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How to Choose a Fish Finder

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On: Fri, Oct 8, 2010 at 10:00AM | By: Lee Clymer

I absolutely love technology. In my life I have watched everything from party line telephones become cell phones to small black and white television screens become plasma screens. It’s been a wonderful ride so far, and it just keeps getting better. All parts of our lives are inundated by new technology that improves exponentially every ten years, sometimes even daily. Boats and fishing have seen technological advancements in spades.

When I learned how a fish finder actually works I was amazed. It was so simple it was absolutely brilliant. Sonar can’t pass through air so when a sonar signal hits a fish’s air bladder it reflects back to the transducer. The bigger the fish, the bigger the air bladder, and thus the bigger the reflected signal. Bear in mind, however, that anything with a different density from water will show up. Seaweed, trash, or just about any submerged object will give a reflection.

There are a few basic attributes to look for in a fish finder. As with anything you buy, the stronger the attributes of the unit you intend to buy, the more it will cost you. If you are nothing but a flats or river fisherman there’s no need to spend a lot of money on a unit that will show your jig sink to six hundred feet. $100 or so will get you a good decent unit that will display the bottom contours and peg fish in a size range reliably.

If you’ve never used a fish finder and are finally making a jump into the technological aspect of fishing, the primary things you want in a fish finder are, of course, the fish themselves, bottom contours, and structure. In order to do this accurately a couple of things are involved. First, the power of the transmit unit comes into play. Second, and naturally, the receive unit’s sensitively and response time. Other variables include the area the signal is transmitted into, known as the cone, or cone angle, and frequencies transmitted. All the other variables are quality enhancements enabling you to use the strength of the unit to your best advantage.

Like Tim the Toolman Taylor always advocated, “more power.” In this case it true. The more water you want to penetrate, the more power you’ll need to do it. Going back to the flats and rivers that are seldom, almost if ever, more than fifty feet, a low power unit will suffice. If you’re an open ocean fisherman, you’ll need something with the juice to find deepwater fish. Simply stated, depth and power are proportional.

While some people believe SONAR is like RADAR in the aspect it uses radio frequency; that’s not true. In fact, it is sound. SONAR is actually an acronym for Sound Navigation and Ranging, where as RADAR is an acronym for Radio Detection and Ranging. I want to make this clear so later you can understand lower frequencies take more power, but also travel further and, of course, remain stronger, longer. That’s why those cars with the loud bass speakers rattle your windows as they drive by. It’s not necessarily a function of the power applied but the frequency.

In fish finder transmitters, although part of the transducer, which also encompasses the receiver, there are two primary frequencies used. The lower frequency is the 50 kilohertz (khz) range and the higher frequency is in the 192 kilohertz (khz) to 200 kilohertz (khz) range. The 50 kilohertz range will go deeper, but not return as much detail, and the upper range will be absorbed by the water quicker, but due to the quicker frequency will return exacting detail. The transducers must be matched to the transmit frequency. Fish finder packages will match the unit, but if you have to replace one, make sure the frequencies match.

Some transducers are dual frequency, and even triple frequency, and the unit itself will switch automatically as needed, or manually if you prefer. With the extra equipment the price logically goes up.

The cone of the transmitter will determine the area of water covered by the signal. A narrow cone window will typically run about 8 degrees as launched by the transducer. The narrow windows are used by the upper frequencies and wide windows are used by the lower frequencies. Many 50 khz transducers launch as wide as 35 degrees. The wide cone areas can help locate fish and the narrow cones will pin down locations and densities. Medium cones run from 12 degrees to 20 degrees and will often be found in single frequency all-around general purpose units.

In order to understand the power needed to penetrate the depths and fill the cone you must know how to translate the specs. Power will be rated in either watts RMS or watts peak–to–peak. It’s very important to distinguish between the two. An easy way to figure this out is multiply RMS power times eight to get peak–to–peak or divide peak–to–peak by eight to get RMS. You must compare apples to apples and oranges to oranges. Any fishing in over a couple of hundred feet will require at least 500 watts RMS and thus 4000 watts peak–to–peak, and it is preferable to be dual frequency. 500 watts will get you about 500 feet in the high frequency ranges and about 1500 feet or more in the low frequency range. Keep in mind though, power isn’t everything, it’s just one of the major factors. It’s important to have a whole package of transmitter, receiver, and screen.

Speaking of screens, the one thing you’ll need to look for in a screen is a high vertical pixel count. This will provide better detail, just like your television. Your fish finder is just a small television and employs the same type of technology. The horizontal pixel count is important, but since you’re looking down and want to see depth, structure, and bottom, you’ll concentrate on vertical pixel count. The two together are expressed as screen resolution and expressed, for example, as 640 x 480, with the 640 representing vertical count and 480 representing horizontal count. This means each vertical line has 640 color dots (presuming it’s a color screen) and each horizontal line has 480 dots, or basically 480 vertical lines to the screen.

So why does this matter? Each dot represents a certain amount of depth. In shallow water the unit will drop its range accordingly. If there’s a twenty foot range, each dot will represent about a third of an inch (20 feet x 12 inches equals 240 inches. 240 inches divided by 640 pixels equals .375 inches per pixel). A small fish close to the bottom will show up nicely.

Now let’s go out to the ocean and take a look at the bottom five hundred feet below the boat. Wow, how things change. Now each of those pixels represent almost ten inches in the water column (500 feet x 12 inches equals 6000 inches divided by 640 pixels equals 9.375 inches). Again, if you’re shallow water fishing you can go with a less expensive unit and still get the same effective quality, but the depths demand more power and clarity to work well for you.

There are a few features that you may want to investigate as frills, although they are nice frills. They are gain, bottom lock, A scope, and zoom. Gain should be adjustable to give an ability to zoom in power-wise. Bottom lock expands the view directly above the bottom. A scope shows in real time what is right under the transducer in the cone. The easiest way to explain this is that it assists in interpreting the vertical lines by displaying the information on the side in a horizontal manner. Finally zoom is just what is sounds like. It allows you to focus on a certain area of the screen and blow it up larger.

I’ve found each manufacturer covers the gamut of typical features pretty well, and it’s all the frills that really run up your costs. Most of those frills are really nice to have, but go with the necessities first, and then with whatever budget you have left over, buy your preferable frills. There’s no sense in having frills on a second rate unit.

May reel zings ring in your ears.


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