Side-by-side comparison
| Parameter | Coaxial | Microstrip Transmission Line |
|---|---|---|
| Structure | Inner conductor, dielectric, outer shield (cylindrical) | Conductor trace on dielectric, ground plane below |
| Characteristic impedance formula | Z₀ = (60/√ε_r)·ln(b/a) Ω | Z₀ ≈ (87/√(ε_r+1.41))·ln(5.98h/(0.8w+t)) for w/h < 1 |
| Typical Z₀ | 50 Ω (RG-58); 75 Ω (RG-59 video) | 50 Ω on FR-4 (ε_r=4.4): trace ≈ 2.9 mm wide on 1.6 mm board |
| Mode | Pure TEM (from DC to GHz) | Quasi-TEM (some fringing; not pure TEM) |
| Radiation loss | Negligible — fully shielded | Non-zero — open structure radiates at bends and discontinuities |
| Attenuation (typical) | RG-316: 1.1 dB/m at 3 GHz | FR-4 microstrip 50 Ω: ~0.8 dB/m at 3 GHz (dielectric loss dominant) |
| Integration with IC | Requires SMA/MCX connectors at PCB edge | Directly routed on PCB; integrates with SMD components |
| Power handling | RG-58: ~500 W at 30 MHz | Limited by trace width and PCB material; typically <10 W |
| Dispersion | Low — TEM mode is non-dispersive | Slight dispersion due to quasi-TEM nature |
| Cost | Higher — cable, connectors, shielding hardware | Lower — just PCB trace, no extra hardware |
Key differences
Coaxial line supports a true TEM mode with zero radiation because the outer shield fully encloses the field — RG-316 attenuates only 1.1 dB/m at 3 GHz. Microstrip is a quasi-TEM structure on an open dielectric (FR-4, ε_r ≈ 4.4); it radiates slightly at bends and discontinuities and its impedance is set by trace width, substrate height, and ε_r — a 50 Ω trace on a 1.6 mm FR-4 board is about 2.9 mm wide. Coax handles hundreds of watts; microstrip is limited to a few watts before dielectric heating dominates.
When to use Coaxial
Use coaxial cable when connecting discrete RF modules or test equipment at frequencies up to 18 GHz where shielding, power handling, or flexible routing is needed, such as a 50 Ω link from a signal generator to a DUT.
When to use Microstrip Transmission Line
Use microstrip when designing a 2.4 GHz LNA PCB, a Wi-Fi antenna matching network, or any circuit where the transmission line must connect directly to SMD components without connectors.
Recommendation
For PCB-level RF design at frequencies up to 6 GHz, choose microstrip — it integrates directly with your components and costs nothing extra. Use coaxial connections only at the board edge or for off-board signal routing where shielding and power handling matter.
Exam tip: Exam questions test the microstrip impedance formula and ask you to calculate trace width for 50 Ω on FR-4 (ε_r = 4.4, h = 1.6 mm) — practice this numerically; the answer is approximately 2.9 mm.
Interview tip: RF hardware interviewers will ask you to explain why microstrip is quasi-TEM while coax is true TEM, and how that affects radiation loss and dispersion — frame your answer around the open vs enclosed field structure.