More theoretical than anything ...

A cold night's discussion evolved into: if my radar is picking up a
target (a buoy in the case of this discussion) at whatever range in a
dead fog, I'll lose it when it gets close enough to where I can
physically see it (50 yards was the distance mentioned, but you can't
see anything at 50 yds in a dead fog .. but you get the idea).

If that's the case; i.e., losing a radar target when you close on it;
is it merely the way radar works, or is the radome mounted incorrectly
or are there other variables I'm unaware of insofar as radar and
minimum range/tuning?

Thanks

Gary

wrote:

A cold night's discussion evolved into: if my radar is picking up a
target ... I'll lose it when it gets close enough to where I can
physically see it ...

If that's the case; i.e., losing a radar target when you close on
it; is it merely the way radar works, or is the radome mounted
incorrectly or are there other variables I'm unaware of insofar as
radar and minimum range/tuning?

The simplest explanation is that the height of the transmitter (aka, radome)
enters into the equation.

--
Good luck and good sailing.
s/v Kerry Deare of Barnegat
http://kerrydeare.home.comcast.net/

On Tue, 18 Jan 2005 14:48:17 -0500, "Armond Perretta"
wrote:

If that's the case; i.e., losing a radar target when you close on
it; is it merely the way radar works, or is the radome mounted
incorrectly or are there other variables I'm unaware of insofar as
radar and minimum range/tuning?

The simplest explanation is that the height of the transmitter (aka, radome)
enters into the equation.

================================

Antenna height is part of the issue but there is also a minimum
turnaround time from transmit to receive which enters into it. 25 to
50 yards is about normal in my experience. At that range you should
be able to guage your required offset and steer a straight course to
the proper side of the buoy, picking it up again after you pass it.

In article .com,
wrote:

More theoretical than anything ...

A cold night's discussion evolved into: if my radar is picking up a
target (a buoy in the case of this discussion) at whatever range in a
dead fog, I'll lose it when it gets close enough to where I can
physically see it (50 yards was the distance mentioned, but you can't
see anything at 50 yds in a dead fog .. but you get the idea).

If that's the case; i.e., losing a radar target when you close on it;
is it merely the way radar works, or is the radome mounted incorrectly
or are there other variables I'm unaware of insofar as radar and
minimum range/tuning?

Thanks

Gary


There are a number of things that make up MRR (Minimum Radar Range)

1. The PRR (Pulse Repition Rate) and PL (Pulse Length) of the
Transmitter, and the RGD (Receiver Gate Delay) of the
Receiver all effect the "Minimum Radar Range". The first
two you have no control over, but the RGD is usually set
when the radar is installed, for the vessels installation.
2. The VB (Vertical Beamwidth) of most Radar Antennas is in
the neighborhood of 25 Degrees. 12.5 above and 12.5 below
the Horizontal center of the antenna. If the antenna is
mounted up HIGH, (Like over 30 meters) like a Large ship
then the 12.5 degree lower Beamwidth will lose the target
much sooner that one mounted at sat 3 meters. So antenna
hight, and antenna VB, need to be taken into consideration
during installation.
3. Antenna Shading can also play a significant roll in MRR.
When a Radar antenna is place at the rear of a vessel,
and must look forward thru all the masts and rigging of
Say, a sailboat, then all that stuff deflects and obscures
the RF Pulse that the radar produces and the return echo
that it needs to show the target. The narrower the HB
(Horozontal Beamwidth) of the antenna, the better it will
decern targets, near or far. This is why Radar Antennas
should be mounted where they have a CLEAR, unobstructed
view, forward.


Hope that helps......


Bruce in alaska
--
add a 2 before @

On Tue, 18 Jan 2005 21:29:16 GMT, Bruce in Alaska
wrote:

In article .com,
wrote:

More theoretical than anything ...

A cold night's discussion evolved into: if my radar is picking up a
target (a buoy in the case of this discussion) at whatever range in a
dead fog, I'll lose it when it gets close enough to where I can
physically see it (50 yards was the distance mentioned, but you can't
see anything at 50 yds in a dead fog .. but you get the idea).

