rf/microwave coaxial cable tutorial

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RF/MICROWAVE COAXIAL CABLE TUTORIAL .

Abstract: A practical understanding of the physical and electrical parameters of coaxial RF and

Microwave Cables is needed in order to design, develop, and fabricate cable assemblies that will

meet the various challenges of their purpose. This tutorial is meant to provide the basic design

concepts and considerations to accomplish this, and is aimed at Applications and Sales engineers

that are interested in furthering their knowledge of the subject. Included is an overview of RF Cables

and their applications, the composition of coaxial cable, key electrical and environmental

specifications, the effects of environmental conditions including humidity and chemical vapors, and

the importance of careful and proper material selection.

© kSARIA Corporation. All rights reserved. 300 Griffin Brook Drive, Methuen, MA 01844 USA | www.ksaria.com | Phone: (866) 457-2742 | 2 P a g e

RF/Microwave Coaxial Cable Tutorial

Table of Contents: Page

Introduction 3

What is RF/Microwaves 3

RF Cable Overview 3

Cable Selection 4

Applications 4

Coaxial Cable composition 4-5

Key Electrical and Environmental Specifications 6

Impedance 6-7

Insertion Loss 8

Voltage Standing Wave Ratio (VSWR) 9

Velocity of Propagation 9

Phase matching/tracking 10-12

Coax cable attenuation with time 13

High power coax cables 14

Environmental Concerns 15

Effect of humidity and water vapor on coax cables 15

Effect of sunlight on coax cables 16

Effect of corrosive vapors on coax cables 16

Coax mechanical dimensions 16

Summary 18

Appendix A - Cable Selection Checklist 19

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Introduction It is important that sales and applications engineers understand the fundamental design constraints and factors that impact the quality and reliability of any cable system. This tutorial is meant to provide an introduction and basic understanding of RF and Microwave Cable design, construction, electrical specifications, and environmental considerations.

What is RF and Microwave?

RF (Radio Frequency) has become synonymous with wireless and high-frequency signals, describing anything from AM radio between 535 kHz and 1605 kHz to computer local area networks (LANs) at 2.4 GHz. Back in the day, RF had defined frequencies with a range from a few kHz to roughly 1 GHz. Today, most engineers also consider microwave and millimeter wave frequencies as RF, extending this range to about 300 GHz. kSARIA is currently equipped to fabricate and test RF Cable Assemblies that operate from 10MHz to 40 GHz (the limit of our test capability).

So, who’s using RF/Microwave signals? The following table shows some of the key RF allocations in the United States:

Frequency Range Allocated Purpose*

Kilohertz (KHz) Telegraph, AM Radio, Public Safety, Amateur Radio

Megahertz (MHz) CB Radio, Taxis, Trucks, Buses, Railroads, Aviation Communications and Radar, Lojack, Amateur Radio, Remote Control, Television VHF/UHF, FM Radio, Cellular Phones, GPS

Gigahertz (GHz) Radars (all types), LANs, Microwave Ovens, Vehicle anti-collision, Police Radar, RFID, Geostationary Satellites, Low Earth Orbiting Satellites, Cable TV Relay, Local Multipoint TV distribution, Direct TV, Fixed microwave communications (public and private)

Table 1 - Frequency Allocations

*Details and specifics can be found in: “FCC ONLINE TABLE OF FREQUENCY ALLOCATIONS 47 C.F.R. § 2.106”

Ravenous worldwide demand for bandwidth has pushed technologists to design systems that operate at ever higher frequencies where the spectrum is less crowded, system size is smaller, and data transfer speeds are as fast as possible. RF Cable Overview

Coaxial transmission lines were first invented in the late 1880s but had essentially no useful application until the late-1930s where they were first used for transmitting television signals between major cities. Today, the coaxial cable format is used extensively within the RF and Microwave communications industry. Modern coaxial cables are designed to simply transfer RF and Microwave signals from one point to another. By design, coaxial transmission lines are precisely matched to the source of the RF signal and to the receiver or load. It is this impedance match that ensures maximum transfer of the signal from source to load, or stated another way, perfectly matched impedance will yield absolute minimum loss of energy due to the insertion of the cable assembly into the system.


