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Old 28-04-2020, 12:55   #46
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Re: Do I need a radar reflector?

Only real way to know is to go on another boat with radar and see what your boat looks like at close range and about 3 miles. If you cant see it then you are radar invisible. If possible check it in fog or light rain as well.
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Old 28-04-2020, 13:02   #47
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Re: Do I need a radar reflector?

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Originally Posted by Chuck Hawley View Post
I think the biggest issue with both the 2" and 4" Mobri reflectors is their size (radius) is near the wavelength of X-band radar and far smaller than S-band radar, making them close to invisible.
S-band wavelength is 10cm, isn't it? So a 2" reflector is too small to reflect it; it should be able to reflect X-band, which I believe is about 3cm. The 4" reflector should reflect both - assuming the proper orientation.
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Old 28-04-2020, 13:46   #48
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Re: Do I need a radar reflector?

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On airplanes we call that a transponder. And I think the same for marine. It detects a radar signal and sends back an amplified signal.
If you are concerned about not being seen, a transponder could put your mind at ease.
Good seamanship and diligently keeping watch has always seemed better to me than depending upon someone else to see me and watch out for me.
I was corrected on that earlier, a transponder encodes additional data, and replies I believe on a different frequency so a special transponder receiver is required to go along with the Radar transceiver.
You do know where the term squak came from?
AIS is of course nearly identical to ADSB. Just a whole lot cheaper.
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Old 28-04-2020, 13:49   #49
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Re: Do I need a radar reflector?

I’ve heard about seeing birds in a Radar for decades, it’s the holy grail for a Fishermen cause of course the birds feed on bait balls, and that’s where you will find the Sportfish too of course, so if you can find birds, you can find fish.
I have never seen a Consumer Radar that would reliably paint birds, even the 6’ open array ones
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Old 28-04-2020, 14:08   #50
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Re: Do I need a radar reflector?

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Originally Posted by Breaking Waves View Post
you need to ask? lol

I could be wrong here because I have not looked at this in years, but my understanding was that us flagged vessels on international voyages were required to meet parts (but not all of) of solas. Vessels only in domestic waters do not need to. And very clearly reflectors are NOT on the USCG required equipment list for small pleasure vessels.
Domestic waters isn't a term I've heard before, did you mean terratorial waters? Quick Google suggests in the US you need to comply apart from great lakes and some rivers. Though could be wrong. Big cultural differences between UK and US as very few on that side of the pond have even heard of solas let alone known that there are a few parts they should comply with, on these forums anyway. Maybe the RYA helps in that respect. Without getting into how good or bad some of the reflectors in the shops actually are...

https://www.rya.org.uk/knowledge-adv...gulations.aspx
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Old 28-04-2020, 14:09   #51
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Re: Do I need a radar reflector?

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I have never seen a Consumer Radar that would reliably paint birds, even the 6’ open array ones
Nothing "reliably" paints birds - but birds will paint in the right conditions - floating on flat calm water or even in flight at the right range. Seen it on little Furunos and Kodens as well as Pathfinder, Bridgemaster and a couple different Kelvin-Hughes models.
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Old 28-04-2020, 14:25   #52
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Re: Do I need a radar reflector?

Some good info here, and some simplistic stuff too. A prudent mariner does not rely on a single system, so optimizing radar and having AIS (not just receiver) are both desirable.

I love it when sailors report that they can pick up birds. I have done that too, as well as outboards on ribs zipping by. In calm water. Unfortunately the ocean is usually not calm. And merchant ships don't usually keep tweaking the radar for ultimate sensitivity; in fact, they tend to de-sensitize the radar so that the collision alarm doesn't alert on those birds... When I first went cruising, heading south out of the PNW, I would hail on VHF the passing merchantmen - what a lesson. Maybe a third wouldn't respond, and another third spoke English so badly that communication was not really possible, but the third that I could talk to were a real wake-up call: in daylight at 3-5 miles they invariably never saw me on radar, even while tweaking the set knowing that I was there. One suggested that I should buy a radar reflector (I had a nice big Firdell Blipper in the rigging). The problem is that in 6-8 ft seas, typical around here, the reflection from the waves (sea clutter) is as large or larger than the reflection from a proper radar reflector. Modern digital radars with DSPs do a good job of separating consistent but intermittent signals from the random sea clutter, but they are not nearly perfect. So when the seas are up do not count on even a radar reflector to make you visible, but do get one for the other times. Fiberglass, as well as wood, boats are invisible to radar without reflectors.

