1,190,495. Microwave filters. STANDARD TELEPHONES & CABLES Ltd. 23 May, 1968, No. 24693/68. Heading H1W. A waveguide bandpass filter utilizes a length of square- or circular-section waveguide having a cut-off frequency above the operating frequency and capable of supporting evanescent H and E waves, the waveguide being provided with obstacles each having a capacitive reactance which is the conjugate of the positive imaginary characteristic impedance which the waveguide presents to evanescent waves. The cut-off waveguide 3 is connected at either end to propagating H 01 mode rectangular guides 1, 2 and consists of three sections of length l. The inner section is provided with a single capacitive screw (or iris) 6 and the two outer sections with opposed capacitive screws 4, 5. Screws 7 are for matching purposes. If each screw 4, 5, 6 is adjusted to provide a capacitive reactance conjugate to the inductive reactance of the respective section, it can be shown that each section behaves as a passband filter. Apart from this, the filter has additional properties caused by the transformation of the incident H 01 (or H 11 ) mode energy into E 11 (or E 01 ) mode energy by the capacitive obstacles 4, 5. It is shown in the Specification that an equivalent circuit which takes into account the effect of the evanescent E mode wave includes a parallel resonant circuit in the series branch, whereby the possibility of a rejection frequency arises. The frequency at which rejection occurs depends upon the H-#E transformation ratio determined by the screws 4, 5 and the sign of this ratio depends upon the polarity of the E mode relative to the incident H mode. This, in turn, depends upon which side of the centre-line of the guide the gap between screws 4, 5 is situated. Thus, the rejection frequency may be shifted by adjusting the relative penetration d 1 , d 2 of the screws 4, 5. Thus, the lowest rejection frequency is achieved with d 2 = 0 and D 1 = max., i.e. with all the screws on one side of the guide. With screws on alternate sides of the guide (d 1 = 0) the rejection frequency is more remote from the passband. To achieve a controllable rejection frequency, two sections are necessary, one with a single obstacle and the other with a pair of opposed obstacles, but the order in which they occur is not significant. Thus screws 4, 5 may be interchanged with screw 6 in the illustrated embodiment.