Signal loss in RF cable – what is the numbers?

Coaxial cables have been the backbone of radio frequency (RF) signal transmission for decades, carrying everything from cable TV signals to cellular data. However, as signals travel through coaxial cables, they experience signal loss – a reduction in signal power. Understanding and accounting for this loss is critical for reliable system design.

In this 2024 update, we’ll cover the key factors affecting signal loss, how to accurately calculate total loss, and techniques to minimize loss in coax cable runs.

What Causes Signal Loss in Coax Cables?

There are several contributing factors to signal loss in coaxial cables:

1. Resistive Losses

The metallic conductors in coax cables have inherent electrical resistance, which causes some signal energy to be converted to heat. Thicker cables with larger conductors like LMR-400 exhibit lower resistive losses.

2. Dielectric Losses

The insulating material separating the inner conductor from the outer shield also absorbs some signal energy, converting it to small amounts of heat. This dielectric loss effect is independent of cable size.

3. Mismatch Losses

Impedance mismatches between cables or devices can cause signal reflections, leading to additional loss. Using properly terminated 50 ohm cables minimizes mismatch loss.

4. Leakage Losses

Imperfect shielding can allow some signal radiation and leakage, although this effect is small in high quality coaxial cables.

Calculating Total End-to-End Loss

The total signal loss in decibels over a cable run is determined by:

Total Loss (dB) = Cable Loss (dB) + Connection Loss (dB)

Cable Loss

Cable loss is specified by the manufacturer in dB per unit length, at particular frequencies. For example:

  • LMR-400: 0.18 dB per meter at 2.4 GHz
  • RG-58: 1.5 dB per meter at 2.4 GHz

The cable loss per length formula is:

Cable Loss (dB) = Cable Loss per Meter (dB/m) x Length (meters)

So a 10 meter run of LMR-400 would have 1.8 dB loss at 2.4 GHz.

Remember that every 3 dB loss corresponds to reducing the signal power by 50%.

Connection Loss

Each connector, adapter, or splice adds 0.1 to 0.5 dB of loss typically. Carefully assembled high quality connections minimize this loss.

Techniques to Reduce Signal Loss

There are several best practices to minimize total end-to-end loss:

  • Use thick, low-loss cables like LMR-400 or LMR-600 for long cable runs
  • Keep cable runs as short as possible
  • Ensure tight screw-on connections with proper torque
  • Use dedicated wireless bridges when feasible to eliminate cable loss

Proper planning and accounting for all sources of loss is key to maximizing signal transmission through coaxial cables. New loss testing devices like the Poynting EPNT-1 analyzer also simplify verifying cable assemblies.

Implementing these tips provides the strongest and most reliable RF signals possible over coax in 2024 and beyond. Let us know if you have any other questions!

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