A helical antenna structure, for connecting to a coaxial transmission line, the coaxial transmission line having an inner conductor and an outer conductor, and the helical antenna structure having: a dielectric support, wherein the dielectric support is substantially cylindrical and has a rounded distal end, or a hemispherical portion at its distal end; a first helical electrode and a second helical electrode, both located on the dielectric support, and electrically isolated from each other; a first connection means for connecting the first helical electrode to the inner conductor of a coaxial transmission line; a second connection means for connecting the second helical electrode to the outer conductor of a coaxial transmission line; wherein: the dielectric support includes a channel or a chamber configured to receive the coaxial transmission line, such that a distal end of the coaxial transmission line is located at or near the distal end of the dielectric support; the dielectric support is configured to embed the distal end of the coaxial transmission line within the dielectric support, and first connection means and the second connection means are located in bores through the dielectric support, the bores arranged to connect, respectively, the first helical electrode to the inner conductor, and the second helical electrode to the outer conductor; at least one of the first helical electrode and the second helical electrode is configured to act as a radiating antenna structure for outwardly emitting a microwave/RF field; and the first helical electrode and the second helical electrode are configured to sustain an electric field in the helical region therebetween. a dielectric support, wherein the dielectric support is substantially cylindrical and has a rounded distal end, or a hemispherical portion at its distal end; a first helical electrode and a second helical electrode, both located on the dielectric support, and electrically isolated from each other; a first connection means for connecting the first helical electrode to the inner conductor of a coaxial transmission line; a second connection means for connecting the second helical electrode to the outer conductor of a coaxial transmission line; wherein: the dielectric support includes a channel or a chamber configured to receive the coaxial transmission line, such that a distal end of the coaxial transmission line is located at or near the distal end of the dielectric support; the dielectric support is configured to embed the distal end of the coaxial transmission line within the dielectric support, and first connection means and the second connection means are located in bores through the dielectric support, the bores arranged to connect, respectively, the first helical electrode to the inner conductor, and the second helical electrode to the outer conductor; at least one of the first helical electrode and the second helical electrode is configured to act as a radiating antenna structure for outwardly emitting a microwave/RF field; and the first helical electrode and the second helical electrode are configured to sustain an electric field in the helical region therebetween. the dielectric support includes a channel or a chamber configured to receive the coaxial transmission line, such that a distal end of the coaxial transmission line is located at or near the distal end of the dielectric support; the dielectric support is configured to embed the distal end of the coaxial transmission line within the dielectric support, and first connection means and the second connection means are located in bores through the dielectric support, the bores arranged to connect, respectively, the first helical electrode to the inner conductor, and the second helical electrode to the outer conductor; at least one of the first helical electrode and the second helical electrode is configured to act as a radiating antenna structure for outwardly emitting a microwave/RF field; and the first helical electrode and the second helical electrode are configured to sustain an electric field in the helical region therebetween.