Thomas Keating and QMC Instruments Ltd have combined their skills to provide Dr Kawahata of NIFS with two
three channel, multi-polarization multiplexers for use in a diagnostic instrument on the Large Helical Device
in Nagoya. These combine co-polarized waves over three waveguide bands 10-26.5 GHz; 26-40 GHz and
40-60 GHz, though the use of in-waveguide low pass filters. Such filters are more associated with free-wave
operation and this is a novel application of this technology.
The solid construction multi-layer filters are shown below
and these are inserted into the precision machined waveguide holders designed and made by TK.
The results are really very good - given the constrained waveguide and high incidence angle of the filter's operation, recalling that the best a neutral density based system could do would be to give -3dB (high band), -6 dB (middle band) and -6 dB (lower band) of loss. This would be worse if you used conventional -3dB waveguide couplers, because part of the guide would be operating way out of band. The performance is shown below:
Our results give losses of -1 dB, around -2 to -3 dB and -1 dB in the Eplane (E field in plane of bend) slightly
worse in the H plane, in those respective filter bands. The benefit to NIFS in this lower loss is doubled, given
that both transmit and receive multiplexers are required