When deciding upon any form of coax cable it is necessary to assess the loss that it will introduce. The loss or attenuation is one of the key issues.
The loss introduced y the coax cable needs to be assessed n terms of what can be tolerated within the system. Obviously the minimum loss is ideal, but as low loss coax is more expensive, there is obviously a balance between loss and cost.
Coax cable attenuation
The power loss caused by a coax cable is referred to as its attenuation.
Obviously the longer the coax cable, the greater the loss, but it is also found that the loss is frequency dependent, broadly rising with frequency, although the actual level of loss is not linearly dependent upon the frequency.
As a result it is specified in datasheets in terms of decibels per unit length, and at a given frequency.
There are three main constituents to the coax cable loss.
- Resistive loss: Resistive losses within the coax cable arise from the resistance of the conductors and the current flowing in the conductors results in heat being dissipated. The actual area through which the current flows in the conductor is limited by the skin effect, which becomes progressively more apparent as the frequency rises. To help overcome this multi-stranded conductors are often used.
To reduce the level of loss due in the coax cable, the conductive area must be increased and this results in low loss coax cables being made larger. Importantly it is found that the resistive losses increase as the square root of the frequency.
- Dielectric loss: The dielectric loss represent another of the major losses arising in most coax cables. Again the power as a result of dielectric loss manifests itself as heat dissipated within the dielectric.
It is found that the dielectric loss is independent of the size of the RF cable, and it increases linearly with frequency. This means that resistive losses normally dominate at lower frequencies. However as resistive losses increase as the square root of frequency, and dielectric losses increase linearly, the dielectric losses dominate at higher frequencies.
- Radiated loss: The radiated loss of a coax cable is normally much less than the resistive and dielectric losses. However some very cheap coax cables may have a very poor outer braid and in these cases it may represent a noticeable element of the loss.
Power radiated, or picked up by a coax cable is more of a problem in terms of interference. Signal radiated by the coax cable may result in high signal levels being present where they are not wanted. For example leakage from a coax cable carrying a feed from a high power transmitter may give rise to interference in sensitive receivers that may be located close to the coax cable. Alternatively a coax cable being used for receiving may pick up interference if it passes through an electrically noisy environment. It is normally for these reasons that additional measures are taken in ensuring the outer screen or conductor is effective. Double, or even triple screened coax cables are available to reduce the levels of leakage to very low levels.
Coax cable attenuation with time
It is often tempting to utilise old cable that may be available. This may not always be advisable.
It is found that the attenuation of coax cables increases over a period of time for a number of reasons.
- Cable loss due to flexing: Although many coax cables are flexible, the level of loss or attenuation will increase, particularly if the RF cable is bent sharply, even if within the makers recommended bend radius. This increase in loss can arise as a result of disruption to the braid or screen, and as a result of changes to the dielectric. At frequencies of 1 GHz with RF cables normally exhibiting a loss of 10 dB, there could be an increase of a decibel or so.
- Increased attenuation due to braid contamination: Even if a cable is not flexed, there can be a gradual degradation in performance over time. This can be caused by contamination of the braid by the plastictisers in the outer sheath.
- Loss due to moisture entry : Moisture penetration can affect both the braid where it causes corrosion, and it may enter the dielectric where the moisture will tend to absorb power. The different types of dielectric affected differently. It is found that some versions of polyethene can absorb moisture more readily than other types. Although foam polyethene offers a lower level of loss or attenuation when new, it absorbs moisture more readily than the solid types. Accordingly coax cables with solid dielectric polyethene are more suited to environments where the level of loss needs to remain constant, or where moisture may be encountered.
- Loss due to degradation of braid: It is found that the loss in coax cables that use either bare copper braid, or tinned copper braid exhibit more degradation than those with silver plated braids, although the later are more expensive. Additionally it is found that braids using tinned copper exhibit about 20% greater loss than those using bare copper, but they are more stable over time.
Moisture entry is a major issue and athough RF cables are enclosed in a plastic sheath, many of the plastics used will allow some moisture to pass through them. For applications where moisture may be encountered, specialized cables should be used otherwise the performance will degrade.
The loss introduced by a coax cable is obviously very important because power lost can never be regained. This can degraded the overall system performance. However any decision about the type of coaxial cable to be used will be a balance of a number of factors including loss, size, weight, long term stability, and also its cost.