Thread: Sonar Quiz #19

Okay, I'm surely not the sharpest knife in the drawer, but I'll give this a shot. I'm just looking at this from the most obvious perspective.

The leading and trailing edge is where the beam is hitting the fish from an angle as the fish enters or departs the cone and the thicker part of the arch is where the beam must pass through(or is blocked) on the fish while the transducer is directly over the fish which in turn gives it the thicker display in the middle. That would show the beam hitting a fish and giving it the arch shape and steadily becoming thicker upon approach and thinning as the fish is moved towards the trailing edge of the cone.

The transducer is both more sensitive and transmits more power in the center of the beam. The sensitivity and power taper off as you move away from being directly under the transducer.

The thin edges of the fish arch are recorded while the fish is just barely detectable by the sonar. As the transducer moves closer to the fish, the fish is closer to the center of the beam resulting in a stronger return.

This also explains why the misaligned transducer in Quiz #18 creates fish arches where the thickest part isn't at the center. When the transducer is tilted, the point where the fish is nearest the center of the beam (the transducer is pointed directly at the fish) is not the point where the fish is closest to the transducer as it is when the transducer is properly aligned.

Don't the air bladder have something to do with it as well??

Yes and no slabbacks. An inflated or partially inflated air bladder will return a stronger sonar echo than other parts of a fish will, but it is still the fact that the fish, in general, is returning a stronger sonar return while it is in the stronger center part of the transducer beam versus the leading/trailing edges of the transducer beam.

Two fish of the same species pass through the center of a sonar beam at the same water depth. If they are of the same size, shape and orientation to the transducer; the one with the more inflated air bladder would have a more pronounced or thicker central part of the sonar return versus the fish with the less inflated air bladder would.

Doppler effect

Originally Posted by RCC

Doppler effect

Unless the the fish or boat is moving at hundreds of miles an hour, Doppler shift is very small. The transducer can't tell if it has been shifted unless it moves outside its sensitive range.

Originally Posted by RCC

Doppler effect

I would be interested in learning more about this and how it relates to sonar...

Is there a link for more info ...??

Rickie