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» Online Classroom   »   » Public Discussion of Marine Radar   » Reflected Radar Hazard?

   
Author Topic: Reflected Radar Hazard?
kdeemer


 - posted May 10, 2006 11:01 AM      Profile for kdeemer           Edit/Delete Post 
I am an operator of a 44 foot power boat with two radar transmitters that are temporarily mounted in a manner that may be hazardous and would like some advice. One unit is a Furuno, rotating antenna, unit. It has a large aluminum housing and is mounted in the front center of the roof above the flybridge of the boat (where the boat is operated). This unit is not currently working. The other unit is a Garmin array (4kw) that is mounted, also on the roof and less than 12 inches directly to the left of the Furuno unit. We realize that this configuration needs to be changed, but the boat needs to be brought back from Mexico before it can be worked on.

My concern is possibility that the Garmin unit will reflect off of the Furuno housing and scatter radiation down into the bridge area with possible harm to those on the bridge. The Furuno housing is bowl shaped and elevated on a pedestal. It looks like just the right shape to reflect a beam down into the bridge. I also realize that there should be a large blind spot caused by this housing, but the Garmin seems to image a continuous coastline.

My partners think that I'm nuts to be concerned about this but, as a former physicist and electrical engineer it seems plausible. Any thoughts would be appreciated.

Ken

From: Los Angeles
David Burch


 - posted May 10, 2006 04:44 PM      Profile for David Burch           Edit/Delete Post 
I assume that you are describing an open array unit next to a radome scanner, as shown below.

 -

My guess is there would not be much reflected intensity on a level below the line of sight.

First, the reflected intensity is weak to begin with,

second, it reflects way more in the direction emitted which is essentially straight out over the boat, and

third the round shape (of either unit) is actually a very poor radar reflector, as described in the book Radar for Mariners in our book catalog.

That book has an extensive section on radiation safety, and for more than a couple meters away from a rotating beam the intensity is very low (from a radiation safety point of view) even when in direct line of the rotating beam.... but see that discussion in the book for more specifics and actual numerical examples, as well as the pattern of emitted radiataion from these antennas.

And finally, microwaves are NOT penetrating the overhead at all. They are very quickly absorbed in solid materials.

Note is the above picture is not correct, it does not matter. The conclusions above would be the same for any combination of units.

Thanks for this excellent question. Radiation safety in radar use has been a mystery for many years, which is why we devoted so much to it in our book. Please follow up if there is more we might add or if we have misinterpreted your question.

From: Starpath, Seattle, WA
kdeemer


 - posted May 11, 2006 10:32 AM      Profile for kdeemer           Edit/Delete Post 
Thanks for your prompt reply. I've ordered your book. The units in the pictures are correct, except that the array mounts flush with the top of the roof and has no housing below it.And it is on a plane with the lower part of the rotating unit's housing. The geometry is such that reflected rays could be directed downward. Also, the roof is a relatively thin plastic, which I wouldn't think would absorb much radiation.
From: Los Angeles


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