Position of radar reflector

[Alex-341]

Its mounted upright currently, not sure the 4m pole will be possible though

[Scary Des]

Originally Posted by codprawn

....Peak pulses may look good on paper but in the real world don't mean so much.

As usual I think you’ve missed the point, sure tri lens has a nice smooth reflection but it is only painting an area 5mtrs square, whereas the Echomax paints a 20mtrs square, albeit with some peeks and nulls. Would you rather present a large but rough image as apposed to a small but well formed one
As always you are arguing in circles, on the one hand you rave about Seame because of the size of its (rough)image now you are saying that small but well formed is better more Cobbledygook Des

[jwalker]

Originally Posted by Scary Des

Would you rather present a large but rough image as apposed to a small but well formed one
As always you are arguing in circles, on the one hand you rave about Seame because of the size of its (rough)image now you are saying that small but well formed is better more Cobbledygook Des

Are you still confused about this, Des? The strength of the return does not determine the size if the image on the screen. It will determine the distance at which the object can be seen and also the brightness/clarity of the image on the screen. But if the return is solid, since the screen has a limited brightness, this is the level at which it will show.

Codders is correct. It is a compromise. Would you rather be seen for some of the time at a greater distance or at only some angles or for all of the time at a limited distance?

I would be concerned about a reflector which had nulls for a good few degrees of arc however, you can have a reflector where the plot is spikey but the lows are still quite acceptable.

I remember reading the article that Cod is talking about and it did seem resonably well carried out but, on the few occasions I've relied on magazine articles for making a decision, I regretted it each time. You need to live with something for a while to appreciate which of it's characteristics are important to you. Often, it's the subtleties that make the thing nice to own and use.

[Jono]

Originally Posted by Scary Des

.... Would you rather present a large but rough image as apposed to a small but well formed one ....

Some of us don't have a choice in this life.....

[jwalker]

Originally Posted by Jono

Some of us don't have a choice in this life.....

Trust you....

[codprawn]

Originally Posted by Scary Des

As usual I think you’ve missed the point, sure tri lens has a nice smooth reflection but it is only painting an area 5mtrs square, whereas the Echomax paints a 20mtrs square, albeit with some peeks and nulls. Would you rather present a large but rough image as apposed to a small but well formed one
As always you are arguing in circles, on the one hand you rave about Seame because of the size of its (rough)image now you are saying that small but well formed is better more Cobbledygook Des

Try to think of it as a laser beam verses a flourescent lamp. The flouro lamp may be a lot dimmer but it is visible FAR more of the time - the laser beam will give a MASSIVE burst but only for a fraction of the time.

2 extremes I know but trying to illustrate a point!!!

[Scary Des]

Originally Posted by jwalker

Are you still confused about this, Des? The strength of the return does not determine the size if the image on the screen. ......

Originally Posted by raymarine e series manual

On-screen targets may be large, small, bright or faint, dependent on the size of the object, its orientation and surface. Strongest target returns are displayed in yellow with weaker returns in two shades of blue. Be aware that the size of a target on screen is dependent on many factors and may not necessarily be proportional to its physical size. Nearby objects may appear to be the same size as a distant larger objects.
With experience, the approximate size of different objects can be determined by the relative size and brightness of the echoes.
You should bear in mind that:
The size of each on-screen target is affected by:
The physical size of the reflecting object.
The material from which the object is made. Metallic surfaces reflect signals better than non-metallic......

this is what Raymarine say about there radar, does yours work differently Des

[Scary Des]

Originally Posted by codprawn

Try to think of it as a laser beam verses a flourescent lamp...........

No, try to think of it as a big return so people can see you or a small return which they won’t If you look at DJL's post (47) you will see that the tri-lens is achieving 4mtr2 at best whereas the echomax is achieving 5mtr2 at worst and 20mtr2 at best Simple choice. Des

[codprawn]

Originally Posted by Scary Des

No, try to think of it as a big return so people can see you or a small return which they won’t If you look at DJL's post (47) you will see that the tri-lens is achieving 4mtr2 at best whereas the echomax is achieving 5mtr2 at worst and 20mtr2 at best Simple choice. Des

It is NOT that simple!!!

