Topic 04 - Searchable angles and areas

Ultrasound is transmitted directly underneath the transducer, which is mounted on the bottom of the boat. The transducer has its own directional characteristics, that is, the characteristics of beams transmitted, which affect the detection capability under the water. The directional characteristics of the transducer are largely determined by the frequency of the ultrasound transmitted and received by the transducer, and this affects the searchable angles and areas.


Frequencies commonly used by a conventional fish finder for recreational boat are 50 kHz (low frequency) and 200 kHz (high frequency). A lower frequency has a wider search angle and area. Generally, the searchable angle of 50 kHz beams is approximately 50 degrees and that of 200 kHz beams is approximately 15 degrees. The searchable areas by both kinds of the beams are illustrated below. As you can see, lower frequency beams are suited for wide-area-search at one instance, and higher frequency beams are suited for narrow-area-search. Anglers can make use of these characteristics to find a fish school.


Commercial fishermen also make use of these characteristics. They conduct a search by low frequency (i.e., 15 kHz) beams first to grasp the general locations of fish schools around the boat. Subsequently, they narrow the search area for a fish school, using high frequency beams (200 kHz), to detect the exact location of the fish school and operate the boat to be directly above the targeted fish school.


Interrelationship between searchable angle and frequency of ultrasound

Searchable area is determined by the frequency of the ultrasound used. While use of low frequency beams (50 kHz) facilitates a search of wider area, use of high frequency beams (200 kHz) allows the operator to narrow down the search area to detect the more precise location of a fish school.


Directional characteristics

Inside the transducers, transducer elements are incorporated. The transducer element transmits ultrasound through its vibration, which is caused when it receives electric signals. How focused the ultrasound can be transmitted wholly depends upon directional characteristics of the transducer used. The figure on the left hand side shows how ultrasound is transmitted from the transducer. The oval shape just below the transducer indicates signal transmission intensity of ultrasound. The signal strength is the strongest on the centerline of the oval shape, and the signal strength becomes weaker towards the edges of the oval shape. Directivity angle of ultrasound can be generated by connecting the transducer and the points where the signal strength is half of the strongest signal (centerline).