112 Kirilenko et al.
Figure 9. Frequency dispersion of K-inverters.
rations which result in reduction of overall size of a diplexer. Figure 8b shows one such configuration.
VII. DIPLEXERS BASED ON E-T
To begin with let us consider the simplest E-T diplexer shown in the inset of Figure 10. The K-scheme has been used to synthesize the diplexer. It consists of two 0.4 GHz bandwidth five resonator channel filters centered at f01 s 27.1 GHz and f02 s 27.8 GHz. This diplexer configuration offers better than 20 dB passband return loss and 40 dB isolation in the lower part of the operating bandwidth. If in the lower part of the operating band one can easily satisfy S1111. f S1122. s S1133. f 13 then in the upper part, where the waveguide height is close to lgr2, the reflection resonance of the TE10 mode incident from the side arm of the E-T lets S1111. tend to unity. Thus, the side and the straight arms are essen-
Figure 10. Frequency response of the conventional E-T diplexer.
tially decoupled and reasonable results cannot be achieved even in the middle part of the operating band by means of the K-scheme. The frequency location of the above mentioned resonance can be shifted up by decreasing the height b1 of the straight arms. Therefore, additional step transitions must be used to restore the height back to the nominal value of b at the output ports of the channel filters. However, these transitions can be easily incorporated into the last two K-inverters of the channel filters. Figure 11 shows that this approach involving double K-inverters offers at least 4 dB extra return loss in the passband. The operating band of the diplexer can be moved up by decreasing the height b1 more, however, at the expense of lower Q-factor and consequently higher passband insertion loss.
The inset of Figure 12 shows a viable remedy to the above problem, where an extra capacitive iris has been used at the input port. Using the Y-scheme and a numerical optimization proce-