# Microstrip Bandpass Filter

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 |
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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 |
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From the results in Figure 2 a spacing of s1 = 0.53mm is chosen for the final design of the filter. The S-parameters for this final design is shown in Figure 3. The filter has an S11 < -20dB passband of about 40 MHz (~1%) and S11 is nearly -40dB at 4.0GHz.

Figure 3: S-parameters of bandpass filter |
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### References

[1] | J. Zhu, N.K. Nikolova, J.W. Bandler, "Self-Adjoint Sensitivity Analysis of High-Frequency Structures with FEKO", ACES 2006. |