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.
Live imaging transducers
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ronje
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Live imaging transducers
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Ronje
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ronje
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Re: Live imaging transducers
Problems observing interaction between lure and fish
The prime intention is to see the interaction between the fish and the lure as it happens.
The angler needs to see both simultaneously and the fish will mostly be easier to see
than the lure ‘cos its bigger with a larger sonar reflection.
The main problem.
Note 2 white circles. Top one is a 7inch Castaic SP swimbait on retrieve. Bottom one is
a 1m barramundi on intercept path.
Look at the difference in reflected signal displayed on the screen. Barra image is “fuzzy”
but large. Lure image can barely be seen (small) and also “fuzzy”.
The lure is practically invisible due to its poor return/reflection so how on earth are we to
see the interaction between fish and lure if we can’t see the bl..dy lure!
We need to make the lure reflection more visible.
Here's a 175mm soft plastic swim bait. Very popular size.
On splashdown following a cast, the water disturbance is substantial and can easily be
seen by the sounder out to 15 metres range depending on how big the splash is.
As the lure sinks headfirst through the water column following a cast, the length of the
lure is exposed to the sounder so it’s large side reflection area is easily seen.
The lure is sinking nose first and “side-on” exposing up to all 2400 sq mm of surface area to the
sounder beam.
Then it reaches the bottom (or the depth you want) and you start retrieving. Which
profile does the sounder see now? The large side profile still?
No, it sees the much smaller cross section (max 250 sq mm) profile as it’s retrieved
towards you nose first.
Now that smaller head-on smaller reflective area is all that the sounder can see and
THAT head-on profile determines how much signal is reflected from it on retrieve. NOT
the length.
As a result, the size of the reflection drops and sometimes disappears until it gets closer
to the boat. The perspective, scout, landscape image above is at 8 metres range, the
lure signal is still poor so we need to make it bigger.
The 7inch lure is almost invisible.
Remember the profile “rule of thumb” for most lures. Side profile = approx. 8-10 times
the x-section (head-on) profile.
THAT’S why fishos go for bigger lures which enable the increased cross-section profile
(as its retrieved towards you) to be seen better by the sounder.
NOT because big lures attract the attention of big fish. They do and that’s nice but its
incidental to the live imaging concept.
So you need thicker lures (not longer ones) if you want the sounder to see them better.
So lets use an even bigger lure. Maybe 9 or 11 inch. Its thicker but only marginally.
That size needs a “treble harness” (with more sharp points than a porcupine) making it a
$50 lure with more exposed metal than a lattice crane on a high-rise building site. I’d be
reluctant to put it in the water (if I ever got it set up with harness and without landing in
hospital with treble in hand) in case a fish or snag took it. Geez!
Some anglers may have been tempted by “bait popping” paste with reflection –
enhancing qualities and “designed for live sonar” fishing. It looks like the tooth fairy has
plenty in stock so get yours while stocks last.
There are very good engineering reasons that counter Mr Tooth Fairy’s claims about live
sonar but he’ll probably continue to catch more fishermen than fish just like in times
past. Shades of moon/tide bite times.
Why not attach something to the nose of the lure to increase the size of the head-on
reflection on retrieve but that isn’t heavy enough to upset the lure swimming stability?
I’ve investigated a spinner blade to do the job. It could easily have been a chatterbait
blade or similar. Fishermen are innovative and resourceful and will come up with a lot
more and better ideas than that.
What do some gurus, experts, retailers and advisers need to take away from this?
To provide better service to your "customers", take the time to undertake some research, gain a bit of relevant knowledge and stop guessing.
The prime intention is to see the interaction between the fish and the lure as it happens.
The angler needs to see both simultaneously and the fish will mostly be easier to see
than the lure ‘cos its bigger with a larger sonar reflection.
The main problem.
Note 2 white circles. Top one is a 7inch Castaic SP swimbait on retrieve. Bottom one is
a 1m barramundi on intercept path.
Look at the difference in reflected signal displayed on the screen. Barra image is “fuzzy”
but large. Lure image can barely be seen (small) and also “fuzzy”.
The lure is practically invisible due to its poor return/reflection so how on earth are we to
see the interaction between fish and lure if we can’t see the bl..dy lure!
We need to make the lure reflection more visible.
Here's a 175mm soft plastic swim bait. Very popular size.
On splashdown following a cast, the water disturbance is substantial and can easily be
seen by the sounder out to 15 metres range depending on how big the splash is.
As the lure sinks headfirst through the water column following a cast, the length of the
lure is exposed to the sounder so it’s large side reflection area is easily seen.
The lure is sinking nose first and “side-on” exposing up to all 2400 sq mm of surface area to the
sounder beam.
Then it reaches the bottom (or the depth you want) and you start retrieving. Which
profile does the sounder see now? The large side profile still?
No, it sees the much smaller cross section (max 250 sq mm) profile as it’s retrieved
towards you nose first.
Now that smaller head-on smaller reflective area is all that the sounder can see and
THAT head-on profile determines how much signal is reflected from it on retrieve. NOT
the length.
As a result, the size of the reflection drops and sometimes disappears until it gets closer
to the boat. The perspective, scout, landscape image above is at 8 metres range, the
lure signal is still poor so we need to make it bigger.
The 7inch lure is almost invisible.
Remember the profile “rule of thumb” for most lures. Side profile = approx. 8-10 times
the x-section (head-on) profile.
THAT’S why fishos go for bigger lures which enable the increased cross-section profile
(as its retrieved towards you) to be seen better by the sounder.
NOT because big lures attract the attention of big fish. They do and that’s nice but its
incidental to the live imaging concept.
So you need thicker lures (not longer ones) if you want the sounder to see them better.
So lets use an even bigger lure. Maybe 9 or 11 inch. Its thicker but only marginally.
That size needs a “treble harness” (with more sharp points than a porcupine) making it a
$50 lure with more exposed metal than a lattice crane on a high-rise building site. I’d be
reluctant to put it in the water (if I ever got it set up with harness and without landing in
hospital with treble in hand) in case a fish or snag took it. Geez!
Some anglers may have been tempted by “bait popping” paste with reflection –
enhancing qualities and “designed for live sonar” fishing. It looks like the tooth fairy has
plenty in stock so get yours while stocks last.
There are very good engineering reasons that counter Mr Tooth Fairy’s claims about live
sonar but he’ll probably continue to catch more fishermen than fish just like in times
past. Shades of moon/tide bite times.
Why not attach something to the nose of the lure to increase the size of the head-on
reflection on retrieve but that isn’t heavy enough to upset the lure swimming stability?
I’ve investigated a spinner blade to do the job. It could easily have been a chatterbait
blade or similar. Fishermen are innovative and resourceful and will come up with a lot
more and better ideas than that.
What do some gurus, experts, retailers and advisers need to take away from this?
To provide better service to your "customers", take the time to undertake some research, gain a bit of relevant knowledge and stop guessing.
You do not have the required permissions to view the files attached to this post.
Regards
Ronje
Ronje
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