Sailaway wrote in news:FX_Rg.4099$KF6.3557
@newsfe09.lga:

Just curious... Don't the marine radars have blanking, an adjustable
point where the radar emits no signal - so a mast wont reflect directly
back into the mast-mounted antenna? Or have masking, where the receiver
ignores returns that are too close - like from the mast or rigging?


otnmbrd wrote back:
no...... in fact, not too many shorebased sets do either although some
will have a setup where the actual picture is blanked out.

Interesting. I brought it up because way back when I was in the Navy *A
school* for avionics I remember learning about those things. Never
worked on radar though, except the ancient ones in the school.

otnmbrd wrote:
Sailaway wrote in news:FX_Rg.4099$KF6.3557
@newsfe09.lga:

Just curious... Don't the marine radars have blanking, an adjustable
point where the radar emits no signal - so a mast wont reflect directly
back into the mast-mounted antenna? Or have masking, where the receiver
ignores returns that are too close - like from the mast or rigging?


no...... in fact, not too many shorebased sets do either although some will
have a setup where the actual picture is blanked out.

My Ray SL70 appears to have blanking set at 25 yards, though I can't
find it in the specs. I'll be talking to a tech soon and I'll ask him
about that.

We may be talking two different things here (and I'll let the more
knowledgeable correct me if I'm wrong).
I "know" of two types of blanking. In one case the transmitter is switched
off during a certain arc of the scanner rotation.
The other "blanking" is done by the computer to blank off the picture over
certain areas of the scan rotation where the radar image would just show a
more or less solid land return of no value to the viewer.
In both cases these are used on shore based sets and to be honest I can see
little if any value for underway shipboard use.
Where I am we are using a shorebased ECDIS system which blanks the radar
image overlay on the chart plotter over land though it does show the radar
image of the offshore islands.

otn


My Furuno 1623 has adjustable blanking, and it's a very inexpensive unit.
I'd be
quite surprised to find a consumer unit that didn't have this ability.

CWM

otnmbrd wrote:
The other "blanking" is done by the computer to blank off the picture
over certain areas of the scan rotation where the radar image would
just show a more or less solid land return of no value to the viewer.
In both cases these are used on shore based sets and to be honest I
can see little if any value for underway shipboard use.

I believe (dredging up details from a distant memory of school) this is
called *masking*, whereas blanking is turning off the transmitter during
part of the rotation. Blanking is to prevent high powered returns
bouncing back into the receiver from metal objects in very close
proximity like mast-mounted antennas, and masking is to remove unwanted
clutter. The masking I remember was performed by the receiver ignoring
returns that were too close in time from being transmitted, like
stern-mounted antenna returns from the mast or rigging. Modern radars
(as opposed to the ones I learned on) also have a computer-controled, I
think processing that can selectively mask several types of returns like
sea returns that cause clutter. Of course this is all from rusty
memories, so it could be incorrect.

In article ,
Sailaway wrote:

Just curious... Don't the marine radars have blanking, an adjustable
point where the radar emits no signal - so a mast wont reflect directly
back into the mast-mounted antenna? Or have masking, where the receiver
ignores returns that are too close - like from the mast or rigging?

They have "Blanking" designed into them that keeps the receiver turned
off, while the transmitter is on, and then allows a very short time
period to allow the reciver to turn back on to receive the echo from
the targets. This doesn't make the mast disappear, it just keeps
the receiver off while the transmit pusle is reflected off the
obstructions. It also doesn't keep the obstructions from attenuating
the returned echos before they get to the receiver, either.
It isn't that the mast is creating targets for the receiver to see, it
is the mast is attenuating the transmit and receive signals.

Bruce in alaska
--
add a 2 before @

On Mon, 25 Sep 2006 22:23:01 -0400, Sailaway
wrote:

Sailaway wrote in news:FX_Rg.4099$KF6.3557
:

Just curious... Don't the marine radars have blanking, an adjustable
point where the radar emits no signal - so a mast wont reflect directly
back into the mast-mounted antenna? Or have masking, where the receiver
ignores returns that are too close - like from the mast or rigging?


otnmbrd wrote back:
no...... in fact, not too many shorebased sets do either although some
will have a setup where the actual picture is blanked out.

