Understanding Fishfinder Frequencies

Which fishfinder frequency works best in what conditions, for what purposes, and why? 

When it comes to finding the best fishfinder for you, you may or may not have considered the frequency or frequencies one unit or another utilizes to ping through the depths. True, this is a function of the transducer as much as it is of the fishfinder itself, but most fishfinders are made to work with a specific set of transducers and the bulk of them come prepackaged with one. So, often frequency isn’t even a factor people consider — even though frequency has a huge impact on how good a unit does or does not perform functions ranging from picking up bottom to differentiating fish from structure. 

Many modern unis, like the new Simrad NSX line, ping through the depth with multiple frequencies and in multiple directions, all at the same time.

How Fishfinder Frequency Works 

In this day and age, just about everyone understands the basics of how a fishfinder works: the transducer sends out sonar waves (the “pings”), listens for their reflections, then communicates the results to the LCD screen at your helm. But those sonar waves, just like the waves in the ocean, vary in size. And their size dictates a lot of different variables, like how far they travel and how easily they get bounced back. This is easy to visualize since we anglers are used to dealing with waves on the ocean, which act in a similar fashion: really big ones can travel great distances, but they may roll right over small objects without being reflected back. Conversely, small waves won’t go terribly far but can be bounced back by tiny little objects. Simple enough, right? 

Pinging at incredibly high frequencies — in this case, a Hummingbird system using 1.150 to 1.275 megahertz — can provide rather mind-blowing detail levels.

The measurement used to express the frequency of these sonar waves, kilohertz (kHz), can seem a bit counter-intuitive because the higher the kHz is, the smaller the waves are. Again, we have an easy visualization: in a 100-by-100 chunk of ocean there’s only room for a couple big, rolling swells. But in the same space there’s room for hundreds or thousands of small ripples to move across the surface. Hence, the really big waves have a very low frequency, while the little ripples have a much higher frequency. Very low frequency sonar waves can punch through tremendous depths, but they offer poor sensitivity since it takes a really large object to bounce them back. Very high frequency sonar waves can’t go nearly as far, but they can provide tremendous detail since the smallest objects will create a return. 

Armed with the above knowledge, it should be easy to see why a modern imaging unit pinging away with a high 800 kHz frequency can deliver shocking detail, showing sprigs of weed and mere minnow swimming around next to them — but only for a hundred or so feet (range will always hinge on environmental conditions, especially with very high frequencies). Meanwhile, a 50 kHz unit may well show you bottom in 1000 feet of water, but detail levels will be lacking. 

Scanners and Side-Finders Vs. Down-Lookers 

Most “scanners” or “imaging” units and side-finders operate on very high frequencies, commonly offering the 400 to 800 kHz range and sometimes even higher. For freshwater and inshore guys, this is great. You can pick out the rockpile with fish hanging around it if it’s within casting distance, and drop your jig down right in front of the fish’s noses. But for offshore anglers, the ability is more or less moot. The structure you’re fishing may well be a temp break, or a pinnacle hundreds of feet down, and those scanners and side-finders will never even spot ‘em. 

This would seem to make choosing the right fishfinder frequency easy. Get high frequency for shallow water, and low frequency for deep water, right? Hold on a sec… 

Trolling on a boat with omnidirectional sonar pinging 360-degrees at 85 kHz, the captain made a sudden swerve after spotting the tuna — and this guy jumped on the line.

How CHIRP Works 

Enter: CHIRP. The use of the term is something of a misnomer when it comes to fishfinders, because “CHIRP,” commonly referred to as compressed high intensity radar pulse, refers to radar waves. But even then it’s not accurate, because it doesn’t have to be high intensity. Marketers being marketers, the term was more or less hijacked years ago. However, what a CHIRP fishfinder does is quite similar to what a CHIRP radar does: it compresses those pings into a rapid-fire burst, which spans a certain range of frequencies. Just what that range is again depends on the system and the transducer, but the bottom line is that multiple sonar waves of multiple frequencies get blasted out in a fraction of a second. Then the transducer listens for returns bounced back by any or all of them, and interprets the results. 

CHIRP can take place through multiple bandwidths; this is a view of Garmin’s GDS 25 showing both high and low frequency CHIRPs at the same time.

Early in the CHIRP game some manufacturers referred to the tech as “spread spectrum,” which is probably a more accurate description. Be that as it may, we’re stuck with the term — and either way, once someone has a clear understanding of how frequencies work the advantage of utilizing multiple frequencies at the same time should become apparent. 

This image shows the difference between a traditional CHIRP and a high frequency imaging CHIRP, on the same unit.

Now mix CHIRP with high frequency scanners and low frequency down-lookers. Oh, and let’s make some transducers with multiple angled elements that look down and around at the same time, and viola: we’ve got 3D fishfinders, too. 

Which Fishfinder Frequency is Best for You? 

When you cut through all the marketing hype and sort out the options, which fishfinder frequency is the best for your needs? That depends on how and where you fish, but the fact of the matter is that in this day and age, if you want to see fish in just about any reasonable depth or direction, there’s gear out there that’s made to accomplish the task. The real question is, how much are you willing to pay? If you’re okay with spending a quarter mil on fishfinding electronics, you could theoretically rig the same boat up with omnidirectional sonar and spot tuna up to 5000’ (advertised range) 360-degrees around your boat in the ocean, see redfish 350’ to either side in the bay, identify both edges of a channel in 3D, and see a two-inch minnow hovering next to a twig in a lake.

Gee, ain’t tech wonderful?  

Lenny Rudow …has been a writer and editor in the marine field for over two decades, and has authored seven books. He is currently the Angler in Chief at Rudow's FishTalk Magazine, is Electronics and Fishing Editor for BoatUS Magazine, and is a contributing editor to several other publications. His...