Coaxial cable

Coaxial cable cutaway model:
1. Core or inner conductor
2. Insulation or dielectric between inner conductor and cable
shield 3. Outer conductor and shield
4. Protective jacket

Coaxial cables , or coaxial cables for short , are two-pole cables with a concentric structure. They consist of an inner conductor (also called a core ), which is surrounded by a hollow cylindrical outer conductor at a constant distance . The outer conductor shields the inner conductor from interfering radiation .

The gap is called an insulator or dielectric . The dielectric can consist partly or completely of air (see air line ). The outer conductor is usually protected from the outside by an insulating, corrosion-resistant and waterproof jacket. The structure of the cable determines the line impedance as well as the frequency-dependent cable attenuation .

“Flying” coaxial cables without permanent installation are often used as antenna cables for radio or television reception or as cinch connections, especially in the audio sector.

Common coaxial cables have an outer diameter of 2 to 15 mm, special shapes from 1 to 100 mm. There is also a coaxial design of overhead lines, the trap line .

construction

Coaxial cable, stripped

Flexible coaxial cables usually have inner conductors made of thin, braided or stranded copper wires and cable shields made of likewise braided copper wires, whereby the shield can be supplemented by a foil. The braid may then have a lower degree of coverage. Rigid coaxial cables for high performance or high shielding factors are constructed with a rigid outer conductor in the form of a tube.

Special forms of coaxial cables have two inner conductors or several coaxially arranged outer conductors. These cables with two outer conductors are available under names such as triaxial cables and are used in video technology, for example, when the shielding function is to be separated from the outer conductor. Another special form is a coaxial cable with ferrite sheathing . The ferrite coating acts as a common mode choke , which supports a push-pull signal in a transformer and has an inductive damping effect on a common mode signal . The transfer impedance as a characteristic of the shielding effect is not influenced by the ferrite, but the shielding attenuation is .

A variant of the coaxial cable is the slotted cable , which is used as an elongated antenna. The split cable is a coaxial cable with incomplete shielding. Its outer conductor has slots or openings through which HF power can be emitted and absorbed in a controlled manner over the entire length of the cable.

Uses

Ferrite-sheathed and common coaxial cable RG-58 in cross section.
Coaxial high-end audio cable with BNC plugs, adapted to cinch.

Coaxial cables are suitable for transmitting high-frequency, broadband signals in the frequency range from a few kHz to a few GHz. These can be high-frequency radio signals, radar signals or simply measurement signals in a test laboratory. Coaxial cables were also used for Ethernet networks until the 1990s. For some applications, for example for microphones, a DC voltage is occasionally also transmitted in order to supply a consumer with electrical energy ( remote supply , tone wire supply , phantom supply ).

Coaxial cables are used to transmit high-frequency asymmetrical signals; the outer conductor usually carries the reference potential, namely the ground, the inner conductor carries the signal voltage or, in the case of remote feeding, also the supply voltage. The ribbon cable is used to transmit high-frequency symmetrical signals .

Coaxial cables find a special application in the generation of high-power pulses in radar technology. No signals are transmitted, but here the cable acts as a high-voltage source with a precisely defined internal resistance , which has released its entire stored charge after a defined time.

Coaxial cables for the electrical transmission of digital stereo - or multi-channel - the audio signals used between different devices. The usual S / PDIF interface is found on CD players, DAT recorders, MiniDiscs , between DVD players and home cinema receivers, audio systems in vehicles and digital audio cards in PCs.

Physical Properties

In coaxial cables, the useful signal power is transmitted in the dielectric between the inner conductor and outer conductor. Mathematically, this is described by the Poynting vector , which ideally only has a value other than zero in the dielectric. In this case there is no electrical field component in the ideal conductor in the direction of wave propagation. In the dielectric, the electric field component for an electromagnetic wave is oriented vertically between the inner and outer conductor, the magnetic field component is oriented cylindrically around the inner conductor and the Poynting vector is oriented in the longitudinal direction of the line. At high frequencies , the coaxial cable can be viewed as a waveguide ; the surfaces of the metallic inner and outer conductor serve as a border for guiding an electromagnetic wave. Since this is usually undesirable, the circumference of the outer conductor must be smaller than the wavelength λ. This limits the usability of coaxial cables at very high frequencies because undesired waveguide modes can then occur.

