Need help with humminbird fish finder beam info!

[jjacobs007]

There i was at lake,As i looked at my fish finder sitting in 25ft of water i began to wonder just how big of a circle it was showing me of the bottom structure.
I have a humminbird 560 with a 20 degree cone,not dual. So the question is, whats the math on figuring out the actual footage of the cone on the lake floor verses the depth. any help would be helpful lol.
For example,if the water is 30ft deep and im looking at the bottom of the fish finder, is the beam 10ft by 10ft on the lake floor or what?

 

[dingbat]

Re: Need help with humminbird fish finder beam info!

(2 x depth) x (tangent of 1/2 cone angle) = diameter of the beam (beamwidth)

Here is the kicker. Hummingbird does not rate their cone angles using the industry standard -3db (half power point). Instead they rate theirs where the power is 1/10 (-10db) of the center axis power which gives the impression of a larger cone angle. Nothing is different in transducer performance; only the system of measurement has changed.

A transducer that has an 20 degree cone angle at -10db (yours) would have a 10 degree cone angle at -3db.

20 degree cone angle rated at industry standard -3 db
30 feet of water = (30 x 2) x (0.3639) = 60 x 0.3639 = 21.83 feet in diameter
25 feet of water = (25 x 2) x (0.3639) = 50 x 0.3639 = 18.19 feet in diameter

20 degree cone angle rated at Hummingbird standard -10 db
30 feet of water = (30 x 2) x (0.1763) = 60 x 0.1763 = 10.57 feet in diameter
25 feet of water = (25 x 2) x (0.1763) = 50 x 0.1763 = 8.81 feet in diameter

 

[TerryMSU]

Re: Need help with humminbird fish finder beam info!

(2 x depth) x (tangent of 1/2 cone angle) = diameter of the beam (beamwidth)

Here is the kicker. Hummingbird does not rate their cone angles using the industry standard -3db (half power point). Instead they rate theirs where the power is 1/10 (-10db) of the center axis power which gives the impression of a larger cone angle. Nothing is different in transducer performance; only the system of measurement has changed.

A transducer that has an 20 degree cone angle at -10db (yours) would have a 10 degree cone angle at -3db.

20 degree cone angle rated at industry standard -3 db
30 feet of water = (30 x 2) x (0.3639) = 60 x 0.3639 = 21.83 feet in diameter
25 feet of water = (25 x 2) x (0.3639) = 50 x 0.3639 = 18.19 feet in diameter

20 degree cone angle rated at Hummingbird standard -10 db
30 feet of water = (30 x 2) x (0.1763) = 60 x 0.1763 = 10.57 feet in diameter
25 feet of water = (25 x 2) x (0.1763) = 50 x 0.1763 = 8.81 feet in diameter

I am not sure the 3dB issue is valid in this case. I know all about 3dB being the half power point, etc. but as long as there is adequate recovered signal, the 3dB should be relatively moot. Thus at relatively extreme of depths, the 3dB vs 10dB may be a valid concern. That said, I do wish that everyone would rate their systems using the same constraints.

TerryMSU

TerryMSU

 

[dingbat]

Re: Need help with humminbird fish finder beam info!

I am not sure the 3dB issue is valid in this case. I know all about 3dB being the half power point, etc. but as long as there is adequate recovered signal, the 3dB should be relatively moot. Thus at relatively extreme of depths, the 3dB vs 10dB may be a valid concern. That said, I do wish that everyone would rate their systems using the same constraints.

TerryMSU

TerryMSU

Twice the signal equates to four times the precision. When is four times less precision adequate?

It's deceptive marketing. Most buyers don't know the difference and buy the largest advertised cone angle they can find.