Coax cable attenuation / loss
- an overview of the effects and causes of attenuation or loss in coax
cable.
Attenuation is a key specification for all coax cables. The
function of a coax cable is to transfer radio frequency power from one point to
another. In doing so, in the ideal world, the same amount of power should exit
from the remote end of the coax cable as enters it. However in the real world
this is not so, and some power is lost along the length of the RF cable, and
less power reaches the remote end than enters the RF cable.
Coax cable attenuation
The power loss caused by a coax cable is referred to as
attenuation. It is defined in terms of decibels per unit length, and at a given
frequency. 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.
For virtually all applications the minimum level of loss is
required. The power is lost in a variety of ways:
- Resistive loss
- Dielectric loss
- Radiated loss
Of all these forms of loss, the radiated loss is generally
the least important as only a very small amount of power is generally radiated
from the cable. Accordingly most of the focus on reducing loss is placed onto
the conductive and dielectric losses.
- 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.
However 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 lost as
dielectric loss is dissipated as heat.
It is found that the dielectric loss is independent of the size of the RF
cable, but it does increase 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 found that the attenuation of coax cables increases
over a period of time for a number of reasons. The main reasons are as a result
of flexing, and moisture entry into the RF cable. As the degradation and
increase in loss depends to some degree on the construction of the coax cable,
this may affect he choice of which cable to employ.
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.
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. Additionally 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.
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.
The dielectric also has an effect. 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.
Although 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.
Summary
The loss introduced by a coax cable is of paramount
importance. Any power that is lost in the RF cable will degrade the performance
of the system in which it is used. However the decision of which RF cable to use
may not just rest in deciding which cable provides the lowest loss, but in a
variety of parameters including its size, weight and also its long term
stability
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