Radar reflectors

I am interested in peoples opinion.

Which type of radar reflector do people think is the most effective, and which type of radar reflector do people prefer to buy?

 

i belive, its said, ive been reading people like to buy electric small rod type reflectors you dont have to place high but dont work very well and on the other hand there are those who swear big ugly clumsy high mounted crossed diamond swinging iron ones are best while big ships often still dont see them...

 

See the September 1995 issue of Practical Sailor for a comparison of radar reflectors.

http://www.ussailing.org/safety/Studies/radar_reflector_test.htm

I've often wondered if it would be possible to paste diploes all along the mast as reflectors. Probably not much gain from each one, but there could be a lot of them, and different lengths could cover both X-band and S-band frequencies.

 

Tspeer:

Please expand on your thoughts. What do you mean by diploes (dipoles ?) ? Please describe them?

 

Just a thought, but has anyone done any work with a reactive transmission device ? With the radar detector / jamming devices on the market it would seem a small leap to develop an instrument that : upon detecting a radar transmission would broadcast it's own return signal that would be infinitly more recognizable than any passive reflector, sort of an IFF (identification friend or foe ) for ships at sea. I used to work on AA radars and can tell you nothing paints brighter than a jamming signal.

 

Please keep me posted on this IFF.

Could this work in a port also, or will this only work at open sea? I spend quite some time hopping through the port of Amsterdam by sail (hey, your an addict or go no where at all ;-)
With only 6m length I don't like the thought of a big baloon in the mast...

 

A dipole antenna (sorry for the dislexic fingers) is as simple as it gets - just a straight conductor. It's typically made to be as long as half the wavelength you're trying to receive/transmit with a feed in the middle. The best known use of dipole antennas as radar reflectors is chaff - the clouds of foil strips dropped from airplanes like electronic smoke screens against radar.

The antenna pattern for a dipole is a maximum at 90 degrees from the axis, and zero parallel to the axis, but is not terribly sensitive to angles near 90 degrees. So it's not that sensitive to heel angle, although this also means it's not a very high gain antenna, either.

Basically I'm proposing turning the mast into a tall, narrow chaff cloud by applying lots of dipoles covering the X and S band frequencies used by marine radar. None of these are going to be very good reflectors. But I'm speculating that a lot of them could add up to something useful. The aperature size would be huge, since it could approach the length of the mast. I'm basically taking the principles for creating stealthy craft (avoiding edges perpendicular to the line-of-sight and abrupt changes in surface conductivity) and turning them on their head to make the boat as un-stealthy as possible.

One way to do this might be to print appropriate aluminized strips on a piece of mylar. The mylar would then be glued to the mast. The length and spacing of the aluminized portions, plus the interference with the aluminum or carbon mast used as a backing, would determine the radar cross section. The weight and windage would be negilible and the cost very low, so you could use this technique as an adjunct to other radar reflectors.

 

Hi Tom Speer

That sounds an excellent idea. What sort of dimensions would you need for each aluminium strip?

Rob

 

I dont know that recieve / transmit antennae principle apply here, you are trying to do neither, instead you are trying to reflect a transmitted signal back towards the reciever. I think that for this purpose maximizing the surface area that is opaque and/or reflective to the given frequency is your only option so you would tune your reflector to the highest freq encountered,this being neccasarily opaque to the freqs below A small grid,relevant to the highest anticipated freq, woven into the sails would be very effective, at the right angle. But for the purpose of omnidirectional reflection it would be hard to improve upon the ever shifting return of a star type reflector that continuously varies it's optimal return presentation in reference to an unknown transmitter vector. An active return, in response to a recieved transmission, would, I think , be much more effective than any passive reflector we could devise.

 

S-band radars have a wavelength of 8 - 15 cm and X-band radars have a wavelength of 2.5 - 4 cm. So the dimensions of the strips will only be a few cm in length and maybe only a few mm in width. Obviously you can pack a lot of them on something the size of a mast, so we're really talking about an antenna array rather than individual dipoles.

I'll repeat again - I'm not by any stretch of the imagination a radar guy or even someone who knows anything about electromagnetics. But speculating about designs outside one's competence doesn't seem to be an obstacle on this board!

BTW, a slot in a conducting surface acts like a dipole, too, so you can make the array by taking a sheet of aluminized mylar and etching away just what you want, or by etching away what you don't want! It might even be possible to have the length of the strips in the vertical direction be tuned to S-band and the circumfrential spacing of the strips be tuned to X-band. The circumfrential spacing might act as an end-fire array.

There are lots of possibilities for printed array patterns. For example, you might want to make a "T" with each arm of the T being half a wavelength, the height of the T 1/8 of a wavelength, and cut the T down the middle so it forms two half-wave dipoles connected by a quarter-wave "U". This will result in the radiation from each arm of the T having the same phase, so they reinforce each other. You could connect adjacent T's with quarter-wave stubs to form a whole line of half-wave emitters the length of the mast, all in phase with each other, known as a colinear array. If these colinear arrays were spaced half a wavelength apart, adjacent arrays would have opposite phase, but when the reflection from one array reached the other array it would have the same phase and the arrays would reinforce each other. This is known as a broadside array, and it has a very strong signal at right angles to the plane of the array elements.

As you add more and more elements to a colinear array the antenna pattern becomes more and more directional. So you might not want to have a continuous line of elements along the whole length of the mast - it might only reflect back at the source when the source was in a narrow range of angles near the perpendicular to the mast. There are several ways one might get around this. One is to use shorter arrays, maybe only 2 - 4 elements each, but lots of them. Another way might be to spiral the array around the mast so there's a large part of it that will be near perpendicular to the line of sight to the source regardless of heel angle.

Using lots of arrays means they can be made in a variety of sizes, covering the range of frequencies in the desired band. Kind of like diversifying a stock portfolio - the gain will be less when you get it right, but the chances of getting it wrong are diminished. Since S-band is what ships at sea use most and it's the hardest to reflect because the longer wavelengths need a bigger reflector, I think the array should be targeted at S-band.

So maybe the most useful form would be a self-adhesive plastic patch roughly 25 cm (10") square. It would have 4 strips of 4 elements each, forming a broadside array of 4 colinear arrays. Each element would be 5.75 cm long, spaced 5.75 cm apart and connected so as to have all the elements in phase and center the array in the S-band. Something like this:

(from http://www.tpub.com/neets/book10/42j.htm)
You could slap the patches on the mast, sails, etc.

This Java applet shows how different spacings and frequencies affect the array. Hopefully the effect of a metal mast is to simply reflect the pattern, doubling the intensity in the direction away from the mast.

Would the array be more reflective than the mast itself? I don't know. There's also the issue of polarization and the effect of wrapping the array around the curved mast instead of being in plane. But the latter might be handled by clever adjustment of the array spacing.

 

Plumbtex
There are reactive transmission devices out there. I have installed them. The last one was four or five years ago so I can't remember the brand name.
The one issue I had with them (at least the ones I installed) is that they were inneffective after the scanning boat got within 1/2 mile. Hopefully other reflective methods would have that covered. The cool thing about them is the target that they paint was an attention getter...i.e. you look big to the other guy. I don't know how well they stood up in use.
I'll see if I can find the info on them.

 

Here's one:
http://www.sailgb.com/p/seame_active_radar_reflector/