Engineering information: RF cables

General cable design criteria

Microwave transmission lines are used to transmit electromagnetic energy in a controlled manner. In contrast to ordinary circuit theory where resistance (R), capacitance (C), conductance (G) and inductance (L) are represented as lumped constant elements, the R, C, G, and L of microwave transmission lines are considered distributed parameters. Hence, the microwave transmission line is a distributed element circuit. The electrical length of the microwave transmission line is a function of the physical length and the Velocity of Propagation. The principal mode of propagation in a coaxial microwave transmission line is the (TEM) Transverse Electro Magnetic mode. This means that the electromagnetic field has only radial components which include the vector electric field (E) and the vector magnetic field (H). TEM can exist in all transmission lines with two or more conductors or in free space. As the frequency increases, the wavelength will decrease. Therefore, the internal dimensions must be proportionally reduced for mode-free propagation in the TEM mode. If frequency increases and the internal radial dimensions remain constant, the next higher order mode may exist. This second mode in the coaxial line is transverse electric mode TE11. In coaxial microwave transmission lines, the TEM mode propagation is preferred because a second mode may cause resonance. A coaxial line may be used at frequencies that are slightly higher than the theoretical cut-off because the cut-off frequency does not mean that resonance will occur, it only means the possibility of resonance.

One of the first things to consider when selecting or designing a coaxial cable is determining the temperature requirements. The dielectric materials selected for the outer jacket and inner core are some of the limiting factors affecting the allowable temperature range.

Cable style (high flexibility, low flexibility or semi-rigid) should be the next determination. Some applications are able to use any of these styles. Since many flexible cables perform to the level of semi-rigid, and have a similar cost to semi-rigid, then the cost of installation should be considered.

High flexibility cables require a careful selection of materials and construction to ensure a long flex life. For low loss applications, a solid centre conductor is -usually preferred. However, a solid centre conductor may limit flexibility and is not always the most cost effective for larger diameter cables.

Always consider the cost limitations when selecting a cable style or design. Overdesign of a cable may drive the cost unnecessarily high. A lower cost cable may appear to meet the requirements initially, but take care to consider the weaknesses of each individual style. For example, additional armour can be supplied over most cable assemblies to provide extra protection, however, it is costly.

In conclusion, specific requirements must be carefully considered with regard to the selection of cable and cable assemblies including but not limited to the frequency range, VSWR, insertion loss, mechanical and electrical requirements along with any environmental or application restrictions. A thoughtful and precise review of requirements will result in an optimal design.

Concepts

Impedance

When impedance is mentioned in reference to coaxial cables, the characteristic impedance is normally implied. Characteristic impedance (Z0) is the ratio of voltage to current in a travelling wave. In low loss coaxial cable, the impedance is directly related to the logarithm of the ratio of the inner and outer diameters, and inversely related to the square root of dielectric constant of the core material. In a low loss coaxial cable, the impedance is always a positive real number. Maximum power transfer results only when the characteristic impedance of the transmitter, RF line, and the receiver (or antenna) are equal to each other or the complex conjugate. If the match is exact, losses are only due to the attenuation of the transmission line. If there is a mismatch, reflection losses will result.

Capacitance

Capacitance is the property which permits electrical energy to be stored in a dielectric between two conductors that are at different potentials. Similar to impedance, capacitance is dependent upon the inner and outer conductor dimensional ratio and the dielectric constant, but in a reciprocal way. For example, in cables with the same dielectric constant, if capacitance decreases then impedance increases. The capacitance of a cable is expressed in picofarads (10-12 farad) per foot, and can be calculated with the following formula:

VSWR/return loss conversion

Reflection can be estimated by reflection coefficient, which is the ratio of reflected wave voltage (current) to incident wave voltage (current). Reflection coefficient has a magnitude and phase and can be represented by complex numbers. Another parameter for reflection is voltage standing wave ratio (VSWR). VSWR is defined from the magnitude of reflection coefficient and, therefore, does not have a phase. Return loss compares the power in the reflected wave with that in the forward wave. The unit for return loss is decibel. Return loss can be calculated from VSWR and vice-versa.

