  Low-loss planar Goubau lines show promising potential for terahertz
applications. However, a single-wire waveguide exhibits less design freedom
than standard multi-conductor lines, which is a significant constraint for
realizing standard components. Existing filters for planar Goubau line lack
clear design procedures preventing the synthesis of an arbitrary filter
response. In this work, we present a design for a bandpass/bandstop filter for
planar Goubau line by periodically loading the line with capacitively coupled
{\lambda}/2 resonators, which can be easily tuned by changing their electrical
length. The filter's working principle is explained by a proposed
transmission-line model. We designed and fabricated a passband filter centered
at 0.9 THz on a 10-{\mu}m silicon-membrane substrate and compared measurement
results between 0.5 THz and 1.1 THz to electromagnetic simulations, showing
excellent agreement in both S11 and S21. The measured passband has an insertion
loss of 7 dB and a 3-dB bandwidth of 31 %. Overall, the proposed filter design
has good performance while having a simple design procedure.
