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-   -   Transducer/Fishfinder freq. question (https://www.thehulltruth.com/marine-electronics-forum/382338-transducer-fishfinder-freq-question.html)

VanW 10-15-2011 09:57 AM

Transducer/Fishfinder freq. question
 
I've done a bit of reading about these over the last couple of months and for some reason I always thought 200khz would show a wider range of fish than 50khz. Like if there was a bunch of fish off of the bottom in 150 feet, you would be able to see them in better definition (for a lack of a better term) in 200 khz.

But the other day when I was out fishing with my newly installed c80 and 1kw m260 I had the fishfinder in split screen mode with 50/200khz and fish were showing on the 50khz side better.

I have everything on auto settings. What a big difference between it and my Garmin 526s though. I don't know if it's just the screen size or what, but I can see fish like nothing compared to with the Garmin.

Anyway, what's the deal? Was I wrong in thinking that 200 khz would be better for me? I normally fish banks in the 130-400 feet range.

DocStressor 10-15-2011 10:45 AM

The 50 kHz frequency provides a much wider cone angle than the 200 kHz.

You are covering a lot more bottom, which is why you are seeing more fish.

The ability to discriminate fish from bottom structure is greater at 200 kHz as long as you have enough power output and transducer sensitivity. The M260 certainly has more than enough of both for 150 ft of water.You should see higher resolution than at 50 kHz, just fewer fish because of the narrower cone angle.

VanW 10-15-2011 11:46 AM

For some reason I thought it was the other way around. So if I see fish at 50khz and at 200khz I should be good to go fishing spot wise for sure then....

icyb 10-15-2011 03:52 PM

unless its a typo but the m260 and the b260 are reversed. the 50khz is 14 degrees and the 200khz is 23 degrees. i would think down to 150 the 200khz would show more definition.

pastaman1234 10-15-2011 05:37 PM

The 50 is wider. If you see fish in 50 and not in 200 move the boat until you see fish in both cones. Then you know they are directly under you.

DocStressor 10-15-2011 05:59 PM

For the M260, the cone width for 200 kHz is 6 degree and the 50 kHz is 19 degrees.

http://www.airmartechnology.com/uplo...hures/m260.pdf

semperfifishing 10-15-2011 06:03 PM

Each frequency has its own particular advantages.

200 khz gives better detail, shows less noise and undesired echoes. Also, target separation is better with the higher 200 kHz frequency.
Waters absorption rate is greater for higher frequencies and shorter wavelength thus a 200 kHz sound wave will have less depth penetration than a 50 kHz signal.

50 kHz sound waves can penetrate far deeper due to the longer wavelength.
thus they will penetrate deeper but also will have less definition and target separation.And 50 kHz sound waves will have greater noise than a 200 kHz signal.

So it boils down to:

200 kHz
Shallower depth
Narrow cone angle
Better definition and target separation and less noise.

50 kHz

Greater depth
Wider cone angle
Less definition and target separation and more noise.

But used together on a split screen , each adds to the overal sonar picture so the fisherman has a wide variety of information of conditions and targets under his keel.

And some sonars such as the Furuno FCV620/FCV585 have the ability to help the user locate the target by utilizing the different frequencies and their respective cones.

When using either of these units if you read a depth number right on the target that means the fish in within the 200 kHz cone.
If you still get a great target but no depth number on the target the fish is outside the 200 kHz cone width but within the 50 kHz cone.



http://img812.imageshack.us/img812/6096/img2579a.jpg

This fish target is inside the 200 kHz cone. Being that this is a 12 degree cone the width at 113' is 24'..this fish is basically under the boat.


http://img545.imageshack.us/img545/5205/img2582y.jpg

Same target but now outside the 200 kHz cone but still within the wider 50 kHz cone. This being a 45 degree cone the width at this fishes depth is 94' wide.

Now you know that the fish is port or starboard and not under your boat..so you make a 360 until you pick up the target with a depth number on it..then it is under the boat.


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