Radar Reflectors

[zeehag]

phil--awesome--same with tubes --pvc or even cardboard, from paper towels, and stick aluminum foil scrunched up some, and make one for each spreader, and one for backstay or shrouds, and one wherever you wish it to be...it will be seen.....a couple together on the mast--easily taped on and remove-able with reasonable ease

[jeremiason]

Radar reflectors have a purpose... As you are aware radar can only see to the horizon. Depending on its height and the height of the target this could be many miles.

The problem with most sailboats is they present a low profile target and a very tail, and for lack of a better word, skinny mast. This is magnified when you get into rough seas and the mast begins to sway, causing it to be even a harder target for radar.

Although there will be arguements from the group, placing a radar reflector or two high in the rigging will make you vessel a better target for radar at far greater distances than the mast will.

We use two 2" Tubular Radar Reflectors on our boat. They are wired tied above the second spreader on the shroads (Port and Starboard), which places then about 50 feet from the water surface. Permenently mounting the reflectors on the mast is easy and saves were and tear on the flag halyards.

Our boat shows up well on radar and has been spotted well over twenty miles away in heavy seas.

Also someone meantioned AIS... You should know that in heavy seas, the roll of the boat will effect both your reception and transmission of the AIS. The techies can explain this concept better than I can, but it has something to due with the VHF signal as the antenna swings back and forth.

[estarzinger]

Originally Posted by jeremiason

We use two 2" Tubular Radar Reflectors on our boat.

Unfortunately, factually, those 2" tubes do essentially nothing for you. They do not meet either the ISAF or the SOLAS standards for minimum cross section. They score the lowest on all tests, and that's among a group that on the whole already score very low. They only exist because they used to meet the racing requirement, and the racers liked them because they were low weight and low drag. But after seeing the test results they were disallowed (by all major authorities) from the racing requirements (by the minimum cross section standard).

Here is what one excellent test said about those specific reflectors: "Mobri reflectors are available in two diameters, neither of which performed well.. With the radar beam exactly at right angles, they act as a series of dihedral reflectors, but even small heel angles cause it to operate in a deep null with little reflection. The series of end plates that would form the third side of each trihedral are too small to be effective, even in X-band, and are operating too close to edge-on at small heel angles. The smaller 2" diameter unit suffers an additional problem in that the 1" radius of each dihedral reflector is less than a
wavelength even at X-band."


You should know that in heavy seas, the roll of the boat will effect both your reception and transmission of the AIS.

This is true in theory, but not important in practice. A VHF antenna is directional and if heeled will in theory radiate better (to a surface ship) in one direction than another. HOWEVER, in practice this is not at all important and AIS is still be far the best 'increase visibility' device and much better than a passive radar reflector in all conditions

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[panthablue]

Well here's our radar reflector. A traditional Octahedral type. You can buy the backstay fitting from this website

Backstay Mount for Octahedral Radar reflector

[Paul Elliott]

Originally Posted by estarzinger

This is true in theory, but not important in practice. A VHF antenna is directional and if heeled will in theory radiate better (to a surface ship) in one direction than another. HOWEVER, in practice this is not at all important and AIS is still be far the best 'increase visibility' device and much better than a passive radar reflector in all conditions

There will be some loss in antenna gain when the boat is heeled, but as Evans says it's not generally an issue. This is one reason why we use shorter, lower gain, VHF antennas on sailboats, and the power boats often have a longer, higher gain antenna. The shorter antenna has a "less focused" pattern and so can perform reasonably well at extreme angles.

[David M]

As the antenna gain increases, the lobe gets wider and flatter.



Radiation pattern - Wikipedia, the free encyclopedia

Boat-Project.Com - VHF Marine Antenna Fundamentals

[panthablue]

Originally Posted by Paul Elliott

There will be some loss in antenna gain when the boat is heeled, but as Evans says it's not generally an issue. This is one reason why we use shorter, lower gain, VHF antennas on sailboats, and the power boats often have a longer, higher gain antenna. The shorter antenna has a "less focused" pattern and so can perform reasonably well at extreme angles.

Hi Paul. Excuse my ignorance on this subject, but I'm interested to know: on a sailboat is the loss in gain due the angle of the aerial when the boat is heeled, or the effective shortening of its height?

[Jim Cate]

Originally Posted by Randyonr3

might be weary of it on our boat as the backstay is also our SSB antenna. at present, our backstay is a prescribed length for the tuner to work with, adding something that would be a varable to the resistance might cause issues........

G'Day RAndy,

Unless you used wire for all the up and downhauls for that overcomplicated device it would have no measureable effect on the tuning of your backstay antenna. Even with the use of wire your tuner would easily be able to match the resultant mess.

But I think that that mounting system is pretty silly!

Cheers,

Jim

[Paul Elliott]

Originally Posted by panthablue

Hi Paul. Excuse my ignorance on this subject, but I'm interested to know: on a sailboat is the loss in gain due the angle of the aerial when the boat is heeled, or the effective shortening of its height?

The loss is strictly due to the antenna pattern. Look at the diagram David M. just posted -- more of the transmit power, or receive gain, is aimed in the wrong direction. Most marine antennas don't have as tight a pattern as the diagram shows, but the concept is valid.

Now that I think of it, some of that antenna pattern is no doubt due to effective height, but it is at least as much due to the phase shift caused by antenna not being perpendicular to the direction of the signal.

[panthablue]

Hi Paul
I've had a look at the links. I found the Wikipedia entry somewhat baffling, but the boat project site made some sense.
I guess then for a heeling boat, the signal is stronger in the fore and aft direction, but slightly less across the boat.

