A system comprising: an electrosurgical microwave or RF energy source; a feedline including an inner conductor, an outer conductor coaxially-disposed about the inner conductor, and a dielectric material between the inner conductor and the outer conductor; and a probe operably coupled to the electrosurgical energy source by the feedline, the probe including: an antenna assembly defining a feed gap ("G") between a balun structure ("B") and a distal radiating portion ("R"); and the balun structure ("B"); characterised in that the balun structure ("B") includes: a balun insulator disposed about the outer conductor; and a conductive balun sleeve disposed about at least a portion of the balun insulator and electrically connected to the outer conductor; and in that the system further comprises: a flexible, fluid-cooled shaft including: an outer tubular member fluidly coupled to an interior lumen of the antenna assembly, and an inner tubular member disposed coaxially around the feedline and inside the outer tubular member, the inner tubular member, and the outer tubular member being adapted to circulate coolant fluid ("F") therethrough. an electrosurgical microwave or RF energy source; a feedline including an inner conductor, an outer conductor coaxially-disposed about the inner conductor, and a dielectric material between the inner conductor and the outer conductor; and a probe operably coupled to the electrosurgical energy source by the feedline, the probe including: an antenna assembly defining a feed gap ("G") between a balun structure ("B") and a distal radiating portion ("R"); and the balun structure ("B"); characterised in that the balun structure ("B") includes: a balun insulator disposed about the outer conductor; and a conductive balun sleeve disposed about at least a portion of the balun insulator and electrically connected to the outer conductor; and in that the system further comprises: a flexible, fluid-cooled shaft including: an outer tubular member fluidly coupled to an interior lumen of the antenna assembly, and an inner tubular member disposed coaxially around the feedline and inside the outer tubular member, the inner tubular member, and the outer tubular member being adapted to circulate coolant fluid ("F") therethrough. an antenna assembly defining a feed gap ("G") between a balun structure ("B") and a distal radiating portion ("R"); and the balun structure ("B"); characterised in that the balun structure ("B") includes: a balun insulator disposed about the outer conductor; and a conductive balun sleeve disposed about at least a portion of the balun insulator and electrically connected to the outer conductor; a balun insulator disposed about the outer conductor; and a conductive balun sleeve disposed about at least a portion of the balun insulator and electrically connected to the outer conductor; and in that the system further comprises: a flexible, fluid-cooled shaft including: an outer tubular member fluidly coupled to an interior lumen of the antenna assembly, and an inner tubular member disposed coaxially around the feedline and inside the a flexible, fluid-cooled shaft including: an outer tubular member fluidly coupled to an interior lumen of the antenna assembly, and an inner tubular member disposed coaxially around the feedline and inside the an outer tubular member fluidly coupled to an interior lumen of the antenna assembly, and an inner tubular member disposed coaxially around the feedline and inside the outer tubular member, the inner tubular member, and the outer tubular member being adapted to circulate coolant fluid ("F") therethrough.