Filters are of the most common components in any communication network and in microstrip form integration with other components is easily achieved.  In [1] a design for a bandpass filter at 4.0GHz in microstrip form is given and its performance is evaluated as one of the design parameters is changed.  Figure 1 shows the layout of the filter and the variable parameter s1.  This filter is modeled in FEKO and a grid search algorithm in OPTFEKO is used to determine a value for s1 resulting in the smallest reflection coefficient at 4.0GHz.

Figure 1:  Microstrip filter layout

Figure 2 shows the input reflection (relative to 50Ohm) of the filter as the spacing s1 is varied.

Figure 2:  Reflection coefficient at 4.0GHz for variable s1

From the results in Figure 2 a spacing of s1 = 0.518mm is chosen for the final design of the filter.  The S-parameters for this final design is shown in Figure 3.  The filter has a S11 < -10dB passband of about 20 MHz (0.5%) and S11 is nearly -40dB at 4.0GHz.

Figure 3:  S-parameters of bandpass filter

References