In DEMO and future nuclear fusion reactors to produce economically competitive electricity, neutral beam (NB) systems will be required, delivering up to 150 MW of high neutral particle energy (1 MeV) with high wall-plug efficiency (η > 60%). In the Siphore NB concept [1], one of the promising candidates to meet these challenging requirements, negative ions will be extracted from a long, thin ion source 3 m high and 15 cm wide, accelerated and subsequently photo-neutralized. This requires the development of a new generation of negative ion sources. At the Swiss Plasma Center (SPC), a novel helicon plasma source, based on a resonant network antenna, is currently under study. The source delivers up to 10 kW at 13.56 MHz, and is installed on a linear (1.8 m long, 0.4 m diameter) vacuum vessel, allowing for full plasma characterization. In this work, the principles of operation of resonant antennas as helicon sources will be introduced, and absolute spectroscopic, Langmuir probe and interferometry measurements will be presented to characterize the performance of the source in terms of hydrogen/deuterium dissociation and negative ion production as a function of the input power at low gas pressure (0.3 Pa) and moderate magnetic field (~100 Gauss), as required by Siphore. This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission.