Component Selection and Comparison Choosing Between Technologies Selection

How do I decide between a waveguide solution and a coaxial solution for a given frequency and power level?

Deciding between a waveguide and a coaxial solution depends on the operating frequency, power level, insertion loss tolerance, bandwidth requirements, and physical size/weight constraints. Waveguide is preferred when: operating frequency is above approximately 18 GHz (where coaxial cable losses become prohibitive, exceeding 1-3 dB/meter), power levels exceed 100 watts CW (where coaxial cables and connectors approach their voltage breakdown and thermal limits), minimum insertion loss is critical (waveguide loss is 5-10x lower than coaxial cable at the same frequency), and phase stability is important (waveguide has better thermal phase stability than coaxial cable). Coaxial cable and connectors are preferred when: operating frequency is below approximately 18 GHz (where coaxial losses are manageable), power levels are moderate (under 100 W CW for most connectors), flexibility is needed (waveguide is rigid; coaxial cable is flexible), size and weight are constrained (coaxial is more compact than waveguide at the same frequency), and broadband operation is needed (coaxial operates from DC to its upper cutoff frequency, while waveguide operates only within its single-mode bandwidth, typically 40-67% fractional bandwidth). At intermediate frequencies (12-40 GHz), both technologies are viable and the choice depends on the specific loss, power, and size requirements.

Category: Component Selection and Comparison

Updated: April 2026

Product Tie-In: All Components

Waveguide vs Coaxial Technology Selection

This is one of the most fundamental technology selection decisions in RF system design. The choice affects loss, power handling, size, weight, cost, and system architecture. Understanding the crossover points between the technologies is essential for practical RF engineering.

Loss Comparison

At 10 GHz: WR-90 waveguide: ~0.03 dB/m. Semi-rigid coax (0.141"): ~0.7 dB/m. Flexible coax (RG-58): not usable. Ratio: waveguide is ~23x lower loss
At 30 GHz: WR-28 waveguide: ~0.1 dB/m. Semi-rigid coax (0.047"): ~3 dB/m. Ratio: waveguide is ~30x lower loss
At 77 GHz: WR-12 waveguide: ~0.3 dB/m. Coax: essentially unusable at any practical length
At 2 GHz: WR-430 waveguide: very large (4.3" x 2.15"). Low-loss coax (LMR-400): ~0.07 dB/m. Coax is practical and far more compact

Power Handling Comparison

WR-90 waveguide (X-band): 120 kW CW at sea level. N-type coaxial connector: approximately 300 W CW at 10 GHz. SMA connector: approximately 100 W CW at 10 GHz. 2.4mm connector: approximately 20 W CW at 40 GHz. The waveguide advantage in power handling is enormous: 100-1000x higher than coaxial connectors at the same frequency.

Decision Matrix

Below 6 GHz: almost always coaxial (unless very high power). 6-18 GHz: coaxial for short runs, waveguide for long runs or high power. 18-40 GHz: waveguide preferred, coaxial for short interconnects. Above 40 GHz: waveguide for all but the shortest interconnects; above 65 GHz, waveguide is essential for any significant distance.

Waveguide vs Coaxial Performance

Coaxial loss: alpha ~ (Rs / (2 pi)) x (1/a + 1/b) / (Z0 x ln(b/a)) [Np/m]
Waveguide loss (TE10): alpha ~ Rs x (2b pi^2 + a^3 k^2) / (a^3 b k Z_TE sqrt(1-(fc/f)^2))
Coax power limit: P_max ~ V_breakdown^2 / (2 Z0) reduced by derating factor
Waveguide power limit: P_max ~ E_breakdown^2 x a x b / (2 Z_TE) [much higher]

Common Questions

Frequently Asked Questions

Can I use flexible waveguide?

Yes. Flexible waveguide uses a corrugated or bellows construction that allows bending while maintaining the waveguide cross-section. It has higher loss than rigid waveguide (typically 2-5x) and lower power handling, but it provides flexibility for connecting to rotating joints, vibrating platforms, and misaligned flanges. Flexible waveguide is commonly used in radar systems for the connection between the transmitter and the rotating antenna pedestal.

What about coax-to-waveguide transitions?

When a system uses both technologies (coax for short interconnects, waveguide for long runs), transitions are needed. A coax-to-waveguide adapter uses a probe (the center conductor of the coax extending into the waveguide) positioned at the proper depth and distance from the waveguide short wall to couple between the TEM mode of the coax and the TE10 mode of the waveguide. Good transitions achieve return loss better than 20 dB over the waveguide band.

What about SIW (substrate integrated waveguide)?

SIW is a planar waveguide structure fabricated in PCB using rows of plated vias to form the sidewalls of the waveguide within the PCB substrate. It combines the low-loss advantages of waveguide with the manufacturing simplicity and integration capability of planar circuits. SIW is increasingly used for mmW circuits above 30 GHz where microstrip losses become excessive.

Keep Reading