A guide to the selection of flexible and semiflexible R.F. coaxial cable -- Times Microwave Systems

Title: A guide to the selection of flexible and semiflexible R.F. coaxial cable

Author: Times Microwave Systems

1. In choosing the appropriate cable construction for a particular application, the cable characteristics listed below are to be considered:

A-Characteristic Impedance
B-Impedance Uniformity (VSWR)
C-Capacitance
D-capacitance & Impedance Stability
E-C.W./. Power Rating
F-Maximum Operating Voltage
G-Attenuation
H-Attenuation Uniformity
I-Attenuation Stability
J-Velocity of Propagation
K-Electrical Length Stability
L-Pulse Response
M-Shielding
N-Cut-Off Frequency
O-Self-Generated Cable Noise
P-Operating Temperature Range
Q-Flexibility
R-Environmental Resistance
S-Gable Strength
T-Qualification & UL Approval

2. It should be noted that the stipulation of materials and dimensions does not guarantee that each lot of cable will have identical mechanical or electrical characteristics. Different types of manufacturing equipment or different manufacturing conditions can give substantially different performance characteristics. For example, VSWR and attenuation uniformity may have to be specified over a band of frequencies. See Table I for formulae describing cable characteristics.

A. CHARACTERISTIC IMPEDANCE: 1. The average characteristic impedance of a coaxial cable is determined by the ratio of the inner diameter of the outer conductor to the outer diameter of the inner conductor and by the dielectric constant of the insulating material between the conductors. Select impedance to match system requirements.; 2. The most common values for coaxial cables are 50, 75, and 95 ohm. Other impedances from 35 to 185 ohms are available in the coaxial configuration.; 3. The most common values for balanced lines are 75, 95, and 125 ohms.; 4. Note that actual input impedance at a particular frequency may be quite different from the characteristic, or surge impedance of the cable due to reflections in the line. The VSWR (Voltage Standing Wave Ratio) of a particular length of cable is an indicator of the difference between the actual input impedance of the cable and its average characteristic impedance.
B. IMPEDANCE (VSWR) UNIFORMITY: 1. The VSWR of a cable assembly is the summation of reflections due to the connectors, the connector termination technique and the cable. The VSWR of the cable is the summation of random and periodic reflections within the cable, most commonly caused by variations within the processing equipment. The VSWR will vary with frequency. A common occurrence is the VSWR "spike" which is illustrated in Figure 1. If required, cables can be procured in specified lengths to a maximum VSWR requirement on a swept basis. If a very tow VSWR is required, it may be necessary to procure complete assemblies verified on a swept basis.

C. CAPACITANCE: 1. Select to meet system requirements.; 2. Capacitance values (shown below) for standard coax lines depend only on cable impedance and dielectric material.

TABLE II NOMINAL CAPACITANCE CABLE TYPES

 _PF/FT_
   30.8                50 Ohm Solid Polyethytene
   25.4                50 Ohm Foam Polyethylene
   29.4                50 Ohm Solid PTFE
   20.6                75 Ohm Solid Polyethylene
   16.9                75 Ohm Foam Polyethylene
   19.5                75 Ohm Solid PTFE
   16.3                95 Ohm Solid Polyethylene
   13.5                95 Ohm Air space Polyethylene
   15.4                95 Ohm Solid PTFE
   10.0               125 Ohm Air space Polyethylene
    6.5                185 Ohm Air space Polyethylene

D. CAPACITANCE & IMPEDANCE STABILITY

1. The capacitance and impedance of long lengths of cable will exhibit very little change over their operating temperature ranges less than 2%. Semiflexible foam dielectric cables normally exhibit the least change. In short cable lengths at frequencies over 1000 MHz, the VSWR can vary significantly if dielectric movement at the connector interface occurs.

E. AVERAGE C.W. POWER RATING

1. Tables III through X serve as a guide to the average power ratings of standard flexible and semiflexible coaxial cables under the conditions noted.

2. These power ratings must be derated by correction factors for the ambient temperature, altitude and VSWR encountered in a particular application. High ambient temperature and high altitude reduce the power rating of a cable by impeding the heat transfer out of the cable. VSWR reduces power rating by causing hot spots.