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A coaxial cable carries current in both the inner and the outer conductors. These currents are equal

and opposite and as a result all the electromagnetic fields are confined within the cable. This means

that the cable operates by propagating an electromagnetic wave inside the cable and by design it

neither radiates nor picks up signals. Since there are no fields outside the coax cable it is not affected

by nearby objects or stray signals, which makes it ideal for applications where the RF cable has to be

routed through or around buildings or close to other objects.

Cable Selection

When choosing a type of coax cable to be used, it is important to consider and understand its

intended performance. This requires a basic understanding of the various electrical and

environmental specifications. Currently industry supports an amazing array of cable choices and it is

critical that the specsmanship involved is well understood at the earliest stages of system

development. As we’ll see, the electrical and environmental specifications are not extremely difficult

to understand. The Cable Selection Checklist Appendix A should be used to capture key

performance areas that must be considered and carefully addressed.

Applications

Coax cable is used in many applications where it is necessary to transfer Radio Frequency (RF)

energy from one point to another. A familiar example of the use of coax cable is for domestic cable

television. Coax Cable is also used extensively for commercial and industrial applications connecting

RF receivers and transmitters to antennas as well as many other areas of system integration.

Regardless of the application, the construction of a coaxial cable is designed to ensure that signal

loss and interference are minimized.

Coaxial Cable composition

Coax cable is made from a number of different elements that, when properly designed and carefully

integrated, enable the coax cable to carry a Radio Frequency signal from point A to point B with bare

minimum loss of incident power. The main elements within a coax cable are:

1. Center conductor

2. Insulating dielectric

3. Outer conductor

4. Outer protective jacket

The overall construction of a coaxial cable is shown in Figure 1. The coaxial cable is built up from a

number of concentric layers. Note that the center conductor and outer conductor share the same

geometric axis and hence the name coaxial. Although there are many varieties of coax cable, the

fundamental elements of their construction are the same.


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Figure 1-Construction of a typical Coaxial Cable

1. Center Conductor - The center conductor of the coax cable is typically copper wire. Although normally a single solid conductor, in some applications, e.g., where the cable is likely to see repeated flexure or where extremely low insertion loss is needed, the coax cables are designed using a multiple strand center conductor. The center conductor is sometimes plated with silver for improved conductivity.

2. Insulating Dielectric - In between the center conductor and the outer conductor of the coax cable there is an insulating dielectric material. This dielectric not only physically holds the two conductors apart; it also establishes some fundamental electrical characteristics of the cable. The dielectric may be solid or as in the case of many low loss cables it may be semi-air spaced. This may be in the form of long tubes in the dielectric, a spiral wrap of dielectric tape, or a foam construction where air forms a major part of the material. Polyethylene, Foam polyethylene, and Teflon are the most common dielectric materials used.

3. Outer Conductor/metallic shield - 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, such as Semi-Rigid Cable. To improve the shielding effectiveness of the cable, double or even triple layered braid is sometimes used. Normally this is accomplished by placing one braid directly over another, or a copper foil or tape may be used. By using additional layers of shielding, the levels of stray pick-up and radiation are considerably reduced.

4. Outer jacket - Lastly, there is often a final cover or outer sheath to the coax cable. This jacket serves no electrical function, but provides protection against dirt and moisture. The outer jacket is also designed to prevent the coax cable from being damaged by other mechanical means such as abrasion. There are a wide variety of materials in use for the outer jacket, with selection based primarily on environmental considerations such as resistance to solvents, abrasion resistance, moisture resistance and thermal environment.


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Key Electrical and Environmental Specifications

Characteristic Impedance

The most important coax cable specification is its characteristic impedance (Zo). This is the impedance seen by a signal source when looking into a theoretical infinitely long length of cable. The physical dimensions of the cable, along with the dielectric used, determine the overall impedance.

For RF coax cable there are two main characteristic impedance standards that have been adopted over the years. The 75 Ohm coax cable is used almost exclusively for domestic TV and VHF/FM applications, and for most commercial RF applications, the 50 Ohm coax cable has been taken as the standard.

