Coaxial Feeder / RF Coax Cable Tutorial

- coax cable or RF coaxial feeder is a form of RF feeder - it

offers a relativly low loss, while remaining rugged and

flexible.

Coax cable tutorial includes:

Coaxial feeder overview Coax specifications Coax impedance Loss or attenuation Coax power rating Coax velocity factor Coax environmental factors Coax types & data Coaxial installation tips

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.

RF coax cable history RF coaxial 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.

What is coax cable? - the 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:

1. Centre conductor

2. Insulating dielectric

3. Outer conductor

4. 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

1. 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.

2. 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.

3. 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.

4. 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.

Coax Cable Specifications & Parameters

- definitions and explanations of the variety of specifications

and parameters used to define the performance of a type of

coax cable.

When choosing a type of coax cable to be used, it is necessary to understand its performance. Coax cable specifications define the performance so that decision can be made about which type to use for a given application.

In order to understand the performance of the coaxial cable it is necessary to understand the specifications for the different parameters.

Characteristic impedance specification Possibly one of the most defining coax cable specifications is its characteristic impedance. This is the impedance seen looking into an infinitely long length of cable by a signal source. The dimensions of the cable along with the dielectric used determine the overall impedance. This specification is measured in ohms and is resistive.

The most common impedance figures are:

50/52 ohms : This cable is the form that is generally used for professional RF applications. 75 ohms: This impedance is more widely used in domestic applications for television and

hi-fi RF signal leads. 93 ohms: Coax with this impedance specification was used in many early computers,

linking the computers themselves and also monitors. It was used because of its low capacitance level.

Other values of impedance are available although they are considerably less widely used. Some searching may be required to locate coaxial cable with an unusual impedance level.

Read more about characteristic impedance

Loss / attenuation specification Another major parameter for coaxial cable is its loss or attenuation. It is found that there is a degree of loss as a signal travels along a coax cable. This arises from a number of factors and is present on all cables. It is also proportional to the length.

The coax loss or attenuation parameter is specified in terms of a loss over a given length. It is generally specified in terms of a loss measured in decibels over a given length, e.g. 0.5dB / 10 metres.

Unfortunately not all manufacturers define the loss over the same length and therefore comparisons take a little more calculation to determine.

Read more about coax cable loss

Power rating specification Although for low level signal applications the power rating is unlikely to be important, where higher power levels are being carried, this specification can be an issue. Normally the limiting factor arises from the heat loss within the cable. If the power in the RF cable is to be pulsed, then it is necessary to check that the operating voltage is not exceeded.

Read more about power rating

Velocity factor specification The velocity factor specifications of a coaxial cable is the speed at which the signal travels within the cable compared to the speed of the signal (i.e. speed of light) in a vacuum.

In some instances, the velocity factor specification for the coax cable may be of importance. For many areas where the coax is simply being used for feeding signals from one point to another, it will not be important.

For applications where the phase of the signal is of importance, the velocity factor needs to be known.

The velocity factor specification is quoted as a figure which is less than "1". It cannot go above unity otherwise signals would be travelling faster than the speed of light.

It is found that cables have very similar velocity factor figures. This is because the dielectric between the two conductors governs the velocity factor. Cables using a solid polyethylene dielectric will have a velocity factor around 0.66, and those using foam polyethylene will have velocity factor figures ranging from about 0.80 to 0.88.

Read more about velocity factor

Capacitance specification For some applications the capacitance specification of the coax cable will be important. As can be imagined, there is a capacitance between the inner and outer conductors of the cable, and this is proportional to the length of cable used as well as the dielectric constant and the inner and outer conductor diameters.

Read more about coax capacitance

Maximum voltage In some applications the voltage may rise to high levels. At some voltage it is possible the cable may break down, causing damage to the cable itself.

Voltages can arise as a result of high levels of standing waves and high power levels. Checks should be made, before selecting a particular type of coax, that it will be able to withstand the level of voltage anticipated.