If that's the case; i.e., losing a radar target when you close on it;
is it merely the way radar works, or is the radome mounted incorrectly
or are there other variables I'm unaware of insofar as radar and
minimum range/tuning?

Thanks

Gary


There are a number of things that make up MRR (Minimum Radar Range)

1. The PRR (Pulse Repition Rate) and PL (Pulse Length) of the
Transmitter, and the RGD (Receiver Gate Delay) of the
Receiver all effect the "Minimum Radar Range". The first
two you have no control over, but the RGD is usually set
when the radar is installed, for the vessels installation.
2. The VB (Vertical Beamwidth) of most Radar Antennas is in
the neighborhood of 25 Degrees. 12.5 above and 12.5 below
the Horizontal center of the antenna. If the antenna is
mounted up HIGH, (Like over 30 meters) like a Large ship
then the 12.5 degree lower Beamwidth will lose the target
much sooner that one mounted at sat 3 meters. So antenna
hight, and antenna VB, need to be taken into consideration
during installation.
3. Antenna Shading can also play a significant roll in MRR.
When a Radar antenna is place at the rear of a vessel,
and must look forward thru all the masts and rigging of
Say, a sailboat, then all that stuff deflects and obscures
the RF Pulse that the radar produces and the return echo
that it needs to show the target. The narrower the HB
(Horozontal Beamwidth) of the antenna, the better it will
decern targets, near or far. This is why Radar Antennas
should be mounted where they have a CLEAR, unobstructed
view, forward.


Hope that helps......


Bruce in alaska


Nice post, Bruce!

Brian W

Bruce in Alaska wrote:
In article .com,
wrote:

More theoretical than anything ...

A cold night's discussion evolved into: if my radar is picking up a
target (a buoy in the case of this discussion) at whatever range in
a
dead fog, I'll lose it when it gets close enough to where I can
physically see it (50 yards was the distance mentioned, but you
can't
see anything at 50 yds in a dead fog .. but you get the idea).

If that's the case; i.e., losing a radar target when you close on
it;
is it merely the way radar works, or is the radome mounted
incorrectly
or are there other variables I'm unaware of insofar as radar and
minimum range/tuning?

Thanks

Gary

Bruce,

Jeez ... you oughta teach ...

Recreational boat, height say 12 feet ... would making a change in the
angle of the radome be wise? In other words, would it make sense it to
have a unit that
could be maneuvered up and down (up being level, down being a degree or
two difference from ideal)?

For that matter, would "up" increase range (obviously losing minimum
read)?

Gary


There are a number of things that make up MRR (Minimum Radar Range)

1. The PRR (Pulse Repition Rate) and PL (Pulse Length) of the
Transmitter, and the RGD (Receiver Gate Delay) of the
Receiver all effect the "Minimum Radar Range". The first
two you have no control over, but the RGD is usually set
when the radar is installed, for the vessels installation.
2. The VB (Vertical Beamwidth) of most Radar Antennas is in
the neighborhood of 25 Degrees. 12.5 above and 12.5 below
the Horizontal center of the antenna. If the antenna is
mounted up HIGH, (Like over 30 meters) like a Large ship
then the 12.5 degree lower Beamwidth will lose the target
much sooner that one mounted at sat 3 meters. So antenna
hight, and antenna VB, need to be taken into consideration
during installation.
3. Antenna Shading can also play a significant roll in MRR.
When a Radar antenna is place at the rear of a vessel,
and must look forward thru all the masts and rigging of
Say, a sailboat, then all that stuff deflects and obscures
the RF Pulse that the radar produces and the return echo
that it needs to show the target. The narrower the HB
(Horozontal Beamwidth) of the antenna, the better it will
decern targets, near or far. This is why Radar Antennas
should be mounted where they have a CLEAR, unobstructed
view, forward.


Hope that helps......


