How does coaxial cable work?

From RF Design Wiki · By the RF Design Wiki editorial team · Published October 21, 2026 · Last reviewed October 21, 2026 · How we research

Coaxial cable carries a TEM wave between a center conductor of diameter d and a shield of inner diameter D. Its impedance is Z₀ = (60/√ε_r)·ln(D/d) ≈ (138/√ε_r)·log₁₀(D/d). The field is confined inside the shield, so coax is self-shielding and immune to nearby objects.^[1]

What limits the usable frequency?

Above a cutoff frequency the TE₁₁ mode can propagate, roughly f_c ≈ c / (π·√ε_r·(D + d)/2). This is why precision connectors shrink as frequency rises: 3.5 mm to about 34 GHz, 2.92 mm to about 40 GHz, 2.4 mm to 50 GHz, 1.85 mm to 67 GHz and 1.0 mm to 110 GHz. See RF connectors.

How is coax loss specified?

In dB per 100 m or per 100 ft at stated frequencies. Loss scales roughly with √f from conductor loss plus f from dielectric loss. Larger diameter and foam dielectric lower loss; semi-rigid and phase-stable test cables trade flexibility for stability.

What cable families are common?

RG-58 (50 Ω), RG-59 and RG-6 (75 Ω): general purpose and video.
RG-316, RG-174: thin, flexible; RG-316 has PTFE dielectric for high temperature.
Semi-rigid (.141, .086, .047 in): fixed interconnects in microwave modules.
Low-loss foam cables (e.g., LMR-type) for antenna feeds.

Always check the manufacturer datasheet for loss, power and bend radius.

Why does shielding effectiveness vary?

Braided shields leak through apertures; double braids, foil-plus-braid and solid outer conductors improve shielding. Leakage matters in EMC-sensitive and multi-channel systems; see EMC basics.^[2]

Frequently asked questions

What D/d gives 50 ohms with PTFE?

With εr = 2.1, ln(D/d) = 50√2.1/60 ≈ 1.21, so D/d ≈ 3.35.

Why is coax loss higher at high frequency?

Skin effect raises conductor resistance and dielectric loss grows with frequency.

Can I use 75 Ω cable for 50 Ω gear?

Short runs often work but introduce a 1.5:1 mismatch.

References

Online sources accessed October 21, 2026; printed works cited by edition. See all sources.

“Microwave Engineering, 4th ed.”. D. M. Pozar, Wiley. 2012.
“Electromagnetic Compatibility Engineering”. H. W. Ott, Wiley. 2009.

Categories: Transmission lines

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