Radar reflectors, aside from the rare and expensive luneburg lens designs (e.g. Lensref), are corner reflectors and are very good if constructed with precision - the angles must be exactly 90° or the reflected signal doesn't return straight back to source. They are equivalent to a flat plate of steel with the same cross section, facing directly at the radar. On the water a flat sheet almost never is facing exactly correctly, and of course doesn't maintain that alignment for but an instant, so is worthless. An aluminum mast generally reflects in a narrow line top to bottom - not a very large area but helpful if motoring in calm water (otherwise the reflection is going all over the place). A corner reflector consistently returns a strong signal except when one plane is parallel to the signal (the plate will mostly not reflect back in the right direction and the parallel plates won't reflect the parallel signal). This is solved by mounting them in the "catch rain" position where the corners effectively overlap. This is also the case in the excellent Firdell Blippers.

I can remember two times when I was very unhappy about the fact that large vessels didn't have radar reflectors. In the first case a large powerboat coming out of San Diego on a mirror-calm morning approached and passed me, and the whole time I was tweaking my radar and never caught him on radar even though it was a close approach. WTF? It wasn't a danger, but a real surprise. The other one was a party boat in a very crowded Southampton Water after dark; he was travelling up and down the channel repeatedly, with lots of lights and loud music, and for the life of me I couldn't get a reflection. And that was after years of experience with radar. Don't be like these guys: please buy a radar reflector.

Greg
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Old 28-04-2020, 14:30   #53
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Re: Do I need a radar reflector?

The best you can afford
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Old 28-04-2020, 15:21   #54
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Re: Do I need a radar reflector?

I want to put my two cents worth in favour of carrying as large a reflector as you can, even if on a temporary basis as suggested earlier.

On GAIA we carry and use constantly, an AIS transponder and a broadband radar. The latter because of its low power draw is on all night. The AIS of course draws practically nothing and is always on. We consider its advent a godsend.

GAIA has a junk schooner rig which means 7 large diameter 16ft long hollow pultruded FG 'battens' for each sail as well as two Aluminum masts and two Aluminum yards. We filled these battens with crumpled up Aluminum foil. Apparently perhaps also in part due to our steel hull we reflect radar very well judging by the comments we have received when communicating with traffic.

Although we consider AIS transceivers the most important we think all three should be carried if possible. AIS has the added advantage of yielding the name of a vessel of interest and using the name vastly increases the likelihood of a response.

My comments are coloured or influenced by the type of sailing we do. If we were mainly coastal sailing or sailing in areas with a lot of recreational traffic we might think differently but I would still carry all three.

Jimsv GAIA
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Old 28-04-2020, 15:52   #55
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Re: Do I need a radar reflector?

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Originally Posted by Lodesman View Post
S-band wavelength is 10cm, isn't it? So a 2" reflector is too small to reflect it; it should be able to reflect X-band, which I believe is about 3cm. The 4" reflector should reflect both - assuming the proper orientation.
I’ve just been reading up on radar corner reflectors since I know little about this subject. I have two radar sets on my boat - an old analog Furuno that came with my boat, antenna mounted on the mast above the spreaders, and a second new Raymarine Quantum (cool stepped-wave technology) on a stern pulpit pole, which took the place of a non-functional radar that was on my boat when I bought it.
I had a reflector in a fibreglas enclosure mounted on my mast below the spreaders, but took it off as I kept fouling spinnaker halyards and such on it.

According to what I read, a corner reflector dimensions have to be several multiples of the wavelength to work. X- band would need a dimension of 9 cm or more, S-band 30 cm (one foot) or more.

I have an AIS receiver only.

Reading through the commentary, it sounds like my idea of buying one of those small diameter tube reflectors is throwing away money for little or no effect.

Now I’m not sure if re-mounting the old fixed reflector makes sense (with the line fouling issues it adds) or buy a ball type reflector to raise on my flag halyard. The downside to this is that if it’s removable, I’ll probably forget to raise it when needed.

How is it that a nearly 60 ft high aluminum mast, probably 12 inches across wouldn’t present enough of a RCS to be easily seen?