The main difference between a coaxial cable and a waveguide is the inner conductor present in the coaxial cable and thus the restriction to the TEM mode of wave propagation in the cable.

Coaxial cables have a defined wave impedance . For radio and television reception technology, it is usually 75 Ω, for other applications it is 50 Ω. The attenuation of a coaxial cable is determined by the loss factor of the insulator material and the resistance layer . The losses in the dielectric, namely the insulating material, are determined by its permittivity ; they are decisive for the conductor coating on the line . With a coaxial cable, the distance between the inner conductor and the outer conductor and the material in this space ( dielectric ) are decisive for the wave resistance.

• There are various reasons why the characteristic impedance of common coaxial cables is between 30 Ω and 75 Ω:
• Line loss (attenuation), depending on the isolator and ohmic resistance of the line
• transferable power
• The line loss per unit length depends on the material separating the inner and outer conductors.
• If air is used as an isolator, the losses are minimal at Z = 75 Ω
• For polyethylene, the optimal value is 50 Ω.

The power that can be transmitted through a coaxial cable depends on the characteristic impedance. The maximum power that can be transmitted is at a wave impedance of 30 ohms.

The wave resistance is therefore selected depending on the application.

• TV and radio technology: 75 Ω to keep losses low. Since these systems do not transmit, the point of least loss is chosen.
• Communication technology: 50 Ω in order to have good transmission properties for both reception and transmission. (Mean value between 30 Ω and 75 Ω)

At higher outputs and to minimize signal losses, the dielectric can be replaced by thin spacers or foam between the inner and outer conductors, the remaining space between the conductors is filled with air. As a dielectric, air enables almost lossless transmission. For air-filled lines, losses occur almost exclusively in the metal of the line. Such coaxial cables are often manufactured with outer conductors made of closed sheet metal and solid inner conductors. However, they are then mechanically less flexible and are only used in fixed installations. Examples are the connection lines between the transmitter and antenna with transmission powers from around 100 kW and cable networks.

Due to their concentric structure and the routing of the reference potential in the outer conductor, coaxial cables offer an electromagnetic shielding effect. The transfer impedance is a measure of this shielding effect and describes the quality of a coaxial cable shield.

Parameters of a coaxial cable

The important parameters of a coaxial cable include:

• the characteristic impedance (cable impedance) Z L - it is independent of the line length and (for high-frequency signals approximately) of the signal frequency, the unit is ohms . Coaxial cables with a characteristic impedance of 50 ohms (general HF technology) or 75 ohms (television technology) are common, rarely 60 ohms (old systems) or 93 ohms. The value can be determined experimentally using time domain reflectometry . The wave impedance is calculated from the ratio of the inner diameter  D of the outer conductor and the diameter  d of the inner conductor of the cable and the dielectric properties ( relative permittivity ) of the insulation material ( dielectric ):${\ displaystyle \ varepsilon _ {\ rm {r}}}$
${\ displaystyle Z_ {L} = {\ frac {Z_ {o}} {2 \ pi {\ sqrt {\ varepsilon _ {\ rm {r}}}}}} \, \ ln \ left ({\ frac { D} {d}} \ right) \ approx {\ frac {60 ~ \ Omega} {\ sqrt {\ varepsilon _ {\ rm {r}}}}} \, \ ln \ left ({\ frac {D} {d}} \ right) = {\ sqrt {\ frac {L '} {C'}}}}$
with the wave resistance of the vacuum ${\ displaystyle Z_ {0}}$
A calculation program can be found in Ref. The above formula and the program neglect the discharge surface G 'and the resistance surface R' of the line. This simplification is permissible in high-frequency operation.

Since the ratio D / d is limited for mechanical reasons and is logarithmically greatly undervalued, the wave resistance of coaxial cables cannot be produced arbitrarily either. Coaxial cables can therefore only be implemented in practice in the range of 30 to 100 ohm wave impedance.

