Waffle-iron filter
It is a variation of the corrugated-waveguide filter but with longitudinal slots cut through the corrugations resulting in an internal structure that has the appearance of a waffle-iron.
Filters with an analogous design are now appearing in photonics, but, due to the higher frequency, at a much smaller scale.
Network synthesis is a more advanced method than image-parameter techniques but the latter can still be used where a simple repeated-pattern design is desired.
The limit is the frequency at which the distance between the metal teeth is greater than half the free-space wavelength of the signal.
The point at which this can start to happen is the frequency at which the height of the slot is greater than half the free-space wavelength of the signal.
[7] Other design criteria will usually result in a filter which does not match the waveguides to which it is to be connected at its input and output.
There are many structures that can be used for matching but a useful one here is the stepped-impedance transformer which has the added advantage of helping to suppress the unwanted slot modes.
[8] A common application of waffle-iron filters is to remove the harmonics of transmitters, such as high power radar, before applying to the antenna.
Legislation in most jurisdictions requires strict limits on out-of-band transmissions since these can cause serious interference with other stations.
For instance, to remove all harmonics up to the fifth it is necessary for a low-pass filter to have a stopband greater than three times the passband.
Steps need to be taken to prevent unsafe levels of microwave radiation escaping from these apertures which are often large to accommodate the product.
The waffle-iron is placed nearest the microwave chamber to first reduce the energy to a level which will not cause the absorbent lining to overheat.
Cohn's original data for the corrugated filter could also be applied to the waffle-iron with only a small adjustment of one parameter.
An alternative approach to using Cohn's empirical data, but still an image parameter design, is due to Marcuvitz who used a waveguide T-junction equivalent circuit to represent corrugations and this method was later extended by others to waffle-irons.
[13] One of the main drawbacks of the image parameter design method in this, as in other, filters is that the impedance match at the terminations is not good.
[14] A small improvement to matching can be had by starting and ending the filter on a half-space instead of a full tooth or space.
Not only does it take better account of the terminal impedances but the designer has additional degrees of freedom allowing improved matching.
The Zolotarev response has a stopband at low frequency, the cutoff of which can be controlled by the designer so it is not detrimental in a waveguide filter.
[17] Another design approach, particularly suitable for CAD because it is a numerical method, is to decompose the filter into a number of finite elements.
Whichever analysis method is used, the final output needed is the scattering parameters matrix for each element.
These spurious modes can be suppressed by fitting thin wires across the width of the filter in the space between the teeth on the vertical centre-line of the waveguide.
The units are connected together with λ/4 impedance transformer sections of waveguide in order of progressively higher frequency operation.
If tapering of the teeth is allowed, a two-unit design can often be reduced to a single unit with the same wide stopband.
For instance Matthaei describes a 1.2–1.64 GHz passband filter with rounded teeth and a wide stopband with a power-handling capability of 1.4 MW.
A filter operating in the 0.1 to 4.0 THz band has been built using parallel-plate waveguide (PPWG[24]) technology with 50 dB of rejection in the stopband.
