How much does the mast effect the picture when the radar is mounted on the
stern?
How big of a slice does the mast block when the radar is mounted to the
mast?
Dang that's two questions.
- Allen
Endeavour 37 sloop
about 22' to mast 8" cross section
budget radar unit undetermined.

"bushman" wrote in message
. ..
How much does the mast effect the picture when the radar is mounted on the
stern?
How big of a slice does the mast block when the radar is mounted to the
mast?
Dang that's two questions.
- Allen
Endeavour 37 sloop
about 22' to mast 8" cross section
budget radar unit undetermined.

Depends. If you have a larger open array, not much, if any. If you have a
very small antenna, perhaps some. However, unless you are are on a
prefectly steady course (not likely), the shadow should not be an issue.

Thanks.
Looks like more stuff on the back rail, but I will have the advantage of
beeing able to trim the array to kep it horizontal.
Thanks again. - Allen

On Mon, 25 Sep 2006 13:00:13 GMT, "bushman" wrote:

How much does the mast effect the picture when the radar is mounted on the
stern?
How big of a slice does the mast block when the radar is mounted to the
mast?
Dang that's two questions.
- Allen
Endeavour 37 sloop
about 22' to mast 8" cross section
budget radar unit undetermined.


Rough estimate:
mast 2/3 ft. distance 22ft that's 1 in 33 or less than
2 degrees masking.

Mast mount - not enough data, but lets suppose an
equivalent scanner width of 2 ft at a distance of 2 ft:
inv cos 0.5 about 60 degrees
mast 2/3 masking 2 ft so 1/3 reduction in signal.

Roughly (You bet!)
a 1/3 reduction in echo strength in a 60 degree segment
(hardly noticeable) but watch out for detector overload

Brian Whatcott Altus OK

Brian Whatcott inscribed in red ink for all to know:
On Mon, 25 Sep 2006 13:00:13 GMT, "bushman" wrote:


How much does the mast effect the picture when the radar is mounted on the
stern?
How big of a slice does the mast block when the radar is mounted to the
mast?
Dang that's two questions.
- Allen
Endeavour 37 sloop
about 22' to mast 8" cross section
budget radar unit undetermined.



Rough estimate:
mast 2/3 ft. distance 22ft that's 1 in 33 or less than
2 degrees masking.

Mast mount - not enough data, but lets suppose an
equivalent scanner width of 2 ft at a distance of 2 ft:
inv cos 0.5 about 60 degrees
mast 2/3 masking 2 ft so 1/3 reduction in signal.

Roughly (You bet!)
a 1/3 reduction in echo strength in a 60 degree segment
(hardly noticeable) but watch out for detector overload

Brian Whatcott Altus OK

My radar is mounted on a mount on the front of my mizzen. There is a
small, permanent artifact directly in front, but there is a huge swath
of missing sensitivity behind me... I once missed a ferry (until I
heard its horm) behind me - completely invisible until it got to one side...

bob

In article ,
"bushman" wrote:

How much does the mast effect the picture when the radar is mounted on the
stern?
How big of a slice does the mast block when the radar is mounted to the
mast?
Dang that's two questions.
- Allen
Endeavour 37 sloop
about 22' to mast 8" cross section
budget radar unit undetermined.



Ok, I like the "Pencil - Eyeball" analogy, but you need to take
into account the Wavelength of the Radar and the Horizontal
Beamwidth of the Antenna.

Small noncommercial Marine Radars will have H. Beamwidths of say
5 or more degrees. This means that this radar can't really
differentiate Targets at the same range that are within 5 degrees of
each other. It also means that your mast shadows and reflects SOME
Transmitted Energy whenever the antenna is within +/- 2.5 Degrees
of the Mast and other Conducting Rigging. Now if the obstruction
is Multiple Wavelengths wide, it will present a much bigger obstruction
and loss of Transmitted Energy, by reflection, as well as a resulting
loss of Receive Sensivity in that direction. Since 99% of Marine Radars
in the Small Vessel Catagory, are XBand, Wavelength, is 3 cm. So if
your mast is more than 12cm wide is presents a measuable loss, or
shadow, in that specific direction.

