Live imaging transducers
Posted: Tue Oct 15, 2024 10:59 am
Beamwidth of forward and down modes of live imaging
Lets get rid of the myth of these nice clean-cut pictures of what beamwidths are used.
Here we go.
The beam on the left is the theoretical image of what we are asked to believe. The reality is the beam on the right. Not so clearcut is it?
Remember that the manufacturers are trying to depict 3 dimension reality on a 2 dimensional screen so one of the dimensions can’t be represented.
For those who remember high school drafting, we needed a isometric view to do that. Like so. A 3 dimensional object.
Does our 3 dimensional image of forward/down mode beams actually look like the isometric view above?
Yep, it sure does. In fact it looks even weirder. It looks like this. Ever seen one of these? The Hindenberg Zeppelin of 1937 before it crashed (told you there was nothing new in this technology).
So..how do we depict a 3D object like a zeppelin onto our 2 dimensional image of beamwidth?
We use a device called a polar diagram.
here's a typical 20 degree sonar beamwidth. Not necessarily exactly the same as the ones used in live imaging but typical.
See the zeppelin shape with “rudders” and fat elongated sausage body? Bit fatter than 20 degrees isn’t it.
These extra “rudder shapes” are called lobes with 12 minor ones (brown numbers) and the main lobe making 13. THAT many? Yep, and THAT’S a whole lot different to the nice clean-cut image of the 20 degree beamwidth quoted.
So what are the 2 red dots? They’re barramundi and they’re outside the 20 degree triangle but they’re showing up on our screen. They’re in minor lobe 6 so will send a return for display.
And that is the reality vs the theoretical and its also what a lot of yanks get their knickers in a knot about. The “ghost fish images outside the supposed beamwidth”.
One of these manufacturers with a new model transducer uses a claimed beamwidth of 20 degrees but has added extra "crystals" to its new model.
The result is a transducer model a little bit different to its predecessor in that the extra crystals result in more radiated sonar power to sharpen the sausage a bit (little bit thinner) and also a little bit longer by increasing the radiated sonar power a bit into the main lobe. 3db more radiated signal is twice radiated power so means less fuzzy but slightly clearer images.
Then claimed that this "new" transducer improved the target resolution by a factor of nearly twice. We know that claim is simply incorrect don't we from last time.[/b]
The perspective/landscape/scout mode beamwidth is much wider so signal lobes are fatter.
The angle is indeed supposed to be 135 so the 2 red radials should be at 067.5 and 292.5 degrees. (Too messy to change the lines).
Polar diagrams are also used for the other beamwidth images like down-scan, side-scan, straight sonar (anywhere that beamwidths are quoted including radar scanners, radiocommunications antennas, UV radiation machines so there’s nothing new in polar diagrams either. Told ya).
Happy to talk about any of this stuff but separately so not to clog the thread.
Lets get rid of the myth of these nice clean-cut pictures of what beamwidths are used.
Here we go.
The beam on the left is the theoretical image of what we are asked to believe. The reality is the beam on the right. Not so clearcut is it?
Remember that the manufacturers are trying to depict 3 dimension reality on a 2 dimensional screen so one of the dimensions can’t be represented.
For those who remember high school drafting, we needed a isometric view to do that. Like so. A 3 dimensional object.
Does our 3 dimensional image of forward/down mode beams actually look like the isometric view above?
Yep, it sure does. In fact it looks even weirder. It looks like this. Ever seen one of these? The Hindenberg Zeppelin of 1937 before it crashed (told you there was nothing new in this technology).
So..how do we depict a 3D object like a zeppelin onto our 2 dimensional image of beamwidth?
We use a device called a polar diagram.
here's a typical 20 degree sonar beamwidth. Not necessarily exactly the same as the ones used in live imaging but typical.
See the zeppelin shape with “rudders” and fat elongated sausage body? Bit fatter than 20 degrees isn’t it.
These extra “rudder shapes” are called lobes with 12 minor ones (brown numbers) and the main lobe making 13. THAT many? Yep, and THAT’S a whole lot different to the nice clean-cut image of the 20 degree beamwidth quoted.
So what are the 2 red dots? They’re barramundi and they’re outside the 20 degree triangle but they’re showing up on our screen. They’re in minor lobe 6 so will send a return for display.
And that is the reality vs the theoretical and its also what a lot of yanks get their knickers in a knot about. The “ghost fish images outside the supposed beamwidth”.
One of these manufacturers with a new model transducer uses a claimed beamwidth of 20 degrees but has added extra "crystals" to its new model.
The result is a transducer model a little bit different to its predecessor in that the extra crystals result in more radiated sonar power to sharpen the sausage a bit (little bit thinner) and also a little bit longer by increasing the radiated sonar power a bit into the main lobe. 3db more radiated signal is twice radiated power so means less fuzzy but slightly clearer images.
Then claimed that this "new" transducer improved the target resolution by a factor of nearly twice. We know that claim is simply incorrect don't we from last time.[/b]
The perspective/landscape/scout mode beamwidth is much wider so signal lobes are fatter.
The angle is indeed supposed to be 135 so the 2 red radials should be at 067.5 and 292.5 degrees. (Too messy to change the lines).
Polar diagrams are also used for the other beamwidth images like down-scan, side-scan, straight sonar (anywhere that beamwidths are quoted including radar scanners, radiocommunications antennas, UV radiation machines so there’s nothing new in polar diagrams either. Told ya).
Happy to talk about any of this stuff but separately so not to clog the thread.