Radar gone Phut !!

MarkJohnson12345

MarkJohnson12345

Well-Known Member
Joined
23 Dec 2004
Messages
1,177
Location
Swansea Wales UK
www.markjohnsonafloat.org.uk
Last w/e I was testing the radar, initially getting a good understandable signal.

Without any reason the screen then produced a load of concentric rings on the screen, with no obvious meaning.

Today I checked the radar, alas the same meaningless rings were on the screen.

Radar is a Furuno 17** integrated with a chart plotter.

I have looked through the maual, but can find no obvious explanation.

Suspect some part has failed, just hope that I do not have to remove the Radome.

Any one else had this problem???

TIA
 
I would suggest it is not the radome as that only delivers a video signal to your RADAR.

Are you still getting targets painted on the screen?

Most modern yachty radar will give a text fail if the dome has a problem or will not time-out.

Give the cables a waggle, especially your GPS, that would be my first approach. In fact disconnect the GPS.

The next thing, try running the radar in demo mode, see if it does the same thing, demo uses the standard radar function but feeds a recorded signal (I think).

Then try a full factory reset.

Then a dealer.
 
Concentric, though inconsistent.

I will try to paste a photo.

The boat is out of the water with some buildings around it, but prior to the rings it was giving a good understandable image.

Going back into the marina this week, which is not so near buildings, so will give everthing a wiggle and reset then.

Thanks
 
These rings may be caused by Side Lobes" i don't know what it means but it is mentioned in the RYA Radar course book.It could also be the effect of returned pulses from very close object/s which the processor fails to interpret.Wait till you are back in the water with a bit of space and check it again going through the distance scale to see if the effect is still there.
 
A sidelobe is a secondary receiption, sort of.

Ok.

Here is my effort, this used to be what I did, but explaining is not always my forté, please no marks out of ten /forums/images/graemlins/frown.gif

sidelobes.gif


This is how the RADAR receiver power looks through an arc of say 60° of rotation through the target. Er, say you were facing a radar reflector, turn your head 30° to the left, rotate to be facing 30° right, as you pass the target your eyes will focus best as you start looking straight at it, at 30° you are looking hard over and not seeing it very well.

Well a radar works slightly in the same way, peak power will return when the antenna is facing the target.

Just for info this is a good way to understand beamwidth, you know the quoted bw of an 18" radome is 5.2° well this can be seen in the above diagram, as the bw is measured 3db down from the peak and the pattern as you see is not a straight line but a dome, the radar is painting the target for the time it is on the bw line I have drawn in.

Thus it becomes obvious that two targets that appear in that 5.2° bw sweep will be seen as one, a narrower beam means a narrower beamwidth and better discrimination of targets. 3db incidentally is the half power point of RF, it just is ok!

Right, now you see the secondary domes along the baseline, these are called sidelobes, an example sort of - on a SW radio this can be heard as a quiet but rubbish reception of the station a little along the dial. These are generally harmless, the sidelobes I have marked A are well below the threshold and will be ignored by the RADAR. I am not sure how a basic yacht radar will look, but this is an interpretation of a good radar, it may be that a yacht radar has a far smaller dome etc. This is only an example.

Sidelobe B is the dangerous one, it has a good chance of being seen as a secondary target, it depends where the radar cuts off the receiver as such. But B could cause a shadow on the screen.

FWIW, I have personally never seen a sidelobe cause concentric circles on a radar, I have seen them cause all kinds of problems, but nothing like the originator described.

Sorry to waffle, just thought some might like a simple overview of beamwidth and sidelobes in a simple english way, it has been a long time since I have done theory so some parts may be fraught with minor factual errors.

Should I hit send and be shot down, ho hum...
 
[ QUOTE ]
Sidelobe B is the dangerous one, it has a good chance of being seen as a secondary target, it depends where the radar cuts off the receiver as such. But B could cause a shadow on the screen.


[/ QUOTE ]

If I understand what you are saying, one real target could be seen as two, or even several, distinct targets?
 