Velocity of propagation

Velocity of propagation is the speed of signal transmission relative to the speed of light. Since it is inversely proportional to the square root of the dielectric constant, a lower dielectric constant will result in an increase in velocity. Velocity of propagation is expressed as a percentage of the speed of light in a vacuum and can be calculated by the following formula:

Delay

Delay time is defined as the duration between the time a signal enters a coaxial line until it emerges from the other end of a coaxial line. The delay time is essentially independent of frequency and is a function of the dielectric constant and the physical length of the transmission line. Delay time is typically indicated in nanoseconds (10-9 seconds) per foot.

Shielding

The shielding effectiveness of a coaxial cable depends on the construction of its outer conductors. Generally, the shielding efficiency is measured by the relative level of the signal leaking from the outer conductor in decibels per one foot of the length. The effectiveness of shielding on microwave cables usually diminishes with increased frequency. In practice, the shielding efficiency of semi-rigid (solid sheath) cables is limited by the leakage of the connectors and the cable/connector junction. Some factors which influence the shielding effectiveness of flexible cable assemblies are:

1. Number of shields: flat braid, round braid and helical wrap

2. Braid style and coverage: a flat braid is usually better than a round braid and a higher percentage of braid coverage normally provides better shielding.

3. Thickness of shield materials and plating of the conductor: cable outer conductors are typically silver plated.

4. Connector and style of attachment: the best shielded connector typically uses a threaded coupling nut with a slotless outer conductor attached to the cable by clamping, soldering or crimping with minimal amount of outer conductor junctions.

Power handling of coaxial assemblies

Two potential failure modes must be considered when determining the power handling capability of an RF coaxial cable:

• Peak power (voltage breakdown)

• Average or CW power

The peak power (voltage breakdown) occurs when the voltage gradient between the cable centre conductor exceeds a limiting value causing the signal to arc across the path of the least resistance. Generally, the path of the least resistance is located at the cable/ connection junction. Catastrophic breakdown is not the only problem: the existence of corona, usually around the centre conductor, produces other deleterious effects. Corona cutting is a concern with PTFE insulators whereupon the PTFE is eroded causing the formation of cavities (usually without carbonisation).

The peak power (voltage breakdown) occurs when the voltage gradient between the cable centre conductor exceeds a limiting value causing the signal to arc across the path of the least resistance. Generally, the path of the least resistance is located at the cable/ connection junction. Catastrophic breakdown is not the only problem: the existence of corona, usually around the centre conductor, produces other deleterious effects. Corona cutting is a concern with PTFE insulators whereupon the PTFE is eroded causing the formation of cavities (usually without carbonisation).

If the transmission line has reflections, the voltage and the current along the line will have maximums and minimums. The cause of this nonuniform distribution is superposition of the incident and reflected waves. Breakdown is a function of the maximum voltage. Higher reflection results in lower voltage handling. Even high-performance assemblies with low VSWR can have poor peak power handling if they are connected to an unmatched load. Peak power handling is dependent on frequency since the typical value of VSWR is proportional to the increase in frequency. The most common breakdown at high altitudes (usually greater than 70 000 ft) is ionisation breakdown in the air path. For vacuum and space applications, the main type of breakdown is multipaction breakdown. For average power rating of a cable with a pulsed signal, multiply the peak power rating by the duty cycle.

Frequency range, ambient temperature, altitude, physical size, and the thermal properties of each layer of construction are the primary factors which determine the average power handling capability of an RF coaxial cable. The average power failure occurs when the level of power transmitted results in resistive and dielectric heating at a rate higher than the rate at which the heat can be conducted away through the different layers of cable and dissipated from the outermost cable layer to the environment. A buildup of heat energy causes the internal cable temperature to exceed the maximum rated dielectric temperature. Convection, conduction, and radiation are methods to remove heat from the cable assembly. Conduction transfer of heat through the outer and inner conductors of a cable is particularly effective for short assemblies. For very high altitudes and space applications, the air is too thin or non-existent and convection cooling is ineffective. Heat from the cable assembly can only be removed by radiant heat and conduction.