Bruce in alaska
--
add a 2 before @

wrote:

Recreational boat, height say 12 feet ... would making a change in the
angle of the radome be wise? In other words, would it make sense it to
have a unit that
could be maneuvered up and down (up being level, down being a degree or
two difference from ideal)?

For that matter, would "up" increase range (obviously losing minimum
read)?

Up will not increase range. At 12 feet, your horizon is about 4 miles
away. At ranges beyond that, object are "falling" below the horizon, so
nothing is gained by aiming up.

The formula for the radar horizon is almost the same as visual, about
1.2 times the square root of the height in feet yields the distance in
nautical miles. The total distance is the sum of the horizons for the
two objects, so a twelve foot radar can see a twelve foot object about 8
miles away.

When your using the radar for navigating, remember that low objects,
like a beach, may not be visible over the horizon. What you think is
the edge of land may be a hill set back a mile from the beach.

FWIW, I set my radar halfway up the mast, at about 23 feet. This gives
me all the range I need, and still lets me see objects in to about 50
yards. I have a slight downward tilt to compensate for squatting under
power. The only reason to have the radar "runtime adjustable" would be
if the boat has an extreme squat at speed. And of course tender
sailboats can benefit from leveling mounts.

In article .com,
wrote:

Bruce,

Jeez ... you oughta teach ...

Recreational boat, height say 12 feet ... would making a change in the
angle of the radome be wise? In other words, would it make sense it to
have a unit that
could be maneuvered up and down (up being level, down being a degree or
two difference from ideal)?

For that matter, would "up" increase range (obviously losing minimum
read)?

Gary

Gary,
I do teach, and have taught many a budding Marine Electronics Techs,
as well as Commercial Operators and Skippers, for many years.

Changing the Pitch of the Antenna is only slightly effective, in lowering
a radars MRR. This is due to the Vertical Beamwidth being 25 or
so degrees. This beamwidth is expressed at the -3db points (Half Power)
of the Main Lobe of the Antenna Pattern. so moving it just a coule of
degrees isn't going to have a significant effect on MRR.

The Largest Factor, that can be adjusted for, in MRR, is RGD (Receiver
Gate Delay) and this is usually set via an internal control at
installation. If it is set to short, Close in targets will curve in
toward the center of the display on the Shortest Range. If it is set to
long, then close in targets will bow out from the center of the display
on the Shortest Range. It is a very small adjustment of only a few
microseconds, one way or the other.
This adjustment is required because the designer can't know how long the
interconnect cable between the Display and the RF Deck is going to be
in any installation and it must therefor be adjusted AFTER the cable is
installed, and cut for length, if nessesary. Some of the later models
of consumer radars, don't allow for the interconnect cable length to be
changed, and therefor the Factory Setting is good enough, but if the
installation requires ANY change in interconnect cable length, then the
RGD setting must be recalibrated for proper operation, as specified in
most installation and user manuals.

Years ago we installed an Xband Furuno Radar on the ForeMast of 260 Ft
Reseach vessel, that had an AftHouse design. This allowed the new Radar
to have a clear, unobstructed, view forward, so as to allow the best
possible sensitivity for seeing small Research Bouys at the langest
ranges. The interconnect cable was 5 or 6 times as long as what the
designers intended, and we had the Factory help us install a modification
of the RGD system to take that fact into account, during installation.

Bruce in alaska
--
add a 2 before @

Are we talking a big midchannel buoy with strong radar return or a mooring
buoy which is lost in the sea return (if ever seen).
G
wrote in message
oups.com...
More theoretical than anything ...

A cold night's discussion evolved into: if my radar is picking up a
target (a buoy in the case of this discussion) at whatever range in a
dead fog, I'll lose it when it gets close enough to where I can
physically see it (50 yards was the distance mentioned, but you can't
see anything at 50 yds in a dead fog .. but you get the idea).

If that's the case; i.e., losing a radar target when you close on it;
is it merely the way radar works, or is the radome mounted incorrectly
or are there other variables I'm unaware of insofar as radar and
minimum range/tuning?

Thanks

Gary