Coaxial feeder or RF coax cable
- an overview of coax cable often called coaxial feeder or RF cable, used to
feed antennas and deliver radio frequency power from one point to another.
The most common type of antenna feeder used today is undoubtedly coaxial
feeder or coax cable. Coax cable, often referred to as RF cable, offers
advantages of convenience of use while being able to provide a good level of
performance. In view of this vast amounts of coax cable, coax feeder are
manufactured each year, and it is also available in a wide variety of forms for
different applications.
Applications of coax cable
Coax cable or coaxial feeder is used in many applications
where it is necessary to transfer radio frequency energy from one point to
another. Possibly the most obvious use of coax cable is for domestic television
down-leads, but it is widely used in many other areas as well. While it is sued
for domestic connections between receivers and aerials, it is likewise also used
for commercial and industrial transmission lines connecting receivers and
transmitters to antennas. However it is also sued where any high frequency
signals need to be carried any distance. Its construction means that signals
that the levels of loss and stray pick-up are minimised. In view of this it is
also used in many computer applications. Coax cable was used for some early
forms of Ethernet local area networks, although now optical fibres are used for
higher data rates, or twisted pairs where frequencies are not so high as these
cables are much cheaper than coax.
History of RF coax cable
RF coax cable is a particularly important part of today's RF
and electronics scene. It is a component that could easily be overlooked with
little thought of how it appeared. In the late 1800s there were a huge number of
basic discoveries being made in the field of electricity. Radio, or wireless as
it was originally called was not understood well, and the first transmissions
were made in the 1890s. Some transmissions were made earlier but not understood.
The first known implementation of coax cable was in 1884 when
Ernst von Siemens (one of the founders of the Siemens empire) patented the idea,
although there were no known applications at this time. It then took until 1929
before the first modern commercial coax cables were patented by Bell
Laboratories, although its use was still relatively small. Nevertheless it was
used in 1934 to relay television pictures of the Berlin Olympics to Leipzig.
Then in 1936 an a coaxial cable was installed between London and Birmingham in
the UK to carry 40 telephone calls, and in the USA an experimental coaxial cable
was installed between New York and Philadelphia to relay television pictures.
With the commercial use of RF coax cable establishing itself,
many other used the cable for shorter runs. It quickly established itself, and
now it is widely used for both commercial and domestic applications.
Coax cable basics
Coax cable, coaxial feeder is normally seen as a thick
electrical cable. The cable is made from a number of different elements that
when together enable the coax cable to carry the radio frequency signals with a
low level of loss from one location to another. The main elements within a coax
cable are:
- Centre conductor
- Insulating dielectric
- Outer conductor
- Outer protecting jacket or sheath
The overall construction of the coax cable or RF cable can be
seen in the diagram below and from this it can be seen that it is built up from
a number of concentric layers. Although there are many varieties of coax cable,
the basic overall construction remains the same:
Cross section though coaxial cable
- Centre conductor The centre conductor of the coax is
almost universally made of copper. Sometimes it may be a single conductor
whilst in other RF cables it may consist of several strands.
- Insulating dielectric Between the two conductors of
the coax cable there is an insulating dielectric. This holds the two
conductors apart and in an ideal world would not introduce any loss,
although it is one of the chief causes of loss in reality. This coax cable
dielectric may be solid or as in the case of many low loss cables it may be
semi-airspaced because it is the dielectric that introduces most of the
loss. This may be in the form of long "tubes" in the dielectric, or a "foam"
construction where air forms a major part of the material.
- Outer conductor The outer conductor of the RF cable is
normally made from a copper braid. This enables the coax cable to be
flexible which would not be the case if the outer conductor was solid,
although in some varieties made for particular applications it is. To
improve the screening double or even triple screened coax cables are
sometimes used. Normally this is accomplished by placing one braid directly
over another although in some instances a copper foil or tape outer may be
used. By using additional layers of screening, the levels of stray pick-up
and radiation are considerably reduced. The loss is marginally lower.
- Outer protecting jacket or sheath Finally there is a
final cover or outer sheath to the coax cable. This serves little electrical
function, but can prevent earth loops forming. It also gives a vital
protection needed to prevent dirt and moisture attacking the cable, and
prevent the coax cable from being damaged by other mechanical means.
How RF coax cable works
A coaxial cable carries current in both the inner and the
outer conductors. These current are equal and opposite and as a result all the
fields are confined within the cable and it neither radiates nor picks up
signals.
This means that the cable operates by propagating an
electromagnetic wave inside the cable. As there are no fields outside the coax
cable it is not affected by nearby objects. Accordingly it is ideal for
applications where the RF cable has to be routed through or around buildings or
close to many other objects. This is a particular advantage of coaxial feeder
when compared with other forms of feeder such as two wire (open wire, or twin)
feeder.
Characteristic impedance
All feeders posses a characteristic impedance. For RF coax
cable there are two main standards that have been adopted over the years, namely
75 and 50 ohms.
75 ohm coax cable is used almost exclusively for domestic TV
and VHF FM applications. However for commercial, amateur and CB applications 50
ohms coax cable has been taken as the standard. The reason for the choice of
these two standards is largely historical but arises from the fact that 75 ohm
coax cable gives the minimum weight for a given loss, while 50 ohm coax cable
gives the minimum loss for a given weight.
These two standards are used for the vast majority of coax
cable which is produced but it is still possible to obtain other impedances for
specialist applications. Higher values are often used for computer
installations, but other values including 25, 95 and 125 ohms are available. 25
ohm miniature RF cable is extensively used in magnetic core broadband
transformers. These values and more are available through specialist coax cable
suppliers.
Impedance determination
The impedance of the RF coax cable is chiefly governed by the
diameters of the inner and outer conductors. On top of this the dielectric
constant of the material between the conductors of the RF coax cable has a
bearing. The relationship needed to calculate the impedance is given simply by
the formula:
D = Inner diameter of the outer conductor
d = Diameter of the inner conductor
Capacitance and inductance
The capacitance of a line varies with the spacing of the
conductors, the dielectric constant, and as a result the impedance of the line.
The lower the impedance, the higher the capacitance for a given length because
the conductor spacing is decreased. The capacitance also increases with
increasing dielectric constant, as in the case of an ordinary capacitor.
It is also often necessary to know the inductance of a line as well.
Further reading
There are further articles and pages about coax cable on this
site. Topics include the velocity factor, coax cable loss, coax cable power
rating, and the environmental considerations for these RF cables. The menu can
be seen at the bottom end of the left hand menu.
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