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:

....
Radar cannot see further than the horizon.
The horizon depends on how far UP the antenna is mounted.

I know - MSEE and lots of experience in this band in military service.

I am more interested in the mechanics of SR arches than electronics.

From your SR31, we're up just above the flybridge. How far can YOU
see
with binoculars...6 miles? 5? Check it out.

If we had a 500KW military surface search radar on your arch, it could
"see" targets 6 miles away....maybe 8 or 9 if they are TALL targets like
lighthouses, radio towers, etc. So, what's the point?

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.

You're not as tall as a containership, who can use a more powerful radar.
So, why waste money on HIS radar transceiver??

From up there, how about the 4KW radome? Loading on both the arch and
your electrical system are lots less, also your wallet. 4KW has no
trouble making a target at 8 miles.

I AM talking about 4 kW, no more. What did you expect?

....
I know the big swinging radar horn impresses the other sailors and looks
cool like you're an aircraft carrier....but why?....(c;

I have no TV antenna, a 6" VHF antenna only, and no satellite dish
compensated for the movements ... sorry, no need to impress. I'd rather
be sailing anyway but it is not possible.


Besides, you're not going 400 knots like a jet. If you can see targets 2
miles away you have MINUTES to turn away...not seconds. That's far
enough for collision avoidance isn't it?

As I said above: It's all about resolution. Like seeing a small boat
without lights when going 25 knots in darkness.

Do you know what the actual permissible mechanical load would be on my
arch?

TIA

Marc

--
Switzerland/Europe
http://www.heusser.com
remove CHEERS and from MERCIAL to get valid e-mail

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.

In article ,
Marc Heusser alid
wrote:

I tried to find any specification throgh the local dealer, and he did
not even get an answer at Sea Ray in the States (neither yes nor no,
essentially along the lines of we do not supply it so we do not know):

I would like to have a 4'/22kg Radar antenna on a Sea Ray 315 Sundancer
Sport Spoiler, Molded Fiberglass (Seamless) w/Overhead Lighting.

OP he
I finally got an answer from SeaRay (my dealer asked a technician there,
so orally only, not in writing):
Yes, it should be ok to mount 22kg on the Spoiler.

Thanks for all your info on radars :-)

Marc

--
Switzerland/Europe
http://www.heusser.com
remove CHEERS and from MERCIAL to get valid e-mail

On Thu, 19 Jul 2007 11:47:10 +0200, Marc Heusser
alid wrote:

Yes, it should be ok to mount 22kg on the Spoiler.

Just as an FYI, on a boat it is known as a "radar arch".