Harbin Osteen wrote:

Though you might not see targets farther, with a larger antenna you would
have a better resolution, and that would be a good thing, don't you think?

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.

I was on an ancient AD (Destroyer Tender, built during WWII, USS
Arcadia) and had a helluva discussion with a WO who was doing the CIC
thing when we entered the Chesapeake Bay and why the old radar wouldn't
enable me to determine where the bogey I was tracking went into and out
of a glob of traffic. Also he was sure I should be able to pickup
dolphins since the radar beam went over the edge of the ship, made a 90
degree down angle turn, another 90 degree angle turn at the surface, and
shot out over the surface. To was repeated for the echo. This on a
ship where the repeater would drop out because the AC volts supplied
dropped below 90.

Bill, who was a ETN2 in the old days and has forgotten most of this kind
of stuff.

On Tue, 17 Jul 2007 16:51:11 -0800, Steve Thrasher
wrote:
.....
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.
....
Bill, who was a ETN2 in the old days and has forgotten most of this kind
of stuff.

Hmmm...not quite.

Here's another try:
The higher the frequency, the smaller the antenna needed to shape the
beam. A wider antenna gives a narrower horizontal beam.

Pulse repetition frequency limits max range but helps angular
depiction on a plan position indicator. Narrow transmit pulse helps
resolution, hinders max range. Bigger antenna aperture increases max
receive range.

Your turn! :-)

Brian W

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!

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

I was on an ancient AD (Destroyer Tender, built during WWII, USS
Arcadia) and had a helluva discussion with a WO who was doing the CIC
thing when we entered the Chesapeake Bay and why the old radar wouldn't
enable me to determine where the bogey I was tracking went into and out
of a glob of traffic. Also he was sure I should be able to pickup
dolphins since the radar beam went over the edge of the ship, made a 90
degree down angle turn, another 90 degree angle turn at the surface,
and
shot out over the surface. To was repeated for the echo. This on a
ship where the repeater would drop out because the AC volts supplied
dropped below 90.

Bill, who was a ETN2 in the old days and has forgotten most of this
kind
of stuff.



Hi, Bill! Nice to meet another tender sailor! Yours would have been an
SPS-21 Pathfinder, too! We also had AN/SPS-6 air search until Navy got
tired of paying to fix it and we turned its nice gyro mount into a
fantastic TV antenna rotator.

Larry ET1 (ET-1598 Cal Tech)
USS Everglades (AD-24)
Started in WW2 but not completed until 1952 for Korean War.
I was aboard her from 1966-1969 Pier Papa, Charleston.
Two Med cruises, Cruises to JAX to do Yellowstone's work for them.
Radio 2 had TBK, TBL, TCS, RBA, RBB, RBO....all mine.
Callsign was Glitter Delta or my ham call at the time WB4THE/MM2 running
phone patch traffic for the crew to K4OKD back home.
KW7, KW37, AN/URC-32, R-390A....real comm equipment of the day.
Can you still hear the 37 beeping taking hits?....(c;

In article ,
Steve Thrasher wrote:

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.

Bzzzt, Wrong answer, would you like to try again for what is behind
Door #2??????

It is a good thing you have all that Military Experiencee, but it really
doesn't translate into Commercial Marine Radar Experience. These two
senerios are Apples and Oranges, as they use similar Technology, but are
doing Totally Different Jobs andd providing Totally Different Data
to the Operator.

In Commercial Marine Radars the Antenna Horozontal and Vertical Beamwidth
ARE very significant factors in the Resolution of Targets on the PPI
type displays used. These two Specs set the Basic Operating ability
of the system to differrentiate between targets, at the same distance,
but sperated by small Azimuth Angles. The PRR (Pulse Repetition Rate)
sets the Maximum Range, along with Pulse Length, and in Commercial
Marine Radar is usually Fixed in two or three settings, determined by
the Range Switch Setting. The "Frequency of the Beam" in Commecrial
Marine Radars is FIXED, by the Spec'd Magnitron and in the USA, only
Xband and SBand, are Licensable under CFR 47 Part 80. Since Sband
Radars are very LARGE, you only find them on Vessels, usually over 1600
Tons or bigger, with 95% of the Commercial Marine Radars opeerating
at 10Ghz in the XBand.

If one wants to discuss the relative merits of PPR, PL, as opposed to
Antenna Gain, Vertical and Horozontal Beamwidths in the designing of
Commercial Marine Radars, Great, but 99% of the folks reading these
posts, don't understand the language, and don't have a clue about the
concepts used in the design of the Equipment that they buy, and install
on their Vessels. They just look at, if the Radar shows them what's
out around them, and hope they aren't missing something coming their way.

Bruce in alaska a Longtime Marine RadioMan, but now just
an Old Fart......
--
add a 2 before @

Bruce in Alaska wrote:
Bzzzt, Wrong answer, would you like to try again for what is behind
Door #2??????

Sure lots of things were wrong, I did say I'd forgotten most of this
stuff. :-) Probably for good reason I might add, didn't put any beans
on my table.

But, the answer to what was being bandied about sort of boils down to
why the dish used for satellite service in Florida is a different size
than the one used in Alaska, ignoring all the boring stuff about
pitch/yaw and things like that.

Bruce in Alaska wrote in news:bruceg-
:

Bruce in alaska a Longtime Marine RadioMan, but now just
an Old Fart......
-

Bruce's radar still operates on a bedspring antenna on VHF....(c;

Now THAT's OLD!

Larry

In article ,
Larry wrote:

Bruce in Alaska wrote in news:bruceg-
:

Bruce in alaska a Longtime Marine RadioMan, but now just
an Old Fart......
-

Bruce's radar still operates on a bedspring antenna on VHF....(c;

Now THAT's OLD!

Larry

I actually DID work on one of those VHF Radars, as very young tech.
It was at a Military Museum, and I replaced the Dynamotor that suppiled
the HV for the Transmitter. I got the Dynamotor from a unit that was
"Scrapped in Place" half way up the Mountain at Alitak, Alaska on the
southern tip of Kodiak Island. It was installed during WWII, to do
Sea Search for the Japanese Navy approching Kodiak Island. I was able to
lug the Dynamotor, down off the mountain, on one of my trips to the top,
to service a Remote Base Marine VHF site I had installed at the summit.
It is amazing how far 25 Watts of VHF will go when the Antenna is at
3500 Ft. That Site was designed to work a Tropo Path, 235 Miles long,
to an associated Salmon Processor at Chignik, Alaska. Really worked
good most of the summer, untill a weather front would come thru and
dissrupt the Temprature Inversion Layer between the two locations.
Then it would go out, and it took a couple of days, to rebuild the
Inversion Layer, before the path would return. Now days, it is all
done by SatPhones, and Cellular. Oh well, MF/HF is just about DEAD,
as well, in the Maritime Community. Good thing I am reTIRED.....

Bruce in alaska
--
add a 2 before @