Catheter-based applicators incorporating a microwave ablation antenna are provided. The applicator can be flexible and suitable for delivering ablation to target tissues via endoscopic approaches as well as intracavitary/intraluminal approaches. The applicators are configured to create large volume thermal ablation zones with minimal radiation losses along the applicator axis traveling proximal to the ablation zone. This effect is accomplished by employing a backward-facing choke element and/or a multi-slotted antenna having a plurality of spaced-apart conducting elements encircling the antenna.