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 produced, but other impedances are available through specialist cable manufacturers.

As shown in Figure 2, a coaxial cable can be considered as a distributed series inductance with a distributed shunt capacitance between the inner and outer conductors. The levels of inductance and capacitance can be calculated as seen below.

Figure 2 Electrical Schematic of a Coaxial Cable


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Coax impedance determination

The impedance of the RF coax cable is governed by the diameters of the inner and outer conductors. On top of this the dielectric constant of the material between the conductors has a bearing. The relationship needed to calculate the impedance is given by the formula:

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

Figure 3 Cross Section of a Coaxial Cable

Coax capacitance The capacitance of a coaxial line varies with the spacing of the conductors and the dielectric constant of the insulator. As in the case of an ordinary capacitor, the coax capacitance increases with increasing dielectric constant. The capacitance can be calculated using the following formula.

Where: C = Capacitance in Picofarads/meter εr = Relative permeability of the dielectric D = Inner diameter of the outer conductor d = Diameter of the inner conductor

C = 24.1 X εr

log (D/d)

Zo = 18 log (D/d)

√ε


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Coax inductance The inductance of the line can also be calculated. Again this is proportional to the length of the line. Note that the inductance is independent of the dielectric constant of the material between the conductors.

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

Insertion Loss / Attenuation In transmitting an RF signal along a transmission line there is an amount of energy lost as the signal travels along the cable. The power loss caused by the insertion of a coax cable is referred to as the cable’s Insertion Loss and sometimes called the assembly’s attenuation. Insertion Loss is specified in terms of decibels per unit length, and at a given frequency. Obviously the longer the coax cable, the greater the Insertion Loss, but the loss is also frequency dependent, broadly increasing with frequency. For virtually all applications the minimum level of loss is required.

If the power transmitted to the load before insertion is PT and the power received by the load after insertion is PR, then the insertion loss in dB is given by:

Insertion Loss = 10 log Pt Pr

There are three main causes of Insertion Loss within a coaxial transmission line:

Resistive loss

Dielectric loss

Radiated loss

Radiated Loss is the least important as only a very small amount of power is radiated from the cable. For this reason most of the focus on reducing Insertion Loss is placed on the resistive and dielectric losses. Radiated loss is primarily controlled by the outer shield techniques used in the design of the cable.

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. Resistive Losses increase as the square root of frequency, a phenomenon referred to as skin effect. To reduce the level of loss, the conductive area must be increased and this results in low loss coax cables often having a larger overall diameter.

Dielectric Loss represents another of the major losses in coax cables. The power lost as Dielectric Loss is also a result of heat being dissipated. Dielectric Loss is independent of the size of the RF cable, but it does increase linearly with frequency. Since resistive losses increase as the square root of frequency, and dielectric losses increase linearly with frequency, the dielectric losses dominate at higher frequencies.

L = 0.459 log (D/d)


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Voltage Standing Wave Ratio (VSWR) VSWR is used as an efficiency measure for transmission lines. Any impedance mismatches or discontinuities in the cable will tend to reflect RF radio waves back toward the source end of the cable, preventing all the power from reaching the destination end. VSWR is a measure of the relative size of these reflections. An ideal transmission line would have a VSWR of 1:1, with all the power reaching the destination and no reflected power. An infinite VSWR represents complete reflection, with all the power reflected back down the cable, such as would be the case if the cable was short circuited, or open circuited.

Extreme care must be taken throughout the cable assembly process to ensure that all transitions such as cable connector soldering or crimping retain the physical characteristic of the cable and connector designs. Improper transitions are seen by the RF signal as discontinuities. These discontinuities result in part of the signal being reflected back to the source. Typical VSWR for a cable assembly for use in the frequency range of 2 – 18 GHz would be 1.35:1 or better. Voltage Standing Wave Ratio simply put is the ratio of the maximum to the minimum voltage of a standing wave (which is the instantaneous sum of incident and reflected waves).

