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Coax In Depth

Coaxial cable structure

Introduction to coaxial cables

A coaxial cable is one that consists of two conductors that share a common axis. The inner conductor is typically a straight wire, either solid or stranded and the outer conductor is typically a shield that might be braided or a foil or, in some 'high end' types, both.

Coaxial cable is a cable type used to carry radio signals, video signals, measurement signals, audio signals and data signals. Coaxial cables exist because we can't run open-wire line near metallic objects (such as ducting) or bury it. We trade signal loss for convenience and flexibility. Coaxial cable consists of an insulated centre conductor which is covered with a shield. The signal is carried between the cable shield and the center conductor. This arrangement gives quite good shielding against noise from outside the cable, keeps the signal inside the cable and keeps cable characteristics stable.

Coaxial cables and the systems connected to them are not ideal. There is always some signal radiating from coaxial cable. The outer conductor functions as a shield to reduce coupling of the signal into adjacent wiring. More shield coverage means less radiation of energy (but it does not necessarily mean less signal attenuation).

Coaxial cables are typically characterized with the impedance and cable loss. The length has nothing to do with a coaxial cable impedance. Characteristic impedance is determined by the size and spacing of the conductors and the type of dielectric used between them. For ordinary coaxial cable used at reasonable frequency, the characteristic impedance depends on the dimensions of the inner and outer conductors. The characteristic impedance of a cable (Zo) is determined by the formula 138 log b/a, where b represents the inside diameter of the outer conductor (shield or braid), and a represents the outside diameter of the inner conductor.

The most common coaxial cable impedances in use are 50 Ohm and 75 Ohm.

50 Ohm cable is used in radio transmitter antenna connections, many measurement devices and in data communications (Ethernet).

75 Ohm coaxial cable is used to carry video signals, TV antenna signals and digital audio signals.

There are also other impedances in use in some special applications (for example 93 Ohm). It is possible to build cables at other impedances, but those mentioned earlier are the standard ones that are easy to get. It is usually no point in trying to get something very little different for some marginal benefit, because standard cables are easy to get, cheap and generally very good.

Different impedances have different characteristics. For example:
For maximum power handling, somewhere between 30 Ohm and 44 Ohm is the optimum.
Impedance somewhere around 77 Ohm gives the lowest loss in a dielectric filled line.
93 Ohm cable gives low capacitance per foot.

It is very hard to find any coaxial cables with impedance much higher than 93 Ohm.

The characteristic impedance of a coaxial cable is determined, as we saw earlier, by the relation of outer conductor diameter to inner conductor diameter and also by the dielectric constant of the insulation. The Impedance of the coaxial cable changes, somewhat, with the frequency. Impedance changes with frequency until resistance is a minor effect and until dielectric dielectric constant is stable. Where it levels out is the "characteristic impedance". The frequency where the impedance matches to the characteristic impedance varies somewhat between different cables, but this generally happens at frequency range of around 100 kHz.

Essential properties of coaxial cables are their characteristic impedance and its regularity, their attenuation as well as their behaviour concerning the electrical separation of cable and environment, i.e. their screening efficiency. In applications where the cable is used to supply voltage for active components in the cabling system, the DC resistance has significance. Also the cable velocity information is needed for some applications. The coaxial cable velocity of propagation is defined by the velocity of the dielectric. It is expressed in percents of speed of light. Here is some data of come common coaxial cable insulation materials and their velocities:
Polyethylene (PE) 66%
Teflon 70%
Foam 78..86%

Return loss is one figure which shows cable performance meaning how well it matches the nominal impedance. Poor cable return loss can show cable manufacturing defects and installation defects (cable damaged on installation). With a good quality coaxial cable in good condition you generally get better than -30 dB return loss, and you should generally not got much worse than -20 dB. Return loss is probably better known to you as VSWR, only expressed differently (-15 dB return loss = 1.43:1 VSWR, -23 dB return loss = 1.15:1 VSWR etc.).

Often used coaxial cable types.

General data on some commonly used coaxial cables compared:
Cable type:RG-6RG-59 B/URG-11RG-11 A/URG-12 A/URG-58 C/URG-213URG-62 A/U
Impedance Ohms 75 75 75 75 75 50 50 93
Conductor material Bare
Conductor strands 1 1 1 7 7 19 7 1
Conductor area (mm2) 0.95 0.58 1.63 0.40 0.40 0.18 0.75 0.64
Conductor diameter 0.028"
n/a 0.048"
n/a 0.035"
Insulation material Foam PE PE Foam PE PE PE PE PE PE (semi-solid)
Insulation diameter 4.6 mm 3.7 mm 7.24 mm 7.25 mm 9.25 mm 2.95 mm 7.25 mm 3.7 mm
Outer conductor Aluminium polyester tape and tin copper braid Bare copper wire braid Aluminium polyester tape and tin copper braid Bare copper wire braid Base copper wire braid Tinned copper wire braid Bare copper wire braid Bare copper wire braid
Coverage Foil 100% braid 61% 95 % Foil 100% Braid 61% 95% 95% 95% 97% 95%
Outside diameter 6.90 mm 6.15 mm 10.3 mm 10.3 mm 14.1 mm 4.95 mm 10.3 mm 6.15 mm
Capacitance per meter 67 pF 67 pF 57 pF 67 pF 67 pF 100 pF 100 pF n/a
Capacitance per foot 18.6 pF 20.5 pF 16.9 pF 20.6 pF 20.6 pF 28.3 pF 30.8 pF 13.5 pF
Velocity 78% 66% 78% 66% 66% 66% 66% 83%
Weight (g/m) 59 56 108 140 220 38 n/a n/a
Attenuation db/100m
50 MHz 5.3 8 3.3 4.6 4.6 n/a n/a 6.3
100 MHz 8.5 12 4.9 7 7 16 7 10
200 MHz 10 18 7.2 10 10 23 9 13
400 MHz 12.5 24 10.5 14 14 33 14 17
500 MHz 16.2 27.5 12.1 16 16 n/a n/a 20
900 MHz 21 39.5 17.1 24 24 n/a n/a 28.5

NOTE: The comparision table above is for information only. There is no guarantee of correctness of data presented. When selecting cable for a certain application, check the cable data supplied by the cable manufacturer. There can be some differences on the performance and specifications of the same type of cable from different manufacturers. For example the insulation rating of cables vary. Many PE insulated coax cables can handle several kilovots voltage, while some foam insulated coax cables can handle only 200 volts or so.

NOTE: Several of cables mentioned above are available with foam insulation material. This changes the capacitances to somewhat lower value and gives higher velocity (typically around 80%).

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