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Oz Jon

A DIY Radar Reflector for Small Boats

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Oz Jon

In response to encouragement from a few LinC members, I've been persuaded to post on my experience with this topic.

 

One of my (endearing?) characteristics is that I talk a lot! - when I start it's hard to stop!

 

So I will input several initial pages to get the thread rolling.

 

To keep it compact and easier to read, please hold off on comments until at least, after the 3rd page.

 

It will take me some time to get the 3rd page up, so please restrain your enthusiasm! - Lol!

 

So here we go!

Edited by Oz Jon
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Oz Jon

Page 1 - intro

 

It seems that a few people are be interested in a high performance, cheap DIY radar reflector for their boat, so I'll provide information on how I made mine, and why I chose the design that I used.

 

Among the serious hazards of the sea, are ships! - big heavy steel things!

 

One ship collision can ruin your whole day! - Lol!

 

Ships are not rapidly maneuverable and need to know of your presence well in advance to give them time to alter course enough to avoid a collision.

And there is the problem! – they probably are not aware of your presence soon enough (or at all) because small boats don't show up well on their radars (particularly wooden or fibreglass boats).

 

A good radar reflector on your boat will produce a strong echo on the ships radar when you are still 10 or more miles away. If it's big enough, it will show up on both the ship's short-range X-band radar and its long range S-band radar.

 

You can buy radar reflectors from marine chandlers starting at about  US$30 each up to several hundred dollars each – some of them work reasonably well and some don't. You will need 2 of those cheaper ones to get  full 360 degrees azimuth around your boat. Single devices to give that  full azimuth cover will set you back over $200. My design will outperform it with change from $30 if labour is free.

 

The best commercial value for money I have seen recently is the Plastimo 16” - you would need 2 of them for full cover . Reasonably good performance, but they don't quite meet the recommended minimum effective performance over the full azimuth range. Lesser performance if your boat is heeling (as yachts do)

 

A very good source of information on commercial radar reflectors can be found in a 2007 test report at  <www.ussailing.org>

 

The most practical form of DIY reflector is an (octahedral) corner reflector array. It has the very desirable characteristic that it reflects the ships radar signal straight back in the direction it came
from – ie. the ship. 

 

Attached  is a photo of an aluminium Plastimo 16” reflector  mounted in “edge-on” configuration.
My design is similar shape, but bigger and  made from different material.

 

 

next  page 2 – how it works and construction details

post-15613-0-82907500-1448082927_thumb.jpg

Edited by Oz Jon

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Oz Jon

page 2 – how it works and construction details

 

Attached is a figure showing how radar signals striking this type of reflector reflect back to where they came from.
(1/8th part of an octahedral reflector shown)

post-15613-0-04168000-1448083514_thumb.jpg

 

You can visualise it like 3 mirrors all fixed at right angles to each other.
If you shine a laser pointer at it, the light reflects off  2 (or 3) surfaces and goes back in the direction it came from.
There is a limit to useful angles though. If you shine the light at too big an angle, it doesn't work at all. You will see this effect in the patterns in a later page.

 

Radar signals aren't thin pencil beams like a laser pointer makes.
The radar signal striking a radar reflector fills the whole thing with radiation.

 

The amount of signal reflected back to it's source will be bigger if the reflector is bigger.
with radar reflectors, size really counts !

Signal is proportional to area squared, so proportional to the fourth power of the dimensions .
Double the dimensions and you get 2x2x2x2=16 times the effectiveness!
The return signal is strongest “on-boresight” and tapers off as the incident angle gets bigger.

 

A key characteristic of a radar reflector is it's “Radar Cross Section” (RCS), usually stated in square metres (M^2).
2.5 square metres is regarded as the minimum requirement for good performance for a small boat reflector.
[Don't worry about the RCS values. For reasons I won't go into, a reflector's RCS is always far bigger that what you would expect by just looking at it]

 

I have two 600mm (24”) reflectors in my catamaran – they work very well!

 

They are made from 1/8” ply, epoxy soaked to preserve the ply and also used as glue to stick ordinary, aluminium kitchen foil on one (or both) sides of the ply.

