Radar reflector

[jwalker]

OK. Thanks for that Des. But, I don't see why the target would look enlarged more than normal. I have a feeling something else was happening in your example. The 3 boats may have given targets large enough to join on the display or the side lobes of the scanner beam were active on the targets or, perhaps, the gain was set a little too high for that situation and the targets smeared into each other. There are many possibilities to consider before presuming the enhancer is the cause.

[Scary Des]

Originally Posted by jwalker

.......There are many possibilities to consider before presuming the enhancer is the cause.

JW I’m confused (isn’t the first time and won’t be the last ) If you look at a large ship on your radar it generally gives you a large bright blob, if you look at an islet of a similar size it gives you a large diffuse blob. A boat with a radar reflector gives you a larger blob than one of a similar size without. The brightness is all down to the quality of the returning signal and not the size. Or have I got it wrong Des

[DGR]

Originally Posted by Scary Des

JW I’m confused (isn’t the first time and won’t be the last ) If you look at a large ship on your radar it generally gives you a large bright blob, if you look at an islet of a similar size it gives you a large diffuse blob. A boat with a radar reflector gives you a larger blob than one of a similar size without. The brightness is all down to the quality of the returning signal and not the size. Or have I got it wrong Des

Des,

It's both. A big flat sided tanker will give a large, high quality return from loads of surfaces and will appear as a bright big blob because it's reflection back to the source is so big, from a load of different places on the hull/superstructure. A smaller ship will reflect less of the energy back to the source - smaller blip.

It's the opposite to some of the stealth technology - think of that Stealth Fighter with all the facets on it to send the radar energy anywhere but at where it came from - or a modern warship where the topsides are canted inwards to reflect surface radar upwards rather than back at the target.

An island wouldn't reflect as well because of what it is made of but would still give you lots of returns from different surfaces just not strong (bright) ones - but gives enough of a return that you can see it on a screen. If the Isle of Wight was made of steel, you'd struggle to see anything else on radar in the entire channel!!

If you look at a coastline on a radar screen, do you get 'speckling' along the coast from stronger returns amongst the weak 'land' returns (i.e. cars, or buildings etc) ?

D...

[Mike B]

Not sure if the following helps clarify at all.

The test specifies the brightness of the blip increased not the size. This was in 1998 but I'm guessing the current models use the same principle.

http://www.ussailing.org/safety/Stud...arenhancer.htm

"Seventeen of these responses rated the improvement as "much brighter," while the six others chose "brighter" as their subjective qualifier."

[jwalker]

Des, I'll reply a wee bit later....just had a big glass of wine with me tea....

[Scary Des]

Originally Posted by jwalker

Des, I'll reply a wee bit later....just had a big glass of wine with me tea....

I think I’ll join you

[jwalker]

Originally Posted by Scary Des

....coming back through Hurst Narrows one afternoon we couldn’t understand why we could see three yachts about a mile ahead of us through the window and only one large one on the radar. Once they passed the narrows they went there separate ways and became separated blips again on the radar even though one remained far bigger than the others, someone suggested that this was due to AR.

Des, if you already no what I'm going to say then just ignore me. Radar scanner beams have a width and they are slightly diverging. Mine has a beam width of approx 4°. Let's say a target is side on at 2 miles and it represents a viewing angle of 6°. The 4° beam will begin to see it 2° early, it will scan by and it will still see it 2° after. Therefore, the total size of the target painted on the screen will be 2+6+2 degrees, 10°.

In your example of the 3 boats getting closer together as they pass through the narrows, there will come a point where the images on the screen will overlap but the boats will still be 4° apart. Obviously, as the boats separate once through, they will become 3 targets again.

A very narrow target, say, a pole, will have a minimum width on the screen which equals the beam width of the scanner and this is what I would expect the radar enhancer to produce.

There are other variables which will change the width of the image on the screen and most of these can be controlled using the unit's settings. One, which I use extensively, is target enhancement and this artificially enlarges the image on the screen and it is used so one's eye will notice an otherwise small blip on the screen. If it is used where traffic is dense, it will plainly cause overlap of the images and confusion.

The wider the beam, the bigger the image on the screen.

The 63sq metre I think is misleading. My understanding is that the enhancer gives a return strength equivalent to a target of that area not necessarily a target of any particular width.

[tim griffin]

Originally Posted by jwalker

The wider the beam, the bigger the image on the screen.

.

Actually a typical Yacht radar will have a horizontal beam width of around 6 degrees .
This will have the following effects on your picture

Less transmitted power
A wider beam will make gaps appear closed due to the example Jw gave
Targets close together will merge Sary Des's 3 Yachts

When I teach my students I mention echo stretch is to be used carefully it does enhance the picture but enlarging the picture can tend to hide details it is not recomended for pilotage and accurate navigation.
The bigger the scanner the narrower the beam the narrower the beam the better picture painted by the scanner transmitting and receiving .
Think of the size of a warship scanner

[jwalker]

Originally Posted by tim

..This will have the following effects on your picture...

Except for the less transmitted power, I agree but your comments make radar appear somewhat less attractive, however, there are other things which have a bearing on the screen image and hence its usefulness.

