Design of A Broadband Bowtie Antenna for RF Spectral Measurements of Alfvén-wave in the KSTAR Tokamak

Abstract During KSTAR plasma experiments, torsional Alfven waves in the frequency of few GHz or below were detected. To understandthis plasma waves during the crash of MHD instabilities, an RF spectrometer has been developed for detection of the radiated RF signalsin the KSTAR Tokamak. It has the capability of broadband RF spectral measurement (50 ~ 400 MHz). To detect the broadband RFsignals which are radiated from the KSTAR systems, a broadband antenna is the key feature of the RF spectrometer. In this paper, abroadband bowtie antenna for detection of Alfven-waves in the KSTAR Tokamak is presented. Planar-type bowtie antenna is designedand fabricated on an FR4 substrate with thickness of 1.6 mm. The antenna consists of bowtie shaped balanced radiators and broadbandplanar balun. The antenna is designed to have an input impedance of 50 Ohm, and a taper-shaped balun is adopted for field and imped-ance matching between 50 Ohm transmission line to 110 Ohm feeding network of balanced radiators. The implemented antenna pro-vides around -3 to 3 dBi of gain for the whole frequency band. The VSWR of the bowtie antenna is less than 12:1 over the frequencybandwidth of 50 to 2000 MHz.Keywords: Bowtie, KSTAR, Alfven-wave, Tokamak, MHD