Our supplier HUBER+SUHNER has developed a unique computer modelling program that accurately predicts the power rating for coaxial cables of varying designs and materials. Power handling is calculated for convection cooling only. Conduction and radiation are included in the safety margins. These charts provide the CW or average power rating for all cables versus frequency. The following calculation shows how to use the CW power charts for non-standard temperature conditions:

Multipaction and ionisation breakdown

Multipactor breakdown is a failure mode of an RF component that only occurs under conditions of high vacuum, where a certain frequency distance product condition exists between the inner and outer conductors and where a sufficiently large RF electric field strength exists. In a high vacuum environment, an electron may have a free path longer than the electrode separation distance. When this electron collides with the electrode it may release secondary electrons. If both frequency and the distance between inner and outer conductor are favourable, the secondary electrons will be accelerated by the electromagnetic field. Large electron densities rapidly build up and breakdown results. At very low and very high frequencies multipactor breakdown is impossible. Multipactor breakdown can also occur between the conductor and the insulator. A multipactor discharge itself adsorbs little power, but once initiated it can cause increased outgassing from materials within components, which may lead to a gas discharge and total failure. To prevent this event, the microwave components should have vent holes of sufficient size to allow the gasses to escape at a known rate. Multipactor breakdown also results in increased heating within the cable or connector, noise generation, harmonic distortion, and intermodulation (when multiple frequency RF signals are applied).

For every vacuum application the power handling should be calculated individually. The worst frequencies for multipactor breakdown are between 500 MHz and 2.5 GHz. At low voltage levels (less than 20 V) and low average power (less than 8 W), multipactor breakdown is theoretically impossible.

In ionisation breakdown, secondary electrons are produced through collisions between electrons and gas molecules. Ionisation breakdown occurs at pressures higher than those for multipaction. Like multipactor breakdown, ionisation breakdown is not possible at very low and very high frequencies and low power levels. However, ionisation breakdown is considerably more complex than multipactor breakdown because of the additional dependence on pressure and the type of gas (if other than air).

Our supplier HUBER+SUHNER has designed and manufactured several high-performance cable assemblies for use in high power, high altitude, and space environments. These products were tested by an independent laboratory to determine if any failures due to ionisation and/or multipactor breakdown would occur.

Connector power handling

The primary factor restricting power handling in the coaxial adaptor or connector is overheating due to restricted heat dissipation. High power cable assemblies, in general, should not exceed 200 ˚C, however dielectric materials used in precision connector interfaces like 7 mm and 3.5 mm are only rated to 90 °C. Our supplier HUBER+SUHNER manufactures a high temperature precision bead for high power applications. The maximum temperature usually occurs on the connector inner conductor. When connectors are employed in a coaxial cable assembly, the connector should have a centre conductor diameter that is equal to or larger than the cable centre conductor diameter in order to maximize the power handling of the assembly.

Although many applications support the use of standard connectors and coaxial transmission lines, recent designs in TWT’s, high power filters and

high-power test equipment have placed a great burden on standard coaxial cable assemblies. Since the internal configuration of the connector termination is a major contributor to heat build-up, we employ a unique dielectric material known as Fluoroloy H® inside connectors used for high power applications. This material has a slightly higher dielectric constant (compared to standard Teflon® dielectric) but has a higher rate of thermal conductivity which allows the heat that is generated in the centre conductor to transfer to the outer conductor more rapidly, thus increasing the power handling capability of the connector or adaptor. The majority of our connectors and adaptors can be produced with Fluoroloy H® dielectric upon request. In addition, we can design special customised high-power interfaces that are mechanically and electrically compatible with standard interfaces. Contact our sales department regarding any high-power requirements.

