Fish Finder Basics.

Understanding the basics of a Fish Finder can help anglers fill their live wells, win tournaments, and keep their boats away from places hazardous to the health of props, lower units and boat hulls.

Fish finders can't find fish. They can only tell you when you have found fish.
(Your fish finder is only as valuable as the amount of time you invest in learning how to use it and using it.)

Newest Lowrance fishfinder the HDS12 Gen 2 - see relation of the fish and the branches on the brush.

If properly installed and functioning it will produce a wealth of information for any angler. Fish Finders work by sending an electrical impulse from the transmitter, converting it into a sound wave by the transducer and then sending that sound wave into the water. It is a Sonar. When the wave strikes an object it rebounds off the object sending an echo back to the transducer. The transducer then converts the echo back into an electrical signal. The receiver amplifies this signal and sends it to the display for you to see.

Fish Finders have 4 basic parts.

  • Transmitter
  • Transducer
  • Receiver
  • Display

The Transmitter:

A small amount of current from your battery is taken by the transmitter and converted into pulses. The Transmitter sends these pulses to the transducer. The computer will continue to tell the screen to light up pixels as long as an object stays in the cone. With this in mind the length of an object on the screen has nothing to do with the size of the object. A fish swimming in the same direction of the boat will stay inside the cone for a long time and produce a long thin line on the screen.
A thick mark, no matter how long is a strong signal, i.e. harder bottom or a bigger fish.

What is a sonar transducer?

 It is the transducer, the heart of the fishfinder, that sends out the sound waves and then receives the echoes, so the main processor can interpret or detect what is below the surface of the water.
No matter what make or model fishfinder unit you have, there is a sonar transducer that makes the whole thing work.
The Best Transducers have High Quality Cultured Piezoelectric Crystals.
Choosing the best Transducer for your unit needs a little understanding of the differences and the features of Transducers.

CHIRP Sonar:
Compressed High Intensity Radar Pulse:

Basic sonar was a joint effort between the U.S. and British in World War I.
Side Imaging was developed by the U.S. Navy Mine Defense Lab in the 1950s.
CHIRP Sonar, the latest in fish-finding technology, is no exception.
It was also developed in the 1950’s as a military technology, and it’s recently been introduced by all the major fishing electronics manufacturers.

Unlike the single frequency of the broadband or digital sonar technology, CHIRP continuously sweeps a spectrum of many frequencies within a long duration pulse. The equivalent sound energy is hundreds of times greater than a fixed, single-frequency pulse, resulting in more energy on target. This provides huge advantages in detail, resolution and accuracy at much greater depths.
Sweeping frequencies improves the quality of the sonar signal by offering:
Better target separation:
Because CHIRP uses a range of frequencies, rather than a single pulse, CHIRP sonar greatly improves the ability to distinguish fish targets that are very close together or on the bottom.
Fish become easier to differentiate from the structure they are holding to.
Less interference from errant noise that would have been picked up by a single frequency sonar.
CHIRP creates a unique range of frequencies and listens for only those sonar returns, this gives CHIRP sonar the ability to distinguish between what is a real echo, and what is just extra disturbances bouncing around underwater.

What frequencies to choose?

 Here is a quick breakdown of some options in the new world that includes high-frequency and CHIRP sonar:
Low CHIRP (25-80kHz) or 50kHz:
Lower frequency means maximum depth penetration for deep-water fishing.
For greatest depth with a traditional 50kHz transducer, choose a sounder with 1-2kW of power.

 Medium CHIRP (80-160kHz) or 83kHz:
Gives the widest coverage area, 83kHz is ideal for watching a bait under the transducer in shallow water.

 High CHIRP (160-800kHz) or 200kHz:
Higher frequencies display a higher resolution image, making it easy to discern fish from structure or structure from the bottom.
The tradeoff is that maximum depth goes down as the frequency goes up.
455kHz allows for scanning of a wide field of view with picture-like detail.
800kHz has a shallower maximum depth range but even greater realism and detail than 455kHz.

Airmar manufactures the vast majority of transducers.

The Screen:

The returning echoes are then run through an amplifier to make the impulse signal stronger. Next the computer measures the length of time it took for the signal to return. The speed of sound waves travelling through water is a constant so the exact depth of the water can be calculated. The computer takes that information and sends it on to the screen. The screen lights up small squares called pixels so you can view it. Pixels are arranged in a grid pattern on the screen. The screen is a certain number of pixels high and a certain number wide. The number of pixels in width is important to the amount of bottom history you can see on the screen and the vertical number of pixels is critical to how much resolution the unit offers.

So how much resolution is enough? Less expensive depth finders have 100 vertical pixels while the high-end units offer 240 or more. Let's compare the two.

In 40 feet of water, a depth finder with 100 vertical pixels, 1 pixels equals 4.8 inches. If a four-pound fish was inside the cone, 1 pixel would light up to represent its girth. So a fish within 5 inches of the bottom would blend in with the bottom on the screen. On the depth finder with 240 vertical, each pixel equals 2 pixels, so the same fish would light up 3 pixels. Since more resolution can be shown, the same fish would be a slight bump on the bottom.

Operation Tips:

  1. The quickest way to learn how to operate your depth finder is to go to a clear lake where you can see bottom to at least 10 feet. Simply compare what you can see visually with how your depth finder reports it. If you can see rocks, watch how your screen records rocks. The same applies to detecting weeds, brush, and even the bottom. This can accelerate your learning process by years.
  2. Do not use the Fish ID feature! This option draws precious power by making the unit interpret the signal for you. Learn to recognize fish by the arcs and you'll be much better off.
  3. With most units the sensitivity or gain control is an automatic feature. The computer interprets the information it receives and adjusts the sensitivity up or down, to give you the clearest picture possible. To get the most from your unit put the sensitivity in manual. Now slowly turn the sensitivity up a little until a second echo or false bottom reading appears on the dial. (The second echo should appear on the dial at twice the actual bottom depth.) When that second echo appears, the unit has been fine-tuned for sensitivity to mark everything between the transducer and the bottom, including tiny minnows, weeds, brush and, of course, walleyes.
  4. Walleye are bottom - hugging fish so use the zoom feature. This option will give you a more detailed view of where the à hang out and allow each pixel to represent more detail.
  5.  When marking fish on your depth finder, don't try to determine what type of fish it is. Consider every fish a walleye and fish it with confidence and determination to make it bite!

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