Small size and ruggedness are two important factors in the selection of
bandpass filters for military and OEM applications. Monolithic ceramic block combline bandpass filters not
only offer a size advantage in UHF through Lband frequencies; they also have other characteristics that make
them extremely attractive when compared to other technologies. The filters are characteristically lower in
cost and have relatively good insertion loss due to their high Q material (Q>10,000). This paper describes the
design technique used for ceramic bandpass filters.
The procedure for designing ceramic bandpass
filters is straightforward and relies on standard filter theory. It is only in the construction stage of the 
realization that the structure becomes unique and the commercial attractiveness
becomes apparent. A design example is provided here together with an equivalent circuit for a Chebyshev
equalripple filter constructed with a material possessing a dielectric constant of 37. Ceramic materials with
low loss tangents (67 x 10^{6}) and high dielectric constants (37 and 78) provide a means to create
small coaxial structures which could be coupled to form combline bandpass filters. The sketch in Figure 1
shows the basic foreshortened quarterwavelength coaxial resonator structure. The resulting filters are
compact, rugged devices with low insertion loss in bandwidths of 0.5 to 6 percent. It is also possible to
realize transmission zeros in these devices and structures. 
Design Procedure
The design procedure for
these combline filters is based on papers by Matthaei (1) and Cristal (2) which include descriptions of the
physical structures required for their realization. It is necessary to determine the order of a filter based
on a given bandwidth, rejection, loss, etc. Using Reference 1, a low pass to bandpass transformation is
performed.
For a Chebyshev response, n is
obtained from:
Since n cannot be a fraction, it will be rounded up to the next highest integer. Once n is calculated, the
low pass prototype element values (or g values) are obtained (1). Using the above information, coupling
coefficients are given by (1):
To excite the TEM mode, resonators are
located in close proximity to one another. In doing that they become electromagnetically coupled via their
associated electric and magnetic fields. While designing such devices, the desired degree of coupling is
usually known, and it is required in order to determine the spacing necessary to achieve this coupling. By
using coupling coefficients (from equation 4), Reference 2, and transmission line theory, coupling
coefficients are adjusted.
