Steve Thrasher wrote in news:469d6401$1
@news.acsalaska.net:

Basically it boils down to: The higher the frequency of the beam, the
higher the frequency of the PRF (Pulse Repetition Frequency) and the
shorter your pulse length the better your resolution. Antenna size
only
affects the signal strength the receiver sees, for the most part.
Which
is assuming it's built for the proper frequency.



What you say IS true when trying to differentiate targets IN LINE with
the radar beam. However, it is the resolution caused by BEAM WIDTH that
allows you to differentiate targets PARALLEL to the sweep of the beam.

Let's look at two obviously extreme cases......on two antennas.

Antenna 1 has a horizontal beamwidth of 25 degrees, rediculously wide.
There are 3 ships visible traveling a course sort of parallel to ours but
only 100 yard apart following each other about 5 miles away. They're
travelling in our direction and to starboard of us. Our antenna rotates
clockwise, painting the front ship first. As the wide beam starts
bouncing off the front ship, a return shows at the 5 mile range ring on
the first sweep. But, due to the rediculously wide beam of our wide
antenna, the beam starts bouncing off the 2nd ship long before it has
stopped bouncing off the first ship, so the radar display now shows a
single long target, not two ships. Because the wide beam is now painting
the stern trailing ship behind ship 2, ship 3, long before the beam stops
bouncing off ship 2, even though it has stopped bouncing off ship 1, the
display shows one LONG ship....not 3 small ships traveling one behind the
other in single file. No matter what the PRF or pulse width or frequency
of the RF, it will show as one ship unless we can stop painting ship 1
before it paints ship 2 and stop painting ship 2 before it paints ship 3.
Pulse width will make the long blob narrower, the narrower the pulse, but
no pulse, no matter how narrow, will cure this resolution problem EXCEPT
a very narrow, focused beam with very little to no side lobes. Side
lobes painting the ships MUST return signals BELOW the radar receiver's
sensitivity setting or the original problem returns. Way too many radar
operators NEVER turn down receiver sensitivity enough to see there are 3
ships, not one, maybe until the blob on the screen becomes really
rediculously long. They always operate sensitivity up to the sea return
threshold.

Antenna 2 has a 1 degree beamwidth from its much better design, which,
unfortunately, makes it HUGE in comparison. That's why the big radar
antennas are SO WIDE spinning around atop the big ships. The wider the
slot or reflector, the narrower their beamwidth horizontally, where it
counts. This antenna starts painting ship 1, comes to the stern and
stops painting ship 1 a few milliseconds before it starts painting ship
2. It will show up the 3 ships as 3 ships, not one big blob, because of
its horizontal beamwidth, which has nothing to do with PRF, pulse width
or RF frequency.

But, do pleasure boats need to see 3 distinct targets at 5 miles? We're
not going to target ship 2 for the potato gun, are we? We only need to
see there's something out there and which way it's moving in relation to
our collision course with it. The watch will see it's 3 ships without
spending another $8000 on a big, wide open antenna array that makes the
boat rock with its mass and momentum....overkill. Doesn't that make
better sense?

Larry
--
AN/SPS-21, the old Raytheon Pathfinder. Sensitivity was awful. When I
pulled the waveguide off her, it FLUSHED! FLOODING THE DECK! Leaky
joints aloft....(c; The bottom joint needed a drain!...hee hee.
Nothing's more fun than 4 guys lifting the antenna off the rotary joint
in the middle of the Atlantic so the 5th guy can CHANGE THE DAMNED V-
BELT, which broke in the storm! I got the T-shirt!