Coaxial Cable
One of the three fundamental wired transmission media — coaxial cable carries cable TV, cable internet, RF test signals
Anatomy of a Coaxial Cable
A coaxial cable (coax) consists of four layers: a central conductor (solid or stranded copper), a dielectric insulator(polyethylene or foam), a shielding braid (copper or aluminum mesh), and an outer jacket(PVC or PE). The central conductor and shield share the same geometric axis — hence "coaxial."
The concentric design provides two key advantages over simple wire pairs:(1) controlled impedance — the geometry defines the characteristic impedance (typically 50 Ω for RF, 75 Ω for video/CATV); and (2) electromagnetic shielding — the outer conductor acts as a Faraday cage, blocking external EMI and preventing radiation from the inner conductor.
Characteristic Impedance
The characteristic impedance of a coaxial cable is determined by the ratio of the outer conductor inner diameter (D) to the inner conductor outer diameter (d), and the dielectric constant (εᵣ) of the insulator:
For air dielectric (εᵣ ≈ 1), Z₀ ≈ 60 × ln(D/d) Ω. The two standard impedances are:
- 50 Ω: Optimized for power handling and low loss — used for RF test equipment, two-way radio, Wi-Fi, cellular, and Ethernet (10BASE2/5)
- 75 Ω: Optimized for minimum attenuation — used for CATV, cable modems, satellite TV (LNB to receiver), and video distribution
Common Coaxial Cable Types
| Type | Impedance | Diameter | Use |
|---|---|---|---|
| RG-6 | 75 Ω | 6.9 mm | CATV, cable modem, satellite TV |
| RG-58 | 50 Ω | 5.0 mm | 10BASE2 Thin Ethernet, amateur radio |
| RG-59 | 75 Ω | 6.1 mm | Baseband video, CCTV, older CATV |
| RG-11 | 75 Ω | 10.3 mm | Long cable runs, trunk lines, headend |
| RG-213 | 50 Ω | 10.3 mm | Military, HF radio, base stations |
| LMR-400 | 50 Ω | 10.3 mm | Cellular, WiFi, low-loss replacement for RG-213 |
Loss Mechanisms
- Conductor loss (skin effect): At high frequencies, current flows only in the outer skin of the conductor. Loss increases as √f. At 1 GHz, the skin depth in copper is ~2 µm.
- Dielectric loss: Energy absorbed by the insulator. Loss increases linearly with frequency. Foam dielectric (εᵣ ≈ 1.5) has lower loss than solid PE (εᵣ ≈ 2.3).
- Connector loss: Poorly installed or damaged connectors create impedance discontinuities causing reflections (VSWR).
Typical attenuation: RG-6 at 1 GHz ≈ 20 dB/100m; LMR-400 at 1 GHz ≈12 dB/100m; LDF5-50A (Heliax) at 1 GHz ≈ 5.5 dB/100m.
Coaxial Connectors
- BNC: Bayonet-lock, 50 Ω — used for test equipment, older Ethernet (10BASE2), video
- N-type: Threaded, 50 Ω — used for cellular base stations, WiFi antennas, RF test
- F-type: Threaded, 75 Ω — used for CATV, cable modems, satellite TV (consumer)
- SMA: Threaded, 50 Ω — used for GPS, WiFi, microwave, compact RF connections
- RP-SMA: Reverse polarity SMA — used for WiFi antennas (deliberate connector incompatibility)
- BNC: Bayonet, 75 Ω — used for SDI video, broadcast