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

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

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 @

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.

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.