Dual Frequency Fish Finders

[Mac_Attack]

What is the benefit of havig a dual frequency FF. Which frequency is for what depth? Thanks Bill Mc ;D

[reelapeelin]

It's a depth thing...unless you're fishin' REAL DEEP waters, 200's all you need...jmho...

I second that, unless your fishing real real deep, you will never see the difference in the 200mhz and the 50mhz. I use them every single workday and I never take it off of 200mhz and I go as deep as 300 feet.

[Skools Out]

yeah i have a single on couple of my boats and a triple angle dual freq. one on the sea ray i can't tell alot of difference. the only reason i have that on the SR is because it came on the boat. *after using both i'd say buy a single. unless you are going out in 600 ft of water.

[Airslot]

Can be usefull for finding structure, or fish. If you run it in split screen, you can sometimes pinpoint where the structure is, or isn't. If the structure shows on low freq but not high, your in the neighborhood but not right over it. Could be fish or a wreck your looking for. Just another way to use the tools on the boat.


Airslot
I figure that once I add radar I can thow the fishpoles away as I won't have time to fish anymore, just play with the boat gadgets all the time

[Blue_Runner]

AS, I try to play between bites ;D

[bigshrimpin]

50kHz is a wider cone, than 200kHz band. 50kHz band will mark fish that you won't see with 200kHz. When you get into deep water 300+ft . . . 200kHz is the way to go for marking Cod.

For example: Tranducers usually tell you the angles of the cones. (14/45deg) I believe that this means the 50khz cone is 45deg and a 200khz cone of 14degrees. So you can imagine that the large cone picks up more fish, but returns a weaker signal when you get into deep water. If you mark a fish and it's a strong signal on both 200 and 50 . . . it means the fish is UNDER THE BOAT. :)

[chesapeake724]

This is more a practical benefit, not so much for performance. :
I'm planning on getting a GPS/FF combo. It will mainly be a GPS, but for a few more bills I'll have a backup sounder. The existing fishfinder has a single 200kHz transducer, but if I buy a dual freq. transducer for the combo unit, I would have the option to run both at the same time if one's at 50kHz and the other is at 200kHz. IIRC, you can't run two transducers at the same time at the same frequency, or else you'll get interference.

[RobsTV]

Bringing up old thread, because topic is my current while searching for depth finder, and new to me.

Looks like conflicting info here.
Frequency goes hand in hand with cone degree (or cone beam). One is useless without knowing the other.

200kHz is best for shallow water, and low kHz for deep?
So far from what I gather, the opposite holds true, because of the cone degrees in use today..

Popular modern Dual Frequency Humminbird's come as 200kHz with 20 degree cone, plus 83kHz with 60 degree cone.

As bigshrimpin might have been suggesting, cone degree seems to be much more of the key thing here than kHz, or at least easier to determine needs.

The wider the cone angle, the less sensitive it will be in deeper water, but it will give you a wider field of view in shallower water.

Formula for size at bottom of cone beam is:
1/2 of the cone angle x 3.14 divided by 180 = tangent. Tangent x depth x 2 = Max Diameter of cone in feet on bottom (above bottom will of course be even smaller all the way to surface).

Using formula:

20 cone (200kHz) at 30' depth = 10' circle is max that depth finder can see on the bottom, or 1/4 the diameter of 20' boat circle when transducer is on transom. At 45' depth still only 15' circle, so if you are in the front of anchored boat, you would be 7' away from outer range of FishFinder, but at 100' depth it is now starting to hit a usable range of about 35' max beam circle. Still, in a 20' boat at 100' depth, cone beam only goes about 17' in either direction (35' total), so front fishing is slightly out of range that tranducer sees (when anchored).

60 cone (83kHz) at 30' depth = about 31' max beam circle on bottom or about 1:1 depth:beam circle. 100' depth is about 105' circle.

So you can see that you would want a lower frequency (larger cone) when in shallow waters of 80' or less, and a higher frequency (smaller cone) when in waters of more than 100'.

Here on the central left coast of Florida, we need to go out 20-30 miles to hit 45' of water, so you can see why this info matters. At 45', with a 200kHz equipped with a standard 20 cone, we would miss much of the bottom more than 7' off either side and only see what we were right on top of. A few feet off target, and we would never see it. When diving we look for water around 45' to 60', so again 200kHZ would not be well suited.

Make sense? If I screwed up and have my facts wrong, please correct my errors.

I was going to get Humminbird Matrix 12 for about $150, with decent specs, except 200kHz and 20 degree cone. Then found the Matrix 17 with better all around specs, plus Dual Frequency as stated above, for only $15 more. Garmin handheld GPS will plug right in. They say with these new versions, you can watch a dropped BB fall through the water. Sure ya can. But at least it is better than the 10 year old stuff I had before. Also, while quality was down the tubes with many humminbird's, since it changed hands a few years ago, they now make some of the highest quality units. Sorta like Merc's had a well deserved bad rap, and now they are O.K., although spending less than $200 to see if something is now O.K. (humminbird) is much easier than spending $20,000 to see if an outboard is any good.