Waveguide Engineering

Corrugated Filter

Q: How does a corrugated filter differ from a waffle-iron filter?

A standard corrugated lowpass filter uses full-width transverse ridges across the broad wall, which gives excellent harmonic rejection but is prone to trapped resonances of higher-order modes such as TE20 and TE30 in the wide full-height sections between ridges. A waffle-iron filter is a corrugated filter modified by cutting longitudinal slots through the ridges, creating a two-dimensional array of bosses that resembles a waffle iron. These transverse breaks raise the cutoff of the unwanted higher-order modes above the operating band, suppressing the spurious passbands that a plain corrugated filter would exhibit. The trade-off is more complex machining and slightly higher fundamental-mode insertion loss, typically 0.1 to 0.3 dB more than an equivalent plain corrugated section.

Q: How wide a stopband can a corrugated waveguide lowpass filter achieve?

A well-designed corrugated or waffle-iron filter can deliver a clean stopband extending from roughly 1.2 times the lowpass corner frequency out to 3 to 4 times the corner, with rejection of 40 to 60 dB across that range. In WR-28 hardware operating around 30 GHz, this means passing the Ka-band fundamental with under 0.4 dB loss while rejecting second and third harmonics at 60 and 90 GHz by more than 50 dB. The wide spurious-free stopband is the main reason corrugated filters are preferred over coupled-resonator topologies for harmonic suppression after frequency multipliers and high-power amplifiers, where coupled-line filters would re-enter passband at integer multiples of the design frequency.

Q: What sets the insertion loss and power handling of a corrugated filter?

Insertion loss in the passband is dominated by conductor loss on the ridge surfaces, which scales with the square root of frequency and inversely with conductivity. Silver or gold plating reduces loss by 15 to 30 percent versus bare aluminum, with a typical Ka-band corrugated filter showing 0.2 to 0.5 dB passband loss. Power handling is governed by the minimum gap between opposing ridges, where the electric field concentrates. A 0.5 mm gap in WR-28 supports several hundred watts CW at sea level, but the breakdown threshold falls sharply at altitude, so space-qualified filters are derated or pressurized. Surface roughness from machining also matters because it increases effective loss above the smooth-wall prediction.

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Built from a periodic series of transverse ridges machined into the broad walls of a rectangular guide, this waveguide filter forms a cascade of high-impedance and low-impedance sections that behaves as a lowpass filter. The fundamental TE₁₀ mode passes with insertion loss as low as 0.2 to 0.5 dB, while frequencies above the design cutoff frequency, especially the second and third harmonics from multipliers and amplifiers, are rejected by 40 to 60 dB across a stopband reaching 3 to 4 times the corner frequency. The waffle-iron variant adds longitudinal slots to suppress trapped higher-order modes, making corrugated filters the standard harmonic cleanup element in Ka-band and millimeter-wave transmit chains.

Category: Waveguide Engineering

Passband Loss: 0.2 to 0.5 dB

Stopband Rejection: 40 to 60 dB

How Periodic Ridges Build a Waveguide Lowpass Response

A corrugated filter is the waveguide realization of the classic stepped-impedance lowpass ladder. Instead of lumped inductors and capacitors, the design alternates short low-impedance sections (the narrow gaps over each transverse ridge) with high-impedance sections (the full-height regions between ridges). Because the TE₁₀ wave impedance scales with guide height, each ridge reduces the local height and lowers the characteristic impedance, while the full-height regions between ridges restore it. Cascading several of these steps, typically 5 to 11 sections, synthesizes a Chebyshev or maximally flat lowpass prototype directly in metal, with no dielectric and no soldered junctions to fail under thermal cycling or high power.

Because the structure is a true waveguide, the fundamental TE₁₀ mode propagates with very low loss in the passband, limited only by conductor resistance on the ridge surfaces. Above the corner frequency the periodic discontinuities present a steeply rising reflection, producing the lowpass skirt. Unlike coupled-resonator filters, which re-enter passband at integer multiples of their design frequency, a properly designed corrugated section maintains rejection across a broad, spurious-free stopband. This makes it the preferred topology for stripping harmonics generated by frequency multipliers, mixers, and saturated power amplifiers in radar and satellite uplink hardware.