You could make them with sheet steel or aluminium sheet (more difficult and expensive, but rugged) or you can use that corrugated plastic sheeting that's used for advertising signs instead of ply.

[even corrugated box cardboard for a very cheap experimental version]

 

The attached figure shows the construction.

post-15613-0-74643500-1448083553_thumb.jpg

 

Make 2 in the diamond configuration.

Dimensions are not critical. You can make smaller ones if you are short of space. 18” is about as small as you want to go for really good performance. Even 12'' is reasonable, but  you will have a few coverage gaps (as you will see from page 3).

 

Small crinkles in the stuck-on Al foil don't matter.
Keeping the assembled pieces close to right-angles to each other is important for best results.

You need some way to keep the assembly together – ordinary spring clothes pegs will do for a start, but you need to invent something bit better! - Heh! -  exercise your imagination! - Lol!

 

next Page 3 – patterns and a bit of tech-stuff


A bit of a delay while I prepare the text for page 3

Hang-on, it gets exciting! - lol!


A bit of a delay while I compose the text for page 3

 

Hang-on guys, it gets exciting on page 3 - lol!

 

Cheers

Edited by Paul
attached images where I believe they go?

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Oz Jon

page 3 – patterns and a bit of tech-stuff

 

Here is the pattern drawing for page 3

 

A thing of beauty! - the descriptive text and some discussion will follow shortly!

 

post-15613-0-57411300-1448084872_thumb.jpg

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Oz Jon

page 3 – patterns and a bit of tech-stuff

 

Ignoring for a moment the dotted lines and the big cyan coloured curve

 

The above drawing shows the radar cross section (RCS) of a single commercial Plastimo 16” diamond shaped reflector, mounted in “edge-on” configuration, versus horizontal angle. [plus other stuff]

 

Only half (180 degrees) of the coverage is drawn from port- ahead – starboard directions – the other half via the stern is identical

 

They are the pattern of a single Plastimo 16. Quite a good value-for-money product, but you can see that it has gaps in it's coverage and it doesn't meet the 2.5M^2 RCS specification at any elevation angle.

 

The bits of immediate interest are the  green, orange and red solid lines, marked  0, 5 and 10-12.
0 means horizontal, 5 is 5 degrees elevation (or depression) and 10-12 is the range of 10-12 degrees.

 

I've also drawn another copy of the single pattern (the dotted curves) showing the performance of a 2nd Plastimo 16 mounted rotated horizontally by 45 degrees to the first one.

 

That combination fills the missing coverage gaps quite well (the horizontal green line) with an RCS of 9 at 0 degrees elevation (ie. no boat heeling) and just a bit short of the spec (the horizontal orange line) with an RCS of 2 at 5 degrees of boat heel. Only an RCS of 1.3 at 10-12 degrees of heel though and worse for even more boat heel.

 

What's the simplest, cheapest solution?  - 2 bigger reflectors – performance increases with the dimensions cubed.

 

So as not to clutter the drawing any further, I've drawn (cyan coloured) just one lobe of the 0 degree elevation pattern for my 600mm (24”) model. It is far bigger than the corresponding Plastimo16 lobes!

You can extrapolate that to see how much better it will do the full coverage job, even at higher elevations.

 

You probably don't need to build a reflector as big as 24” if you are limited for space.

An 18” one would be a pretty good compromise size.

You don't need to mount 2 reflectors close to each other either. Anywhere on the boat will do.

 

And if you have a non-metallic hull (ie. Not steel, aluminium or concrete) then you can put them out of the way and out of the weather somewhere convenient in your hull.

 

When not in use, (or if you want be stealthy?) you can dismantle them and store them flat.

 

Enjoy!

                                                So! - any comments or question?

Edited by Oz Jon
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Oz Jon

In the article I published several years ago, I recommended mounting reflectors in the "catch-rain" orientation.

 

When I recently re-drew patterns in the above format, I copied them from a reliable test article which labled them as "edge-on" orientation.

 

That article also had labled plots for other orientations.

 

I'm now convinced that they got the captions mixed in the publication - and I fell into their trap!

