[Alberg30] Radar Antenna Mounting

[Bob Johns]

>Are self leveling Radar Antenna mounts a necessity.
>    George K.
>    #259 Omniscient ( named by former owner)

George,

We've had radar on our Alberg since 1990 (Raytheon R10). If you are
concerned with getting the last bit of performance under extreme heel  you
might consider a self leveling antenna.  I don't think it is worth all of
the expense and complication.

Antenna patterns on marine radars have a large vertical beamwidth compared
to the horizontal beamwidth. (Check the beamwidth specifications of the
radar you are considering.) I think that the R10 that I have has a vertical
BW of 20 degrees, but the book is on the boat so I can't check it now. In
any case, the BW of an antenna does not cut off sharply and objects just
outside the edge of so-called beamwidth are visible, depending on size and
reflectivity of the object. If you consistently sail your boat on its ear
you might want want the self leveling mount. The type of sailing we have
done when cruising usually does not put the rail far down and we have never
been aware of a performance loss due to heeling.

There are a lot of things that affect radar performance. We did long
distance coastal cruising so one thing we were concerned about was range
which we could see when making a landfall. That is why we installed the
antenna on the mast just above the spreaders. I calculated that spreader
height would give the maximun range of a target of similar height at about
16  miles, which is the instrumented range of the R10. I   haven't looked
at self leveling mounts, but I believe that most of them mount on a post at
the stern. I wouldn't want more weight on the mast at spreader height than
a simple mount and the antenna itself because of the effect on boat
performance. (I don't know the weights of the different mounts, but a self
leveling mount has to be heavier.)  But keeping the weight low restricts
maximun range of the radar.

One of the things that a lot of people don't realize is that wave height is
one of the biggest factors in radar performance, especially  at close
ranges where you have to use the sea clutter control to reduce the returns
from the waves. In smooth water you can see very tiny objects (One time we
saw 2 feet of bamboo pole sticking up above the water at several hundred
yards with the radar carefully adjusted and the water glassy). With waves
you may not be able to  see a reasonable sized boat that doesn't have a
radar reflector. You want to adjust the radar very carefully when you are
in an area where most boats are using reflectors because you may not see
the  boats without reflectors due to the wave returns, even though the
radar appears to be set up properly because you see all  of the reflector
equipped boats.

Everything on a boat  is a compromise and this applies to radar as well.
One disadvantage of the mast mounted antenna that I didn't anticipate was
that with the antenna in front of the mast I get echoes that appear to be
astern of the boat but they originate from forward of the boat from large
land masses ot large ships. This effect might not occur with a stern post
mounted antenna.

Time to stop.

 Regards,
	Bob Johns, Wind Call #397



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