Intermodulation distortion

Intermodulation distortion in passive microwave components is caused by internal nonlinearities. In a truly linear system, the output is directly proportional to the input.

In a nonlinear system, the output signal is distorted by changes in the amplitude of the input signal. Intermodulation distortion creates new output signals from the nonlinear combinations of two or more input signals mixed together. A nonlinear circuit will create an infinite number of harmonics from two fundamental frequencies (f1 and f2). A particular concern for telecommunication systems engineers is the intermodulation product of the third order (such as 2f1 – f2 and 2f2 – f1), especially if f1 and f2 are closely spaced. With certain system designs and bandwidth allocations, the third order intermodulation products can be generated at the same frequencies as the receive channels of the system. In general, intermodulation products increase system noise and reduce the number of available channels.

Intermodulation distortion is most pronounced in systems where the high-power transmission and low power receiver signals are carried simultaneously in the same transmission line, such as in the cable between the duplexer and the antenna in GSM base stations and in certain space applications. For low power levels, the effects of intermodulation distortion are significantly less. Our supplier HUBER+SUHNER is involved in the research of the intermodulation problem as a participant in the IEC TC46 WG6 passive intermodulation working group. 

Coaxial cable assemblies have often been viewed as linear components. However, pure linear components do not exist. There are small nonlinearities in the connectors and in the cable to connector junctions. Intermodulation distortion in connectors is usually caused by thin surface oxide layers at the connector junctions or by insufficient

contact pressure when the current-carrying contact zones become separated. Separation is usually microscopic and can be caused by either electron tunnelling or microscopic arcing. The presence of ferromagnetic materials in the current path may also contribute to intermodulation distortion.

Some simple design rules can help avoid intermodulation distortions in coaxial cable assemblies:

• Use of semi-rigid cable with a seamless outer conductor in place of flexible cable.

• Use of a solid centre conductor in place of a stranded centre conductor.

• Directly attach the outer conductor to the connector body by soldering or clamping in lieu of crimping.

• Limit the number of parts in the current path.

• Eliminate contaminants in the current path.

• Use high quality machining in the connector parts with a smooth surface finish.

• Avoid contaminants in the plating solutions.

• Ensure adequate and uniform plating thickness.

• Avoid use of magnetic materials in the current-carrying path.

• Ensure adequate contact pressure.

• Contact surface of female contact fingers should cover as close to 360 as possible (i. e. narrow slots or slotless).

• Use connector interfaces with radial dimensions as large as possible (7/16 over N, N over SMA)

Space applications

Every space application is unique and requires careful consideration before selecting the components to be used. A space environment subjects components and assemblies to severe environmental stress:

• Low earth orbit spacecraft subject solder joints, welds, brazements and mechanical connections to continuous hot/cold thermal cycling every 90 minutes. The manufacturing process must be carefully controlled per NASA STD-8739 requirements to assure consistent, reliable connections and assemblies. Solder connections must be 100 % X-rayed to assure their integrity and reliability.

• There is no atmosphere, so convection cooling does not occur. Excess heat must be removed by radiation, which requires the surface of the connectors to be an infrared emitter, or by conduction which requires a secondary heat sink.

• Certain materials “outgas” in the extreme vacuum of space which requires the designer to select materials and components that meet NASA requirements for Total Mass Loss (TML) and Collected Volatile Condensable Material (CVCM) to avoid contamination of optics and other sensitive equipment on board the spacecraft.

• Materials must be carefully chosen so that ionising radiation does not destroy the connector or cable dielectric or the cable jacket.

• Multipaction failure (described in more detail herein) is a concern for high power applications.

• Intermodulation distortion (described in more detail herein) is a concern within systems where high-power transmitting and low power receiving signals need to be carried simultaneously in the same transmission line.

