Fish-Hawk FSR channel ?

[saskie]

It could be the radio receiver picking up the electrical pulses your finder uses to create the accoustic pulses that actually sound the bottom.

They're pretty low power and shouldn't affect anything out side of a few feet. Try moving a little farther away from the finder/transducer. Even a couple feet might make a big difference.

As for others talking about range - the range will be reduced more over water than over dry land as there is always a layer of higher-moisture air over water, which will impede and absorb radio waves. This layer will likely only be a few feet, but it does make a difference. Even standing up your transmitter and receiver will be at most 6ft above surface and the evapouration layer can really effect reception.