Article by Capt. Steve Katz
CHIRP
Sonar Basics
With today’s sales and marketing efforts from marine electronics manufacturer, what boater wouldn’t want a CHIRP (Compressed High Intensity Radar Pulse) sonar system? They offer improved detail over traditional systems and depending on frequency, can provide a lifelike image of the bottom, including detail of the underwater structure unseen by traditional sonar systems, All this at a price comparable to traditional transducers.
What’s the best way to operate a CHIRP sonar system? Years ago, with traditional fixed frequency 50/200 kilohertz(kHz) or similar, many captains adjusted the sonar before they left the dock and never changed it throughout the day. Some captains did adjust the gain (power) settings to compensate for deeper or shallower water, but many were happy as long as they had the bottom and/or fish echo blips on the screen.
With the increased capabilities of today’s CHIRP systems, many Captains find it beneficial to make constant adjustments to get the best picture, and therefore, the most accurate representation of the water column below their boat.
Frequency
Different frequencies can show different levels of detail. For example, a high-frequency such as 200 kHz setting shows excellent detail in shallow water, but its range is limited in deeper water. A low-frequency such as 50 kHz penetrates deeper into the water column, but shows less detail. The transitional depth at which one should use high frequency versus low frequency is dependent on many factors, mainly transducer type, placement, sensitivity and power output. For boats with high performance CHIRP transducers and top-of-the-line hardware, 300-feet is a good starting point for the crossover point from high frequency to low frequency. One of the best ways to determine what is best for your system is to set your machine on split screen using the high frequency and low frequency at the same time. This will allow you to compare side-by-side results in real time and the performance of your system at varying depths.
CHIRP Frequency
A CHIRP system sweeps a frequency range, not just a single frequency like a traditional tone burst sonar system. Instead of a 50 kHz or 200 kHz tone burst signal, a CHIRP transducer sweeps 42 kHz to 65 kHz on the low frequency and 150 kHz to 250 kHz for the high frequency. This example of CHIRP frequency range is for the popular Airmar B275LHW transduce. Other transducers have different, but similar ranges.
Not all fish are created equal when it comes to sonar. The fish composition, swim bladder size, muscle mass, etc. all contribute to a sonar echo that may be more visible at a given sonar frequency. Using a CHIRP sonar allows your system to scan a wide band of frequency range, as much as 100 kHz range on the B275LHW transducers high frequency, giving the sonar system the information to create a detailed echo image comprised of a wide range of frequencies.
Most CHIRP systems also allow you to dial in and set a specific frequency of your choice, within the range of operation for that transducer model. In the example above, you could set the high frequency at 180 kHz if that turns out to be a good frequency for your needs. Even setting a CHIRP traducer at a traditional fixed frequency of 50 kHz or 200 kHz is generally better than a tone burst transducer due to the low Q factor of a CHIRP transducer. The Q factor, or quality factor, is a calculation involving the bandwidth and acoustic pulse accuracy of the ceramics. The lower the Q factor, the better the performance of the transducer.
In general, a higher frequency shows more detail, but has limited working depth. A low frequency travels deeper but shows lower detail.
Cone Angle
The ceramic elements in a transducer are similar in function to a stereo speaker, sending out a sound wave in the direction that it is pointed. Some transducers send out a narrow signal, such as a 6-degree beam angle on high frequency and a wide signal, such as a 25-degree cone angle on low frequency. Why does this matter? With a narrow beam width angle, you can be more assured that a fish echo on your sonar screen represents a fish under your boat. With a wide beam width angle, you cover much more area, but a fish in the cone could be far away from your boat. If you know the beam width angle of your transducer, you can calculate your coverage area. For example, in 100 fathoms a transducer beam width angle of 10 degrees gives you bottom coverage area of 105-foot diameter circle, where as a 25-degree beam width gives you a 266-foot diameter circle of bottom coverage and increased water column coverage due to the larger cone. Most anglers like a wide beam angle since it lets them cover much more area in a single pass and if a fish echo is seen on the screen they know the depth and approximate lateral distance, and can therefore make multiple passes in that area in the attempt to catch that fish.
It is important to experiment with your system, try different frequencies, adjust the gain, TVG, scroll speed, noise rejection, interference, color gain and other variables to see what works best for the type of fishing you are doing that day. Different depths, sea state, salinity, water temperature etc., may require different settings. Learning about your sonar and understanding the echoes on the sonar screen will help you spend more time catching and less time fishing.
Captain Steve Katz is the owner of Steve’s Marine Service and holds NMEA, AMEI and NMEA2000 certificates along with ABYC Master Technician certification and factory training from many manufacturers. To reach Steve, call 410-231-3191.