Coaxial Cables

Coaxial Cables: The Shielded Highway for Signals

Coaxial cable — or simply "coax" — has been a workhorse of communication systems for over a century. From the cable connecting your TV antenna to the massive submarine cables crossing ocean floors, coaxial cables provide a reliable, shielded path for signal transmission. Their unique concentric geometry gives them excellent electromagnetic shielding and consistent impedance characteristics that make them ideal for high-frequency signal transport.

Construction and Geometry

Think of it this way: a coaxial cable is like a pipe within a pipe, with insulation between them. This concentric geometry creates a self-shielding transmission line where the outer conductor acts as both a return path and an electromagnetic barrier.

From inside out:

1. Center conductor: Solid or stranded copper wire (carries the signal)
2. Dielectric insulator: Polyethylene, PTFE (Teflon), foam, or air (maintains spacing)
3. Outer conductor (shield): Braided copper, aluminum foil, or solid copper tube (carries return current and provides shielding)
4. Outer jacket: PVC or polyethylene (mechanical and environmental protection)

The geometry is defined by two key dimensions: inner conductor diameter (d) and outer conductor inner diameter (D). These determine the cable's electrical characteristics.

Characteristic Impedance

The characteristic impedance depends on the conductor dimensions and dielectric material:

Z₀ = (138/√εᵣ) × log₁₀(D/d) ohms

Where εᵣ is the relative permittivity (dielectric constant) of the insulating material.

Standard impedances:

• 50 Ω: RF/microwave equipment, cellular base stations, laboratory instruments
• 75 Ω: Video and TV distribution, cable television, satellite receivers
• 93 Ω: Early computer networks (now obsolete)

Why two different standards? 50 Ω minimizes attenuation loss for air-dielectric cables. 75 Ω minimizes signal reflections (best for wideband signals like video). The choice was made decades ago and standardized.

Signal Propagation

Signals travel through coax as TEM (Transverse Electromagnetic) waves:

Velocity of propagation: v = c/√εᵣ (typically 66-85% of the speed of light)

• Solid polyethylene (εᵣ = 2.3): v ≈ 66% of c
• Foam polyethylene (εᵣ = 1.5): v ≈ 82% of c
• Air dielectric (εᵣ ≈ 1.0): v ≈ 95% of c

Velocity factor (VF = v/c) is important for determining electrical length in antenna systems and delay calculations.

Attenuation

Signal loss in coaxial cable has two main components:

Conductor loss: Increases as √f (skin effect concentrates current at higher frequencies) Dielectric loss: Increases linearly with frequency

Total attenuation (dB/100m) at various frequencies:

Higher-quality cables with larger diameter, foam dielectric, and solid outer conductor achieve lower loss — essential for long cable runs and high frequencies.

Bandwidth and Frequency Range

Coaxial cable supports an extremely wide frequency range:

• Lower limit: DC (0 Hz) — coax can carry DC power + signals simultaneously
• Upper limit: Determined by when higher-order modes can propagate (cutoff frequency)
• Cutoff frequency: fc = c/(π(D+d)√εᵣ) — typically several GHz for standard cables

Practical frequency limits:

• RG-58 (5mm): Useful to ~1 GHz
• RG-6 (7mm): Useful to ~3 GHz
• Semi-rigid (3.5mm): Useful to ~18 GHz
• Precision lab cables: 40+ GHz with proper connectors

Shielding Effectiveness

The outer conductor provides electromagnetic shielding — protecting the inner signal from external interference and preventing signal leakage:

• Single braid: 60-80 dB shielding (adequate for most applications)
• Double braid: 80-100 dB (broadcast quality)
• Solid copper tube: >120 dB (CATV trunk lines, laboratory)
• Tri-shield (foil + braid + foil): 90-110 dB

For cable TV systems, shielding effectiveness is critical — inadequate shielding causes signal ingress (external signals leaking in, causing interference) and egress (cable signals leaking out, violating FCC emissions limits).

Applications

1. Cable Television (CATV): RG-6/RG-11 distributing 5-1000 MHz to homes
2. Cellular base stations: Low-loss cables connecting radio equipment to tower-mounted antennas
3. Broadcast: Connecting transmitters to antennas (high-power rigid coax)
4. Laboratory: Test equipment connections (precision 50Ω cables with SMA/N connectors)
5. Ethernet (historical): 10BASE2 (thin coax) and 10BASE5 (thick coax) — now replaced by twisted pair
6. Satellite receivers: RG-6 from dish to indoor receiver (950-2150 MHz IF signal)
7. CCTV/surveillance: Video signal distribution

Connectors

Key Takeaways

1. Coaxial cable's concentric geometry creates a self-shielding transmission line with consistent impedance, supporting DC to multi-GHz frequencies.
2. Characteristic impedance Z₀ = 138/√εᵣ × log₁₀(D/d) — standardized at 50Ω (RF) and 75Ω (video/broadcast).
3. Attenuation increases with frequency (√f for conductor loss) and decreases with cable diameter — larger cables mean lower loss.
4. Shielding effectiveness (60-120+ dB) prevents external interference from corrupting signals and prevents signal leakage.
5. The upper frequency limit depends on cable diameter — smaller cables support higher frequencies before higher-order modes propagate.
6. Despite competition from fiber optics and twisted pair, coax remains essential for cable TV, cellular tower feeds, broadcast, and laboratory applications.