• the attenuation per length, expressed in decibels per meter or per kilometer - it depends on the frequency. Low-loss coaxial cables have the largest possible diameter, the conductors are silver-plated ( skin effect ), the dielectric is made of Teflon or foamed material (high air content). Low- loss cables have a spiral of insulating material to support the inner conductor, the dielectric then mainly consists of air or a protective gas (SF6, sulfur hexafluoride ).
• the capacitance per unit length for a 50 ohm coaxial cable is approximately
100  pF / m
• the inductance coating for a 50-ohm coaxial cable is approximately
250  nH / m
• Propagation speed and shortening factor . The maximum possible speed of propagation is given by the speed of light in a vacuum and is 299,792.458 km / s. This corresponds to around 30 cm per nanosecond (30 cm / ns; see also: light foot ). In the earth's atmosphere , the permittivity of the air reduces the speed to around 299,700 km / s. The speed of propagation in cables is further reduced due to the permittivity of the dielectric used. For the calculation using the so-called reduction factor, that is the reciprocal of the square root of the permittivity of the cable dielectric, ie . For polyethylene (PE), which is often used as a cable dielectric , there is a shortening factor of just under 0.67. This means that the speed of propagation is around 200,000 km / s and the delay time is calculated at around 5 ns per meter of cable (for comparison: only around 3.33 ns / m in a vacuum). Also widely used as an insulating material is Teflon with a , which leads to a delay time of around 4.7 ns per meter.${\ displaystyle {\ varepsilon _ {\ rm {r}}}}$${\ displaystyle {\ frac {1} {\ sqrt {\ varepsilon _ {\ rm {r}}}}}}$${\ displaystyle \ varepsilon _ {\ rm {r}} = 2 {,} 25}$${\ displaystyle \ varepsilon _ {\ rm {r}} \ approx 2}$
• Shielding attenuation in decibels or transfer impedance in mOhm / m. The shielding is not used for cables. The transfer impedance is the usual measured variable. The measurement methods for transfer impedance are standardized.

Line matching and reflections

Coaxial cables for high frequency applications are generally operated with line matching . The load resistance of the cable should correspond as exactly as possible to the wave resistance so that no reflections occur at the end of the line that could cause standing waves and increased losses. The degree of mismatch is determined with standing wave measuring devices or time domain reflectometry . For signals with a low bandwidth , the value of the load resistance can be changed using a resonance transformer .

Reflections and frequency-dependent properties of the dielectric also change the slope of digital signals (see dispersion and pulse schedule ).

Reflections occur at all points where the wave resistance changes, even with unsuitable connection points (plugs) at higher frequencies.

Signal interference

Ingress measurement without notch filter 5–18 MHz
Ingress measurement with notch filter 5–18 MHz, indicated by the absence of the signal in the left area of ​​the diagram

Under Ingress ( engl. , Infiltration '), also called exposure, refers to electromagnetic disturbances in coaxial cables by transmitting equipment , household appliances, power lines, switching power supplies , etc. will. The disturbances mainly occur when the cable or its shielding is damaged, plug connections defective (or poorly shielded) or the shielding dimension (at least 85 dB) of the cable itself is too low. Even if the amplifiers are overdriven or a defect in the transfer point or on the incoming cable is an outer jacket break. If a branch or distributor in the ground is damaged, ingress can also occur. Ingress can also arise from poorly shielded antenna sockets , connectors and distributors. Therefore, only class A components should be used. The interference radiation should not exceed 40 dB.

Egress means the opposite phenomenon in which the signal comes out. In a weaker signal, this can lead to HF interference and interference radiation from neighboring devices. Therefore, only class A components should be used here as well.

Connectors

The design and outer diameter as well as the desired operating frequency range determine the coaxial connection pieces that can be used, the HF connectors. A distinction is made between plugs ("male connector" or "plug") and sockets ("female connector" or "jack"). There are also “genderless” connectors, such as APC connections . The plug connectors differ in the inner diameter D of the outer conductor, the size and homogeneity of their line wave resistance and the insulating materials used. This as well as the homogeneity of the wave impedance determine the maximum operating frequency (cutoff frequency) to a large extent. Common ones are the BNC connectors used on laboratory and radio equipment and previously on network cables . They are available with line impedances of 50 ohms and 75 ohms.

