Mega Beam Angles

[Whistler]

Does anyone happen to know what the SI and DI 1.2MHz beam angles are and or what a plot of that looks like on a graph (side lobes etc.)? I'm just curious about the technical specs of it.

-jason

[tweakjr]

[Whistler]

Thanks tweak. I've seen that diagram, but it is deceptive. Unless something has changed, the beam angle for 455KHz is 86 degrees but the beam angle for 800KHz is not 86 degrees. It is something less (don't remember the exact angle). I suspect that 1.2MHz must also be even less than 800KHz but I haven't seen anything that shows what that angle is.

The resulting shape of the 800KHz beam angle beyond its nominal angle (something less than 86 degrees) results in null areas of beam/cone coverage where sensitivity to the send and return signal is down something like 10dB or more. My guess is that a 1.2MHz beam has a much smaller beam angle with many more null areas. But they may be close be tight enough to each other that the null areas are small enough not to have a significant negative impact on what is picked up by the sonar. Or they may blend the three different beams together to ensure full 86 degree coverage. However, the transducer can only be pulsed with one frequency at a time. So there is a chance things could get missed when the 800 and 1.2MHz pulses occur.

[Wayne P.]

Thanks tweak. I've seen that diagram, but it is deceptive. Unless something has changed, the beam angle for 455KHz is 86 degrees but the beam angle for 800KHz is not 86 degrees. It is something less (don't remember the exact angle). I suspect that 1.2MHz must also be even less than 800KHz but I haven't seen anything that shows what that angle is.

The resulting shape of the 800KHz beam angle beyond its nominal angle (something less than 86 degrees) results in null areas of beam/cone coverage where sensitivity to the send and return signal is down something like 10dB or more. My guess is that a 1.2MHz beam has a much smaller beam angle with many more null areas. But they may be close be tight enough to each other that the null areas are small enough not to have a significant negative impact on what is picked up by the sonar. Or they may blend the three different beams together to ensure full 86 degree coverage. However, the transducer can only be pulsed with one frequency at a time. So there is a chance things could get missed when the 800 and 1.2MHz pulses occur.

The spec coverage for the sonar pulses are measured away from dead center until the loss is 10 db (-10db). The actual coverage of any sound is a lot more at lesser strengths. That is why the SI pulses overlap under the transducer and a subject is recorded on both sides=no gap.
455 kHz physical coverage is more than 180 degrees left to right since each side covers from the water's surface to past vertical on both sides and the transducer is submerged. Ever seen jackplate echos on SI even when the transducer is 6"+ below the jackplate?

You are correct that the coverage of 800 and 1200 is less, same as 200 kHz has less coverage than 83 kHz 2D sonar and 83 has less coverage than 50 kHz.

The specs for the first generation SI units is 86 degrees per side for 455 kHz and 55 degrees per side for 800 kHz at the -10db strength.

[Whistler]

Ok.. So 86 and 55 degrees. Any idea what angle the 1.2MHz is?

[Wayne P.]

Ok.. So 86 and 55 degrees. Any idea what angle the 1.2MHz is?

Have not seen that in any specs.

[Hollada]

How much bottom will you see with Mega DI at a given water depth?

[OThessalonikios]

Beam angle specs are tricky. Never think of a sonar beam like a laser which has its energy focused in one area and then abruptly shuts off.
Instead envision it like a flashlight, bright in the center and slowly darkening as you move outward

Manufacturers spec the beam at a set point where the signal drops by a certain amount.
There is no universal standard for what that amount should be which is why comparing beam angles amongst manufacturers isn't easy.
-3dB, -6dB, -10dB are all used and I've seen calculations based on both 1 way and 2 way beam angles.
That means there could be 6 different calculated beam angles for the same sonar...

This isn't a horrible thing. What is horrible is getting so caught up into specs and forgetting that sound exists well outside of what we call "the beam".
In the flashlight analogy, it would be like telling someone "you can't see that object because it's at a place in the light 1/2 the intensity of the center", even though you can CLEARLY see it. Makes no sense right?
Sonar is no different. Objects outside the beam will appear if they are acoustically reflective enough and show in your image.
It's why you can pick up on the faint outline of boat hulls on the surface and why you shouldn't think that your SI can't see below you.
Beam angle drawings are a general representation of where the MAJORITY of the sound is.

Knowing this really simplifies how to set up your sonar.
People often try to make rules and equations (i.e: You want to always set your sonar to 5x the running depth, etc) but this isn't ideal because there are too many variables in sonar, nevermind the variables in calculating your beam.
Temperature, salinity, thermoclines, bottom hardness, etc, all play a massive role in how much range your sonar will achieve on a given day.
Instead of thinking about how much bottom you should see, just use your eyes and look at how much bottom you're actually seeing.
This will tell you where to set your range on any given day. Make the most of your screen, what echoes come back to the boat and what you want from the picture.

