In article ,
none wrote:

On Sat, 03 Apr 2004 18:40:53 GMT, Bruce in Alaska
wrote:

In article ,
bruce wrote:


Well, i thought the width of the beam was such that narrow objects allow
you t see 'thru', in a virtual sense, things like masts and poles? with
all the boats that have the radome on the mast they can se directly behnd
them. the actual mast won't be seen becaue the transmitt delay prevents
any returns shorter than 100' or so from registering on the screen (called
main bang supression)

Actually Main Bang Supression doesn't curb the shading effects of the
mast at all. It only sets the minimum distance that the radar can
resolve. Shading and beam redirection happen due to the RF being
deflected by a "Near Field Object". This happens with any object
within a meter or two of the antenna. Horozontal beamwidth will
tend to mitigate shading if the beamwidth is very narrow (3 degrees
or less), and the antenna is wide, and tend to enhance Redirection.


Since as you say, being hit from the aft, is not a big problem,
this is just a bit of a nusense. The height above the deck,
is not really significant, except in the determination of the Maximum
range of the unit and the amount of healing that can be tolerated by the
radar.



in fact. this is the option that you choose best suited to your purpose.
If you want max range you mount as high as possible, if you want max
sensativity for close in work like harbour bouys and 'ejectae' mount low
and angle down a few degrees even.


Actually, on small vessels, where the antenna is mounted less than 20
meters off the water, The ability to resolve targets has nothing to do
with elevation of the antenna, but everything to do with Horozontal
beamwidth. Commercial Marine Radar Antennas are specifically designed
to have a wide Vertical Beamwidth, so as to illuminate the surrounding
area with, in the neighborhood of 25 degrees @ the 3db Powerpoints.
This gives 12.5 Degrees above and below the horizon, which is will
allow for both closein and radar horizon targeting.


Just make sure that if the antenna is an open slotline type,
that it won't hit you in the head while turning. (ouch) Most modern
radars have a 25 degree vertical beamwidth. This allows for healing in
most all Powerboat installations. Sail installations may require some
antihealing considerations.

not to be argumentative but the actual beam verticle width seems to be
less than that. experiment by mount on a pole on the aft quarter. most
boats are mounting this way now for practicle considerations.

I haven't seen any published spec's for a Commercial Marine Radar
that has less than a 20 degree Vertical Beamwidth. I have been in the
Marine Electronics Field for better than 30 years. Can you privide
some documentation of a Commercial Marine Radar Antenna with a Vertical
Beamwidth of less than 20 degrees? "Most Boats" are Power Vessels and,
those that support Radars don't mount them on a pole in the aft quarter.
I will agree that, it is common these days to mount radar antennas on a
pole, for Sail Powered craft. It isn't the best place for radar
antennas to be mounted, just easier, and cheaper, for lazy installers.



rick


Bruce in alaska
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