3. To select the cable construction for a particular requirement, determine the average input power at the highest frequency from system requirements. Then determine the effective average input power as follows:

Effective Power = (Average Power x (VSWR correction))/((Temp. correction)x(Alt. correction))

Temperature and altitude corrections are shown on Figures2 and 3.

VSWR correction factor =
1/2(VSWR + (1/VSWR)) + 1/2 K1 (VSWR (1/VSWR))

where k1 is shown in Figure 4. Select a cable from Table III rated at this effective power level.

F. MAXIMUM OPERATING VOLTAGE (A.C.)

1. A cable cannot operate continuously with corona which causes noise generation, dielectric damage and eventual breakdown. The maximum operating voltage must be less than the corona level (extinction voltage) of the cable. This should not be confused with the dielectric strength of the cable, which is a test voltage which is applied for one minute only during manufacture .

2. Maximum operating A.C. (RMS) voltage levels or peak voltage are given for each construction in the Cable Data Section of this catalog. The maximum permissible D.C. voltage level is conservatively 3 times the A.C. level.

3. To select a cable for a particular application, determine the actual RMS (peakl.4) or peak voltage (RMS x 1.4) from system requirements. Then determine the effective input voltage by multiplying the actual input voltage by the square root of the VSVVR. Effective voltage = (Actual voltage x VSWR^1/2) Select a cable with a maximum operating voltage greater than the effective RMS voltage. Note that the maximum operating voltages are listed in the cable data section.

4. The maximum operating voltage of airspace dielectric tubular sheathed cables can be increased by pressurizing with dry air or high dielectric strength gases. See Figure 5 for the ratio improvement which can be obtained with dry air and sulphur hexafluoride.

G. ATTENUATION

1. Tables III through X serve as a guide to the attenuation characteristics of the different standard coaxes at a temperature of 20 degrees C.

2. Attenuation must be modified by a correction factor for the ambient temperature. Elevated temperature increases cable attenuation by increasing the resistivity of the conductors and by increasing the power factor of the dielectric (see Figure 6 for correction factor).

3. To select a cable construction for a particular application, determine the desired attenuation at the highest frequency from system requirements. Determine the corrected attenuation by dividing the desired attenuation by the temperature correction factor. Choose the smallest cable meeting the corrected attenuation value from the tables.

H. ATTENUATION UNIFORMITY

1. The attenuation of any cable may not change uniformly as the frequency changes. Random and periodic impedance variations give rise to random and periodic attenuation responses. Narrow-band attenuation "spikes" such as that shown in Figure 8 can occur. If required, cables can be procured in various lengths where a maximum attenuation variation from nominal is specified on a swept basis.

I. ATTENUATION STABILITY

1. The attenuation of braided cables can increase with time and flexure. The change with time can be caused by corrosion of the braided shield, by contamination of the primary insulation due to jacket plasticizers, and by moisture penetration through the jacket. (Vapor penetration occurs through ail plastic and elastomeric materials.) Attenuation degradation is more pronounced at frequencies above 1 GHz. Cables having bare copper and tinned copper braids exhibit far greater attenuation degradation than do cables having silver plated copper braids. These effects are illustrated in Figures 7, 9 and 10.

2. The following guidelines apply:

a. Tin plated braids--Below 1 GHz cables manufactured with tin plated braids have t 5-20% more attenuation than bare copper braids in the "as manufactured" condition, but are more stable than bare copper braided cables.

b. Foam polyethylene--flexible braided cables with foam polyethylene dielectrics have approximately 15% less attenuation than solid polyethylene cables of the same core size and impedance. However, the attenuation of foam polyethylene cables will increase if moisture is absorbed. In high humidity environments, the foam dielectric flexible cables should, not be used above 200 MHz. Avoid the use of cables with the Type I PVC jackets as they have contaminating type plasticizers which will cause an attenuation increase. All of the above problems are minimized or eliminated by the use of the semiflexible cables. Foam polyethylene dielectrics may be used up to 12 GHz when protected from moisture by the seamless aluminum tube sheath construction.

3. For flexible cables in extreme environmental conditions, a protected braid is recommended.

J. VELOCITY OF PROPAGATION

1. The velocity of propagation of cable is determined primarily by the dielectric constant of the insulating material between the conductors. This property is usually expressed as a percentage of the velocity of light in free space.

