Waveguide Design and Selection Circular and Other Waveguide Types Informational

What is a substrate integrated waveguide and how does it compare to traditional waveguide?

Q: What via spacing do I need?

Via pitch should be less than λ/5 (guided wavelength in the dielectric) to prevent radiation leakage between vias. At 30 GHz on εr=3.5 substrate: λg ≈ 5.3mm, so via pitch should be less than 1.1mm. Via diameter is typically 0.2-0.4mm with standard PCB processes.

Q: Can I transition from SIW to microstrip?

Yes. SIW-to-microstrip transitions are standard and well-documented. The most common is a tapered microstrip line that gradually widens into the SIW aperture. Optimized transitions achieve better than 20 dB return loss over the full waveguide bandwidth.

Q: What frequencies are practical for SIW?

SIW is most beneficial above 20 GHz where traditional waveguide becomes expensive and microstrip radiation becomes problematic. It is widely used at 24 GHz (automotive radar), 28-39 GHz (5G mmWave), and 60-77 GHz (radar and communications). Below 10 GHz, the SIW dimensions become large and microstrip is usually preferred.

Substrate integrated waveguide (SIW) creates a waveguide structure within a PCB using two rows of metallic vias as the sidewalls and the top and bottom copper layers as the broad walls. The via rows confine the TE10-like mode, mimicking a dielectric-filled rectangular waveguide. SIW advantages: fabricated using standard PCB processes, integrates directly with planar circuits (microstrip, CPW), compact size (dielectric filling reduces dimensions by √εr), and low radiation. Tradeoffs: higher loss than metal waveguide (dielectric and conductor loss), limited to moderate power levels, and requires careful via spacing (pitch < λ/5) to prevent radiation leakage.

Category: Waveguide Design and Selection

Updated: April 2026

Product Tie-In: Waveguide, Horn Antennas, OMTs

SIW Design and Comparison

Substrate integrated waveguide bridges the gap between planar transmission lines (microstrip, stripline) and traditional metallic waveguide. It provides waveguide-like performance (good isolation, no radiation, defined mode structure) while being fabricated entirely within a standard printed circuit board. This eliminates the cost and complexity of machined metal waveguide components at millimeter wave frequencies.

Parameter	Standard Rect.	Ridged	Circular
Single-Mode BW	40% (1.25-1.9 fc)	50-150%	26% (1.31:1 ratio)
Attenuation	Low	Moderate (3-5x)	Low to very low
Power Handling	High (kW-class)	Moderate	High
Polarization	Single	Single	Dual (TE11)
Cost	Low (commodity)	Medium	High (specialty)

Mode Selection

The equivalent waveguide width of an SIW is calculated from the via parameters: Weff = Wvia_center - d²/(0.95·p), where Wvia_center is the center-to-center distance between via rows, d is the via diameter, and p is the via pitch. This effective width determines the cutoff frequency and the guided wavelength, just as the broad wall dimension does for metal waveguide.

Performance verification: confirm specifications against the application requirements before finalizing the design
Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
Margin allocation: include sufficient design margin to account for manufacturing tolerances and aging effects

Dimensional Constraints

SIW loss is typically 2-5× higher than metal waveguide loss at the same frequency because the dielectric (PCB substrate) contributes significant loss, and the via walls have higher effective surface resistance than smooth metal walls. On a low-loss substrate (Rogers RO4003C), SIW loss at 30 GHz is approximately 0.15-0.25 dB/cm, compared to 0.02-0.05 dB/cm for metal WR-28 waveguide.

Common Questions

Frequently Asked Questions

What via spacing do I need?

Via pitch should be less than λ/5 (guided wavelength in the dielectric) to prevent radiation leakage between vias. At 30 GHz on εr=3.5 substrate: λg ≈ 5.3mm, so via pitch should be less than 1.1mm. Via diameter is typically 0.2-0.4mm with standard PCB processes.

Can I transition from SIW to microstrip?

Yes. SIW-to-microstrip transitions are standard and well-documented. The most common is a tapered microstrip line that gradually widens into the SIW aperture. Optimized transitions achieve better than 20 dB return loss over the full waveguide bandwidth.

What frequencies are practical for SIW?

SIW is most beneficial above 20 GHz where traditional waveguide becomes expensive and microstrip radiation becomes problematic. It is widely used at 24 GHz (automotive radar), 28-39 GHz (5G mmWave), and 60-77 GHz (radar and communications). Below 10 GHz, the SIW dimensions become large and microstrip is usually preferred.

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