• Intermodulation distortion (described in more detail herein) is a concern within systems where high power transmitting and low power receiving signals need to be carried simultaneously in the same transmission line.

Processes and controls used for procurement, manufacture, assembly, soldering, X-ray, inspection, and testing have been certified by NASA for use in spacecraft applications. We have the design, manufacturing, testing, and applications experience and expertise to supply your needs for passive microwave devices for use in any space environment.

Phase

Phase stability vs. temperature

Phase stability vs. temperature is a measure of the signal speed variation when the cable is exposed to different temperatures. The values are specified in parts per million (ppm) or in degrees per gigahertz and meters (deg/(GHz*m)). They usually refer to the difference between maximum and minimum values in a certain temperature range. They can be converted to each other using the following formula:

For frequencies in the low single-digit GHz range, the phase change is not proportional to the frequency anymore but for higher frequencies it is.

Main influences are the materials used and the construction of the cable. Most cables have different behaviours depending on the temperature range considered. There are sections with a linear or a non-linear behaviour. Linear behaviour show the influence of the regular length and volume expansion of the cable components. Non-linear sections originated from phase changes in materials or special mutual reactions between single elements of the cable. An example for a phase changes in materials is the devitrification of PTFE at 20 °C. At this temperature the crystal structure changes from triclinic to hexagonal. This leads to a rapid change of phase, the so called “Teflon® knee”.

Phase stability with flexure

Phase stability vs. flexure is a measure of the phase change of a cable as a result of flexing. The manner of flexure will affect insertion phase.

Reducing the bend radius or increasing the bend angle will increase the phase change. Similarly, as the number of flexures increases the phase change will increase. Increasing the ratio of cable diameter to bend diameter will decrease the phase changes. Phase changes over frequency can be considered a linear response, although with some cables change can be more significant at higher frequencies. A microporous dielectric cable will typically have better phase stability than a solid dielectric

Phase tracking

Phase tracking is the ability of multiple assemblies to closely reproduce their phase relative to each other over a range of temperature, flexure, or both. Phase tracking is essentially a measure of the assemblies’ mechanical repeatability and consistency. Thermal conditioning of coaxial cable enhances tracking characteristics.

Phase matching

Phase matching is a term generally used to describe two or more cable assemblies with the same phase length. A more precise term is electrical length matching since phase measurements are from 0 ° to 360 ° of phase, with repeating cycles of 360 ° phase. The mechanical lengths of phase matched cable may not always be equal due to slight variations in the cable velocity of propagation. There are two distinctly different versions of phase matching: 1) absolute phase matched cables are matched to a predetermined phase value, and 2) relative phase matched cables are matched to each other. In either case, the tolerance of phase matching is frequency dependent although cable length and type may affect the matching capabilities.

Phase matching of cable assemblies

Definition

The term "phase matching" refers to the relative electrical length of an assembly compared with a reference cable or a given electrical length.

Absolute length

Relative length

Guarantee of phase matching

Our supplier HUBER+SUHNER guarantees phase matching ex-factory. The relevant measurement logs are included in the supply. It is essential during installation and service to ensure that all assemblies of a phase matched set are exposed to the identical thermal and mechanical stresses.

Reference cables

Usually, a reference cable is produced for each phase matched assembly set when an initial production run takes place. The absolute electrical length measured is internally saved. The reference cables are stored during 10 to 20 years under controlled conditions (temperature, humidity) together with the order data to allow individual assemblies to be replaced whenever the need arises.

Attenuation (insertion loss)

Attenuation is a measure of the ability of a component to carry an RF signal efficiently. Coaxial cable loss is the sum of the dielectric and conductor losses and is a function of the materials used to manufacture the cable. Attenuation stability with flexure will have similar response characteristics as” phase vs. flexure” described previously, as will the “tracking” characteristics. Attenuation matching will not be as dependent upon the dielectric style, although for long lengths the insertion loss stability vs. flexure is critical.