Coax mechanical dimensions specification The mechanical dimensions specification of the coax is important for a variety of reasons. The dimensions of different coax cables are obviously often different. Larger diameter coax cables often tend to have lower loss levels and higher power ratings.

As cable size may be important to ensure that it fits apertures etc this may be an issue. However one of the major reasons to know the size is to ensure that correct terminating connectors can be used. As connectors need to have the correct dimensions to ensure the cable will fit with the connector correctly, it is necessary to know the dimensions of cable. Often connectors will be made specifically for a popular size of cable.

Coax Impedance / Coaxial Cable

Impedance

- details of the calculation, application and determination of

coax impedance - coax cable impedance, inductance and

capacitance.

All forms of feeder including coax cable have a characteristic impedance.

The coax impedance is one of the main parameters in its specification, one that governs which type of coax cable is obtained.

Coax impedance / characteristic impedance All feeders possess 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 most commercial RF applications 50 ohms coax cable has been taken as the standard for many years.

The reason for the choice of these two impedance standards is largely historical but arises from the properties provided by the two impedance levels:

75 ohm coax cable gives the minimum weight for a given loss 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.

Coax impedance background When analysed a coaxial cable can be considered as a distributed series inductance with a distributed capacitance between the inner and outer conductors. The levels of inductance can be calculated as seen below.

Coax capacitance The capacitance of a coaxial 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 coax capacitance for a given length because the conductor spacing is decreased. The coax capacitance also increases with increasing dielectric constant, as in the case of an ordinary capacitor.

Where: C = Capacitance in pF / metre r = Relative permeability of the dielectric D = Inner diameter of the outer conductor d = Diameter of the inner conductor

Coax inductance The inductance of the line can also be calculated. Again this is proportional to the length of the line.

However the inductance is independent of the dielectric constant for the material between the conductors.

Where: L = Inductance in H / metre D = Inner diameter of the outer conductor d = Diameter of the inner conductor

Coax 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:

Where: Zo = Characteristic impedance in r = Relative permeability of the dielectric D = Inner diameter of the outer conductor d = Diameter of the inner conductor

Note: The units of the inner and outer diameters can be anything provided they are the same, because the equation uses a ratio.

Coax Impedance Calculator

Enter Values: Outer diameter:

Inner diameter: same units as above.

r:

Clear Data

Results: Impedance:

Importance of coax impedance The coax impedance is one of the major specifications associated with any piece of coax cable. As it will determine the matching within the system and hence the level of standing waves and power transfer, it is a crucial element. It is therefore necessary to ensure that the correct coax impedance is chosen for any system.

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.

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

RF coax cable power rating

- an overview of the maximum power levels that an RF coax

cable may be able to carry.

Although for receiver applications the level of power that coax cable can handle is not an issue,

when it comes to medium or high power transmitters, the power handling capacity of the coax

cable becomes very important. If the incorrect type of coax cable is used, it can result in a failure

of the cable, and possible damage to the transmitter.

For most applications where the power is applied continuously, the limiting factor arises from the heat loss within the cable. If the power in the RF cable is to be pulsed, then it is necessary to check that the operating voltage is not exceeded.

RF coax cable power loss

The major root cause for the limit in power handling capacity of an RF cable is the level of heat caused by the power losses occurring in the cable. If the temperature of the coax cable rises too high, the cable my become deformed and be permanently damaged.

For the resistive losses in the coax cable, it is found that most of the heat is generated in the centre conductor. Additionally any heat generated as a result of dielectric losses will be dissipated within the dielectric. It is therefore the construction of the dielectric that is of key importance in determining the power handling capability of the coax cable. Its maximum operating temperature, and its heat transfer coefficient both have a major effect.

It can be seen that the lower the losses of the cable the smaller the temperature rise, and the greater the power handling capability is for the cable. As a broad rule of thumb, lower loss cables will have a higher power rating than higher loss RF cables.

RF coax cable derating

Although a power rating may be given for a particular coax cable, it is often necessary to de-rate it to cater for non-optimal operating conditions.

