Chirp transducer stories, the middle kid

Your fish finder can only show you things it sees in its sonic cone. Think of it as a search light shining into the dark. You could be feet away from the Spanish galleon filled with gold doubloons. But if it's not in the beam, you won't see it. This all begs the question, how much do you really get to see with your sonar system?

I'll start with the long cut, and I'll be brief. There is some math here, but through the miracle of the calculator you can do it. For both PC and Apple users locate your built in calculator and set it to "Scientific" mode. Make sure "degrees" are selected. Apple users will likely have to download their free scientific calculator. (Dashboard/Widgets/More Widgets/Calculate and Convert).

You only need a couple of pieces of information here. The water depth, and beam angle. The calculation is easy to do. Take your water depth and double it. Let's say it's 75 feet, double it and you get 150 feet. Now take the beam angle (in this case it's 26 degrees) and divide it by 2. You end up with 13 degrees.

Now enter 13 into the calculator, and the push the "tan" (tangent) button. What you get is a long number, but we are just going to use a couple of digits to the right of the decimal point. We end up with 0.23. Take the doubled depth number (150') and multiply it by the .023 and you get 34.6 feet. 

That wasn't so hard, was it? Now you know the diameter of the sonar cone base at the seabed in 75' of water. You also used some trigonometry for the first time in maybe decades and discovered you already had a scientific calculator in your computer. Go figure.

Here is the short cut. Go to Furuno's Beam Angle Calculator. Enter the water depth, and the beam angle, and push the calculate button. I thought it was better to teach a man to calculate so he learns to fish, than to just give him a grouper, or something like that. 

What, you don't know what your beam angle is? It's statistically likely you have an Airmar transducer. The way you find out is to look at the label on the transducer.

For through hull and in-hull units it's typically within a couple of feet of the transducer. For transom mounted units look near the plug end. Take the part number and search Airmar's website for the unit. The beam angle and frequency/s will be in their product information.

Middle frequency CHIRP transducers, as I obliquely referred to them in the title are not well understood. We know the low frequency units are for deep water, and the high ones are for shallow waters, So what's up with the middle kid?

The exercise in calculating the size of the transducer cone base had a real purpose. Pictured above is the Airmar B175 tilted element CHIRP transducer. This is available in the three flavors of high, medium and low frequencies.

We are only going to compare the high and medium versions of the B175 transducers today.

When using the cone angle calculator we are going to use the larger degree number, not the smaller one. The CHIRP starts with the low frequency and sweeps up towards the high frequency. Physics tells us the lower frequencies will generate a wider cone at the beginning of the CHIRP than the higher frequencies at the end of the CHIRP. You see all of the cone, but the higher the frequency, the more inwardly the cone is moving.

Here is a simple example. If you're in 10' of water, and using the higher frequency B175 transducer the Furuno cone calculator says you're seeing a circle on the bottom that is 2' in diameter. Whoa, that's not very large. If we were using the medium transducer we would see a circle that is 3' in diameter. Still not a whole lot. The Furuno calculator rounds up the numbers. The cones are really 1.74' and 2.81' respectively.

Now how about 100' of water. The B175H now has a cone that is 17' in diameter, and the medium unit has a cone that is 28' in diameter. How about 500'. High is now at a diameter of 87', and the medium is 141'.

At 100 feet using the B175 high you see 227 square feet of the bottom. With the B175 medium you see 615 square feet of the bottom. That's a little shy of three times more bottom area seen with lower frequency B175.

This exploration started with hearing an anecdotal report that fishermen were marking more fish with the medium frequency transducer than with their high frequency transducer. I think this is true.

If you are just installing a single frequency range CHIRP transducer there are some advantages if you pick the medium frequency transducer. The sonar cone is wider letting you see more. In the case of the B175 you see about three times more area of the bottom with the medium frequency version than the higher frequency version. Depth range is also increased, but this isn't a big factor if you're using it in shallower waters.

The big trade off however is you lose target resolution. So the real issue is how do you use your fish finding system? If you want sharp bottom resolution and are trying to separate fish targets from the bottom, or looking for that small ledge, the higher frequency unit is going to be better for you. If you're looking for that school of tuna, or a wreck, since the sonar cone is much larger you will see more of the bottom and fish in the larger water column with the medium frequency,  but with less resolution.

So what is the resolution difference between the higher frequency CHIRP transducer and the medium frequency, and the even low frequencies CHIRP transducers? Ah, this is not such a simple question to answer. The easy out is to say the resolution is dependent on bandwidth. This is generically true. The B175H sweeps from 130-210kHz. That's a bandwidth of 80kHz. The B175M sweeps from 85-135kHz for a total of 50kHz. The resolution of the medium transducer is lower because it's CHIRP bandwidth is less. 

It sounds straight forward, but I have come to believe in reality this isn't simple at all. There are many variables involved here. What is the resolution of the analog to digital data conversion? Does the CHIRP pulse length have a bearing on this? What is the impact of the transducer's Q value? Do more transducer elements give you better resolution? Is the way the software is written important? I'm going to revisit this subject in the future and see if we can get a better handle on this subject, or at least learn more about it.

Is CHIRP better? There is no doubt at all in my mind. I think the medium frequency B175 alone in shallower waters provides much better resolution, meaning target separation than the best of the typical marine 50/200kHz fish finders today. So maybe depending on your needs, you might want to give the middle kid a chance. We are also going to take a look at Airmar's new B275 high frequency CHIRP transducer with a 25 degree beam angle. It appears to have some new and interesting characteristics. For those with both high and medium CHIRP transducers capabilities, I would be interested in your opinion about this especially when used in shallower waters.