Reflected power has three main implications in radio transmitters: Radio Frequency (RF) energy losses increase, distortion on transmitter due to reflected power from load, and damage to the transmitter can occur from excessive RF voltage.

Power rating Depending on the application, RF Power levels can range from a fraction of a watt to kilowatts. For low level signal applications the power rating is unlikely to be important. Where higher power levels are being carried, this specification becomes very important. Normally the limiting factor is the cable’s ability to dissipate the heat generated within the cable. Beyond that, if the power in the RF cable is to be pulsed, then it is necessary to check that the operating voltage is not exceeded.

Maximum voltage In some applications such as high power antenna feed lines, the RF voltage can be at very high levels and if the maximum voltage rating is exceeded, the cable may break down, causing damage to the cable itself. High voltages can also arise as a result of high levels of standing waves caused by an impedance mismatch (defect or damage) somewhere along the length of the cable. It is important that before selecting a particular type of coax, the design engineer ensures that it will be able to withstand the level of voltage anticipated.

Velocity of Propagation The velocity of propagation (Vp) through a coaxial cable is defined as the speed at which the RF signal travels within the cable relative to the speed of the same signal traveling in a vacuum (i.e. speed of light). For many applications where the coax is 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. In some instances coax cables are cut to a specific electrical length to act as an impedance transformer or a resonant circuit. In this case, the effect of Vp on the electrical length needs to be taken into consideration when determining the required physical length of coax cable.


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© kSARIA Corporation. All rights reserved. 300 Griffin Brook Drive, Methuen, MA 01844 USA | www.ksaria.com | Phone: (866) 457-2742 | P a g e10

The Vp of a cable is quoted as a figure which is less than "1" and is defined essentially by the dielectric material between the two conductors. Cables using a solid dielectric will have a low Vp, typically in the order of around 0.66, and those using foam will have Vp figures ranging from about 0.80 to 0.88. The Vp for the foam dielectric cable is closer to 1 (air) because of the air content in the foam.

The velocity factor (Vp) of the cable is defined as the reciprocal of the square root of the dielectric constant:

For phase matched cables, the Vp is a critical parameter as it, along with physical length, establishes the electrical phase at the output of the cable relative to the phase at the input of the cable. Lower Vp results in shorter wavelengths. Cables with identical physical length but varying Vp factors will have significantly different electrical phase length. It is critical therefore to carefully control the Vp when fabricating phase matched cable assemblies. This is normally achieved with lot control and by selecting cable stock from the same manufacturing run if possible.

In general, higher Vp cable would be used for phase matched cables because the wavelength of the signal would be at its maximum length, resulting in the highest value of inches per degree.

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

Foam polyethylene 1.3 - 1.6 0.88 - 0.79

Solid PTFE (Teflon) 2.07 0.695

Table 2 – Dielectric Materials

Vp = 1

√ε


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Effects of Variation in Velocity of Propagation

For long assemblies, adjustment of the physical length is required to achieve desired phase match. Variations in Velocity of Propagation (Vp) can result in a significant physical length variation. Typical Customer Drawings specify the electrical phase match and the minimum mechanical length, but it is necessary to specify the finished length with additional length being the variable to achieve the desired phase match. For very long cables, that additional length should be at least 5% of the total length.

Consider as an example a set of 4 matched 371 inch cable assemblies using Emerson cable PN LA290S, for use up to 18 GHz. The nominal Vp of the LA290S cable is 82%, but due to manufacturing or material variations, the Vp can range from 81.0% to 83.0%.

Table 3 below shows the change in physical length required, based on changes in the Vp of the cable, to get equally matched cables. It should be noted that this does not include any manufacturing variability related to installation of the connectors.

VP Wavelength

in Inches

Inches per

Degree

Degrees Per Inch

Length in Degrees at 371 inches

Phase delta

Difference in length

1.0 (Air) 0.657 0.001825 547.95 203,288 ref

81% 0.532 0.001478 676.48 250,972 0.0 0.00

82% 0.539 0.001497 668.23 247,912 -3060.6 -4.58

83% 0.545 0.001515 660.17 244,925 -6047.5 -9.16

Effective Wavelength through the LA290S = Vp * Wavelength in Air

Table 3 – Effects of Vp on Phase

For any phase matched cable assemblies, we must work with the cable manufacturer to ensure that all cable for this specific application is lot controlled such that all materials come from the same manufacturing lot. This will minimize the chances of a wide range of Vp values.


