Power, Linearity, and Distortion Practical Power Topics Informational

What is the radial power combiner and when would I use it at millimeter wave frequencies?

The radial power combiner is a cylindrical structure where multiple PA outputs are arranged around the circumference and feed into a radial waveguide that combines all signals into a single coaxial or waveguide output at the center. The radial combiner provides: very low combining loss (0.1-0.3 dB for 8-32 way combining, because all signals travel the same electrical length from the periphery to the center), natural impedance transformation (the radial waveguide provides a smooth impedance taper from the high-impedance periphery to the low-impedance center output), and scalable power combining (4 to 60+ devices can be combined in a single structure). The radial combiner is particularly useful at millimeter wave frequencies because: transmission line losses increase with frequency (a 4-level binary Wilkinson combiner at 30 GHz would have 1.5-2.5 dB total loss from the transmission line sections; the radial combiner achieves 0.3-0.5 dB), the radial structure's dimensions scale naturally with the wavelength (at 30 GHz: the diameter is approximately 30-60 mm for a 16-way combiner), and the single-stage combining eliminates the multiple cascade levels of a binary tree (which accumulate loss). Applications include: wideband millimeter-wave EW transmitters, 5G mmW base station power amplifiers, satellite Ka-band and V-band transponders, and radar T/R module power combining.

Category: Power, Linearity, and Distortion

Updated: April 2026

Product Tie-In: Power Amplifiers, Combiners, Loads

Radial Power Combiner at mmW

The radial combiner was developed to overcome the combiner loss problem at high frequencies. At mmW: every 0.1 dB of combiner loss is significant because generating power is difficult and expensive.

Design Elements

Probe design: Each PA connects to the combiner through a probe (a small antenna) that couples to the radial waveguide mode. The probes are equally spaced around the circumference. The probe impedance must match the PA output impedance (typically 50 ohms) to the radial waveguide impedance at the probe location
Center extraction: At the center of the combiner: the combined signal is extracted through a coaxial probe or waveguide transition. The center impedance is very low (Z₀/N for N probes), requiring an impedance transformer to match to the 50-ohm output
Mode suppression: The radial waveguide can support higher-order modes that cause amplitude and phase imbalance. Use: mode-suppressing resistive sheets, slot patterns, or shape modifications to ensure only the desired TEM mode propagates

Radial Combiner Parameters

Radial combiner loss: L ≈ 0.1-0.5 dB (8-32 way)
vs. Binary Wilkinson: L ≈ 0.2×log₂(N) dB
For N=16: Radial ≈ 0.3 dB, Binary ≈ 0.8 dB
For N=32: Radial ≈ 0.5 dB, Binary ≈ 1.0 dB
Diameter: D ≈ N × λ/(2π) for peripheral probe spacing ≈ λ/2

Common Questions

Frequently Asked Questions

Who manufactures radial combiners?

Commercial radial combiner manufacturers: Microwave Solutions Inc. (MSI): standard and custom radial combiners to 40 GHz. Spatial Corporation: integrated spatial/radial combiners for mmW. Custom designs: many military and aerospace companies design radial combiners in-house for specific programs. At research level: university groups have demonstrated radial combiners at 94 GHz, 140 GHz, and 220 GHz for THz power combining.

What is the bandwidth?

Radial combiners are inherently wideband because: the radial waveguide mode is non-dispersive (TEM mode), and the probe design can be broadband (use a ridged or tapered probe). Typical bandwidth: 2:1 to 4:1 (an octave to two octaves) for well-designed radial combiners. At mmW: the bandwidth may be limited by the probe-to-waveguide transition (which becomes narrowband for small probes). A typical Ka-band radial combiner: 26-40 GHz (42% bandwidth) with 0.3 dB combining loss.

How does it compare to spatial combining?

Spatial (quasi-optical) combining: the PAs radiate into free space and the outputs combine in the propagation medium. Advantages: zero combiner structure loss (no metallic walls or transmission lines), scales to very large N (100+ devices demonstrated). Disadvantages: requires a re-focusing structure (lens or reflector) to capture the combined power, is physically larger, and is more complex to align. Radial combiner: more compact and self-contained. Better suited for: packaged commercial products and systems where a single coaxial output is needed. Spatial combiner: better for: highest N (100+) and highest frequency (above 100 GHz) where even the radial combiner's losses become significant.

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