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it only needs to have the air removed from the bottom to work.being high in the hole also helps reject others pings if others are fishing. like an added ir feature. keep it centered all the time tho. if you move it to the side, the side it's moved to will/can reduce the ping return and blind it on that side of the area.being high also helps reduce tangles landing fish. this helps from cutting the cable too when no fishing string is yanking on it.they can work on top of the ice too if you put some water on the ice first and if the ice is not full of air bubbles. this will at least let you see bottom.
^^x2 Sonar will shoot through clear ice like its water. No need to go below the ice. I know Marcum says to have it just under the water. I'm not sure what the other manufacturers recommend. My take is Marcum says that because their cords are cheap and get damaged when down the hole. If i have 2 feet of ice and the xducer is only a couple inches down how is it going to produce a cone.To the OP, drop it below the ice and see if it works better.
What is happening ,there are days when everything is working great ,then the next day I go to same hole and I can not see my hook .