Coaxial Cable Loss
Calculate signal attenuation in RF cables.
Understanding Coaxial Cable Loss
Coaxial Cable Loss (or attenuation) is the reduction in signal strength as it travels through a cable. No cable is perfect; they all resisting the flow of electrons to some degree. This energy is lost as heat.
Loss increases drastically with frequency. A cheap cable that works fine for a CB radio (27 MHz) might block almost 100% of a WiFi signal (2.4 GHz).
Calculating Signal Loss
Total attenuation depends on the cable type (dielectric material and shield quality), frequency, and length.
Loss (dB) = Length × (k₁√f + k₂f)
This is the "classic" model where:
- Resistive Loss (k₁√f): Due to Skin Effect (current crowdin on the surface). dominates at lower frequencies.
- Dielectric Loss (k₂f): Energy absorbed by the insulation material. Dominates at microwave frequencies.
Power Calculation
Once you know the loss in dB, you can find the remaining power:
Pout = Pin × 10^(-Loss_dB / 10)
Practical Applications
- WiFi Extensions: Moving an antenna 10 meters away using cheap RG-58 cable can result in 10dB of loss, meaning only 10% of your power reaches the antenna! You generally need LMR-400 for anything over 3 meters at 2.4 GHz.
- Ham Radio: Ensuring your 100W transmitter actually delivers 100W to the rooftop antenna, not just heating up the coax in your wall.
- Testing: Using a known length of cable to attenuate a signal that is too strong for a sensitive receiver.
FAQ
RG-58 vs LMR-400?
RG-58 is thin, flexible, and cheap, but has high loss (good for HF/VHF). LMR-400 is thick, rigid, and expensive, but has very low loss (essential for UHF/Microwave). For WiFi, always use LMR-series or equivalent.
Does SWR affect loss?
Yes. High SWR (Standing Wave Ratio) means power is reflected back and forth in the cable. Each pass through the cable incurs more loss. A "matched" line (Low SWR) is always the most efficient.
What is "Velocity Factor"?
Signals inside a cable travel slower than light. The Velocity Factor (VF) is the percentage of speed of light. Solid Polyethylene cables have VF ≈ 0.66, while Foam PE cables have VF ≈ 0.82. This matters for timing and phasing, but not directly for loss.