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Phase Tracking

Although Phase Tracking over temperature is not typically specified, it is important, especially on long cable assemblies that a basic understanding of phase tracking is clear and that efforts are made to mitigate risk of inadequate phase tracking.

Phase tracking can be influenced by three parameters, which are to a large degree beyond kSARIA’s control:

Temperature

Bends

Preconditioning

Temperature changes

The overall phase tracking due to temperature changes depends primarily on whether all cables within an assembly are exposed to the same thermal environment. The absolute phase change is dependent primarily upon the velocity of propagation. In general, the less the absolute phase changes, the better the phase tracking over temperature. Higher Vp cables, such as the Emerson LA290S, are less sensitive to temperature changes and track better.

Bends

The overall phase tracking due to bends is extremely difficult to predict. For static installations, it will depend on the number of bends, the angular arc they encompass and the proximity to other bends. For dynamic installations, it depends on the similarity of the flexure cycle each cable experiences. If individual cables within a given assembly are routed differently, it should be expected that phase tracking, and possibly phase matching, will be impacted.

Preconditioning

Prior to matching the cables of a phase-matched set it is necessary to thermally stress relieve them to assure good tracking. For example, assume that the first time a cable assembly is exposed to 125°C its phase shift changes by 10 degrees. The second time this might be reduced to 8 degrees; the third time, 7.5 degrees; the fourth time, 7.2 degrees; etc. From a practical standpoint, Thermal Cycling artificially ages or stabilizes the assembly. Although the effects of preconditioning are minimal on very long cables, in order to minimize phase tracking variations, the manufacturing/fabrication process should include a preconditioning step just prior to installing connectors.

Coax cable attenuation

Attenuation, or Insertion Loss, is a key specification for all coax cables. The function of a coax cable is to transfer RF power from one point to another. Ideally, the same amount of power should exit from the remote end of the coax cable as enters it. In reality, some power is lost along the length of the RF cable, and less power reaches the remote end than enters the RF cable.


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The power loss caused by the insertion of a coax cable is referred to as attenuation and/or the cable assembly’s Insertion Loss. 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 important to note that the loss is also frequency dependent, broadly rising with frequency. For virtually all applications the minimum level of Insertion 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 radiated from the cable. Accordingly, most of the focus on reducing loss is placed on the resistive and dielectric losses.

Resistive loss: Resistive losses within the coax cable are due to the resistance of the conductors. 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 reduce the level of resistive loss in the coax cable, the conductive area must be increased and this in general results in low loss coax cables being made larger in diameter. It should also be noted that Resistive losses increase as the square root of the frequency.

Dielectric loss: The dielectric loss represents another of the major losses in most coax cables. Again the power lost as dielectric loss is dissipated as heat. The dielectric loss is independent of the size of the RF cable, but it does increase linearly with frequency. Solid dielectrics present more loss than foam or tape dielectrics.

Radiated loss: The radiated loss of a coax cable is normally much less than the resistive and dielectric losses. 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 braid or conductor is effective. Double and triple braided coax cables are available and are used to reduce the levels of leakage to very low levels.

Coax cable attenuation with time The attenuation or insertion loss 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 ingress into the RF cable. Although many coax cables are flexible, the level of insertion loss will increase if the RF cable is bent sharply, even if within the makers recommended minimum bend radius. This increase in loss can happen as a result of disruption to the braid, altered concentricity of the center conductor, or as a result of compression on one part of the dielectric.


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Even if a cable is not flexed, there can be a gradual degradation in performance over time. Moisture penetration can affect both the braid where it causes corrosion, and it may, given enough time, enter the dielectric where the moisture will tend to absorb power. The insertion loss in coax cables that use either bare copper braid or tinned copper braid exhibit more degradation than the more expensive cables that use silver plated braids.

