Choosing a High Frequency DownScan Sonar

[Fishton]

It has been my experience that high frequency sonar beam coverage is very similar to that a of vehicle's high beams on the road at night. 





There is what we can call the primary, or high power area, and then the peripheral area.



It is because of this peripheral light that we can see the reflective material of road signs so clearly at night.  Now if fish were covered in the sonar version of reflective tape, then this peripheral 'light reflection' would be fine.  But they are not.



Would you feel safe driving at night with your high power beams shining off to the left and right leaving only the peripheral light shining dead ahead of you?




   
So when you are looking for a high frequency Side / Down Scan sonar, make sure your Hi-Def transducer is capable of 'High Beam' in the Left, Right and Down Scan.



 

[tsmith35]

Interesting pitch, but the high beam analogy technically only works if your boat is traveling toward the bottom...

[Fishton]

Interesting pitch, but the high beam analogy technically only works if your boat is traveling toward the bottom...

If your transducer was mounted to the bow of your boat facing forward, instead of your transom pointing down, then I would have to agree with you tsmith35? 

[Mike Cork]

This works, obviously your not trying to physically hit the fish with your boat but look over the area you are fishing.

I'm starting to wrap my head around this  Thanks to Fishton, DougV and Triton Mike 

[tsmith35]

The use of down/side scan sonar in fishing is almost the opposite of the use of headlights while driving. The power used for down imaging can be relatively weak since it covers a relatively small field of view. By comparison, the power used for side imaging must be stronger since it covers a much larger field of view. Using your image, and remembering that the "car" here is pointed at the bottom (let's use the bottom edge of the image) and water level is about the middle of the hood:



You can see that the bottom is quite close. You don't need high power. But let's look at the sides, labeled peripheral. That area is much larger. Remember that the actual peripheral area covers all of the area in front of the "car" (90° left to 90° right, as required for sidescan in shallow water), minus the area covered by the "high power" area. To get the same resolution as with your down imaging, the power used must be higher. Side imaging covers a larger "slice" of water.

Let's say you're in 10 feet of water, traveling down the center of a channel 200 feet wide. Your downscan FOV is 60° (much larger in this example than it would be for an actual transducer). The area of the slice covered by the downscan is 115 square feet. By comparison, the area of the slice covered by one side of the sidescan (60°) is about 942 square feet. Total downscan area is 115 square feet. Total sidescan area is about 1885 square feet. The max distance to the bottom for downscan is about 11.5 feet. Max distance to bottom for sidescan is about 100 feet. The deeper the water, the more power downscan will need relative to sidescan, eventually reaching a ratio of 2:1 for sidescan:downscan power in infinitely deep water. Since most freshwater fishing is done in relatively shallow water, the ration of sidescan power to downscan power must be relatively large. In water less than 10 feet, the ratio is very large.

[Mike Cork]

 
So when you are looking for a high frequency Side / Down Scan sonar, make sure your Hi-Def transducer is capable of ‘High Beam’ in the Left, Right and Down Scan.

I think that is what he was pointing out here Tsmith and his last statement is that you need to make sure you have power in all directions. Something to watch for when purchasing your units and ducers.

[coldfront]

I think that is what he was pointing out here Tsmith and his last statement is that you need to make sure you have power in all directions. Something to watch for when purchasing your units and ducers.

there's a lot going on with the 'new' technology...that's for sure.  And the 'big two' go at it in slightly different ways...but arrive at a pretty similar end point.

trying to say one is decidedly 'better' than another appears almost harder than actually splitting a hair...can it be done?  yes...but then again, what's the point?  (it can be fun/entertaining to try on forums however... )

I know which manufacturer I am most comfortable with...but that's been a result of working with a product and the company over the years...and I mean 'working with' in terms of having them on my boat...and calling customer service as well as occasionally reading the manual to figure out how to work it 'best' for the way I use the units...

Personally, I do appreciate the true downscan function of the Lowrance product...but realistically, it's the sidescan that gets the lions-share of the use...as I roam a lake looking out to the sides for strategically located structure breaks...this function alone allows a guy to break down a lake so much more quickly.

[Fishton]

True High Frequency Down looking sonar welcomes the announcement by Humminbird that we will be seeing a True DI range available in 2011.  Well done Humminbird.