2. The following table shows the velocity of propagation and time delay of cables insulated with the most widely used dielectrics.

3. Modified coaxial structures using a helical wrapped center conductor are available with time delays ranging from 10 to 100 nanoseconds per foot. Balanced helical structures are also available with time delays, up to 50 nanoseconds per foot.

K. ELECTRICAL LENGTH STABILITY Applications such as antenna feed systems require many cable assemblies that are trimmed to specific electrical length. In these applications the change of the electrical length of the cable with temperature, flexure, tension and other environmental factors is critical. The variation of electrical length with temperature for standard flexible cables is shown in Figure 11.

For polyethylene insulated cables:--100 to --250 parts/million/degree C.

For TFE insulated cables:-50 to -100 parts/million/degreeC.

The variation of electrical length with temperature for the standard foam dielectric semiflexible cables is --20 to --30 parts/million/degree C.

L. PULSE RESPONSE OF COAXIAL CABLES

1. The following characteristics must be considered when analyzing the Time Domain response of cable to pulses or step functions: a--impedance and Reflection; b--Rise Time; c--Amplitude; d-Overshoot or Preshoot; e--Pulse Echoes.

a. Impedance and Reflection

1. Select impedance to match system requirements.

2. The impedance will vary along the length of cable. Variations of +5% are not uncommon. Cables can be produced to tolerances of +2%. Tighter tolerances are not recommended .

b. & c. Rise Time and Amplitude

1. The output rise time is a function of input rise time, pulse width and cable attenuation. A typical pulse response is shown in Figures 12 and 13, while a typical step response is shown in Figure 14. Increased cable temperature causes an increase in rise time and decrease in amplitude.

d. Overshoot or Preshoot

1. Figure 14 shows the overshoot which can be encountered with a 0.1 ns input pulse rise time in cables due to finite reflections. Such overshoot is not common in cables with longitudinally extruded dielectrics or spline cables.

2. Preshoot is encountered in some balanced delay lines and can be minimized by cable design.

e. Pulse echoes

When a narrow pulse is placed on a cable, the distortions noted above will occur. In addition a small pulse of energy may emerge after the initial pulse has arrived. This pulse echo is caused by finite periodic reflections within the cable. Normally the echo level can be neglected.

M. SHIELDING AND CROSS-TALK (OR ISOLATION)

1. The shielding efficiency of a coaxial cable depends on the construction of its outer conductor. The most common constructions available are:

Single Braid - Consisting of bare, tinned, or silver plated round copper wires (85-90% coverage).

Double Braid - Consisting of two single braids as described above with no insulation between them.

Triaxial - Consisting of two single braids as described above with a layer of insulation between them.

Strip Braids - Consists of flat strips of copper rather than round wires (90% coverage).

Solid Sheath - Consisting of aluminum or copper tubing.

2. The relative shielding efficiencies of these constructions are illustrated in Figure 15 over the frequency range from 10 MHz to 8 GHz. This graph shows the level of signal which leaks through the outer shield of a one foot sample of each construction. The curves describing the performance of the flexible cables, i.e., the triax braid, double braid, and single braid construction are based on measured data. To estimate leakage in cables under 1100 ft. long, add 20 log L from the figure read from the graph (where L is the cable length in feet). The curve showing the typical performance of the semi-flexible (or solid sheath) cables is based on theory. In practice the shielding efficiency of interconnections made using semiflexible (solid sheath) cables is limited by the leakage at the connectors.

3. The total isolation (or cross talk) between two coax cable runs is the sum of the isolation factors of the two outer conductors of the adjacent cables and the isolation due to the "coupling factor" between the runs. This coupling factor will depend on the relative spacing, positioning and environment of the cable runs and on the grounding practices employed. The coupling factor can substantially affect the isolation between the cable runs.

4. Measurements show that the RF(1-30 MHz) cross talk between two single braided coaxes over a 20 foot run length is approximately 80 db down from the signal level inside the cables. The coaxes were laid side-by-side over the 20 foot test length.