Some types of dielectric material can absorb moisture more readily than other types. Although foam Polyethylene offers a lower level of loss or attenuation when new, it absorbs moisture more readily than the solid types. Coax cables with solid dielectric Polyethylene are more suited to environments where the level of loss needs to remain constant, or where moisture may be encountered.

Many RF cables are enclosed in a plastic sheath to serve as protection from abrasion, chemical vapor, and moisture, 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 over time.

Polyurethane is a common jacket material and provides excellent resistance to abrasion, solvents, UV rays, radiation, fungus and it is very flexible.

Polyethylene is also common and known for its low coefficient of friction and insulation resistance. In abrasion resistant grades, Polyethylene is rather stiff.

Flouropolymers such as FEP, PFA, and engineered PTFE are excellent jacket material. The dielectric withstanding voltage of flouropolymers is among the highest of any insulation material. Flouropolymers are naturally flame retardant, very flexible, resist aggressive chemicals and solvents, and can withstand extreme temperature conditions. PTFE is chemically inert and does not contain any process additives, oils, lubricants, or plasticizers, which makes it the best choice for applications where outgassing would be a concern. The one weakness of flouropolymers is that they are not very resistant to abrasion and cut-through.

High power coax cables

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 handling capability of RF coax cable needs to be carefully considered. For coax cables where high powers are likely to be used, specially constructed cables are needed.

If cables using ordinary polyethylene dielectrics were used, the higher temperatures encountered would quickly melt and distort the cable. For very high power applications, air dielectric cables are used. The center 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.

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


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Environmental Concerns

Coax cable is often required to withstand some harsh environmental conditions. There are many factors that affect coax cables to greater or lesser degrees:

Humidity and water vapor Sunlight Corrosive vapors and liquids

Effect of humidity and water vapor on coax cables

Moisture causes two main effects which will increase the level of attenuation or loss in the cable. The first is an increase in resistive loss due to oxidation of the braid which increases its resistance. The second is an increase in the dielectric loss. Water absorbed into the dielectric heats up when power is passed along the coax cable. This heat is a result of power loss in the cable.

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

1. Through the connectors of the coax cable 2. Through voids in the outer jacket 3. By water vapor penetrating through the jacket.

1. Moisture entry through coax cable termination: The most obvious method of humidity entering a coax cable is through its connectors. Very few connectors are weather proofed, and even if they are supposedly weatherproof, it is common to use a protective weatherproof boot at the cable and connector interface.

2. Moisture entry through pin holes in coax cable jacket: 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 outer protective jacket. Moisture can and 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. Care must be taken when installing a cable, and in particular when the coax cable is passed through a wall or other barrier.

3. Water vapor transmission through the coax cable jacket: All materials exhibit a finite vapor transmission rate. If a coax cable is constantly in contact with moisture, then this moisture can and will permeate through the jacket. In some applications large temperature extremes encountered can cause water condensation in the coax cables. This moisture can collect in low areas of the cable causing increased insertion loss and worse case, local areas of corrosion. There are many choices in cable jacket materials, and assembly methods, available to mitigate this risk.


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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 Polyethylene. Polyvinylchloride (PVC) jackets exhibit less than half the life expectancy of the high molecular weight Polyethylene.

Effect of corrosive vapors on coax cables

Using a coax cable in the vicinity of corrosive liquids and vapors 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 vapors may be present on other installations requiring coax cables. Although the rigors of the weather can be very tough, some vapors 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.

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.

Coax mechanical dimensions

The mechanical dimensions of the coaxial cable are important for a variety of reasons. Larger diameter coax cables often tend to have lower insertion loss levels and higher power ratings. Small diameter cables may be required where weight or fit may be of importance. It is very important that the cable designer selects cables based on the availability of the connectors needed to complete the assembly. Again, there are a huge number of cable and connector combinations available, but not everyone’s connectors will fit on everyone else’s cables. System designers will often trade off size (cable diameter) for lowest achievable insertion loss. Of course, there are applications such as satellite systems where extremely low weight and mass may be the critical design feature.

