A medical apparatus, including a probe having a distal end configured for insertion into a body cavity and containing a lumen that opens through the distal end, and an inflatable balloon deployable through the lumen into the body cavity such that when the balloon is deployed through the lumen and inflated, a distal pole on a distal side of the balloon is located opposite the lumen. In embodiments of the present invention, the medical apparatus also includes an electrode attached to the distal side of the inflatable balloon and extending over at least 50% of an area of the distal side of the balloon that is within 30° of arc from the distal pole.
