Conventional Waveguide Harmonic Filters
Harmonic low-pass filters are often used in various RF/microwave/millimeter-wave systems as important components rejecting unwanted frequency spectrum. Three types of waveguide harmonic filters are known in the field of microwave engineering. The types are corrugated, waffle-iron and ridged evanescent mode filters. However all three types of conventional filters have disadvantages, which limiting their suitability for space applications because of several problems listed below.
The corrugated waveguide filter was firstly introduced by Cohn  in late 1940s. Later design methods using modern circuit theory and synthesis techniques have been fundamentally developed by Levy  and generalized for wide class of tapered corrugated waveguide filters . The problem of spurious pass-bands corresponding to high order modes has been reported by Levy . By Levy the spurious pass-bands cannot be eliminated although they can be moved up or down the frequency axis by changing width of corrugated waveguide. It is also stated that the spurious is due to E-plane uniformity of structure, which is a base line of the design. Some modifications have been applied to corrugated filters (for example [10,11]) since Levy’s patent was published, but those modifications are mostly applied to modify pass-band features without changing E-plane uniformity of corrugated structure and therefore the problem of spurious pass-bands still exists.
Waffle Iron Filters
The waffle-iron filter was firstly introduced by Cohn  in 1962. The conventional structure was represented as uniform two-dimensional periodic structure of rectangular teeth put into a waveguide and coupled with interface by E-plane stepped transformers. A design method based on representing the waffle iron structure as uniform waveguide was presented in . Limited power handling is considered as major disadvantage of the waffle-iron filter of Cohn’s design. In addition to power handling limitation spurious spikes in pass-band , roll-off and stop-band are reported . Those spikes are caused by excitation of waveguide modes of higher order, which are not taken into account in known design methods. Later modifications applied by Levy  and Sharp  do not considerably overcome those disadvantages.
Ridged Evanescent-Mode Filters
The first ridged evanescent-mode filter was introduced by Chapell in 1976, though similar comb-line filters were known earlier. The filter structure is based on ridges put in series in rectangular waveguide of evanescent dominant mode. In order to provide propagation of useful bandwidth through the filter, the gaps between the ridges (double ridged) or between the ridges and opposite surface are to be kept very tiny. Therefore peak power handling capacity of the ridged filter is very low comparing with non-evanescent type of filters. Small gaps also cause some quality and reliability problems. Therefore tuning can be often required to compensate production inaccuracy. Some modifications are proposed by Saad  in order to increase power handling by exciting the second evanescent mode. Nevertheless, low power handling of such filters remains being their major disadvantage.
Other Harmonic Filters
Besides to well known harmonic filters listed above other types of filters based on resonance irises, absorption or cascades of usual filters are sometimes used to suppress “harmonics”. On state of my knowledge all those methods are also not free from disadvantages such as high insertion loss, low power, large size or incompatibility to simple production methods.
Reasons of Failure of Conventional Filters
· Conventional filters are based on wave guiding structures having small cross sections with high strength of electrical and magnetic field where electrical breakdown is likely to happen, if transmitted power is high.
· Conventional filters are based on wave guiding structures susceptible of propagation of some other waveguide modes of order higher than the dominant.
· Some of conventional harmonic filters (waffle-iron) are based on structures susceptive of propagation of high order modes in the pass-band.
· Conventional design methods based on distributed or lumped circuits do not usually take the higher modes into account. Those design methods generally purpose to synthesize filter by pass-band and roll-off requirements rather than by upper stop-band where other modes exist and the original model is not valid.
 S.B. Conh “A theoretical and experimental study of a waveguide filter structure”, Office Naval Res., Cruft Lab., Harvard Univ., Cambridge, Mass., Rep. 39, Apr. 25, 1948.
C.G. Matthaei, L. Young, and E. M. T. Jones,
“Microwave Filters, Impedance Matching Networks, and Coupling Structures”,
 R. Levy “Tapered Corrugated Waveguide Low-Pass Filter”, IEEE Trans. Microwave Theory Tech., MTT-21, August 1973, pp. 526-532.
Levy, “Aperiodic Tapered Corrugated Waveguide
Filter”, US Patent 3,597,710,
 S.B. Cohn, US Patent 3,046,503, July 1962
 E.D. Sharp “A High-Power Wide-Band Waffle-Iron Filter”, IEEE, Trans. Microwave Theory and Tech., March, 1963, pp. 111-119
 H. Chapell “Waveguide Low-Pass Filter”, US Patent 3,949,327, 1976
Rodgers, Y. Carmel, P. O’Shea “Electromagnetic Effects on Integrated Circuits
and Systems at Microwave Frequencies”, Institute for Research in Electronics
and Applied Physics,
 Saad “Low Pass Filters with Finite Transmission zeros in Evanescent Modes”, US Patent 4,646,039, 1987
 R. Goulouev, “Corrugated Waveguide Filter Having Coupled Resonator Cavities”, US Patent 6,169,466
 R. Goulouev, “Waveguide Filter Having Asymmetrically Corrugated Resonators”, US Patent 6,232,853