How are microstrip filters designed?

From RF Design Wiki · By the RF Design Wiki editorial team · Published October 21, 2026 · Last reviewed October 21, 2026 · How we research

Above roughly 1–2 GHz, filters are commonly built from transmission-line sections. Richards’ transformation maps lumped L and C to short- and open-circuited stubs, and Kuroda identities convert series stubs into realizable shunt stubs. Band-pass filters are usually built from coupled resonators: parallel-coupled lines, hairpin, interdigital or combline.^[1]^[2]

What common topologies are there?

Stepped-impedance low-pass: alternating high-Z (inductive) and low-Z (capacitive) sections; simple, moderate rejection, spurious passbands at multiples.
Open-stub low-pass/band-stop: shunt stubs from Richards’ transformation.
Parallel (edge)-coupled band-pass: λ/2 resonators overlapping by λ/4; long but well understood.
Hairpin: folded coupled-line resonators, compact.
Interdigital and combline: grounded λ/4 or shorter resonators; compact, good stopband.

^[2]

How does the design proceed?

Compute prototype values, then the required inter-resonator coupling coefficients k_i,i+1 = Δ/√(g_ig_i+1) and external Q values Q_e = g₀g₁/Δ. Extract physical spacings and tap positions from EM simulation of coupled-resonator pairs, then optimize the full layout in an EM simulator.^[2]

What limits distributed filter performance?

Spurious passbands at harmonics of the resonator frequency (2f₀ for λ/2 resonators).
Unequal even- and odd-mode phase velocities in microstrip degrade coupled-line filters; stripline avoids this.
Etching tolerance and laminate Dk variation shift center frequency; choose stable materials.
Radiation and box resonances; enclosures must not resonate in band.

Frequently asked questions

Why use EM simulation for microstrip filters?

Coupling, discontinuities and dispersion are hard to capture with closed-form models.

What is a Kuroda identity?

An equivalence that moves series stubs into shunt form using unit elements.

Are SIW filters distributed?

Yes; substrate-integrated waveguide cavities are used for low-loss mmWave filters.

References

Online sources accessed October 21, 2026; printed works cited by edition. See all sources.

“Microwave Engineering, 4th ed.”. D. M. Pozar, Wiley. 2012.
“Microstrip Filters for RF/Microwave Applications, 2nd ed.”. J.-S. Hong, Wiley. 2011.

Categories: Filters and passives

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