[casual]

While waiting out an appointment in a DR. office I,struck up a conversation with an older Gent.Turn's out he was a retired Sea Going Commerical Captian.I,questioned him about being seen at sea.He told me in detail that one radar reflector barelly presented a target,however when two were on a single Vessell the target became present 8 to 10 miles away.He spent 50 plus years at sea.A graduate from the Merchant Marine Accademy.FWIW

[jeremiason]

Originally Posted by estarzinger

This is true in theory, but not important in practice. A VHFantenna is directional and if heeled will in theory radiate better (to a surface ship) in one direction than another. HOWEVER, in practice this is not at all important and AIS is still be far the best 'increase visibility' device and much better than a passive radar reflector in all conditions

You are wrong about this not being important in practice and misread my my post....

First, in my orginal post, I was not speaking to a normal heel of a sailboat effecting the AIS transmission.

If the boat is in heavy seas and is rolling side to side, both reception and transmission of the AIS will be degraded badly.

I have lost targets as little as 4 miles away in large quartering or beam seas due to the effect on the VHF lobe.

This is common phenomenon, when VHF radio antennas swing back and forth in rough seas. This is further maginfied when the antenna is on top of the mast.
.

Second, I assuming you are taking about a transmiting AIS being better than a passive radar reflector in all conditions, which isn't true.


Yes, I would agree that AIS is good, but NOT ALL BOATS have AIS capabilities and currently more boats have radar than AIS.

Either way, in all conditions AIS might not being seen, either because of heavy weather or becasue the boat trying to see you doesn't have the equipment.

The same could be said about radar, but with more radars on the water, you have a higher chance at being seen by RADAR than AIS.

I am not knocking AIS... I have a CLass B on my boat and use it everytime I am underway. Normally, I am seen at 8-10 miles and receive targets well over 40 miles.

My point to the orginal post was, AIS is one tool in the box and like everything else has its limitations, just like all other marine safety equipment.

As far as the tubular radar reflectors, I would love to see the Report you reference for my own information.

Yes, the tubular reflectors are not as effective as other larger ones, but for me, in practice, they work just fine. On my boat, they are permenently mounted and require no maintenance, which in my world suits me well.

I have had reports of good radar returns in flat seas out to 30 miles and 10 miles in heavy seas. Is it the reflectors or the boat making the returns? Who knows... but I am seen on radar!

Also, there is a huge difference between the Plastimo and Mobri tubular radar reflectors, besides the price!

We use the Mobri and are happy with them

[chrisjs]

There are a lot of half baked (bacofoil) ideas on this website!!
It is a couple of years since I researched reflectors, shortly after nearly being run down by a steel fishing boat in thick fog. We settled on a Trilens (right price, easier to install) though it was clear from several studies (check out MAIB UK reports) that active reflectors were the best bet. Other devices were next to useless - including the ones made of various foils. As regards radar signatures, I am often amazed at how poor to non-existent the signature can be even from larger vessels, so you should definitely check out yours before relying on it to protect you at night or in the fog. We also added AIS and the loudest airhorn foghorn we could find.

[dan360]

An MAIB report on radar reflectors - active sea-me recommended as the best and Plastimo not considered suitable.

http://www.maib.gov.uk/cms_resources/Radar reflectors report.pdf

[estarzinger]

Originally Posted by jeremiason

As far as the tubular radar reflectors, I would love to see the Report you reference for my own information.

Here:

http://www.bethandevans.com/pdf/Rada...tor%20test.pdf

Here is another report that tested the 4" Plastimo reflector (Which would perform much better than your 2" tube). It was the lowest performing reflector tested and the summery was simply: "The 4” tube reflector performed very poorly."

All the various safety bodies (ISAF, US Sailing, SOLAS, etc) have written rules which do not allow these (small diameter) tube reflectors because of their very minimal radar cross section in the normal ship radar bands.I happen to sit on the US Sailing Safety at Sea committee and happen to be the committee subject matter expert on this. I was the safety equipment expert on the two USCG/US Sailing sailing accident investigations this summer.


If the boat is in heavy seas and is rolling side to side, both reception and transmission of the AIS will be degraded badly.

While this is true for a high gain antennae (say 9db), it is simply technically not true for a 3db antennae, which is what is universally recommended for sailboat use.

A 9db antennae has an (approximately) 20 degree beam angle. So heel beyond 10 degrees starts to degrade performance. However a 3db antennae has an 80 degree beam angle. Thus is not degraded until past 40 degrees of heel. (edit: note - these are the essentially no loss angles. The antennae do transmit some signal wider than these angles and a 3db antenna transmits a significant fraction of its signal 360 degrees)

Perhaps you have a high gain antenna - but most sail boats do not and they are not recommended for exactly this reason, or perhaps you often roll or heel more than 40 degrees in heavy seas - but most sail boats do not.

I have some modest amount of heavy weather experience and I have never noticed any significant AIS range degradation.

Second, I assuming you are taking about a transmiting AIS being better than a passive radar reflector in all conditions, which isn't true.

Well . . . First, ALL ships today have AIS. Second, AIS works extremely well in almost all conditions, while passive reflectors work extremely poorly and even worse in poor conditions (sea clutter and heavy moisture). Third, fishing boats which may not have AIS are also often busy fishing and NOT looking at their radar and not keeping any sort of proper watch at all in fact. You simply have to stay out of their way in that case no matter what equipment you have.

I will completely agree that my statement "in all conditions" was overly strongly worded, and I should have said something more like 'in the vast majority of conditions'.

The two points above, about the radar cross section of the tube reflectors and the performance of heeled antenna, are simply facts. This point about AIS vs passive reflectors is a judgement based on all the facts. I have one opinion based on all those facts, which is, stated baldly, that AIS works well while passive radar reflectors are little more than good luck charms. But that's my judgement and I respect that you have come to a different conclusion.

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