A microwave surgical ablation probe having an arrangement of coolant channels in fluid communication with a cooling chamber disposed within the distal end of the probe is disclosed. A hypotube having one or more longitudinal ribs extending radially inward from an inner surface thereof is coaxially disposed around a coaxial feedline. The longitudinal ribs of the hypotube engage an outer sheath of the feedline to define a fluid inflow channel to deliver coolant to the cooling chamber, and a fluid outflow channel to receive fluid from the cooling chamber. The cooling chamber may be formed from porous ceramic or porous metallic material that provides structural support to the probe while permitting coolant to circulate therethrough. The probe includes dielectric and choke members that are adapted to control the microwave radiation pattern (e.g., ablation shape), and which may provide improved coupling of the probe to tissue.