5. Special Constructions - Stripflex cable. Times has developed a special shield construction made up of wide strips of silver plated copper which are braided together into the familiar basket-weave construction. This differs from the standard braid construction where small diameter round copper wires are laid side by side to make up a braid "Carrier," which is then woven into the familiar basket-weave construction. The strip braid cables are only manufactured in silver plated strip construction.

6. Reference "Analysis and Measurement of CATV Drop Cable RF Leakage" for coaxial shield transfer impedance measurements and characteristics.

N. CUT-OFF FREQUENCIES The cut-off frequency of a coaxial cable is that frequency at which modes of energy transmission other than the TEM mode can be generated. It does not mean that the TEM mode becomes highly attenuated. This frequency is a function of the mean diameter of the conductors and the velocity of propagation of the cable. The higher modes are only generated at impedance discontinuities and in many situations the cable can be operated above cut-off without substantial VSWR or insertion loss increase. However, it is recommended that in general usage the cut-off frequency not be exceeded.

O. SELF-GENERATED CABLE NOISE A phenomenon noted in cable is that when flexed, the cable generates an acoustical noise and an electrical noise. The acoustical noise is a function of mechanical motion within the cable. Such noise can be minimized by proper cable design. Electrical noise generation is attributed to an electrostatic effect, which has exhibited more than 500 millivolts in RG cable. This noise voltage can be minimized by preventing motion between dielectrics and conductors or dissipating electrostatic charges between conductor and dielectrics with semiconducting layers. Low noise constructions must take into account the life expectancy and environmental conditions to which they are subjected. Times manufactures low noise cables for special applications.

P. OPERATING TEMPERATURE RANGE

1. The operating temperature range of flexible coaxial cable is determined primarily by the operating temperature range of the dielectric and jacketing materials. Note that only silver plated conductors are suitable for long term use at temperatures over 80"C.

2. Operating temperature limits of the most commonly used dielectrics and jacket types are given in the following table:

MATERIAL                                             Primary Dielectrics
Polytetraflouroethylene (PTFE).............. -250 degrees C to +250 degrees C
Polyethylene................................. -65 degrees C to + 80 degrees C
Foamed Polyethylene ...........................65 degrees C to + 80 degrees C
Foamed or Solid Ethylene Propylene ........... 40 degrees C to + 105 degrees C

                                              Jackets
Fluorinated Ethylene Propylene (FEP) ...... 70 degrees C to +200 degrees C
Polyvinylchloride (PVC) ................... 50 degrees C to + 85 degrees C
Ethylene Ct7loro Trifluoroethylene (ECTFE). 65 degrees C to + 150 degrees C
Polyurethane. ............................-100 degrees C to +125 degrees C
Perfluoroalkoxy (PFA) .....................-65 degrees C to +260 degrees C
Nylon .....................................-60 degrees C to + 120 degrees C
Ethylene Propylene ........................-40 degrees C to + 105 degrees C
High Molecular Weight Polyethylene......... 55 degrees C to + 85 degrees C
Crosslinked Polyolefin .................... 40 degrees C to + 85 degrees C
Silicone Rubber ........................... 70 degrees C to +200 degrees C
Silicone Impregnated Fiberglass ........... 70 degrees C to +250 degrees C
High Temperature Nylon Fiber ............. 100 degrees C to + 250 degrees C

Q. FLEXIBILITY Coaxial cables with stranded center conductor and braided outer conductors are intended for use in those applications where the cable must flex repeatedly while in service. Standard braid constructions will withstand over 1000 flexes through 180 degrees if bent over a bend radius equal to 20 times the O.D. of the cable. Flexible cables may be stored and are normally shipped on reels with a hub radius equal to 10 times the O.D. of the cable. If a flexible cable is to be installed in a fixed, bent configuration, the minimum bend radius recommended is 5 times the cable O.D. Tighter bends can be made. Special braid designs are available for improved flex-life.

Coaxial cables with a tubular aluminum outer conductor, commonly referred to as semiflexible cables. will not withstand more than ten 180" bends over a bend radius equal to 20 times the O.D. of the cable. Semi-flex cables are normally shipped on reels having a hub radius of 20 times the O.D. of the cable.

Semi-flex cables may be field bent for installation. The minimum recommended bend radius is equal to in times the O.D. of the cable. Cables bent on a bend radius of 5 times the O.D. of the cable may exhibit mechanical and electrical degradation. Times has developed special bending techniques where bend radius of 5 times cable O.D. may be used with negligible change in electrical characteristics.