The temperature of the environment is one factor. If the coax cable is operating in a high temperature environment, it will not be able to dissipate as much heat, and therefore the operating temperature will rise. Even at the highest foreseeable environmental operating temperature, the RF cable must be able to remain within its maximum internal temperature. Accordingly a de-rating factor is normally applied if the coax cable is to be used at high temperatures.

If the coax cable is operated under conditions where the VSWR is high, the cable rating needs to be reduced. The reason for this is that when there is a high level of VSWR, there are positions of high and low current along the coax cable. These may be such that they cause the power dissipation to rise significantly in some areas causing higher levels of power to be dissipated locally.

Altitude also has an effect, although at significant heights. If the cable is to be operated at altitude and hence under reduced pressure, the any cooling will be less effective. Therefore the temperature rise within the cable will be greater.

High power coax cables For coax cables where high powers are likely to be used, specially constructed cables are needed. If cables using ordinary polyethylene were used, then the might higher temperatures encountered would soon melt and distort the cable. This would then lead to its destruction.

For very high power applications, typically cables are used that possess an air dielectric and employed. The centre conductor is then held in place by a form of coil that runs along the length of the cable.

For medium to high power coax cables a Teflon dielectric can be used. This can withstand high temperatures of typically up to around 160C.

When considering which cable to use, it should be remembered that as the frequency increases, so the skin effect becomes more pronounced, and coupled with increased losses in the dielectric, this limits the power handling capacity.

Although the power handling capability of RF coax cable may not be an issue for many installations, when using medium or high power transmitters the power rating or handling capability of RF coax cable needs to be carefully considered.

Coax Cable Velocity Factor

- the velocity factor of a coax cable is the speed an

electromagnetic wave travels along a coax cable relative to the

speed in a vacuum.

The speed at which a signal travels within a coax cable is not the same as an electromagnetic

wave travelling in free space.

Instead it is affected by the dielectric that is used within the coax cable, and this has the effect of slowing the signal down.

The velocity factor can be of great importance in some applications, although for many purposes it does not need to be known.

What is velocity factor? The speed at which the signal travels is normally given the designation Vp or Vg and this is the faction of the speed at which the signal travels when compared to a signal travelling in free space. Thus Vp for a signal travelling at the speed of light would be 1.0, and for one travelling at half the speed of light it would be 0.5.

The velocity factor of the cable is found to the reciprocal of the square root of the dielectric constant:

Coax cable electrical length One important factor of a coax cable in some applications is the wavelength of the signals travelling in it. In the same way that the wavelength of a signal is the speed of light divided by the frequency for free space, the same is also true in any other medium. As the speed of the wave has been reduced, so too is the wavelength reduced by the same factor. Thus if the velocity factor of the coax cable is 0.66, then the wavelength is 0.66 times the wavelength in free space.

In some instances lengths of coax cable are cut to a specific length to act as an impedance transformed or a resonant circuit, then this needs to be taken into consideration when determining the required length of coax cable.

The advantage of using a coax cable with a low velocity factor is that the length of coax cable required for the resonant length is shorter than if it had a figure approaching 1. Not only does this save on cost, but it can also be significantly more convenient to use and house.

Dielectric materials There is a variety of materials that can be successfully used as dielectrics in coax cables. Each has its own dielectric constant, and as a result, coax cables that use different dielectric materials will exhibit different velocity factors.

Dielectric constants and velocity factors

of some common dielectric materials used in

coax cables

Material Dielectric

constant

Velocity

factor

Polyethylene 2.3 0.659

Dielectric constants and velocity factors

of some common dielectric materials used in

coax cables

Material Dielectric

constant

Velocity

factor

Foam polyethylene 1.3 - 1.6 0.88 - 0.79

Solid PTFE 2.07 0.695

From this it can be seen that the velocity factor of coax cables that use a polyethylene dielectric will have a velocity factor of 0.66 or thereabouts.

If resonant lengths of RF coax cable are to be used, then it is necessary to know the velocity factor of the coax cable. It is often possible to determine this to a sufficient degree of accuracy from a knowledge of the dielectric material.

