General Design Criteria

Specific requirements must be carefully considered for the proper selection of cable assemblies, including, but not limited to the frequency range, VSWR, insertion loss, mechanical requirements, and any environmental or application restrictions. Cost considerations should also be kept in mind at all times when designing and selecting a cable assembly. Over design may cause the cost to be unnecessarily high while low cost may be an indication of poor reliability and false economy. A careful and precise evaluation of performance and cost trade-offs will result in the optimal choice.

 

Impedance

※Characteristic impedance§ is generally implied when speaking of the impedance of a cable, connector, or cable assembly. Maximum power transfer and minimum signal reflection occurs when the characteristic impedance of a cable assembly matches that of the other components in the system. If the impedances all match, losses are due only to the attenuation of the transmission line; otherwise there will be additional reflection losses. The characteristic impedance (Zo) is directly related to the ratio of the ratio of the inner and outer conductor diameters, and inversely related to the dielectric constant of the core material (汍). Due to the ※skin effect§ of RF energy transfer, the important dimensions are the outer diameter of the center conductor, d, and the inner diameter of the outer conductor, D.

                    Z ﹝ (ohms) = (138 / ﹟ 汍) ℅ (log D/d)

A 50-ohm (次) standard has been chosen as the characteristic impedance of most commonly available components. While other impedances might offer better characteristics in particular situations (for instance, 75次 for lowest attenuation, 35次 for best power handling), the need for nonstandard characteristic impedances in small in most applications. Components with Impedances other than50次, from35次 to 185次, are available but normally at a higher cost.
 

VSWR/Return Loss

Reflections back toward the input end of a cable assembly are caused by variations in impedance along the length of a cable assembly and cause energy loss by interaction with the incident wave. Typically, the connectors and the cable/connector interface will be the major contributors to reflection loss. Cable manufacturing variations and assembly techniques may also affect such loss and will show up at a specific frequency as a spike. The magnitude of reflection can be expressed by the voltage standing wave ratio, VSWR, which is defined as the ratio of the sum to the difference of the incident and reflected voltages. A low VSWR is indicative of uniformity along the length of a cable, good design and attachment of connectors, and proper compensation in the connectors for transitions in the cable/connector interfaces. An equivalent parameter is either the reflection coefficient or the re-turn loss, which compares the power in the reflected wave with that in the incident wave, expressed in decibels. Return loss can be calculated from VSWR, and vice-versa. Note that as VSWR in-creases with grater reflection, the equivalent return loss value de-creases. Typical microwave cable assembly VSWRs range from 1.1 to 1.5, or return losses of 26.4 to 14.0 dB, respectively, indicating power transmission efficiencies of 99.8% to 96.0%.

Figure 1 每 VSWR vs. Frequency
Table1
Attenuation (Insertion Loss)

Attenuation is a measure of the ability of a component to carry an RF signal efficiently, and is the sum of the dielectric loss, conductor loss (copper loss), and radiation loss. Most of such power losses will be noted as an increase in heat in the specified material. Larger diameter conductors will provide less conductor loss; higher frequency will cause greater dielectric loss, Since conductor loss increases by the square root of frequency, and dielectric loss increases linearly with frequency, dielectric losses become a larger proportion of total attenuation as frequency increases. Higher temperature increases attenuation by increasing the resistance of the conductors and the power factor of the dielectric.(Figure 2) gives a correction factor for attenuation versus ambient temperature.

  Figure 2 每 Attenuation Temperature

汐 i (dB per 100' )=0.435﹟f/(Z o Xd)

汐 o (dB per 100' )=0.435﹟f/(Z o XD)

汐 d (dB per 100' )=0.278老﹟汍Xf

汐 total =汐 i +汐 o +汐 d

(where 汐 = the attenuation of the centre or inner (i) conductor, outer (o) conductor, and the dielectric (d), f 每 frequency in MHz, and 老 = power factor (loss tangent) )

Maximum attenuation variations can be specified over a defined frequency range, as attenuation of a cable assembly may not change uniformly with frequency change. Random and periodic impedance variations may produce irregular attenuation spikes, as seen in Figure 3. Attenuation of braided cables can also increase with time and flexure, although closed cell foam and non-contaminating dielectrics may prevent this type of degradation. (Figure 4)

Figure 3 每 Attenuation vs.Frequency Figure 4 每 Attenuation vs. Flexure
The total attenuation form from all sources plus any reflection losses over a specific length is termed the insertion loss, expressed in dB per foot or 100 feet. Insertion loss of a network can slao be defined as the difference in power arriving at a load with and without the network in the circuit.

To select a cable for a particular application, determine an acceptable attenuation ( after temperature correction ) at the highest frequency in the system requirements. Then choose the smallest diameter cable meeting the corrected attenuation value from the cable specifications.

Average Power Handling

The power handling capability of a cable assembly is related to the ability of the assembly to dissipate heat generated by resistive and dielectric losses .Thermal expansion and/or breakdown can be the materials used in the cable assembly, especially the dielectric. It should be noted that 70% or more of the copper loss, which is normally the largest component of insertion loss, is dissipated in the center conductor, In general, the power handling capability of a given cable is inversely proportional to its attenuation, and is directly related to its size. Other factors are the heat transfer properties or the cable components, particularly the dielectric.

