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My Radar question.
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. |
My Radar question.
"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. |
My Radar question.
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 |
My Radar question.
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 question.
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 |
My Radar question.
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 @ |
My Radar question.
"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. |
My Radar question.
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? |
My Radar question.
Sailaway wrote in news:FX_Rg.4099$KF6.3557
@newsfe09.lga: Just curious... Don't the marine radars have blanking, an adjustable point where the radar emits no signal - so a mast wont reflect directly back into the mast-mounted antenna? Or have masking, where the receiver ignores returns that are too close - like from the mast or rigging? no...... in fact, not too many shorebased sets do either although some will have a setup where the actual picture is blanked out. |
My Radar question.
On Mon, 25 Sep 2006 10:52:50 -0700, RW Salnick
wrote: Brian Whatcott inscribed in red ink for all to know: On Mon, 25 Sep 2006 13:00:13 GMT, "bushman" wrote: How much does the mast effect the picture when the radar is mounted on the stern? How big of a slice does the mast block when the radar is mounted to the mast? Dang that's two questions. - Allen Endeavour 37 sloop about 22' to mast 8" cross section budget radar unit undetermined. Rough estimate: mast 2/3 ft. distance 22ft that's 1 in 33 or less than 2 degrees masking. Mast mount - not enough data, but lets suppose an equivalent scanner width of 2 ft at a distance of 2 ft: inv cos 0.5 about 60 degrees mast 2/3 masking 2 ft so 1/3 reduction in signal. Roughly (You bet!) a 1/3 reduction in echo strength in a 60 degree segment (hardly noticeable) but watch out for detector overload Brian Whatcott Altus OK My radar is mounted on a mount on the front of my mizzen. There is a small, permanent artifact directly in front, but there is a huge swath of missing sensitivity behind me... I once missed a ferry (until I heard its horm) behind me - completely invisible until it got to one side... bob A small effective scanner aperture and/or a big close mast diameter could do that to you, no doubt! Ouch.... Brian Whatcott Altus OK |
My Radar question.
Sailaway wrote in news:FX_Rg.4099$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. |
My Radar question.
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. |
My Radar question.
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 |
My Radar question.
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. |
My Radar question.
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 @ |
My Radar question.
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 |
My Radar question.
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 |
My Radar question.
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 @ |
My Radar question.
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. |
My Radar question.
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. |
My Radar question.
In article ,
Charlie Morgan wrote: On Wed, 27 Sep 2006 15:19:41 -0400, Wayne.B wrote: 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. In the manual for my RADAR they indicate that this feature is to turn the transmit off during the period when it would be transmitting directly into the mast on a mast mounted radome. That's all I can go by. That's what the people who made it say about it. The manuals for Furuno RADARs are available on their website for download if you want to see it. Mine is model 1623. The transmit blanking feature is on page 16 or 17, I think. CWM Furuno writes those USER Manuals, for USERS, not for the guy who actually has to FIX them, or understand how they work, and why. Transmitter Blanking is just another "Feature" that Furuno has incorporated into their design, to give the digital displays a way to do what wasn't really nessesary with the older second and third generation analog Radar displays. It is just like when we went from 2nd generation to 3rd generation, with Ring Circulators, replacing the older technology Radioactive T/R Cells, and when we went from Crystal Mixers to MMIC Technology in the 3rd to 4th generation change. You can thank Uncle Sam for all this wonderous advancment, as it all came from Military and Space Technology. Bruce in alaska -- add a 2 before @ |
My Radar question.
Bushman,
I have a mast mounted radar, Raymarine 2kw, on a swivel to keep it level while heeled. I don't experience any blank spots behind me. For example, if there is land behind me, it is uninterrupted. I do experience that many boats don't at all appear on my radar, from any direction, (because they don't reflect enough energy) unless I manually turn up my gain to 60%, which causes other things to appear. If I had it to do again, I would get a 4kw radar so I can get a better return from some of the smaller boats in our local waters. I would also consider mounting the radar off my backstay. Then my jib sail and spinnaker halyards would have one less thing to get tangled around |
My Radar question.
Thanks Capt.
I was considering the hanging backstay mount but my backstay will soon be an antenna. I would think that is a bad combo, maybe not. - Allen "b393capt" wrote in message oups.com... Bushman, I have a mast mounted radar, Raymarine 2kw, on a swivel to keep it level while heeled. I don't experience any blank spots behind me. For example, if there is land behind me, it is uninterrupted. I do experience that many boats don't at all appear on my radar, from any direction, (because they don't reflect enough energy) unless I manually turn up my gain to 60%, which causes other things to appear. If I had it to do again, I would get a 4kw radar so I can get a better return from some of the smaller boats in our local waters. I would also consider mounting the radar off my backstay. Then my jib sail and spinnaker halyards would have one less thing to get tangled around |
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