Coax cable environmental resistance

- an overview of the environmental elements associated with

coaxial cables and the precautions to take when using them.

Coax cable, or as it is sometimes called RF cable can be expensive, but it is also required to

withstand some harsh environmental conditions. In view of its cost, care needs to be taken

otherwise the performance of the coax cable will deteriorate and the RF cable will require

replacement. By taking suitable precautions, the life of the coax cable can be maximised and the

periodic replacement costs for RF cables can be reduced.

There are many factors that affect coax cables to greater or lesser degrees:

Humidity and water vapour Sunlight Corrosive vapours and liquids

Effect of humidity and water vapour on coax

cables One of the biggest enemies for coaxial cable is that of water vapour. If it enters a coax cable then it can significantly degrade its performance, requiring the cable to be replaced. Moisture causes two main effects that give rise to an increase in the level of attenuation or loss in the cable. The first is an increase in resistive loss arising from oxidation of the braid that gives rise to an increase in the resistance of the braid or outer conductor in the coax cable. The second is an increase in the loss arising in the dielectric. Water absorbed into the dielectric heats up when power is passed along the coax cable. This heat is as a result of power loss in the cable.

Water vapour, or even water itself can enter the coax cable through a number of ways:

1. Through the termination of the coax cable (i.e. through connector or other termination method)

2. Through pin-holes in the jacket 3. By water vapour transmission through the jacket.

1. Moisture entry through coax cable termination The most obvious method of humidity entering a coax cable is through the termination. One very good example is the small termination box provided with many TV antennas. When used externally these termination enclosures provide little protection against the elements and the coax will quickly deteriorate. Even when a connector is used to terminate the coax cable there will be problems if the coax is used externally. Very few connectors are weather proofed, and even if they are supposedly weatherproof, then it is wise to take additional precautions.

Normally the best method is to use self-amalgamating tape. This tape comes in the form of a roll and appears like thick PVC tape but it has a thin paper backing on one side to keep each layer separate and prevents it amalgamating with itself before use. It is used in a similar way to insulating tape. The backing strip is peeled off and then it is wrapped around whatever it is to be waterproofed - in this case a coax cable termination - overlapping each winding by about 50% of its width to ensure a good seal. When applying the tape keep it stretched so that it is applied under tension. Also it is best to start from the thinner end of the job, i.e. where the diameter of whatever it is being applied to is smallest. Where there is a connector on a cable, start on the cable and work towards the connector. Also when winding ensure that there are no holes of voids in which water could condense or enter. Keep the self-amalgamating tape in intimate contact with whatever it is to be waterproofed.

2. Moisture entry though pin holes in coax cable jacket Most coax cables have some small holes in their jacket along their length. It is therefore quite possible that moisture will enter through any imperfection in the coax cable jacket. If the pin holes are located externally where they can be affected by the weather then moisture will enter. Unfortunately it is very easy for small abrasions to occur during the installation of a cable and these can include small pin holes right through the jacket. Great care must therefore be taken when installing a cable, and in particular when the coax cable is passed trough a wall or other barrier.

3. Water vapour transmission through the coax cable jacket All materials exhibit a finite vapour transmission rate. Accordingly if a coax cable is constantly in contact with moisture, then this will permeate through the jacket. In view of this coax cable should never be buried directly in the ground. Either use some external protection such as a waterproof pipe, and ensure that no water enters it so that small patches of water form in it. Alternatively use the "bury direct" cables that are available. It is also found in airborne applications that the large temperature extremes encountered cause water condensation in the coax cables. This moisture can collect in low areas of the cable causing local areas of corrosion. One method of overcoming this is to fill any voids in the aircraft where coax cable are carried with non-hardening moisture-proof compound.

Effect of sunlight on coax cables Sunlight has an effect on many substances, and the same is true of coax cable jackets or sheaths. It is particularly the ultra-violet light that causes the degradation to the cables. To increase the life of coax cables, manufacturers use high molecular weight polythene. Polyvinylchloride (PVC) jackets exhibit less than half the life expectancy of the high molecular weight polythene.

