P.S. I presume the salient point is infrasonic radio wave...which is in the audio frequency range, but electrical.

It is still speed of light versus speed of sound. They are two different things -- aren't they?

 

I don't know why you would want to bring EM (radio) far fields and near fields into the mix.

A ultrasonic wave is a sound wave. It's a pressure wave. It REQUIRES a media to propagate thru.
A sound wave will not propagate thru space.

An EM wave requires no media to propagate.
And a media will usually deter an EM wave. It depends on a lot of things. Some EM frequencies don't mind air, but hate water.
Some EM frequencies can't pass thru air. Which turns out to be very lucky for life on earth.

Is this not taught any longer?

 

It was the TS/OP that conflated the two from the get go. Thinking that an ultrasonic wave would be received by the same type of antennas as an EM wave.

 

"extreme" shouldn't be used to describe a simple standard. Even though ANY standard is considered extreme today.

Oh...you forgot to read the word "water" in my post.

It should have been obvious from the topic that I was referring to waves of the electromagnetic variety.

 

P.S. I presume the salient point is infrasonic radio wave...which is in the audio frequency range, but electrical.

The "antenna" is the transducer. Ultrasound transducers "ping" at a one or more frequencies, then wait for the echo at the same frequency. The same piezoelectric transducer that uses an electrical impulse to generate a pressure wave front in water then waits to receive the echo. The pressure wave of the echo strikes the same piezoelectric transducer and generates a small electrical current that is amplified and filtered to constitute the signal seen on screen. The transducers are typically either a linear array or circular array of individual piezoelectrodes. The frequency of the electric signal used to generate the "pings" controls the frequency of the pressure wave. The sequence that individual elements in the array are fired can be used to shape and steer the pressure wave front created by the transducer. There is no one frequency. Lower frequencies penetrate deeper as they are less attenuated by depth. Higher frequencies have better spatial resolution, but less penetration. Remember that pressure waves are affected both by reflection and refraction. Thermal layers can cause a change in the refractive index and bend the pressure waves, just like light refraction. Also, the return echo is dependent on the amount of surface reflecting back to the transducers. Similar to radar cross-sections. The top of a fish is not a very efficient reflector. As for refraction, the wider the "field of view", the more attenuation from refraction and reflection. For example, at a 30 degree interface, 50% loss of reflected return signal, independent of any other source of attenuation. At 45 degrees, 100% loss of reflected sound energy. The long towed arrays used in military underwater sonar are much more efficient at receiving reflected signals. A point source of reflected sound that spreads out over a given angle, say 30 degrees gives only a minute signal if it has to be "heard" at a single point, but much stronger signal if the transducer spreads over a longer length and thus broader angle of return signal.

seehttp://www.westmarine.com/WestAdvisor/Selecting-a-Fishfinder