Using Dual Frequency Fish Finders

General

Question

​My dual frequency fish finder sometimes displays fish on the low frequency, but not on the high frequency. What could be the problem?

Solution

​The low frequency beam angle is wider than the high frequency, giving the low frequency a larger "footprint" or viewable area beneath the boat. 
 
The higher frequency beam width is narrower and may not simultaneously "see" mid water targets outside of its narrower field of view. While the lower frequency has a wider beam angle, the nature of the lower frequency allows it to travel further in the water column for better deep water sounding. 
 
The high frequency has a narrower beam angle but provides better resolution of bottom structure directly beneath the boat in shallower water.