Effect of corrosive vapours on coax cables Using a coax cable in the vicinity of corrosive liquids and vapours can reduce the life of a cable faster than if it was used externally. Salt water is a common problem on sea going vessels, and chemical vapours may be present on other installations requiring coax cables. Although the rigours of the weather can be very tough, some vapours and liquids can speed the deterioration of the coax cable even faster. The use of tin or silver coatings can provide some additional protection but this is not permanent. However it is recommended that specially environmentally hardened cables be used where extreme conditions are anticipated.

Coax cables are normally quite tolerant to being used in a variety of conditions. However to ensure the longest operational life it is best to ensure that they are not exposed to environmental conditions that would cause their performance to deteriorate. If they are then it is necessary to adopt a few precautions to ensure that the coax cable life is maintained for as long as possible.

RF Coax Cable Types & Data

- table data including size, impedance, loss propagation

constant, etc for the more commonly used types of RF coax

cable.

There is a variety of different types of coax or coaxial cable that are in widespread use. Different types of coax cable or feeder are needed for different purposes and applications and accordingly it is necessary have specifications and data to be able to determine the required coax type or RF cable type easily.

While it would be possible to manufacture an infinite variety of RF cables, standard varieties are specified. There are two basic systems that are used for defining RF cables. One originated in the United Kingdom and its type numbers all start with UR. The other system is American with type numbers commencing with the letters RG.

The RG series was originally used to specify the types of coax cables for military use, and the specification took the form RG (RG from Radio Guide) plus two numbers. In some instances these numbers were followed by the letter U which indicated it was for multiple uses. These types of coax cable were all listed in the MIL-HDBK-216 which is now obsolete. Although full MIL specifications are now officially used for specifying most components for military use, the RG series of RF cables continued to be used because of its widespread acceptance. However it should be noted that the RG specifications are no longer maintained so there is no complete guarantee to the exact specification for the particular type of coax cable.

A summary of data for some of the more commonly used types of coax or coaxial cable is given below. Most of these RF cables are easily available from RF cable stockists.:

TABLE OF DATA FOR COMMON COAX CABLE TYPES

COAX

TYPE

CHARACTERISTIC

IMPEDANCE

OUTSIDE

DIAMETER

VELOCITY

FACTOR

ATTEN

@ 100

MHZ

ATTEN

@

1000

MHZ

COMMENTS

RG5/U 52.5 8.4 0.66 1.0 3.8

RG6A/U 75 8.4 0.66 1.0 3.7

RG9/U 51.0 10.7 0.66 0.66 2.4

RG10A/U 50 12.1 0.66 0.66 2.6

RG11A/U 75 10.3 0.66 0.76 2.6

RG12A/U 75 12.1 0.66 0.76 2.6

RG20A/U 50 30.4 0.66 0.22 1.2

RG22 95 10.7 0.75 1.5

RG23 125 24.0 0.52 2.0

RG24 125 25.5 0.52 2.0

RG34 75 16.0 0.46 1.8

RG58C/U 50 5.0 0.66 1.8 7.6

RG59B/U 75 6.1 0.66 1.2 4.6

RG62A/U 93 6.1 0.84 0.9 2.8

RG63 125 10.3 0.6 2.1

RG79 125 12.1 0.6 2.1

RG108 78 6.0 1.1 3.8

RG111 95 12.1 0.75 2.6

RG114 185 10.3 1.1 3.8

RG119 50 11.8 0.5 1.8

TABLE OF DATA FOR COMMON COAX CABLE TYPES

COAX

TYPE

CHARACTERISTIC

IMPEDANCE

OUTSIDE

DIAMETER

VELOCITY

FACTOR

ATTEN

@ 100

MHZ

ATTEN

@

1000

MHZ

COMMENTS

RG120 50 13.3 0.5 1.8

RG122 50 4.1 1.7 5.5

RG213/U 50 10.3 0.66 0.62 2.6 Polythene

dielectric

RG214/U 50 10.8 0.66 0.76 2.9 Double

screened,

silver plated

copper wire

RG223/U 50 5.5 0.66 1.58 5.4

UR43 50 5 0.66 1.3 4.46

UR57 75 10.2 0.66 0.63 2.3 Similar to

RG11A/U

UR67 50 10.3 0.66 0.66 2.52 Similar to

RG213/U

UR74 51 22.1 0.66 0.33 1.4

UR76 51 5 0.66 1.7 7.3 Similar to

RG58C/U

UR77 75 22.1 0.66 0.33 1.4

UR79 50 21.7 0.96 0.17 0.6

UR90 75 6.1 0.66 1.2 4.1 Similar to

RG59B/U

Data for attenuation figures are typical figures and measured in dB / 10 metres dimensions in mm

The RF cables described above are all flexible cable types. For microwave applications where very low loss is needed, semi rigid coaxial RF cable using a solid copper outer sheath may be used. This type of coax offers superior screening compared to RF cables with a braided outer

conductor, especially at microwave frequencies. As the name implies, though, it is not particularly flexible and is not intended to be flexed after it has been formed to the required shape.

This RF cable data has been presented as a guide and no liability can be taken for any errors or mistakes in the data. Naturally every care has been taken to ensure the data concerning these RF cables is correct.

RF Coax Cable Installation Guide

- essential points about how to install RF coax cable and the

key points to note to ensure the best performance is obtained

from the coaxial cable installation.

RF coax cable is widely used for a variety of professional RF applications where RF power either from a transmitter or to a receiver needs to be transferred from one point to another. While RF coaxial cable is very easy to install, it is necessary to ensure that a number of points are observed to ensure the coax cable installation is satisfactory initially, and then lasts. This is particularly important because many coax cable installations are external and need to withstand the rigours of the environment.

While coax cable may perform perfectly well when first installed, factors such as the ingress of moisture may cause the performance to degrade over time. Accordingly this performance reduction may pass un-noticed until the performance has reduced to a point where it may not be usable. By adopting a few simple precautions, the performance of the RF coaxial cable installation can be preserved and a much slower rate of degradation seen.

Coax cable areas to address The hints and tips to help install coax cable can be grouped into a number of categories:

1. Choosing the right coaxial cable 2. Weatherproofing the coax 3. General installation 4. Terminations / connections for the RF cable

Choosing the right coax cable There is an enormous variety of coax cables on the market, and at first sight the choice may not appear easy. The first decision to make, prior to any installation is to choose the required impedance. Domestic hi-fi and video antenna feeds use 75 ohm coax cable. Professional, CB, and amateur radio standardise on 50 ohm cable. Once this choice has been made the next decision will probably be made on the level of signal loss that is acceptable. Typically the lower the loss, the greater the diameter of the cable, and also its cost. Typically there are several cables with similar performance figures and often the decision of the exact type number will

depend on the stock position of suppliers. Once a suitable cable has been found then it can be purchased and installed.

Weatherproofing the coaxial cable When installing coax cable externally it is very important to ensure the cable is adequately weatherproofed. This is critical because any moisture entering the RF coax cable will produce a considerable increase in the level of loss. If any moisture passes into the dielectric material spacing the inner and outer conductors, this will impair the performance of the dielectric, and increase the level of loss. Moisture will also cause the outer braid to oxidise, and reduce the conductivity between the small conductors making up the braid.

It is therefore very important to seal the end of the cable if it is to be used externally, and ensure that no moisture enters. It is also necessary to ensure that the outer sheath of the cable remains intact and is not damaged during installation or further use.

An additional method of preventing large amounts of moisture entering the cable is to loop it up and down. In this way it is more difficult for water to enter the cable and then move along it. However if some moister enters the cable it will move into it by capillary action, so it is always best to ensure that the ends are properly sealed and protected.

General installation tips for RF coax cable All cables have a bend radius. In order to prevent damage they should not be bent into curves tighter than this. If RF coax cable is bent beyond its limit then damage to the inner construction of the cable may result. In turn this can lead to much higher levels of loss.

