Q: What is Attenuation?
A: Attenuation is the sum of losses in the conductor and in the dielectric that determine the exponential loss occurring to a signal during a transmission in a cable. In other words, it is the gradual extinction of the energy through the transmission medium or a ratio that compares input power to output. It is measured in decibels per unit length, typically db/ft or db/m. Attenuation is highly effected by frequency. If a cable works well at low speed, then it may not with higher frequencies. The greater the attenuation in a circuit, then more signal will be lost. So lower attenuation cable is always better but it comes at a price. Designers must make tradeoffs between cost, availability and “good enough” designs that make sense for everyone.
However, not all attenuation is bad. In the medical realm ultrasound makes heavy use of attenuation to differentiate or characterize various structures in the human body. The attenuation is viewed as a reduction in amplitude of the ultrasound beam as a function of distance through the imaging medium.
Power (attenuation loss) can be lost in a variety of ways:
- Conductive Resistive loss – (dominate at lower frequencies)
- Dielectric loss – (dominate at higher frequencies)
- Radiated loss – (important but less dominant than the two above)
- Product usage and life – (attenuation increases with the life of the cable)
Dielectric Loss: To reduce dielectric loss, which is seen also as dissipated heat, properly understanding the frequency running through the cable is critical. Knowing the frequency range, be it high or low, will aid in the dielectric selection. Please refer to the 48 AWG low-capacitance coax in our catalog which has been optimized for higher frequencies with lower attenuation all because of the dielectric.
Radiated Loss: To reduce radiated loss focus needs to be place on the proper shielding. Radiated loss is typically more from interference than from heat loss, although heat being generated from a high speed signal is still possible. So there are two scenarios that shielding minimizes. These are radiated power or received power. Signals radiated from a cable are not desirable if there are sensitive receivers nearby. Likewise, received power is not desirable if there is a highly sensitive signal running through the coax. To prevent this many shield techniques can be employed including single layer spiral shield, foil tape, braided shields, tape and spiral/or braid, double and even triple shields, all in addition to changing the wire pitch which can have the effect of improving the shield coverage.
Product usage and life: Attenuation can increase over the lifespan of a cable. The primary reason is flexing with very small radii and moisture intrusion. Flexing losses can arise from bending due to disruptions in the braid material and the type of braid employed. If the braid is bent, small gaps are opened and there is an opportunity for radiated or received loss. Too tight of a bend and the dielectric may also be affected from the non-uniformity generated.
Loss due to moisture can occur when the shielding is corroded or the dielectric absorbs water, some dielectric materials more than others. Also, the presence of plastictisers in the outer sheath can lead to degradation and moisture intrusion. In terms of shielding:
- Bare copper braid – is the general standard.
- Tinned copper braid – creates 20% greater loss than bare copper but is more stable over time.
- Silver plated braids – have less loss than bare or tinned copper but are more expensive.