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.

In article ,
"Steve Lusardi" wrote:

All marine scanners have a 30 degree verticle radiation pattern,

Well not quite "ALL"....actually the Furuno Spec is 25 Degrees, and has
been for Years....

Me

"Steve Lusardi" wrote in
:

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


Before the water in the dome rots the hell out of the Raymarine radar on
Lionheart, that little sucker can see the 4th boat down our dock on the
1/8 mile range! It even plots the dock correctly from our 20' antenna
on the mizzen. Pulse width must be picoseconds. I don't think it ever
gets very wide to keep resolution high and current drain low. Hell, the
scanner cable to the RL70CRC display where it gets its power from has
very small, long power conductors and most of the power has got to be
heating up the maggie filaments. I had a helluva time explaining to
some captains why a 2KW radar didn't draw more than 2KW off their
batteries. Some of them were afraid to turn 'em on without the engine
charging all that power!...(c;

AIS to the rescue! Need shore fixed stations with all up-to-date
obstruction data coming out of them....

On Fri, 06 Jan 2006 00:34:49 -0500, Larry wrote:

AIS to the rescue! Need shore fixed stations with all up-to-date
obstruction data coming out of them....

And that will tell you about the 16 ft Boston Whaler fishing in the
fog bank right in front of you?

Wayne.B wrote in
:

And that will tell you about the 16 ft Boston Whaler fishing in the
fog bank right in front of you?



No, and neither will the radar scanner at 55 ft as some suggest to get long
range. Boston Whalers with little metal are hard to detect.

Of course, if we were to make $99 AIS transponders MANDATORY, problem
solved.

On Fri, 06 Jan 2006 23:38:05 -0500, Larry wrote:

No, and neither will the radar scanner at 55 ft as some suggest to get long
range. Boston Whalers with little metal are hard to detect.

======================================

We have no problem picking up small boats with the scanner at 24 ft.

It is unlikely that mandatory AIS will ever become a reality for boats
under 30 ft or so, perhaps even larger.

No, and neither will the radar scanner at 55 ft as some suggest to get long
range. Boston Whalers with little metal are hard to detect.




Wayne.B wrote:
We have no problem picking up small boats with the scanner at 24 ft.

It is unlikely that mandatory AIS will ever become a reality for boats
under 30 ft or so, perhaps even larger.


And if it is made mandatory for pleasure boats, how many
people will still not have it, or forget to turn it on, or
leave it broken?

DSK

i am reminded of the guy with an early handheld plotting GPS walking around
in ten foot circles saying, "look, it tracks me exactly".....pre SA! I am a
relative newcomer to radar and use a digital RADAR-PC setup and during the
day i can imagine i am seeing all manner of things that show up on my
screen...if it is flat calm and the gain is high enough and the sewa clutter
is OFF....but at night in the fog healed 20Deg that dot that appears for one
scan...then mis and then two scans again then gone will have you staring at
the screen instead of looking ahead to see if it is a contact or a wave or a
ghost reflection from your rigging or even the bouy at 90degrees from the
blip. What about sidelobe reflections which are again reflected and
recieved...they are only interpretable after the fact...not a priori..? they
are lower in intensity and can look like a small contact in any place. The
receiver knows only when it switched from transmit to receive [time] and the
radial angle of the antenae at that time so a reflected signal appears only
to have been recieved from a distance equivelent to the total pathlength and
in a direction in a straight line perp to the face of the antenae at that
moment of capture...a ghost image. Then when you get a circular series of
large contacts you may well wonder what semi circular beast is ahead of
you.... read a book about the propagation of radar microwaves and see all
the ways a blip can mislead you and thank God the guy you almost mowed over
didn;t have radar and was keeping a lookout. Radar assisted collision are a
significant reality. Real life radar is a tool that must be interpreted and
i am finding out it takes a LOT of interpretation and experience to be able
to rely on it more so than your eyes. The mainbang is suppressed so you
don;t see the big donut around your boat extending for 200ft on a 1/2 mile
range...if you are at 1/8th mile you might see a target at 50ft but only if
the mainbang is not supressed and the gain turned way down and the sea
clutter way up to exponentially deminish the gain applied to close returns.
As for styrofoam cups....the intensity of an electromagnetic wave falls off
in a cubic [3rd power] manner relative to distance and the reflected wave
similarly diminishes but the part reflected is only that portion perfectly
perpendicular to the antenae...as it dips and turns on a weaving mast even
less of it is oriented in a 'perfect' manner. The intensity of the emmitted
electromagnetic field recieived by the antenna is so small it is a marvel
that modern electronics can even discriminate it from the background noise.

Now the clincher....what portion on the emitted signal would a round
styrofoam cup reflect from half amile away? hint, styrofoam is not a
reflector of electromagnetic energy..is it an insulator and absorbs
microwave energy. the only reflection would be from moisture in a thin
lhorizontal line...perpendicular to the antena and the relfected signal is
likely a billionth of the emmitted signal at best. Granted there are
galenium arsenide semiconductor equiped ultra low noise receivers that could
discriminate that SNR but at a few thousand dollars in the hands of a
relatively untrained operator the pleasure boat operators radar.....it makes
for good bench racing stories but little more. AND...if you really are
detecting the water on a birds wings i suggest you tune and adjust the radar
to pick up and discriminate larger targets...else they will be lost in the
clutter
In the process of ruining a 'story' i hope to have saved someones life by
stimulating you to really learn what a radar can and can't do...repeatably.
Quod erat...you know the rest of the story.
rick

"DSK" wrote in message
...
No, and neither will the radar scanner at 55 ft as some suggest to get
long range. Boston Whalers with little metal are hard to detect.




Wayne.B wrote:
We have no problem picking up small boats with the scanner at 24 ft.

It is unlikely that mandatory AIS will ever become a reality for boats
under 30 ft or so, perhaps even larger.


And if it is made mandatory for pleasure boats, how many people will still
not have it, or forget to turn it on, or leave it broken?

DSK

Wayne.B wrote:
On Fri, 06 Jan 2006 23:38:05 -0500, Larry wrote:


No, and neither will the radar scanner at 55 ft as some suggest to get long
range. Boston Whalers with little metal are hard to detect.


======================================

We have no problem picking up small boats with the scanner at 24 ft.

It is unlikely that mandatory AIS will ever become a reality for boats
under 30 ft or so, perhaps even larger.

It won't be mandatory for everything. Kayaks, 14' alu boats, logs.
Radar is still better.

Gaz