Imagine someone in the water with a chemical lightstick and someone with a laser.

IF the laser happens to shine straight into your eyes then fine you can't miss them - on the other hand the light stick is nowhere near as bright but is visible far more often!!!

[jwalker]

The size of each on-screen target is affected by:
The physical size of the reflecting object.
The material from which the object is made. Metallic surfaces reflect signals better than non-metallic......

Originally Posted by Scary Des

this is what Raymarine say about there radar, does yours work differently Des

Nope, mine works just like that.

Please note,

The size of each on-screen target is affected by the physical size of the reflecting object.

So, a radar reflector, which is likely to be in the region of 300-400mm across, is never going to look like a tanker on the radar screen.

Also please note,

The material from which the object is made. Metallic surfaces reflect signals better than non-metallic.

This indicates that a metallic object is more likely to represent its approximate horizontal length than, say, a rib because of the likelyhood of the rib not returning a sufficient echo over its whole length. It does not imply that a metallic object will in some way amplify the return.

Lets consider the case of, for example, a sheet of metal 50mtrs long my 1 metre wide. I'm going to ignore the fact that it is flat and therefore some of the radar energy will not return.

If the sheet was standing on end, it would give a return which represented 1mtr wide. If it was on its side, it would give a return which represented 50mtrs wide.

In both cases, the return would contain errors because of the radar beam width. The amount of error would depend upon the radar in question.

As your quote indicates, there is a degree of learning necessary to interpret a radar screen. Particularly, but not entirely, because irregular shapes will modify the returns. However, it's not overly difficult to get to grips with.

[Scary Des]

Originally Posted by jwalker

......So, a radar reflector, which is likely to be in the region of 300-400mm across, is never going to look like a tanker on the radar screen.....

Sure it won’t but is does ‘paint’ a larger area than its physical size hence the 20mtr2 peak performance of the echomax.

Incidentally you have always said that ‘The strength of the return does not determine the size of the image on the screen’, but, unless I’m getting the wrong end of the stick, you have changed your view on this Des

[jwalker]

Originally Posted by Scary Des

Sure it won’t but is does ‘paint’ a larger area than its physical size hence the 20mtr2 peak performance of the echomax.

Except for the few degrees of the radar beam each side of centre, no it doesn't. The use of the equivalent area to indicate the approximate strength of the return is maybe misleading you.

Incidentally you have always said that ‘The strength of the return does not determine the size of the image on the screen’, but, unless I’m getting the wrong end of the stick, you have changed your view on this Des

Nope, I've not changed my view. Hence the example I gave of the 50 x 1mtr target.


Perhaps I need to get me pencil out again.

Des, I apologise for being abrupt in my reply to you last evening. I'm not so tolerant towards the end of the day.

[codprawn]

To show how meaning less the maximum RCS figures are the Plastimo Tube type has an RCS of 9.30 - the standard Tri lens is 5.39 and yet the Plastimo came out bottom in the review.

Maybe it is best to look at the MINIMUM RCS figures!!! The Tube gets 0.10 - the Echomax gets 1.09 and the Tri Lens gets 1.86m which I think goes to show WHY they place the Tri Lens 2nd only to the SeaMe.

There is a bigger Tri Lens quite sutiable for a RIB that costs £300 and gave 8.95m peak and a minimum of 5.13m.

BTW the Sea Me gave a maximum of 308.27 and a more important minimum of 20.46m. Clearly the winner by a massive margin - and it's British!!!

[jwalker]

Here's a wee diagram to clarify how the scanner collects the radar image and how this is represented on the display screen. The strength of the image cannot influence it's width on screen except, maybe, by a degree of flair on the display. I would guess this to be more likely with CRT displays.

A small target will be expanded proportionally more by the beam width. I've used 4° because my scanner has a beam width of 3.9° so that seemed reasonable.

