Marc Heusser alid
wrote in :

In a word: Resolution. A radome antenna has 4 to 6 degrees horizontal
beam width, a 4' slot antenna less has 1.8 degrees. Makes all the
difference in tight places like rivers. Like seeing one or two ships.
Or
a small ship or sea clutter.
I would go as far as saying that horizontal resolution is the only
specification to worry about. What has not been captured by the antenna
is never going to show up on the display.


Ok, let's talk about the low vs high resolution antennas....

Low is simple, it has a huge beamwidth, relatively speaking. When the
boat is rolled over and the antenna towards the target is pointing,
mostly, into the sea, its wide vertical antenna pattern is more likely to
see "something" out 4 miles on this rotation. Because the target is "in
the beam" for a much longer length of time, as the beam rotates around
during this rolling/pitching motion of the boat, the receiver is more
likely to see the target for that brief instant the antenna isn't pointed
at the moon or Davy Jones' Locker, than it would be if the beam were
"really narrow" where, even if the antenna were mounted on a oil rig and
not pitching and rolling on a Sea Ray...31' or not. Every rotation of a
really narrow beam antenna only leaves the target illuminated with RF and
looking through that really narrow "slot" with the receiver for a much
shorter instance.

Try it. Touch your middle finger to your thumb making a big hole to look
through. Stand rigid on the rolling/pitching deck without trying to keep
your "sight" on the "target" way off in your vision. The target shows up
in the big hole all the time you're looking through it....and would
return our radar RF to the receiver looking through that hole so we'd get
a return on the screen.

Now, roll your forefinger up as tight as you can in the crook of your
thumb, making a small hole to simulate looking at the same target through
this massive, high resolution, antenna array. Point this hole at the
target, but stand rigid not trying to follow the target around with the
pitching and rolling of the boat. You can hardly see the target any more
as it rolls in and out of the hole. The only time the receiver will see
the target is when its rotating by the target synchronizes with the boat
being somewhat level, which happens less and less often as the hole gets
smaller.

Actually, the hole, the window the radar looks through is a narrow
vertical slot, narrow horizontally but, we hope with all that
rolling/pitching, WIDE vertically. But, you get the idea of what I'm
talking about.

Another low vs high phenomenon that's pretty easy to grasp is in waves
and troughs. The low-res antenna, the big hole, paints the target far
longer than the high-res, narrow beam antenna. Being wide, it has a far
better chance of seeing "something" if the waves are high because it
paints the target LONGER on each rotation. The very short painting the
high res big antenna does, has less chance of seeing the target around
the coincidental wave.

All this is why surface search radars on big ships, even Navy/CG ships,
is a relatively small, large aperture antenna, not some huge, high-gain
array with really high resolution....which is the targeting antenna for
the missiles/gunnery.

There's still that CLOSE IN target, the one you are about to run over....

It's ALWAYS much better to paint all the targets in that 1/8th mile
range, no matter whether it's the return from a 40hp Yamaha outboard, the
only radar reflector on his plastic fishing boat, or the little nun bouys
in the channel. The CLOSE IN targets really need to make a blob in the
rainstorm and fog....to hell with containerships 12 miles away. That's
why it's VERY important to keep the antenna LOW, not shooting over the
targets just so you can see that containership and brag about it to
"her".

Larry
--
While in Mexico, I didn't have to press 1 for Spanish.
While in Iran, I didn't have to press 1 for Farsi, either.
While in Florida, I had to press 2 for English.
It just isn't fair.