Sonar Buyer's Guide
Author: Sean Sutherland and Frank Ross
The advances in sonar technology have been phenomenal in the past few years, and innovations seem to come at an escalating pace. Before you tackle the buying decision for an electronics purchase, you should have an understanding of some basic terms and the way sonar works.
Four elements determine a sonar unit’s ability to help you in your quest for fish: transmitter power, transducer efficiency, receiver sensitivity and display options.
Let’s look at power first. The term peak-to-peak is normally used to describe the output power of a sonar transmitter. This term describes the results of a measurement of the total swing of AC voltage from its peak negative value to its peak positive value. Power on sonar units is generally given in watts.
RMS is another term that is used in evaluating sonar. RMS is an acronym for "root mean square," or a DC voltage that will produce the same heating effect (power output in watts) as the AC voltage.
The depth and quality of your home waters will determine what amount of power is required. If you routinely fish relatively shallow lakes, high frequency (which we’ll discuss later) is more of an issue than power. In murky water and greater depths you’ll need more power to penetrate all the way to the bottom. Power is also an issue when it comes to the speed of the signal in relation to your boat speed. A very weak or slow signal will return information after you are long gone.
To select the transducer that’s best suited to your needs, you need to consider the transducer’s operating frequency, cone angle and type of installation.
The transducer functions like an antenna to send out sonar signals (sound waves) and receive return echoes. Transducers are available for mounting on the transom, inside the hull of a boat (for small fiberglass boats), or even to a trolling motor using a special mounting. A true thru-hull option is available for large fiberglass boats.
To help you decide what cone angle will best suit your needs for the type of fishing you do, consider this: the wider the cone angle, the less sensitive it will be in deeper water, but it will give you a wider field of view in more shallow water.
In general, a single-high-frequency, 20°-cone-angle transducer is preferable for inland freshwater lakes, mounted to the transom or trolling motor. For Great Lakes and saltwater use, a dual-frequency transducer with transom mount, puck-style or thru-hull installation is preferred.
For the end user, the display is probably the most important aspect of selecting a fish finder. Monochrome, color, contrast and detail are critical decisions that will have the most dramatic effect on what you are able to discern from a returning sonar signal. The most important factor, relative to detail, is the tiny pixel. A pixel is simply a dot. If you’ve ever looked at a newspaper picture under a magnifying glass, you can easily understand pixels and their role in displaying information. In much the same manner, thousands of tiny dots make up a display image. Units with fewer pixels display images that appear blocky and very crude. The more pixels, or dots, that your display has will determine how smoothly it is able to display lines, curves, a thermocline and subtle differences in structure. A higher resolution (more vertical pixels) and greater contrast will give you more discernable images, especially in direct sunlight.
The revolution in color monitors takes object identification to a whole new level. With high-resolution color displays, fish show up in different colors and almost jump off the screen. A color display is a great idea for the detail-conscious angler who wants to easily differentiate fish, trees, weed lines and baitfish as they slip across the screen. Typically, color units utilize a 16-bit color TFT (Thin Film Transistor) or a 256-color VGA (Video Graphics Array). Higher definition units will sometimes be equipped with a SVGA (Super Video Graphics Array).
Most sonar units operate at a low 50kHz (kilohertz) or at a high 192kHz or 200kHz. There are distinct advantages to frequency that you need to consider before making a purchase, and some units operate at both low and high frequency, depending on the task that is assigned.
Water has a natural ability to absorb higher frequency signals more than those on the lower end so the 50kHz units penetrate deeper depths; however, they have a wide cone angle and produce less definition and target separation. They are also more susceptible to noise and distortion.
Higher frequency units, in the range of 192 to 200, are very effective in shallower depths, have a narrower cone angle, and produce better definition and target separation. These frequencies are less susceptible to noise and distortion.
Deep saltwater environments will require a 50kHz transducer or a dual frequency transducer that can be switched between 200kHz and 50kHz. Downrigger fishermen can track downrigger balls with the wide cone angle of the 50kHz transducer.
Once you’ve found the fish, finding them again is the next challenge. The ability to store points and routes comes in handy on both large and small water bodies.
The decision between marine GPS units with internal and external GPS receivers is truly personal, though some manufacturers recommend against installing internal GPS units in cabins and behind windshields. The wisdom behind this practice is to give the receiver a clear shot to the maximum number of satellites. External GPS units, however, require a more thoughtful installation, as wiring and drilling are necessary.
Two terms thrown around in relation to GPS are DGPS and WAAS. DGPS (Differential GPS) calculates and transmits the necessary correction factors to mobile GPS receivers in the area. WAAS is an acronym for the Wide Area Augmentation System. WAAS improves GPS accuracy using ground towers to more accurately triangulate your position.
Mapping and Charting
Mapping and charting capabilities may be expanded with plug-in SD cards from companies such as Navionics, LakeMaster, Garmin and Lowrance. These systems deliver incredible detail for anglers wanting to know as much as possible about the lakes they are fishing. Some units come conveniently preloaded with mapping and charting software.
NMEA 2000® is the interface standard developed by the National Marine Electronics Association. Equipment designed to this standard will have the ability to share data over a single channel. This networking streamlines a boat’s electronics package by allowing multiple sonar/GPS units to run off of a single transducer or GPS module.
Making the Purchase
The best fish finder for you is the unit that has the features your fishing style requires. When choosing, know that there is a learning curve for every fish finder. Learning the ins and outs of your fish finder may take a bit of practice and patience, but you’ll quickly realize its value in fish found and fish caught.
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