One point Larry missed: That big array will focus the transmit power available.
So 4 KW with a small array will get the same range as 2KW with a small array.

Right Larry?

Also: even in typical power boat, how often do you care about traffic more than
a mile a or two away? I'd rather have really good results close in and NOTHING
more than a couple of miles away (I do boat in an area where there's very little BIG
boat traffic. Maybe a dozen cruise ships a year, and they come in after my boat's
on the trailer on the way home, and leave while I'm diving.


"Larry" wrote in message ...
"luc" wrote in news:1162924151.581326.150280
@m7g2000cwm.googlegroups.com:

A few questions. Is there another small radar that is good?

You get what you pay for. It's that simple. More power has more
range..IF it's up high. It can't see over the horizon. Its horizon is
the same as yours on a clear day. The difference is it can see through
fog and in the dark. The stronger its transmitter, the stronger its
returns from the targets and the strong transmitter making stronger
returns can see more at the same distance in bad weather. Read that,
smaller targets, too.

Is the
quality of the radar directly related to the size of the radome?

The larger the antenna array, the narrower its beamwidth (horizontally).
The narrower the beamwidth, the better it is at resolving the target's
position. A wider beam makes an arc of a target on the display, from
angle it starts seeing the target to the angle it loses the signal from
the target. A bigger antenna, with a much narrower beamwidth, cause a
smaller target on the display, raising the accuracy of the bearing of the
target. The narrower beamwidth also turns one wide target on Radar A
into 4 distinct targets from the big antenna on Radar B. B simply
resolves the targets as individuals because the beam is narrow enough to
stop receiving one target before starting to receive the next as it
sweeps by. So, instead of a blob, you get 4 dots showing better
positions. It's all about horizontal beamwidth. We want narrow
horizontal beamwidth but WIDE vertical beamwidth. If the vertical
beamwidth is too narrow, when your boat heels over or pitches and rolls,
none of the beam that's so narrow lights up the target, so no signal
returns and he doesn't show up on the screen. These tilting mounts sound
nice and do, slightly, improve target painting, but the radars' vertical
beamwidth is so wide, by design, you'll see the target out the beams
heeled over 35 degrees, anyways. The tilting mount centers the RANGE of
the allowable vertical tilt it will tolerate.

What
are the pros and cons of locating a radar on short mast aft, as many
cruisers have, or on the main mast of a sloop?


That's easy. Climb up to where you think you're going to put the radar
while drifting in the harbor. How far can you see? That's how far the
radar can see. It sees targets over the horizon that are tall enough to
come up above the horizon, like tall TV towers, big buildings, water
towers, lighthouses. If you can't see it, the radar can't, either, no
matter what its antenna size and power. It's not magic or clairvoyant.

There's another problem. As you raise up the antenna higher and higher
to see that ship 32 miles away, the vertical beamwidth ends at a higher
and higher altitude, farther and farther from the boat CLOSE IN. From
the top of the mast, that big bouy you're about to run into disappears
from the screen in the fog because the bottom of the radome is a radar
shield to keep from cooking the kids' brains on deck. So, the higher the
radar is located, the further out from the boat the close in targets
disappear because you're shooting the signal right over the top of them.
In a sailboat, I don't get too excited seeing a target over 6 miles away
unless it's doing over 100 miles per hour. What I get excited about is
seeing that damned Bouy in the middle where the two channels intersect in
the fog....you know....so I don't run over it and scrape up the gelcoat,
looking like a complete fool...(c; I always thought it unfortunate
someone doesn't make a sector scanning radar to mount on the bow that can
see only 1000 yards in an arc of 120 degrees, straight ahead. It would
have a very short pulse length, which is the other limit on how close the
radar can see the target from the boat. Radar travels at 300 meters per
microsecond. During the time the transmitter is transmitting, the
receiver is shorted out to protect its sensitive receiver
electronics...on every pulse, we hope. If the transmitter is on for 1
microsecond, any target's echo less than 150 meters away (out, reflect,
come back), comes back while the transmitter is on and the receiver is
off. So, no target is received. If the pulse width (transmitter on
time) is .1 microseconds, the distance is 15 meters and way too late to
turn...(c; Modern radars adjust their pulse widths and the number of
pulses per second (repetition rate) as you reduce range, because close
targets don't need so much RF power so wide to see them with the display
set so close. (Your sonar also works this way, but at sound speed in
water, lots slower.) The pulse width also determines how much resolution
your radar sees on targets close together in line with your signal. If
two boats are in line with your sight and 100 meters apart, the wide
pulsewidth shows one thick target. Narrow pulsewidths resolve them as
two targets because the signal that bounces back shuts down from the
close target before the signal from the outer target starts, leaving a
gap in signals, and a resulting gap in display blip.

In a slow sailboat, compared to something going 30+ knots draining the
tanks, I think 25 ft up is a good compromise between range out 6 miles
and range close in on that nasty bouy with the gelcoat cutting
barnacles....(shudder)

Of course, once the proper short transmitter is beaconing all the bouys
and obstructions, and the boats are forced to either beacon their AIS
data or stay at the dock, all this becomes moot....Watch this:
http://www.aisliverpool.org.uk/currentmap.php?map=48
Just move your mouse pointer over any ship in the Irish Sea and look at
what AIS is all about. Radar's obsolete...It's time America joined the
21st Century. If you can afford to go to sea, you can afford an AIS
transponder, which will get cheaper and cheaper if the market were
expanded rapidly.



Larry
3rd mate engineering, S/V "Lionheart"....
A boat can never have too many electronic gadgets....(c;