As you get closer to your 3 boats they will separate. Similarly, approaching a gap will unmerge the edges. Lowering the gain can separate targets by dropping the weaker edge returns. Adjusting the clutter settings can also enhance the image. You do need to be careful not to mis-adjust so that things which are actually there disappear.

Because radar it essentially a ranging device gaps can be seen through, eg. a harbour can still be seen even if the front edge appears closed at that particular range. A couple of apparently joined boats can be seen past and, say, a third boat is able to be seen behind the gap of the apparently joined pair and the gap will open up as you approach.

It takes a bit of learning but it is my favourite instrument.

One other thing, which I don't understand, although I said the minimum target width will equal the beam width, this was for simplicity. I frequently have targets which are considerably less wide than the beam width. Possibly the gain level just drops off the edges but I'm guessing.

Radar can be very sensitive and it is often easy to see birds flying buy at a mile or so.

It doesn't always see the curvy, rubbery bow of a police rib following behind......

[tim griffin]

Originally Posted by jwalker

Except for the less transmitted power

A antennas job is to focus the transmitted signal into a narrow beam the bigger the scanner the more focused the beam wil be, it gives the effect of greater power. A narrower beam will be better for target discrimination less chance of things merging i.e entrance to rivers and targets close together buoy's and yachts.
Big Girls Rarely Talk
Brilliance
Gain
Range (typical 2/3 miles and focus on a weak target)
Tuning
is te correct way to set up your radar most people just use the auto control but you get better results if you fine tune the controls your self abit like a photographer doing manual focus
Gain set to high screen is too fuzzy turn it down to low and you risk blotting out weak targets
Sea clutter adjustment will weaken the signals around the centre of the picture .
and Rain clutter will adjust the whole of the screen and can weaken the return from gently shelving coastlines .Depending on the age of your set Rain clutter is either on or off
You are absolutley right it is a very useful aid to navigation but it is important to learn how to interpret the picture . I prefer to teach this afloat rather than in the classroom as we have lots of places in the Solent that can demonstrate beam width , Racons and parrallel indexing and of course rule 19 and the use of it and how the rules of the road change if you have radar fitted . RTE are excellent but they work on the x band and some of the big boys only have s band .

[jwalker]

Originally Posted by tim griffin

A antennas job is to focus the transmitted signal into a narrow beam the bigger the scanner the more focused the beam wil be,

Yeh, that's what I though you meant. But it isn't more or less power, it's just the focussing of the energy.

Your big girls thing I don't agree with. It may have been the case, especially with CRT's. On the two radars I have owned (different makes) I've not been able to improve on the auto tuning and certainly not while bouncing in a rib. Range is changed as required during the journey.

but you get better results if you fine tune the controls your self abit like a photographer doing manual focus

You would probably also need to readjust each time you changed the range which would be impractical on a rib.

Brilliance is no longer adjustable using TFT's but backlighting is and it is also adjusted as required, eg. into sun, following sun, running into an overcast area, falling darkness etc.

[tim griffin]

Originally Posted by jwalker

Yeh, that's what I though you meant. But it isn't more or less power, it's just the focussing of the energy.

Your big girls thing I don't agree with. It may have been the case, especially with CRT's. On the two radars I have owned (different makes) I've not been able to improve on the auto tuning and certainly not while bouncing in a rib. Range is changed as required during the journey. Brilliance is no longer adjustable using TFT's but backlighting is and it is also adjusted as required, eg. into sun, following sun, running into an overcast area, falling darkness etc.

Try adjusting the brilliance then contrast then gain so that the speckles just start to appear ( bit like squelch on VHF) always keep a few speckles, Range is determined by the hieght of your scanner and it's line of sight to the horizon ie a radar scanner mounted 4 metres above sea level
it will have a range of 4.5 miles square root of height x 2.2 works on line of sight as does your VHF. if using course up remember to readjust your set after each big course change.
And we still teach
BGRTeven on the new LCD sets try playing on a calm day and see if you get better results.

[jwalker]

Originally Posted by tim griffin

Try adjusting the brilliance then contrast then gain so that the speckles just start to appear ( bit like squelch on VHF)

Neither contrast nor brilliance is available for adjustment on any of my colour TFT's. Gain is adjusted as required.

always keep a few speckles,

Yep.

Range is determined by the hieght of your scanner.....

Yes, but targets also vary in height. As you suggested, I also use 3 miles as my general range setting because I can usually get returns from boats which are poor targets and it's plenty of distance to take evasive action but I also set the range out to see what vessels in the distance are doing and I probably do this at about 8-12 miles off. If a vessel is sitting on the same bearing as we close each other, I will keep it on screen by altering range to monitor it. Closing a harbour I'll range in to see how far away other boats are and in which direction they are moving.

Personally, I feel a rigid procol is too inflexible. I always try to keep an open mind where navigation is concerned. It's easy to fix a thought and not think of alternatives, even ones which seem obvious in hindsight.

[waverunner]

Originally Posted by Top banana

Could any body enlighten me on fitting a radar reflector to my rib. and to what benefit. Thanks

Buy yourself a stealth rib next time!