There’s an old saying: If it’s a bit too long, can’t go wrong. If it’s a bit too short, must abort.

Cable Connectors

Obviously, an RF Cable Assembly is not complete without a set of connectors to mate with the system. Just as there are a wide variety of cable constructions to meet the many different system demands, there are also many types of RF Connectors specifically designed for RF cables. Connectors have evolved side by side with advances in communication system development and for any given frequency range of operation, there exist a specific connector design which is optimized for lowest possible insertion loss and VSWR within that range. Figure 4 shows the most common connector types in use today.


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As is the case with coaxial cable, there are a large number of connector manufacturers, and care needs to be taken to ensure that a chosen connector is compatible with a specific cable group. Quality, reliability, and cost vary significantly amongst the top tier of connector and cable suppliers. Procurement lead times vary wildly depending on the choice of connectors as well, with SMA connectors being readily available while others such as GPO and SSMA are generally not stock items.

Similar to the RF Coaxial cable, the connectors are also coaxial and the manufacturing tolerance plays a huge role in the electrical performance (quality) of the connector. For the more or less standard cable designs, there are many connectors to choose from and many options for manufacturers. Although it is usually not critical to use cable and connectors from the same source, it is often an advantage as the manufacturer of the cable and the connector has invested significant resources into cable/connector compatibility and performance optimization.

Figure 4 Standard RF/Microwave Connector types

Demonstrating compliance to electrical specifications (Test)

Compliance to customer electrical specifications is achieved by performing 100% testing over the desired range of frequencies. There are three specifications that dominate the electrical requirements, Insertion Loss, VSWR, and Phase. These measurements can all be easily made using a Vector Network Analyzer such as the Anritsu model 4644A VNA which includes Frequency and Time Domain capability.

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Connector Type

RF/Mirowave Connector Limits


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Summary

Coaxial transmission lines were first invented in the late 1880s but had essentially no useful application until the late-1930s where they were first used for transmitting television signals between major cities. Today, coaxial cables are used extensively to transmit RF and Microwave signals ranging from AM radio and Cellular Phones, to Radar systems, GPS, RFID, Direct TV and many other communications systems throughout the world.

An understanding of the fundamental composition and construction of RF Coax Cable, and the electrical and environmental conditions they are apt to be used in is very useful when working with a customer to develop an extremely reliable cable assembly at a reasonable and competitive cost. Proper selection of cable assembly materials must be based on the electrical requirements as well as the environment that the assembly will be used in. Careful and thorough consideration of the type of materials used in the construction will prevent potential problems ranging from moisture ingress to excessive signal reflections causing damage to system components.

Sales and Applications engineers are encouraged to visit cable and connector manufacturer websites for a more thorough understanding of what cable assembly components are available to choose from.

Cable Selection Checklist follows on the next page.


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Appendix A

Cable Selection Checklist

Type of Application

□ In-flight space: __________________________ □ Ground test space _________________________

□ Military Aircraft _________________________ □ Commercial Aircraft ________________________

□ Military Equipment ______________________ □ Microwave/RF ____________________________

□ Geophysical ____________________________ □ Cleanroom _______________________________

□ Other: _________________________________________________________________________________

General Requirements

Cable Length: ____________________________ Maximum Cable Diameter: _____________________

Total number of cables: ___________________ Minimum Cable Diameter: _____________________

Data Transmission: □ Digital □ Analog Protocol/Data Rate: __________________________

Other: ___________________________________________________________________________________

Electrical Considerations

Frequency Range: ____________________________ Maximum Insertion Loss: _____________________

VSWR: _____________________________________ EMI: _______________________________________

Phase Matching □ Absolute □ Relative Phase Tracking ______________________________

Environmental

Operating Temperature ______________________ Humidity □ Yes □ No

Shock/Vibration □Yes □ No Radiation □ Yes □ No

Sharp Edges □Yes □No outgassing □Yes □ No

Liquid Exposure type _________________________ Gas Exposure Type ___________________________

Other: ___________________________________________________________________________________


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rf/microwave coaxial cable tutorial

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