Examples:
1) If you are set to 125ft range and are only getting signal to 75ft, adjust your range to 75ft so not to have wasted screen. Even if all your rules tell you that 125 is what you should have at that depth, the variables above (especially bottom hardness) will have an effect on how much signal comes back.

2) If you are set to 100ft and have energy all the way to the edge of the screen, open it up some and take in more data. You'll clearly see where it drops off and keep it there

3) You're looking for big boulders over a massive flat with 455kHz. You get data to 300ft but it gets really smudgy. Set your sonar to where it is most useful to you. It's ok to shorten the range and leave signal off the screen if it's not of use for what you need. It's a sin to have a screen full of black on the outer edge though... Don't ever set a range too far if you're getting nothing back.


Don't limit yourself by thinking up beam angle calculations or placing rules on yourself for how to operate sonar.
In the end, it's all visual and you make adjustments based on what you actually see, not what you SHOULD see.

The name of the game is catching fish and having the best tools to do-so.
This is why you run a Humminbird in the first place.
Pay attention to how much signal The 'Bird gives you and make your range adjustments based on that.

I hope this helps.

Tyson

[bobcoy]

You have provided a great look at the way we need to use these tools. Except for the engineers and mathematicians we all overanalyze all this great technology. I have always tried to make things simpler for myself, but can get way too involved in the tech. Your explanation of use what is useful to you and what looks good to you without trying to "make it work" at the expense of practical usage. Thanks for you insight. Bob

[CSF]

Does anyone happen to know what the SI and DI 1.2MHz beam angles are and or what a plot of that looks like on a graph (side lobes etc.)? I'm just curious about the technical specs of it.

-jason

Mega SI is advertised as a 86 deg beam.Here is a screen shot of a limb hung on a channel marker at the surface, you can see it and the anchoring cables in the return.

[Whistler]

Nice. That's a pretty big target so makes sense that it would be visible. I'll try to find a link to an old thread from another forum where beam angle is discussed. Practically speaking, the beam angle really isn't that important, but the geek in me just likes to know the tech specs.

This first post talks about the beam angle and lobes for DI. And the second post talks about the SI beam. In this case, it is specific to the Lowrance transducer, but I believe the HB transducer it quite similar.

http://forums.sideimagingsoft.com/in...15138#msg15138
http://forums.sideimagingsoft.com/in...36134#msg36134

Again, this information isn't all that practical, but it is interesting from a technical standpoint nonetheless.

[BassTracker175]

Very good info here.

So there’s no way to mount another transducer (of the same length) close to a humminbird Helix Mega SI transducer??? Since the HB SI transducer covers more than 180 degrees, therefore its beam occupies ALMOST the whole slice of water in the transom.

[puhlw]

Is there a depth/SI ratio maximum for each frequency? Mostly thinking mega.

[Wayne P.]

Is there a depth/SI ratio maximum for each frequency? Mostly thinking mega.

[MCIPinkie]

Jesus, Do any of you guys ever fish?

I guess it's the time of year when all you students are working on Master's or Doctor's thesis's.

I'm setting here hoping the ice is off the lake so I can get in the water. When I do, I'll look at my depth finders and see what I can see. 86 degrees, 55 degrees, 120 degrees. Who cares?

Get in the water and see what's there.

[BigFishSteve]

Whistler cares. I also think that the info is pretty cool. I bought side image to look for stuff. DI is great too. It's interesting to know if I am close enough to something for DI to see it or should I be on the SI screen.

[BassTracker175]

When wind blows, you are stuck, LOL.

Jesus, Do any of you guys ever fish?

I guess it's the time of year when all you students are working on Master's or Doctor's thesis's.

I'm setting here hoping the ice is off the lake so I can get in the water. When I do, I'll look at my depth finders and see what I can see. 86 degrees, 55 degrees, 120 degrees. Who cares?

Get in the water and see what's there.

[skid00skid00]

Jesus, Do any of you guys ever fish?

I guess it's the time of year when all you students are working on Master's or Doctor's thesis's.

If you don't care enough to learn how to use the tools, just spend a buck on a lead weight and ball of twine. You'll get far better value for your money.

[ezfishn]

Provides another good excuse for not catching any fish...LOL Never could figure out why I wasn't catching all those fish showing up on the fishfinder and then someone told me to take it out of Simulator Mode....LOL

[BigFishSteve]

I love simulator mode. Gives me hope.