Cable power ratings must be derated by correction factors for the ambient temperature, altitude. And VSWR encountered in a particular situation. High ambient temperature and altitude reduce the power rating of a cable by impeding heat transfer. Reflection loss or VSWR reduces power rating by causing localized hot spots in the cable .To select a cable that will meet the power requirements, determine the average input power at the highest required frequency; then determine the effective input power from the following equation;

Effective Power = Avg. Power x VSWR correction (K) x Temp. corr. X Alt. corr. (Figure 5,6, and 7)

Figure 5 - VSWR Correction Multiplier K
Figure 6 每 Power temperature Correction Factor

The primary factor restricting connector power handling is overheating of the connector center conductor, and thus the connector center conductor should always be equal or larger than the cable center conductor.

Figure 7 每 Power Altitude Correction Factor.
 

Maximum Operating Voltage(Peak Power)

Care must taken that the continuous voltage, and the peak voltage related to pulsed power situations, applied to a cable assembly is held below its maximum voltage rating. There are two separate voltage ratings for a cable: corona voltage and dielectric withstanding voltage:

ㄝ Corona is a voltage related phenomenon that causes noise generation, long term dielectric degradation, and eventual failure. Thus, the maximum operating voltage must be less than the corona extinction level(extinction voltage) of the cable.

ㄝ The dielectric withstanding voltage, or dielectric strength of a cable, is a measure of the voltage level required to abruptly break down the cable dielectric.

To choose a cable for a particular application, determine the actual RMS voltage: Actual RMS Voltage= Peak Voltage Value/ 1.4. Then determine the effective RMS voltage by: Effective RMS Voltage= Actual RMS Voltage ℅ ﹟ VSWR. Select a cable with a maximum operating voltage value greater than the effective voltage.
As a cable assembly is used in a high altitude environment the maximum operating voltage is reduced as the lower atmospheric pressure leads to a reduction in dielectric strength in the termination.

Capacitance

Capacitance is the property that permits electrical energy to be stored in a dielectric between two conductors that are at different potentials. Capacitance is dependent on the ratio of the inner and outer conductor dimensions and the dielectric constant, but in an opposing way from impedance. Thus as capacitance decreases in cables of equal dielectric constant, impedance increases. Capacitance is expressed in picofarads (10 -12 farad) per foot.

C pf/ft = 7.354 汍 /logD/d

Velocity of Propagation

Velocity of propagation is the speed of the transmitted signal as compared to the speed of light and is inversely proportional to the square root of the dielectric constant. Material with a lower dielectric constant will yield a higher velocity of propagation. Such low-density dielectrics will offer lower attenuation, particularly at higher frequencies (see Cable Types).

V 老(%) = (1/﹟汍) ℅ 100

Velocity of Propagation

Velocity of propagation is the speed of the transmitted signal as compared to the speed of light and is inversely proportional to the square root of the dielectric constant. Material with a lower dielectric constant will yield a higher velocity of propagation. Such low-density dielectrics will offer lower attenuation, particularly at higher frequencies (see Cable Types).

V 老(%) = (1/﹟汍) ℅ 100

Delay Time

Delay time is the duration of time that a signal takes from entrance to exit in a coaxial line. The delay time is independent of the frequency and is a function of the dielectric constant and physical length of the transmission line, usually expressed in nanoseconds(10 -9 seconds) per foot.

Delay = Tns = 1.0167﹟汍

Electrical Length (Phase stability)

Applications may require cable assemblies that are trimmed to a special electrical length, which may change with temperature, flexure, or other environmental factors. Solid Teflon dielectric, for instance, has a much more pronounced phase shift(change in electrical length) around room temperature than a micro-porous or air-articulated Teflon dielectric.(Figure 8) A tighter bend radius will increase phase change, and increasing the number of flexures will also increase phase change. Phase changes over frequency can be considered a linear response, although some cables will have more significant change at higher frequencies.
Figure 8 - Phase vs. Temperature Relationship
   
Phase Matching
Phase matching denotes two or more cable assemblies with the same phase length, or electrical length. Phase matching can be absolute, as compared to a predetermined value, or relative, where the assemblies are matched to each other. The physical lengths of cable assemblies may differ due to slight variations within cable stock and connectors. The tolerance of phase matching is frequency dependent, although cable length and type may affect the matching capabilities. Due to the inherent variations in cable and connector manufacturing and assembly, phase matching can be more an art than technology, and may involve a good amount of ※tweaking.§ Phase matching increases labor time, and thus cost.

Operating Frequency

The operating frequency will determine the size of cable. A large diameter cable will not operate in as high a frequency range as a smaller diameter cable, but if the conductor materials and dielectric are the same, the larger cable will have lower attenuation .(See Table 3, Cable Specifications and Table 4, Connector Frequency Range)

Cut-off Frequency

The cut-off frequency of a coaxial cable is that frequency at which modes of energy transmission other than TEM(Transverse Electro-Magnetic) 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 the cut-off frequency without substantial VSWR or insertion loss increase.

