I. Awai et T. Yamashita, A DUAL-MODE DIELECTRIC WAVE-GUIDE RESONATOR AND ITS APPLICATION TO BANDPASS-FILTERS, IEICE transactions on electronics, E78C(8), 1995, pp. 1018-1025
The fundamental TE(10) mode in a rectangular waveguide of a square cro
ss section is degenerate with TE(01) mode. A quarter wavelength resona
tor made of a dielectric square waveguide is, therefore, applied for a
small-sized bandpass filter,just like dual mode filters for base stat
ions in the mobile communication. In this paper, the methods to couple
the two modes are first studied, including cutting a corner of the re
sonator and adding some metal electrodes on its end face. Both methods
help to flow the rf current of the odd mode at the corner, resulting
in decrease of the series inductance and thus increase of the resonant
frequency. The coupling constant, that is proportional to the differe
nce of the odd and even-mode's resonant frequency, can be controlled b
y the perturbations mentioned above. The coupling to the external circ
uit is adjusted by an electrode fabricated also on the end face. It is
connected to a microstrip line and capacitively couples to the resona
nt modes. The coupling strength increases with the dimension of the el
ectrode. The adjustment of the resonant frequency is carried out by th
e similar electrode on the end face and connected to the center of the
side of the square cross section. The frequency decreases with the le
ngth of the electrode. The unloaded Q is measured to be of around 500
for 5 x 5 x 10 mm resonator of epsilon(r)=93. The optimum aspect ratio
for the resonator is found in terms of the Q value. The simplest band
pass filter, i.e., a two-stage bandpass filter is designed and fabrica
ted using 5 x 5 x 10 mm resonator. It is mounted in a square hole made
in a printed circuit board and excited by a microstrip line. The freq
uency characteristics are in good agreement with the expected values.