Black spot in center Furuno radarscreen

Blue Fox

Blue Fox

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I have installed a Furuno 1623 on my boat. Seems to work OK, the screenshot below shows 2 islands, a boat and a couple of buoys that exist in reality. But that black spot in the centre, is that normal? It doesn't disappear when I manually tune down the gain to next to zero and is visible in all ranges, although different in size. Antenna is at the mast, on the official bracket, at around 5 m above sealevel. Could it be the echo of furler, halyards and mast?
Chris

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I have the same .My radar(same model and make) is mounted on a pole on the transom and this is probably caused by the mast.Not a problem in pratice.
 
CRT tubes suffer from this but assume same thing happens to the modern types as well, nothing that you can do. Mast shadow should appear as a tear-drop shape if I am correct, I now expect to be shot down. /forums/images/graemlins/blush.gif
 
It looks very like sea-clutter and may be unavoidable depending on where the antenna is fitted. It appears to extend to 300 feet from the boat so it would be interesting to see what the extent is at minimum range. I don't know what that is on the Furuno; it's 1/8 miles on mine I think.

You can't have too much practice with radar. SWMBO used to think I was mad with all ther time I spent playing with our set - until we encountered fog on passage and she was then duly grateful.
 
[ QUOTE ]
No difference with seaclutter either - well, probably nothing one can do.

[/ QUOTE ]I think it's worth looking at your Furuno 1623 manual, which warns specifically about the danger of a "dark zone" near the centre of the screen caused by incorrect sea clutter settings.
 
I used to have the same on my old steam driven Furuno (only my spot was bigger than yours). I read somewhere that it was all to do with tuning. Advice was to find a long straight wall and tune the set until it appeared straight on the screen. Did all that and still had the spot. Eventually the set went wrong and I replaced it with a state of the art Koden. The only difference is that the spot is smaller (like yours) and red instead of green. In practice I have never found it to be a problem.
 
This is normally coz the radar has not been comissioned properly and the zero timing adjust is incorrect, in the trade it is often known as a black hole or doughnut.

All you returns will be further or closer than they should be, so VRM values will be in error.

Get the timing right and it should go away.
 
This is quite normal. Its reflected signals from your own boat rigging etc.

For best operation I find it best to set the gain and sea state to auto mid range and use the rain filter to adjust the sensitivity ( Keep the setting as low as possible ). I know it seems daft but it does work. You can then pick up crab pot markers and seagulls from 1/2 mile if you want.
 
The receiver sensitivity at short range is affected by several settings so it is worth spending some time experimenting. I have found that the auto gain control will usually set the short range gain too high once it has done its averaging, but I have got the best results by setting the various gain parameters with the range set to a "practical" max working range of around 6 NM. I haven't got a record to hand of the settings on my system but the pic below was taken with the sea clutter at moderate, manual gain at around 30%. Auto gain gives similar results but not consistently, the averaging does not seem consistent /forums/images/graemlins/confused.gif There is no indication in the manual of the algorithm used for this. Piccie shows plotter and radar screens in high clutter in marina, it is still possible to make out the individual pontoons. Radar range 0.25NM. Note that the radar is on a mobo with a clear all round RF view, no masts and w.h.y.

DSC00052.jpg


Comparison at 1NM radar range:

DSC00043.jpg
 
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