Fiberglass hulls are apparently transparent to microwave radiation, but diesel engines, cook stoves, bbq’s on the railings, winches, anchor chain and anchors, etc have to be good reflectors, at least when you are at the crest of a swell, rather than a trough.

Thoughts?
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Old 28-04-2020, 16:38   #56
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Re: Do I need a radar reflector?

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How is it that a nearly 60 ft high aluminum mast, probably 12 inches across wouldn’t present enough of a RCS to be easily seen?
Because the mast is round, or nearly so, so scatters its reflection in all directions. So very little of the energy that hits it goes back to the radar receiver.

A single flat surface would be even worse, it would reflect all the energy away from the radar receiver ... this is how stealth technology works to make sure that no reflection goes back to the receiver.

But a radar reflector has a second and third flat reflector at right angles which then re-reflect all the energy straight back to the radar receiver, giving a much stronger signal.

The same reason that roadside reflectors shine back at you in your headlights so much more than just white paint.
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Old 28-04-2020, 16:42   #57
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Re: Do I need a radar reflector?

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Domestic waters isn't a term I've heard before, did you mean terratorial waters?
No, I do not know the technical term, but what I meant was NOT an international voyage. There is a significant distinction in various US maritime laws and regulations between ones which include an international port of call and ones which have all purely US ports of call.

In any case, it is quite clear that the USCG does not see any 'requirement' for radar reflectors for 'small' (eg our size) domestic pleasure vessels.

I am aware of the UK position - are there other countries which take the same position as the UK?
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Old 28-04-2020, 16:45   #58
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Re: Do I need a radar reflector?

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According to what I read, a corner reflector dimensions have to be several multiples of the wavelength to work. X- band would need a dimension of 9 cm or more, S-band 30 cm (one foot) or more.
yea - all the edges have to be 3 or more wavelength and 'ideally' greater than 10 wavelengths.

there is some reflection possible in smaller dimensions but it is vanishingly small and exceedingly sensitive to angle - as the test results show.
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Old 28-04-2020, 16:49   #59
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Re: Do I need a radar reflector?

Maybe the Air Force could have saved billions by having Catalina build the B1 instead of Boeing. If they can make a 45' yacht with 50' mast, diesel engine, and a ton of lead, radar invisible why not an airplane? Just a random thought I had on a late night watch.
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Old 28-04-2020, 17:07   #60
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Re: Do I need a radar reflector?

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Originally Posted by Kelkara View Post
Because the mast is round, or nearly so, so scatters its reflection in all directions. So very little of the energy that hits it goes back to the radar receiver.

A single flat surface would be even worse, it would reflect all the energy away from the radar receiver ... this is how stealth technology works to make sure that no reflection goes back to the receiver.

But a radar reflector has a second and third flat reflector at right angles which then re-reflect all the energy straight back to the radar receiver, giving a much stronger signal.

The same reason that roadside reflectors shine back at you in your headlights so much more than just white paint.
Excellent graphic Kelkara, thanks.

I get that a corner reflector is far more efficient than just some rather random pieces of metal. But a reflector practical to raise up a sailboat mast can’t be too large because of windage, durability, weight aloft, and because of the propensity to foul lines and tear sails.
A reflector has to have a Radar Cross Section (RCS) far greater than its nominal dimensions. My question is whether this RCS is actually greater than the reflective surface of a mast 60 ft long and 12 inches wide. Most masts are also elliptical rather than round, so they should reflect better on their major axis. Sail track, fittings, blocks, sail headboards, masthead lights, all should add to the rigging RCS, especially for the shorter wavelength X-band.
Say the corner reflector has a (totally made up) RCS of 5 sq ft. One might think a mast 60 ft long by 12 inches on its long axis might have an RCS approaching 15 to 60 sq ft (the curved surface won’t reflect back very well). That doesn’t include any of the larger rigging components attached to the mast.

I guess what has me a bit perturbed is that radar manufacturers boast how their radars can pick up kayakers - which have next to no metal reflectors at all - albeit at pretty short range - and yet an overall length 50 ft sailboat is apparently essentially invisible to the same radar unless one mounts a corner reflector. I know what PT Barnum said, but both positions of now you see it, now you don’t are disconcerting.

Like I said, I have next to no practical experience with radar, though my Quantum radar was excellent in coming into a familiar anchorage to me on a moonless dark night.

What are the experiences of more accomplished users or maybe our military veterans in practical uses of radar?
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