Attenuation (Insertion Loss)

Attenuation (Insertion Loss)

Attenuation is a measure of the ability of a component to carry an RF signal efficiently, and is the sum of the dielectric loss, conductor loss (copper loss), and radiation loss. Most of such power losses will be noted as an increase in heat in the specified material. Larger diameter conductors will provide less conductor loss; higher frequency will cause greater dielectric loss.

Since conductor loss increases by the square root of frequency, and dielectric loss increases linearly with frequency, dielectric losses become a larger proportion of total attenuation as frequency increases. Higher temperature increases attenuation by increasing the resistance of the conductors and the power factor of the dielectric. (Figure 2) gives a correction factor for attenuation versus ambient temperature.

αi (dB per 100’) = 0.435√ ƒ/(Zo x d)
αo (dB per 100’) = 0.435√ƒ /(Zo x D)
αd (dB per 100’) = 0.278ρ√ε x ƒ
αtotal = αi + αo + αd
(where α = the attenuation of the center or inner (i) conductor, outer (o) conductor, and the dielectric (d), ƒ = frequency in MHz, and ρ = power factor (loss tangent))

Figure 2 - Attenuation Temperature

Maximum attenuation variations can be specified over a defined frequency arrange, as attenuation of a cable assembly may not change uniformly with frequency range. Random and periodic impedance variations may produce irregular attenuation spikes, as soon in Figure 3. Attenuation of braided cables can also increase with time and flexure, although closed cell foam and non-contaminating dielectrics may prevent this type of degradation. (Figure 4)

The total attenuation from all sources plus any reflection losses over a specific length is called the insertion loss, expressed in dB per foot or 100 feet. Insertion loss of a network can also be defined as the difference in power arriving at the load with and without the network in the circuit.V

To select a cable for a particular application, determine an acceptable attenuation (after temperature correction) at the highest frequency in the system requirements. Then choose the smallest diameter cable meeting the corrected attenuation value from the cable specification.

Figure 3 - Attenuation vs. Frequency

Figure 4 - Attenuation vs. Flexure