The main weakness of a plain corrugated filter is the excitation of higher-order modes such as TE₂₀ and TE₃₀ inside the wide full-height sections between ridges, which can create narrow spurious passbands within the intended stopband. The waffle-iron filter solves this by cutting longitudinal slots through the ridges, breaking the broad gaps into an array of small bosses. The longitudinal slots raise the cutoff of the unwanted modes above the operating band at the cost of marginally higher fundamental-mode loss and more demanding machining tolerances.

Stepped-Impedance Design Equations

Section Electrical Length (commensurate line):
θ = βℓ = (2π / λ_g) × ℓ

High / Low Impedance Ratio (ripple driven):
Z_H / Z_L ≈ (b / b_gap) , with b_gap < b

Guide Wavelength:
λ_g = λ₀ / √(1 − (f_c / f)²)

Where β = phase constant, ℓ = section length, λ_g = guide wavelength, b = guide height, b_gap = ridge gap height, f_c = TE₁₀ waveguide cutoff (the lower band edge), and f_co = lowpass corner set by the ridge dimensions (placed just above the operating band). Example: WR-28 (a = 7.112 mm) gives f_c ≈ 21.1 GHz, so the guide carries the TE₁₀ mode from 26.5 GHz upward; a 9-section design with f_co ≈ 42 GHz passes the 26.5 to 40 GHz Ka-band and rejects the second and third harmonics near 60 to 90 GHz by > 50 dB.

Corrugated Filter vs. Other Harmonic-Rejection Topologies

Topology	Passband Loss	Stopband Width	Spurious Behavior	Power Handling	Best Application
Corrugated (ridged)	0.2 to 0.5 dB	1.2 to 4 × f_co	TE₂₀/TE₃₀ traps possible	Hundreds of W CW	Multiplier harmonic cleanup
Waffle-iron	0.3 to 0.7 dB	Up to 6 × f_co	Very clean, modes suppressed	Hundreds of W CW	Wideband, high-purity stopband
Coupled-line (planar)	1 to 3 dB	Re-enters at 2f₀	Spurious passbands	Low (watts)	Low-cost integrated PCB
Evanescent-mode	0.5 to 1.5 dB	Moderate	Compact, mode dependent	Moderate	Miniaturized lowpass
Coaxial stepped-Z	0.3 to 0.8 dB	2 to 3 × f_co	TEM, no waveguide modes	Tens to hundreds W	Broadband coax lowpass

Common Questions

Frequently Asked Questions

How does a corrugated filter differ from a waffle-iron filter?

A plain corrugated lowpass filter uses full-width transverse ridges, giving strong harmonic rejection but allowing trapped TE₂₀ and TE₃₀ resonances in the wide gaps. A waffle-iron filter cuts longitudinal slots through those ridges, creating an array of bosses that raises the unwanted modes' cutoff above the band. The result is a cleaner stopband at the cost of harder machining and roughly 0.1 to 0.3 dB more passband loss.

How wide a stopband can a corrugated waveguide lowpass filter achieve?

A good design holds 40 to 60 dB of rejection from about 1.2 times the lowpass corner out to 3 to 4 times the corner. In WR-28 near 30 GHz it passes the Ka-band fundamental with under 0.4 dB loss while rejecting the second and third harmonics at 60 and 90 GHz by more than 50 dB. That wide, spurious-free stopband is why it beats coupled-line filters after multipliers and amplifiers.

What sets the insertion loss and power handling of a corrugated filter?

Passband loss is dominated by conductor loss on the ridge surfaces, scaling with √f; silver or gold plating cuts it 15 to 30 percent below bare aluminum, giving 0.2 to 0.5 dB at Ka-band. Power handling is limited by the minimum ridge gap where the field concentrates; a 0.5 mm WR-28 gap supports several hundred watts CW at sea level but derates sharply at altitude unless pressurized.

Related Terms

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