referring to fig1 there is shown generally at 10 a straight standard waveguide bandpass filter of the prior art . it comprises a waveguide tube 11 having slits 12 formed in a sidewall 13 and broadwall 14 thereof . inductive iris plates 15 of various lengths are positioned within the slits and secured therein . the filter is open at opposed ends and provided with an input securing flange 16 and an output securing flange 17 to interconnect the waveguide to associated electronic radio communication hardware ( not shown ). a disadvantage of such bandpass filters is that they require a long space for their installation , particularly at lower microwave frequencies , and this is sometimes undesirable . referring now to fig2 and 3 there is shown the construction of the temperature - stable folded waveguide bandpass filter 20 of the present invention . the folded waveguide bandpass filter is comprised of two straight waveguide sections 21 and 22 . each section is of a rectangular cross - section , and is defined by opposed parallel sidewalls 21 &# 39 ; and 22 &# 39 ;, respectively , and opposed parallel broadwalls 21 &# 34 ; and 22 &# 34 ;, respectively . one end of each of the waveguide sections is an open end 23 and 24 , respectively , and the opposite end is provided with an end wall 25 and 26 , respectively . a connecting flange 27 and 27 &# 39 ; is also provided about the open ends 23 and 24 , respectively , to interconnect the folded waveguide . when the waveguide sections are interconnected as shown in fig3 one of their broadwalls is an outer broadwall and the superposed broadwalls are the inner broadwalls . as clearly shown in these figures , each of the waveguide sections 21 and 22 have transverse slits 28 and 29 formed transversely in one of their sidewalls 21 &# 39 ; and 22 &# 39 ; and one of their broadwalls 21 &# 34 ; and 22 &# 34 ;. transverse slit 29 extends across the broadwall while transverse slit 28 extends only a predetermined distance across the broadwall , and this is to receive shunt inductive iris plates 30 in the slits 28 and cavity wall plates 31 within the slits 29 , the latter to segment the inner space of the waveguide sections , and to form resonating cavities 32 in both the waveguide sections . one of the broadwalls 21 &# 34 ; and 22 &# 34 ; of each waveguide section 21 and 22 are provided with coupling holes 33 which are positioned at predetermined locations with respect to the cavities 32 . these holes are elongated rectangular holes having rounded ends and are of a predetermined size to provide the proper coupling of the resonating cavities . these holes 33 are formed in a coupling broadwall of each of the waveguide sections . the shunt inductive iris plates 30 and the cavity wall plates 31 are soldered or brazed in position along the slits 28 and 29 respectively . this soldering or brazing is effected with care , and the surface of the broadwall and side wall , where the slits are positioned , is polished so that this interconnection is flush with at least the coupling broadwall which is intended to be interconnected . as shown in fig3 the coupling broadwalls 21 &# 34 ; and 22 &# 34 ; having the coupling holes therein are positioned juxtaposed with one another with the coupling holes in alignment . this can be done by suitable aligning pins ( not shown ) to provide substantially perfect alignment of the holes 33 . the open ends 23 and 24 of the waveguides are positioned at opposed ends of the juxtaposed sections , and these sections are soldered or brazed with one another to form the folded waveguide 20 of the present invention . it is pointed out that one of the waveguide sections 21 or 22 may not have a couping broadwall so that when the sections 21 or 22 are connected , a single coupling broadwall serves as a mutual coupling broadwall . the shunt inductive iris plates 30 and cavity wall plates 31 are preferably , but not exclusively , formed from a metal identified by the trademark &# 34 ; invar &# 34 ;. the end walls 25 and 26 of the sections 21 and 22 are also formed by plates of invar metal soldered or brazed to the waveguide tube sections . these invar plates provide for temperature stability and material compatibility of the waveguide . to reduce losses caused by solder joints of the invar plates forming the irises , copper or precious metal plating can be used to cover the inside of the filter . each cavity and coupling aperture is adjusted with a silver - plated stainless steel screw 35 which is positioned in the broadwalls and the sidewalls of the waveguide sections to optimize the filter performance . as shown in fig2 the center cavity has two coupling holes . this dual coupling system permits to have two identical waveguide sections thus reducing the number of different parts and simplifying the assembly . this approach is not possible with a single hole , since the hole cannot be positioned in the center of the cavity due to electrical properties of the coupling structure . the connecting flanges 27 and 27 &# 39 ; are also constructed of invar material to mate to associated radio interface circuitry . as can be seen from the six - cavity structure defined by the two juxtaposed waveguide sections , shown in fig2 and 3 , each waveguide section delineates two cavities in the space defined between the end walls 25 and 26 and the intermediate cavity wall plate 31 and in the space defined between the intermediate cavity wall plate and the open ends 23 and 24 . the two cavities are delineated by the position of one of the iris plates 30 . the cavity defined between the iris plate and the intermediate cavity wall in the space between the end wall and the intermediate cavity wall has two coupling holes in their broadwall . once these cavities are coupled together and after juxtaposing the sections , the tuning screws 35 are adjusted to tune the frequency of the resonating cavities . accordingly , it can be seen that the signal will follow a serpentine path through these juxtaposed waveguide sections to form a folded waveguide wherein the signal path is equivalent in length to a much longer straight standard waveguide bandpass filter , as shown in fig1 . the invention also envisages the method of constructing the temperature - stable folded waveguide filter of reduced length while maintaining the performance of the straight waveguide filter shown in fig1 . the method comprises providing the two waveguide sections of a construction described herein with transverse slits formed therein and inductive iris plates and cavity wall plates secured in these slits with the coupling holes aligned and the waveguide sections interconnected to one another on juxtaposed coupling broadwalls . as earlier stated , one of the waveguide sections may be void of a coupling broadwall . the filter is implemented following a design method known in the art , and as described by matthaei g ., l . young , and e . m . t . jones , in the publication &# 34 ; microwave filters , impedance matching networks , and coupling structures &# 34 ;, mcgraw - hill book company , new york , 1964 . reprinted by artech house , 1980 . the guided wavelength at the center frequency is given by : ## equ1 ## where λ0 is the free - space wavelength at the center frequency ƒ0 , a is the wider dimension of the waveguide . we can calculate the k - inverter parameters kj , j + 1 by the following mathematical analysis : ## equ2 ## where g0 to gn + 1 are the lowpass prototype element values , ω a is the number of cavities and ## equ3 ## δƒ is the filter bandwith . from the k - inverter values we can calculate the normalized shunt reactance x p / z 0 ## equ4 ## the length of each resonator is then given by l j where : ## equ5 ## a 4 . 8 ghz six - cavity folded filter was designed , built and tested . its frequency response is shown in fig4 . with a bandwidth of 52 mhz , the measured insertion loss was 0 . 34 db at the center frequency . in temperature variation from 0 ° to + 50 ° c ., we could notice a frequency shift of about 200 khz , but no changes in return loss or insertion loss performance was observed . the superposed constructed folded filter of this invention has very competitive performances compared to a standard waveguide filter with half its total length . it is within the ambit of the present invention to cover any obvious modifications of the preferred embodiment described herein , provided such modifications fall within the scope of the appended claims .