8 pole combline L-band cavity filter

A tx filter to be part of a duplexer for an Inmarsat satellite terminal has been analyzed, synthesized by use of the CMS tool - and finally manufactured..
By experimenting a little with filter order and transmission zeroes it is found that in order to fulfill the specifications an 8 pole combline filter with two transmission zeroes on the low side of the pass band will do the job.
The wanted response is shown below to the left

Mechanical constraints have dictated that the two transmission zeroes must be implemented as two triplets in series, e.g. cross couplings between resonator 2&4 and between 5&7.

This topology is easily defined by the topology matrix feature in the CMS tool and the resulting coupling matrix is shown below to the right.

 Wanted response

 Corresponding coupling matrix

With the coupling coefficients now defined a prototype of the filter may be physically designed.

To find a relationship between resonator distances and coupling coefficients a couple of 2-pole test boxes may either be  manufactured - or modeled in a 3D EM simulator like HFSS. The negative cross couplings are implemented as capacitive rods which are also analyzed in HFSS. Once these relationships are established the first prototype can be manufactured.

The final result is shown below where the measured response is compared to the response obtained from the CMS tool.
It is seen that very good agreement is obtained.


Measured response

CMS simulated response



The final duplexer, where
the Tx filter is in the front.