More likely to lose discrimination, a target could start painting as the beam hits it, if this was rotating from left to right, it would start painting at 3db on the right hand side of the dome and keep painting until it has passed sidelobe B.

This can cause a long curve on the screen (if I am remembering right) and an increased bw, lose more targets in it. Large sidelobes on the base line will cause a secondary target.

I am confused by concentric circles, I would need to see it in operation, but I am unhappy with sidelobes being the cause.

EDIT>> I wonder if he has his STC right up, which may bring sidelobes A into play, but this usually blots the screen rather than causes concentric circles. Try dropping your rain and clutter (harbour) right down and see what happens.


p.s. I meant beamwidth not bandwith, though very similar sort of.
 
10 out of ten

I hope the problem is being too close to buildings.

At sea, a large ship close by (1 mile) will echo its position in echos around the radar screen, but as individual echos rather than rings, but significantly at precisely the same distance

But the radar did give meaningful images for a while before it was overpowered or blew something.

see www.markjohnsonafloat.org.uk/radar01.htm (in a short while)

Back in the water this week so will post results then,

Many thanks for time and thought.
 
Of the two diagrams, I am just trying initially to work out what I am looking at.

Is the figure in the top left the range? What does it refer to, 500 or 250 ?

If it is range, it is interesting that the rings are painted, continual and consistent, i.e. they are at permanent range.

This is nothing to do with the position of your boat (watch me eat my hat!). I think we are looking at a noisy cable here, noise is like the hum you get on an amplifier if you touch the input wires.

This is why I suggested a waggle, it may be a dry or broken joint on the video cable. Do you have a join through the bulkhead? Have you got any tight bends in the radar cable, or recently put something electrical near the cable, especially the junction box if you have it broken by the mast.

Does your dome have a drain hole?

Was your fridge or eberspacher turned on or a fluorescent tube, flickering?

Try cleaning all the pins with IPP (isoprop), head cleaner.

I think, this is your cable, which may or may not be good news. The worst case scenario is that it has got water in it, it may be just seriously damp in the dome, but I am not convinced about that.

Does Furuno have a self test mode?

Did you try the demo?

EDIT>> I didn't include my working out.. The reason I think it is your cable and not the RADAR receiver is that the rings are consistent, that leads me to believe that the RADAR is working properly. I am not convinced it is the dome, the magnetron or whichever amplifier they use is a very simple machine, it just takes the RF output and amplifies it up, generally they work or don't. Another outside possibility is the automatic switch that prevents high power coming back down the cable from the transmitted pulse when the RADAR is in receive. But this is more likely to show if smoke is coming out of your RADAR unit. No, I am certain it is not that, I am not sure how yacht radars cope with this process, but I am sure they must have a failsafe which would shut down the unit to protect it. I am sticking to a noisy cable for now...

If you could borrow a cable from Furuno and hang it from the dome through the cockpit door for testing purposes it would be handy. You could also take your base unit and test it on another's boat who has a Furuno RADAR.

First rule of fault finding is to split the fault, you have 3 components, with use of a mates boat you should be able to get it to one or two possibles.

J
 
The picture is on its side here, turn it 90' to the right.

500 and 250 are range in metres

Inside the rings the 'blobs' move as the radar turns. The ranges would not appear to be significant, these buildings are about 30 metres and 60 metres away, on two sides. The rest of the horizon is fairly open.

As said before it went phut (no noise nor smake!) I could see the outline of the marina and pontoons quite clearly.

When in working mode, targets are red, the other colours are unusual.
The cable is continious from the radome, with perhaps one sharpish bend.

Only electrics on were the battery charger (2 pphase type) and there was a small 12v flourescent close to the radar/chart plotter. This has not had any effect to date as far as I know.

Not sure about the dome drain hole, its been up there three years.

Furuno have a simulator mode, which I have not tried.

But I will definelty give it all a waggle. Hopefully all will be well when she is back in the water.

regards and thanks
 
You must log in or register to reply here.

Share this page

Top