The following table lists examples of connectors with a high limit frequency:

diameter designation Cutoff frequency
7.00 mm APC-7, N 018 GHz
3.50 mm (SMA) 034 GHz
2.92 mm K 040 GHz
2.40 mm - 050 GHz
1.85 mm V 067 GHz
1.00 mm W. 110 GHz

Cable types

Coaxial cable for high transmission capacities. Most of the dielectric is air. Spacers are used between the inner and outer conductors to ensure the mechanical dimensions
Rigid coaxial cable with 1.5  inch outer conductor diameter

Cable designation

In the Joint Electronics Type Designation System (JETDS, MIL-STD-196), a system for naming electronic equipment developed by the US Department of War during the Second World War, coaxial cables were designated with the letters RG for Radio Guide. With the revision D in January 1985, the designation was deleted. Because of this, cables sold under the RG-xx label today do not necessarily meet military specifications.

For bus topology in the baseband

• 10BASE5: 10 Mbit / s, base band (baseband), 500 m
• RG-8 - Thick Ethernet or YellowCable
• Characteristic impedance 50 Ω
• Max. Length 500 m per segment
• Max. 100 connections per segment
• min. Distance between connections 2.5 m
• min. Bending radius 0.2 m
• 5-4-3 rule :
• Max. five segments
• Max. four repeaters
• Max. three segments with computer connections (populated segments)
• Diameter 1.27 cm
• Connection of the computer with an invasive plug (also called vampire clamp, vampire branch or vampire tap)
• 10BASE2: 10 Mbit / s, base band (baseband), approx. 185 m
• RG-58 - Thin Ethernet or CheaperNet
• Characteristic impedance 50 Ω
• Max. Length 185 m per segment
• Max. 30 connections per segment
• min. Distance between connections 0.5 m
• min. Bending radius 0.05 m (= 5 cm)
• 5-4-3 rule :
• Max. five segments
• Max. four repeaters
• Max. three segments with computer connections (populated segments)
• Diameter 0.64 cm
• Connection of the computer with T-piece
• RG-58 U - solid copper inner conductor
• RG-58 A / U - Inner conductor stranded copper
• RG-58 C / U - military spec. From RG-58 A / U

For star topology in the baseband

• ARCNET :
• RG-62
• Characteristic impedance 93 Ω
• Max. Length 300 m

• for example cable TV, satellite TV
• RG-59
• Characteristic impedance 75 Ω
• Diameter 0.25 inch (6.4 mm)
• S-video cable.

Low noise

This cable family was specially developed for applications in which mechanical forces such as B. vibrations, bending or torsional movements act on the cable. With conventional cables, considerable interference can arise from external forces. Low-noise cables, on the other hand, have a special semiconducting dielectric to minimize this interference.

Technical specifications

The technical data of some selected cable types:

designation Foreign
transit
diameter
(mm)
min.
Bending
(mm)
Line
wave
resistance
Attenuation at (dB / 100 m) Comparison
of reduction
factor
Screen
dimension a
145
MHz
432
MHz
1.3
GHz
RG174A / U 2.60 15th 50 ± 2 Ω 38.4 68.5 > 104.2 0.66
RG58C / U 4.95 25th 17.8 33.2 64.5
RG213 / U 10.30 50 8.5 15.8 30.0 60 dB
Aircell 5 5.00 30th 11.9 20.9 39.0 0.82
Aircell 7 7.30 25th 7.9 14.1 26.1 0.83 83 dB
Aircom Plus 10.30 55 4.5 8.2 15.2 0.85 85 dB
Ecoflex 10 10.20 44 4.8 8.9 16.5 0.86 > 90 dB
Ecoflex 15 14.60 150 3.4 6.1 11.4 > 90 dB
Ecoflex 15 Plus 14.60 140 3.2 5.8 10.5 > 90 dB
H1000 10.30 75 4.3 b 9.1 c 18.3 0.83 > 85 dB
aSpecifying a shielding dimension without specifying the frequency or the selected reference values ​​(e.g. current strength, voltage or field strength) is not clear. The shielding effect of a coaxial cable is heavily dependent on frequency. More information about the shielding effect of coaxial cables and their standard-compliant measurement can be found in the article Transfer impedance .
b at 100 MHz
c at 400 MHz