Waveguide-to-Coax Transitions: Design, Loss Mechanisms, and Selection Guide

Every RF system that uses waveguide must eventually interface with coaxial cable. Test equipment has coaxial ports. Amplifiers have coaxial connectors. Mixers, filters, and switches are available in both coaxial and waveguide packages, but the interconnect between them requires a transition. The waveguide-to-coax transition (also called a waveguide adapter or waveguide-to-coax converter) is the component that converts the TE10 mode in a rectangular waveguide to the TEM mode in a coaxial transmission line.

This transition is one of the most critical components in a waveguide system because every dB of insertion loss and every mismatch ripple it introduces is multiplied by the number of transitions in the signal path. A typical test setup has four transitions (two on the DUT, two on the VNA cables). A poorly designed or damaged transition can render an entire measurement useless.

1. How It Works: Probe Coupling

The most common waveguide-to-coax transition uses a probe (also called a monopole or E-field probe) that extends from the center conductor of the coaxial connector into the waveguide cavity. The probe is positioned at a quarter-wavelength distance from the shorted back wall of the waveguide.

The Physics

• The coaxial center conductor extends through an aperture in the broad wall of the waveguide, forming a monopole antenna inside the waveguide.
• The probe couples to the electric field (E-field) of the TE10 mode, which is maximum at the center of the broad wall.
• The shorted back wall creates a standing wave that reinforces the coupling at the probe location (quarter-wave transformer effect).
• The probe depth, diameter, and distance from the back wall are tuned to achieve the best impedance match across the operating band.

2. Alternative: Loop Coupling

Loop-coupled transitions use a small wire loop that extends from the coaxial center conductor into the waveguide. The loop couples to the magnetic field (H-field) of the TE10 mode. Loop coupling is less common than probe coupling for standard transitions but is used in some specialized applications where the probe geometry is impractical (very small waveguide sizes, pressurized systems).

3. Loss Mechanisms

Loss Source	Magnitude	Controllable?	Mitigation
Mismatch loss	0.01 - 0.1 dB	Yes	Optimize probe geometry, tuning screws
Conductor loss (waveguide)	0.01 - 0.05 dB	Partially	Silver plating, polished surfaces
Conductor loss (probe)	0.01 - 0.03 dB	Partially	Gold or silver plated probe
Dielectric loss (bead)	0.01 - 0.05 dB	Partially	Low-loss dielectric (PTFE, air-line)
Radiation loss	0 - 0.02 dB	Yes	Proper flange seal, no air gaps
Total typical	0.05 - 0.2 dB

4. Selection Guide by Waveguide Band

Waveguide	Frequency	Common Coax Connector	Typical Insertion Loss	VSWR
WR-284	2.6-3.95 GHz	N-Type	0.05 dB	1.10:1
WR-137	5.85-8.2 GHz	N-Type or SMA	0.08 dB	1.15:1
WR-90	8.2-12.4 GHz	SMA or N-Type	0.10 dB	1.15:1
WR-62	12.4-18.0 GHz	SMA	0.12 dB	1.20:1
WR-42	18.0-26.5 GHz	2.92mm (K)	0.15 dB	1.25:1
WR-28	26.5-40.0 GHz	2.92mm (K)	0.20 dB	1.30:1
WR-15	50.0-75.0 GHz	1.85mm (V)	0.30 dB	1.35:1
WR-10	75.0-110.0 GHz	1.0mm (W)	0.40 dB	1.40:1

5. Specifying a Transition

When ordering a waveguide-to-coax transition, specify:

1. Waveguide size: WR-XX designation (e.g., WR-90)
2. Flange type: UG cover flange, UG choke flange, or CPR/CMR grooved flange
3. Coaxial connector: SMA, N-Type, 2.92mm, 1.85mm, or 1.0mm
4. Connector gender: Male or female (female SMA is most common for test applications)
5. Connector orientation: End-launch or right-angle
6. Performance spec: Maximum VSWR across the band, maximum insertion loss
7. Power handling: CW and peak power rating