. If I remember rightly, the modern sub's attack periscope is only raised for seconds to minimise the possibility of detection. The idea (obviously) is that it gets lost in the sea clutter. There may be other reasons but I suspect that it would not be prudent to discuss them here.
 
alanwilson said:
I have an Echomax up the main mast: called a ship off the west coast of Ireland in quite heavy weather when I saw them visually at 3nm; they had seen me on radar at 5nm but still hadn't seen me visually.

A radar reflector of some kind is a definite requirement: (1) SOLAS (yes, that does apply to small boats), (2) insurance: a friend on starboard collided with a boat on port tack that hadn't seen him due to not looking round the genoa; daylight, good visibility. His insurance claim was rejected because he didn't have a radar reflector up (the other boat took a photo of his boat specifically to prove no reflector).
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Fortunately ours is pretty prominent

http://www.steersman.net/Images/radrelf2012.jpg
 
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