Larry,
You brought up a good point, but your reasoning is incorrect. All marine
scanners have a 30 degree verticle radiation pattern, This is too compensate
for roll and heel. So, radiating a target dead in front is not an issue. The
restriction at close range is Pulse width and receiver turn on time. A RADAR
mile is 6.36 micro seconds. If you want to see a target 100 yards in front,
the RADAR set must transmit a pulse and turn on the receiver to catch the
echo in less than .31 micro seconds. That's a very tall order with a
magnetron, as they are not gated. They operate by dumping high voltage on
the cathode, which rings the hell out of the cavity. They turn off when the
cavity decides it no longer is excited and the receiver can not turn on
until there is no more energy being emitted from the magnetron. This is
becoming a very big issue in Europe at the moment. There now is a new
commercial regulation as of Jan. '06 specifically pointed at canal traffic
that stipulates that all new RADAR sets work at 50 meters. For exactly the
reason you mentioned in your post. Now that's tough to do.
Steve

"Larry" wrote in message
...
"Nicholas Walsh" wrote in
:

I've just bought a new Raymarine radar in the winter sales (hurray!).
Can anybody advise the correct height to mount the scanner on the
mast? My mast height is about 20m.



First my condolences. I've just replaced ours on "Lionheart" with the
4TH one in 3 years. The condensation INSIDE the radome just eats the
cheap potmetal the radar receiver box is mounted in, rotting all the
unsealed boards inside with copper-to-potmetal electrolysis. Raymarine
has replaced them free....but is this the way to make radars??

As to mounting it, there's a trade. You are a sailboat so nothing
happens very fast. 15 mile range is overkill at 8 knots as you won't be
there for 2 hours, yet. If you mount it high up, you get excellent
range. Sounds good, eh? Unfortunately, high up also has a tradeoff in
how CLOSE to the boat you can see that big, heavy, CG bouy in the
whiteout fog bank. High up, the radar's beam goes OVER the top of low-
down items, like bouys, and the closer they are, the worse they display.
So, I consider putting the radar antenna DOWN much more important to
safety, where the range is only 4-5 miles, but you can see the bouy 12'
in front of the bow just fine in the fog. About 10' off the water, no
more than 15' up is ideal.

Your cheap Raymarine uses a phased array scanner antenna made out of a
cheap piece of printed circuit board just etched with the antenna
phasing elements and stripline matching sections, all on the board. It
has a quite narrow horizontal beamwidth, but a quite wide vertical
beamwidth, which is great for sailboats because this antenna works well
heeled over to 20 degrees without being leveled by some gimbal
mechanism. We had one on a post mounted on the port corner of the stern
on an Endeavour 35 sloop and I could never see any range difference by
tilting the mount to level the antenna, much. The waves offshore are
what screw up the targets on the other side of them....

AIS is gonna fix all this....soon, I hope. Everyone needs a
transponder!....

http://www.aislive.com/
take a look.