[Scary Des]

JW, I think that one issue you have overlooked with your diagram is that it takes no account of the relative angles of the targets surface and the radar transmitter. In theory, a radar can ‘see’ only when the targets surface is at 90 degrees to it and the beam gets bounced straight back, obviously because most surfaces aren’t really smooth you can still see things because of scatter.
What a radar reflector does is overcome this misalignment and gives you a 90 degree style image even though the radar beam hasn’t reached that angle this is why it can painting a larger area than it physical size.

This is interesting

and so is thisDes

[jwalker]

Originally Posted by Scary Des

JW, I think that one issue you have overlooked with your diagram is that it takes no account of the relative angles of the targets surface and the radar transmitter.

Jeez, Des. Yes I have. I drew the targets curved so they would give the screen images I drew. Anything flat or curved the other way will have fall-off towards the edges as the returns weeken and, therefore, they appear smaller.

In theory, a radar can ‘see’ only when the targets surface is at 90 degrees to it and the beam gets bounced straight back, obviously because most surfaces aren’t really smooth you can still see things because of scatter.

Correct .

What a radar reflector does is overcome this misalignment and gives you a 90 degree style image even though the radar beam hasn’t reached that angle this is why it can painting a larger area than it physical size.

And how do you reckon it does that?

A corner reflector is simply a way of sending the wave back to where it came from irrespective of the angle of incidence to the reflector. The reflected wave is displaced in the horizontal plane if that is the orientation of the reflector but only by a small amount. Remember, big boat scanners are many feet across and much larger than the reflector. Also remember that the scanner has to recognise the returning wave as the one it sent out in order for it to measure the target's distance, which is what radar essentially does. It wouldn't be any good if it responed to every incoming radar wave.

Of course, there are lots of unusual situations where returns may reflect off surfaces and produce erroneous images. Also, the the scanner will have side lobes to it's beam which can cause problems. But recognising these and reading and setting the radar is all part of its usage.
It is even possible for a large, close target to produce an image which appears to completely surround one's boat but it's a limitation of radar. You could say there's nothing wrong with the radar, the target boat is too big!

[Scary Des]

Originally Posted by jwalker

........ I drew the targets curved so they would give the screen images I drew. ......

Had another look and yep they are curved But the point I was trying to make was that in real life you always get less of a target than what is really there and that drop off to the sides is not an issue with an corner reflector, it will reflect all that hits it.

Originally Posted by jwalker

.....A corner reflector is simply a way of sending the wave back to where it came from irrespective of the angle of incidence to the reflector. The reflected wave is displaced in the horizontal plane if that is the orientation of the reflector but only by a small amount......

One on its own yes, but a good reflectors use a stack of catch rain ones which cover all possible plans.

Originally Posted by jwalker

Jeez, Des.....

Not only the evenings Des

[jwalker]

Originally Posted by Scary Des

Had another look and yep they are curved But the point I was trying to make was that in real life you always get less of a target than what is really there

Cool, cos you were originally saying that the target could look substantially bigger than reality.

One on its own yes, but a good reflectors use a stack of catch rain ones which cover all possible plans.

Take care with that one cos the catch rain type are a set of 3 sided corner reflectors and when they are fitted together the top forms the catch rain bit, not the reflector. Some folk don't understand how they work and they fit them in the wrong orientation.

[Scary Des]

Originally Posted by jwalker

Cool, cos you were originally saying that the target could look substantially bigger than reality.
......

Not target, reflector. That is what i understand a radar reflector does, it makes you look bigger than you would without a reflector, in effect It is the pit-bull terrier of the sea

This is what i mean by multi catch rain type. Des

[Scary Des]

Originally Posted by From Echomax site

Radar Cross Section (RCS) explained
1. A sphere operates with a weak signal at all angles of incident
radiation.
2. A flat plate is an extremely efficient reflector but has a very sharp angle of response.

3. RCS may, for practical purposes, be defined as the cross section area of a conducting sphere of such a size that it would return an echo equal in strength to that of an equivalent flat plate oriented so as to be perpendicular to the direction of the incident radiation.

4. One metre squared is the cross section of a sphere radius 0.565 metre (R2xPi - 1 metre 2)

so 20mtrs 2 is what the radar sees Des