In a similar line, care should be taken to ensure that the cable is not crushed, or likely to be crushed. If the RF cable does suffer damage in this way, the dimensions of the cable will be changed and it will not maintain its characteristic impedance. Additionally if the dielectric between the two concentric conductors in the coax cable is damaged, then there is the likelihood of an increase in the level of loss.

While on the subject of physical damage to the cable, it is necessary to ensure that the sheath of the cable remains intact. If it is broken in any place, then this may allow moisture to enter if it is used externally, and this will cause oxidation and moisture retention in the dielectric that will increase the level of loss.

On some occasions it is necessary to bury coaxial cable. Ideally, normal cable should not be buried directly as this relies purely on the outer sheath for protection and it is not designed for these conditions. Instead it can be run through buried conduit manufactured for carrying buried cables. This has the advantage that it is easy to replace. However ensure that the conduit does not become water logged. Alternatively solution to using some form of conduit is to use a form of coax cable known as "bury direct". This is designed for being buried, and its outer sheath can withstand these conditions.

Coax terminations / connections When installing RF coax cable, it is important to terminate the cable correctly. In most instances the coax cable will be physically terminated using an RF connector, the electrical termination

being either at the antenna or in the receiver. Accordingly the connections to the connectors must be made correctly and the right quality RF connectors should be used.

Although connectors for domestic installations are often poor in terms of their electrical radio frequency performance, there is little alternative to using them in view of the fact that they have to mate with the RF connectors on the equipment. For professional applications, RF connectors can be far better, although it is necessary to ensure that the connectors are suitable for the frequencies used. Some cheap versions of RF connectors may not meet the full specification and can thereby impair the performance of the RF coax cable. It is therefore wide to always buy connectors from reputable sources.

By correctly installing a RF coax cable it can provide many years of satisfactory service. However wear, and exposure to the elements will mean that after some time it may be prudent to replace the RF coax cable. As the degradation in performance will be slow, it may mean that this is not noticed. Only when it is ultimately replaced will a major difference be seen.

Coaxial Feeder / RF Coax Cable Tutorial- coax cable or RF coaxial feeder is a form of RF feeder - it offers a relativly low loss, while remaining rugged and flexible.Applications of coax cableRF coax cable historyWhat is coax cable? - the basicsHow RF coax cable works

Coax Cable Specifications & Parameters- definitions and explanations of the variety of specifications and parameters used to define the performance of a type of coax cable.Characteristic impedance specificationLoss / attenuation specificationPower rating specificationVelocity factor specificationCapacitance specificationMaximum voltageCoax mechanical dimensions specification

Coax Impedance / Coaxial Cable Impedance- details of the calculation, application and determination of coax impedance - coax cable impedance, inductance and capacitance.Coax impedance / characteristic impedanceCoax impedance backgroundCoax capacitanceCoax inductanceCoax impedance determinationCoax Impedance CalculatorImportance of coax impedance

Coax cable attenuation / loss- an overview of the effects and causes of attenuation or loss in coax cable.Coax cable attenuationCoax cable attenuation with time

RF coax cable power rating- an overview of the maximum power levels that an RF coax cable may be able to carry.RF coax cable power lossRF coax cable deratingHigh power coax cables

Coax Cable Velocity Factor- the velocity factor of a coax cable is the speed an electromagnetic wave travels along a coax cable relative to the speed in a vacuum.What is velocity factor?Coax cable electrical lengthDielectric materials

Coax cable environmental resistance- an overview of the environmental elements associated with coaxial cables and the precautions to take when using them.Effect of humidity and water vapour on coax cablesEffect of sunlight on coax cablesEffect of corrosive vapours on coax cables

RF Coax Cable Types & Data- table data including size, impedance, loss propagation constant, etc for the more commonly used types of RF coax cable.

RF Coax Cable Installation Guide- essential points about how to install RF coax cable and the key points to note to ensure the best performance is obtained from the coaxial cable installation.Coax cable areas to addressChoosing the right coax cableWeatherproofing the coaxial cableGeneral installation tips for RF coax cableCoax terminations / connections