Typically, targets within 4 miles, of ANY 3Kw or higher power, Marine
XBand Radar have a much higher SNR (Signal to Noise Ratio) than
what is considered the minimum required to paint a target. This
Means that it can stand a significant loss of transmitted power and
receive sensitivity and still paint a target in the directrion of the
obstruction. Where things get dicey, is when you trying to paint
a bouy out at the horizon, that has minimal Radar Crosssection, and the
mast is in the way.

Bruce in alaska
--
add a 2 before @

"bushman" wrote in
:

How much does the mast effect the picture when the radar is mounted on
the stern?
How big of a slice does the mast block when the radar is mounted to
the mast?
Dang that's two questions.
- Allen
Endeavour 37 sloop
about 22' to mast 8" cross section
budget radar unit undetermined.


The mast and rigging might reduce range a bit looking forward, but it
doesn't "BLOCK" the view because the antenna, unlike your eye, is not a
point source and receptor of RF energy...it has width.

Hold a pencil 6" in front of your nose. Look at something behind the
pencil. Can you see it? Is your view blocked so at some point you can't
see it? No. When one eye is blocked, the other eye can see around the
pencil to the object. Radar antennas, even the little cheap ones rotting
away inside the leaky radome, are like your eyes. When one side of the
antenna can't see through the mast, the other side can see around the
side of it. A weak target, one way off, might not get enough RF bouncing
off it around the mast to "see" it, but on a sailboat, any target over 5
miles away might as well be on the Moon it takes so long to get to it.
Worrying about targets at 16 miles is hilarious...(c; You need more to
see the bouy 400' ahead in the FOG.

My captain's old boat, now belonging to another friend of mine, is an
Endeavour 35 B plan. When it had a radar on it, I mounted it on a
tiltable platform mount on the port side where that rear handrail stacion
is just forward of the stern. The mast wasn't dead ahead of it, which is
why I put it as far to port as I could get it, looking around the
shrouds, a much less blocking target than the mast. Targets ahead were
visible from 22' off the water out 12 miles just fine.

His newer boat, an Amel Sharki 41 ketch, has a radar mount dead aft of
the roller furling main mast beast on the leading edge of the mizzen
mast. I can see a little dead zone from this arrangement, but with the
boat rolling around at sea, it just doesn't paint a target dead ahead all
the time. As soon as the yacht pitches port or starboard, the target
shows up fine until the autopilot has time to react and oversteer it past
the target once again...



--
There's amazing intelligence in the Universe.
You can tell because none of them ever called Earth.

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?

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.

On Mon, 25 Sep 2006 10:52:50 -0700, RW Salnick
wrote:

Brian Whatcott inscribed in red ink for all to know:
On Mon, 25 Sep 2006 13:00:13 GMT, "bushman" wrote:


How much does the mast effect the picture when the radar is mounted on the
stern?
How big of a slice does the mast block when the radar is mounted to the
mast?
Dang that's two questions.
- Allen
Endeavour 37 sloop
about 22' to mast 8" cross section
budget radar unit undetermined.



Rough estimate:
mast 2/3 ft. distance 22ft that's 1 in 33 or less than
2 degrees masking.

Mast mount - not enough data, but lets suppose an
equivalent scanner width of 2 ft at a distance of 2 ft:
inv cos 0.5 about 60 degrees
mast 2/3 masking 2 ft so 1/3 reduction in signal.

Roughly (You bet!)
a 1/3 reduction in echo strength in a 60 degree segment
(hardly noticeable) but watch out for detector overload

Brian Whatcott Altus OK

My radar is mounted on a mount on the front of my mizzen. There is a
small, permanent artifact directly in front, but there is a huge swath
of missing sensitivity behind me... I once missed a ferry (until I
heard its horm) behind me - completely invisible until it got to one side...

bob

A small effective scanner aperture and/or a big close mast diameter
could do that to you, no doubt! Ouch....

Brian Whatcott Altus OK