R. ENVIRONMENTAL RESISTANCE The life of a coaxial cable depends on many factors. The materials used, ultra-violet exposure, high humidity, galvanic action, salt-water and corrosive vapors are prime causes of cable failure. Resistance to flame must also be considered. The following guidelines apply:

a. Sunlight--For low temperature cables exposed to sunlight (ultra-violet), the use of high molecular weight polyethylene, with a specific carbon black particle size, % by weight and particle distribution, is recommended for maximum life expectancy. Polyvinyl chloride jackets exhibit a life expectancy of less than '/2 that of properly compounded polyethylene.

b. High Humidity -- Vapor can enter flexible cables through pin-holes in the jacket, the terminations or by vapor transmission through the plastic or elastomeric jacket. All materials exhibit a finite vapor transmission rate. For example, a ten foot length of cable would exhibit a helium leak rate of approximately 10-4 cc/sec. Because of extrusion techniques even FEP does not offer any improvement over that figure. The combination of finite vapor transmission rates and large temperature extremes appears to cause condensation in cables, if the cables were tilted, this moisture could collect in a low area causing rapid corrosion or shorting of a connector. One method of preventing moisture accumulation in cables is to fill all voids with a moisture-proofing compound which will not harden with age.

c. Salt-water Immersion---The electrical characteristics of cable will be rapidly affected if the conductors are exposed to salt-water. Unless an immersion test is performed on the jacket, there is a good possibility of one pinhole per 1000 feet. Even if sufficient tests could be performed, damage during installation or due to animal damage normally will cause leakage. Cables are recommended which withstand the pressure at the cable depth.

d. Corrosive Vapors The use of tin and silver coatings does afford some protection against corrosive vapors. However, such protection is short-lived. For installation near salt-water or chemical plants, a filled cable such as Imperveon is recommended.

e. Underground Burial & Galvanic Action--Underground moisture which comes in contact with any cable metals, will cause rapid corrosion. Tubular aluminum outer conductors have been almost destroyed in 90 days. Therefore, any cables installed underground should have pinhole free jackets. Since jacket damage due to installation techniques and rodents could occur, cables filled with a flooding compound should be used. In plastic jacketed tubular aluminum cables, a flooding compound should be used under the jacket. For maximum reliability against rodents, a steel tape armor with overjacketing is recommended.

f. Flame Resistance--

1. Flame resistance of cables is a characteristic which should be understood. Cables have different degrees of flame resistance. "Flame retardant" cables are cables over which a flame will not travel when one end is subjected to a flame. Cables with PVC jackets are normally considered flame retardant.

2. Flame retardant jackets which are actually within the flame will burn. If the flame is removed, they will cease to burn. PVC will not drip burning material. However, if the dielectric is polyethylene, the dielectric may drip ignited materials. PTFE and FEP are the most commonly used materials which will not support combustion, drip or burn.

S. CABLE STRENGTH The break strength of the cable depends primarily on the strength of the outer conductor. The cables will normally achieve at least 70% of the break strength of the outer conductor, if the center conductor will stretch up to 10% before breakage. Caution must be taken with cables with copper-covered steel or alloy center conductors where breakage would occur with only 1 to 10% elongation. Conductor sizes less than 26 AWG can easily be broken during assembly operation. Special alloy conductors are available which can achieve a tensile strength of 110,000 psi and 10% elongation.

T. QUALIF1CATION APPROVAL

1. Military--Most military applications require that cable conform to particular specifications. Many of these specifications require the manufacturer to qualify product by conducting a series of tests on a length of cable with a military representative present as a witness. MIL-C-17, the basic specification for most coaxial cables, requires a Qualified Products List (QPL).

2. Commercial (UL) approval---the building codes of many cities require that cables installed in their buildings be approved by the Underwriters Laboratories (UL). With UL service, the cable is subjected to a clearly defined series of tests and examinations, and has met the quality and safety standards imposed by Underwriters Laboratories. A large variety of UL approved designs (style numbers) are available. Approval of new designs meeting UL standards normally can be made in a relatively short period of time.