Cone Angle

Cone angle measured in degrees describes water “column” beneath the transducer where the sonar is active. It is important to understand the nuances of cone angle operation. If not the hardest decision, cone angle is one of the hardest decisions to make when buying a fish finder.

Wide cone angles are typically used with low frequency units, while narrow cone angles are used with high frequency units. Generally, angles above 12 degrees are considered wide angle, and below 12 degrees are considered narrow cone angle.

Unless experienced or specific needs are required, sometimes it’s best to go with the manufacturer included or recommended transducer.

Man-made cylindrical crystals distort and vibrate when a voltage is applied. Cone angle is a function of the diameter of the crystal, which is embedded within the transducer.

For the same frequency a larger diameter crystal will produce a smaller cone angle, and a smaller diameter crystal will produce a larger cone angle.

Precise sound waves are transmitted through water. If waves are measured at 50% signal strength (-3db) from the vertical axis of the cone, the resulting distance defines outer edges of the cone.

A 192 KHZ transducer with a 20-degree cone angle requires (approximately) a one-inch diameter crystal. Using the same frequency, an 8-degree cone angle requires a two-inch diameter crystal.

It is critical to understand cone angle concepts in order to use a fish finder correctly. Fish and structures found within a 3 dimensional cone are reported on a 2 dimensional screen.

This means a fish directly below the transducer is correctly reported as being directly below the transducer, while a fish 20 ft starboard and 3ft below the original fish may be incorrectly reported as being directly beneath the transducer and the 3 ft below original fish.

Dual frequency units can assist with this problem by rendering the more distant fish in light shades, hollow images, or different colors.

A narrow cone angle should be used for all saltwater applications when using high frequency e.g. 192-200 KHZ fish finders. A wide cone angle should be used for most shallow freshwater applications.

When a fish finder looks at 17 ft (or any wide) diameter bottom section, the strongest return signal may be the highest point of the cross section, but interpreted as bottom. The fish finder may ignore structure and any fish underneath the false signal point. This is known as a dead zone. Wide cone angles may cause dead zones, and another reason why dual cone angle units are advantageous.

Cone Angle is a function of crystal diameter, which is non adjustable but the transducer can be replaced to test or change cone angle.

Manufacturers specify cone angles differently. The strongest signal is always at the cone center. It was standard to specify the outer limits of the cone by establishing where (in the water) the signal was at half power (-3 db). However, some manufacturers are using one-tenth power (-10db) to specify the outer cone limits, because weaker signals are received and processed from greater distances.

According to Lowrance® LCX104C user manuals 80% of fishing needs can be meet by a 200 KHZ unit using a 12-degree cone angle (beam). The other 20% of usage is in saltwater from 300 – 500 feet or more, and by downrigger users who want to know where their baits are located. Presumably, because it is the Lowrance default angle, the optimal cone angle is 20 degrees on single frequency 200 KHZ units.