A new theoretical analysis based on the contribution to the dual mode
filter response of each HEM(11) mode polarisation and of higher order
modes is presented. This analysis can be applied to any type of microw
ave dual mode filter. A description is given for a DR filter. The resu
lts enable the presence or absence of two transmission zeros in the fi
lter responses to be explained. It is shown how the frequency position
of the transmission zeros can be adjusted independently, to produce s
ymmetrical dual mode filter responses.