Intermodulation Distortion

Coaxial cable assemblies have often been viewed as components. But there are small non-linearities in the connectors and the cable/connector junctions, usually caused by thin-surface oxide layers at the connector junctions or by separation of current-carrying contact zones. There are some simple design rules to prevent intermodulation distortions:

﹞Use of semi-rigid cable in place of flexible cable.

﹞Use of a solid center conductor in lieu of a stranded one.

﹞Solder or weld the outer connector to the connector body in lieu of crimping.

﹞Use high quality machined connector parts with a smooth finish.

﹞Ensure adequate and uniform plating thickness.

﹞Use connector interfaces with radial dimensions as large as possible.

﹞Ensure adequate contact pressure.

﹞Eliminate use of ferromagnetic materials(steel, nickel).

 

Dielectrics

The most common dielectrics are polyethylene (PE) and Polytetrafluoroethylene (PTFE or Teflon). PTFE and polyethylene is available in solid and low-density forms: foamed, splined , and perforated or expanded tape wrap (micro porous).Low-density dielectrics are used for low loss and/or phase stable applications. If the application is for a specific time delay, low loss cables may not be advantageous since the velocity of propagation is higher, requiring additional length for a given delay time. (Table 2)

Table 2
 

Temperature Limitations

Variations in temperature always affect the electrical performance of a cable assembly; higher temperatures increase conductor resistivity, hence copper losses. Temperature variations can affect dielectric properties significantly, so the dielectric is generally the temperature-limiting factor (temperature range for polyethylene is -65∼C to + 80∼C , PTFE is -75∼C to + 250∼C ). solid Teflon cabbies are generally not recommended for extended applications above 150∼C . Splined and Microporous cables improve the thermal limitations of the solid Teflon dielectric, but require larger bend radii.

 

Mechanical & Environmental

Mechanical specifications are normally straightforward: size, length, and shape of the cable, connector types ,materials ,processes to be used, and labeling are the major requirements. The major concerns are that the mechanical requirements do not conflict with the electrical specifications, and that the overall configuration is realistic and feasible. When choosing a cable/connector combination, matching cable and connector sizes offer better results. Good matching will yield low magnitude reflections, resulting in low VSWR and low phase deviations over frequency.

Improperly controlled bending will result in ovalization or other irregular deformations of the cable, potentially increasing the insertion loss and degrading the performance. A rough rule-lf-thumb is that a cable can be bent with a bend radius twice the diameter of the cable, although the use of bend dies and tooling may allow tighter bend radii. Note that using a 90-degree bend in a cable with a straight connector in lieu of a right angle connector will give better performance and lower cost. SSI uses computer controlled bending methods and equipment to insure tight bends without sacrificing quality and performance. Bending semi-rigid cable in radii less than one inch is not recommended with-out special equipment and knowledge.

Environmental requirements for cable assemblies depend to some extent on the application ,but are usually fairly stringent. The most important requirements noted are temperature ranges (operating and non-operating),humidity or immersion, shock and vibration, and corrosion. Satisfactory solutions in these environments generally depend on materials and assembly techniques and are essentially mechanical design specifications. Note that SSI' s stainless steel coaxial cable offer a superb answer to most environmentally hazardous situations.

It should be noted that plating(normally silver, sometimes gold or nickel) of cable or connector components should always be considered to prevent loss of signal quality and strength due to oxidation. Copper oxide(Cu O)has a magnetic susceptibility of +267 while silver oxide(Ag O)has a value of-19;meaning that the ※skin depth§ for Cu O is very thin and thus skin resistivity is very high in comparison to Ag O. Remember also that ※skin effect §becomes increasingly important at higher frequencies and thus silver plating may be accordingly more advantageous.

 

Cost and Quality Considerations

Often times, minor details can substantially increase the cost of a cable assembly, and may not be necessary. Here are some ways to keep quality high and hold costs down:

ㄝ﹛ Supply full-scale drawings, particularly where there are compound bends in two or more planes.

ㄝ Give precise dimensions with loose tolerances. Keep tight tolerances to a minimum. Measure dimensional requirements to three decimal places with a tolerance of ㊣.030 inches.

ㄝ Avoid female terminations since the connector may require special orientation.

ㄝ Specify all dimensions from the connector reference plane.

ㄝ Avoid direct marking; use shrink tube markers or self-laminating labels.

ㄝ Specify center and outer conductor material if there are any special requirements.

ㄝ Design bend radii as large as possible and identical if possible. Incorporate a ※service loop§ on short cable lengths.

ㄝ Specify the cable or connector, but state ※or equivalent§ if possible. Different manufacturers often make identical parts but with wide variance in cost.

ㄝ Avoid right angle connectors if possible.

ㄝ As connector insertion loss becomes more critical (higher frequency, shorter assemblies),match connector and cable impedances.

ㄝ Specify electrical properties only across desired frequency ranges, as over-testing can be expensive.

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