seminggu diberi masa.....semalam baru siap (malam sebelum hantar pagi esok)......
punya susah la baru dapat maklumat yng En Hatta (BAND HUHA) nak minta ni...
siapa lagi kalau bukan big boss kami yang dapat maklumat ni...(tak pasti dia ambik kat mana)...
apa lagi..salin la..
kepada geng DKE 6A yng nak maklumat ni..ambik jak...
Coax cable attenuation / loss
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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