Reply
 
Thread Tools Search this Thread
 
Old 20 April 2006, 10:03   #71
Member
 
Country: UK - England
Town: nr Lymington
Boat name: JU-JU
Make: Halmatic PAC22
Length: 6m +
Engine: 140.5 Mermaid
Join Date: Sep 2003
Posts: 1,400
Quote:
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
__________________

__________________
Scary Des is offline   Reply With Quote
Old 20 April 2006, 10:22   #72
Member
 
Country: UK - Scotland
Make: HumberOceanOffshore
Length: 8m +
Engine: Volvo KAD300/DPX
Join Date: Oct 2002
Posts: 4,962
Quote:
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.

Quote:
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.
__________________

__________________
JW.
jwalker is offline   Reply With Quote
Old 20 April 2006, 10:40   #73
Member
 
Country: UK - Wales
Town: swansea
Boat name: Too Blue
Make: BLANK
Length: 8m +
Engine: Suzuki DT225
Join Date: Mar 2004
Posts: 12,791
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!!!
__________________
codprawn is offline   Reply With Quote
Old 20 April 2006, 11:08   #74
Member
 
Country: UK - Scotland
Make: HumberOceanOffshore
Length: 8m +
Engine: Volvo KAD300/DPX
Join Date: Oct 2002
Posts: 4,962
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.
Attached Thumbnails
Click image for larger version

Name:	Radar scan x 592vert.jpg
Views:	126
Size:	24.5 KB
ID:	18590  
__________________
JW.
jwalker is offline   Reply With Quote
Old 24 April 2006, 09:10   #75
Member
 
Country: UK - England
Town: nr Lymington
Boat name: JU-JU
Make: Halmatic PAC22
Length: 6m +
Engine: 140.5 Mermaid
Join Date: Sep 2003
Posts: 1,400
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
__________________
Scary Des is offline   Reply With Quote
Old 24 April 2006, 10:54   #76
Member
 
Country: UK - Scotland
Make: HumberOceanOffshore
Length: 8m +
Engine: Volvo KAD300/DPX
Join Date: Oct 2002
Posts: 4,962
Quote:
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.
Quote:
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 .
Quote:
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!
__________________
JW.
jwalker is offline   Reply With Quote
Old 24 April 2006, 11:33   #77
Member
 
Country: UK - England
Town: nr Lymington
Boat name: JU-JU
Make: Halmatic PAC22
Length: 6m +
Engine: 140.5 Mermaid
Join Date: Sep 2003
Posts: 1,400
Quote:
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.

Quote:
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.
Quote:
Originally Posted by jwalker
Jeez, Des.....
Not only the evenings Des
__________________
Scary Des is offline   Reply With Quote
Old 24 April 2006, 12:57   #78
Member
 
Country: UK - Scotland
Make: HumberOceanOffshore
Length: 8m +
Engine: Volvo KAD300/DPX
Join Date: Oct 2002
Posts: 4,962
Quote:
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.
Quote:
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.
__________________
JW.
jwalker is offline   Reply With Quote
Old 24 April 2006, 14:09   #79
Member
 
Country: UK - England
Town: nr Lymington
Boat name: JU-JU
Make: Halmatic PAC22
Length: 6m +
Engine: 140.5 Mermaid
Join Date: Sep 2003
Posts: 1,400
Quote:
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
Attached Thumbnails
Click image for larger version

Name:	EM400.png
Views:	103
Size:	121.0 KB
ID:	18657  
__________________
Scary Des is offline   Reply With Quote
Old 24 April 2006, 14:39   #80
Member
 
Country: UK - England
Town: nr Lymington
Boat name: JU-JU
Make: Halmatic PAC22
Length: 6m +
Engine: 140.5 Mermaid
Join Date: Sep 2003
Posts: 1,400
Quote:
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
__________________

__________________
Scary Des is offline   Reply With Quote
Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off





Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -5. The time now is 09:27.


Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.