Interesting. I brought it up because way back when I was in the Navy *A
school* for avionics I remember learning about those things. Never
worked on radar though, except the ancient ones in the school.

You may be thinking of the T/R cell which shorts the receiver out
during transmitter pulse time.
Then a swept gain control shapes the receiver gain with distance -
low for near targets, high for distant ones

Brian Whatcott Altus OK

Interesting, but I have to wonder.... why bother?.... and I don't doubt
there may be a "techie" answer, since if your radar is behind a mast,
without "blanking" you will get a shadow which is exactly what you'll get
"with" blanking (most ships have "shadow" diagrams and although I know
blanking wasn't done in the past I can't guarantee it's not being done now).
I have to wonder if your set may have this ability so that if it's used in a
shorebased application then "blanking" can be employed.

otn

"Charlie Morgan" wrote in message
...


My Furuno 1623 has two settings in the "system menu" one is for
"Transmit blanking start", and the other is for "transmit blanking
stop". The settings are in degrees of a circle. You set these on a
RADAR with a mast mounted array to stop transmitting during the arc
where the antenna is aimed at the mast, which is only a few inches
away from the antenna array.

http://www.furuno.com/Furuno/Doc/0/0IJR6NM0RIRK19KV99NH090O68/1623%20Operator%27s%20%20Manual.pdf

Pages 16 and 17 of the manual mention it

CWM

In article ,
Charlie Morgan wrote:

On Tue, 26 Sep 2006 19:40:02 GMT, Bruce in Alaska wrote:

In article ,
Sailaway wrote:

Just curious... Don't the marine radars have blanking, an adjustable
point where the radar emits no signal - so a mast wont reflect directly
back into the mast-mounted antenna? Or have masking, where the receiver
ignores returns that are too close - like from the mast or rigging?

They have "Blanking" designed into them that keeps the receiver turned
off, while the transmitter is on, and then allows a very short time
period to allow the reciver to turn back on to receive the echo from
the targets. This doesn't make the mast disappear, it just keeps
the receiver off while the transmit pusle is reflected off the
obstructions. It also doesn't keep the obstructions from attenuating
the returned echos before they get to the receiver, either.
It isn't that the mast is creating targets for the receiver to see, it
is the mast is attenuating the transmit and receive signals.

Bruce in alaska

My Furuno 1623 turns the transmit off and on, not the receiver.

CWM

Actually that is not true...The receiver is turned off, (disconnected)
from the antenna by the Transmit Pulse, as it goes thru the Ring
Circulator that connects the Receiver and Transmitter (Magnitron)
to the Antenna. Also, the Receiver LNB just behind the Ring Circulator,
on some XBand Marine Radars, is shut down by the the Transmitter Timing
Pulse, and doesn't come back on untill the Transmitted Pulse has died
off. Depends on the particular Radar Design.

Bruce in alaska
--
add a 2 before @

On Wed, 27 Sep 2006 14:11:43 -0400, Charlie Morgan wrote:

All I can tell you is what the manufacturer says, and they call the
two settings on my RADAR "Transmit blanking on" and "Transmit blanking
off"

It has always been my understanding (perhaps incorrectly) that
transmit blanking was offered as a safety feature where the scanner
was located too close to the helm position. If so, that would surely
indicate that the transmitter was being turned off, not the receiver.

On Wed, 27 Sep 2006 14:11:43 -0400, Charlie Morgan wrote:


All I can tell you is what the manufacturer says, and they call the
two settings on my RADAR "Transmit blanking on" and "Transmit blanking
off"

Wayne B wrote:
It has always been my understanding (perhaps incorrectly) that
transmit blanking was offered as a safety feature where the scanner
was located too close to the helm position. If so, that would surely
indicate that the transmitter was being turned off, not the receiver.


While Bruce sure sounds like he knows what he's talking about with some
modern units I remember from school it being transmit off during
blanking, but I learned on really old stuff and my memory of it is just
too rusty to trust. I will have to look for my books on it and get back
to you on this - may take a few weeks to find 'em though...
Wish I still knew the radar tech on the ship I worked on in Africa
(circa 1974), now *that* was a radar! It was one of the big weather
radars with an enormous antenna in a huge *golf ball* radome, an a
pretty large room full of transmitter.