 

"catch-rain" orientation, does, in fact, gives the best performance!  and that's what the above pattern captions should say.

 

Sorry about that! - I blame the impetuosity of youth for not recognising the error earlier - Lol!

Edited by Oz Jon
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thebob

So if I were to use a pair in both the catch rain orientation and a pair in the edge on orientation would this improve performance when my yacht is heeled over?

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Ricky

Good post. If you really want to be seen though, AIS Class B Transponder for <$600 is probably the best investment ever on a boat, particularly if you are sailing at night. Ships will almost certainly spot you. 

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Dragonladies.org

What happened with loading up the inside of aluminum masts on sailboats with rolls of tin foil?     Used to be popular cheap solution?

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Oz Jon

So if I were to use a pair in both the catch rain orientation and a pair in the edge on orientation would this improve performance when my yacht is heeled over?

Just 2 in the catch-rain orientation are fine (if you make them 24"). They are OK up to nearly 45 degrees heel - more than that, you probably have other problems to worry about - Lol!

 

I haven't run the patterns, but I suspect that 18" ones would be OK too.

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Oz Jon

What happened with loading up the inside of aluminum masts on sailboats with rolls of tin foil?     Used to be popular cheap solution?

I've never heard of that!

 

It doesn't make sense technically

 

Radar signals would bounce (skattered in all directions) off the outside surface of a (basically a fully enclosed metal cylindrical) Al mast - no signal would get inside the mast (look up "Faraday cage" on Google), so Al foil would do nothing useful.

 

You need a reflector as I've described (preferarably 2, or a much more expensive active device) to be effective  - technical black-magic - Lol!

Edited by Oz Jon

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Oz Jon

Good post. If you really want to be seen though, AIS Class B Transponder for <$600 is probably the best investment ever on a boat, particularly if you are sailing at night. Ships will almost certainly spot you. 

That's true in many places, but I must say that I have my doubts whether very many ferries, etc in the Phils carry appropriate receivers ( or they are working).

 

They seem to have problems with even the most basic safety measures.

 

I recall that it was only last year? that they were mandated to carry a life jacket for every passenger on board!

 

If I ever travel on a Phil ferry, I'll carry my own (inflatable version) and travel on-deck, or sit near the door! - Lol

Edited by Oz Jon

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miles-high

I have my doubts whether very many ferries, etc in the Phils carry appropriate receivers (or they are working)

We were once invited to the deck (or whatever you call the room with a steering wheel and the engine control thing... bridge? is it?) of one of the ferries before… It was one of the largest ferries carrying trucks, busses, cars and cargo as well as passengers… All they had were 1 handheld Garmin GPS and a VHF transceiver… wouldn’t make any difference if your boat had a reflector, transponder or AIS unless you got a radar with proximity warning set… I don’t know much about animals and don’t know if those birds are for cockfighting or for dinner…

 

CIMG0059.JPG

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Oz Jon

We were once invited to the deck (or whatever you call the room with a steering wheel and the engine control thing... bridge? is it?) of one of the ferries before… It was one of the largest ferries carrying trucks, busses, cars and cargo as well as passengers… All they had were 1 handheld Garmin GPS and a VHF transceiver… wouldn’t make any difference if your boat had a reflector, transponder or AIS unless you got a radar with proximity warning set…

Absolutely! ... and I bet they've got spare batteries for them too!

 

Why am I not surprised?

 

No doubt the ship has a clearance certificate from Marina? or the CG? or whoever is supposed to inspect them.

 

As I said in the boat building thread message #87 :-

 

"

I've found from sea-going experience, that the biggest problem you face, is that some ships are not keeping an adequate watch (either visually or by radar).

 

In a yacht, at sea, you need to be very vigilant and get out of the way of ships (notwithstanding that a yacht has "right-of-way").

"

Big, heavy, steel, inattention/incompetence .... always trumps a right-of-way yacht

Edited by Oz Jon

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towboat72

A yacht does not always have the right of way,the most encumbered has the right of way

Because he has limited mobility

I enjoyed the article by the way

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