An ablation treatment apparatus has a multiple antenna device. The multiple antenna device includes a primary antenna with a lumen, a longitudinal axis and an ablative surface area of length L.sub.1. The multiple antenna device also includes a secondary antenna that is positioned in the primary antenna as the primary antenna is introduced through tissue. A secondary antenna distal end is deployed at a selected tissue site from the primary antenna lumen in a lateral direction relative to the longitudinal axis. A sensor is at least partially positioned at an exterior of the secondary antenna distal end at a distance L.sub.2 from the primary antenna along the secondary antenna distal end. L.sub.2 is at least equal to 1/3 L.sub.1. An energy source is coupled to the primary antenna.