Coaxial cables

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.

Coaxial cable is a cable type used to carry radio signals, video signals, measurement signals and data signals. Coaxial cables exists 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 ceter conductor which is covered with a shield. The signal is carried between the cable shield and the center conductor. This arrangement give quite good shielding agains noise from outside cable, keeps the signal well inside the cable and keeps cable characteristics stable.

Coaxial cables and systems connected to them are not ideal. There is always some signal radiating from coaxial cable. Hence, the outer conductor also 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 cable 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 (read: shield or braid), and a represents the outside diameter of the inner conductor.

Most common coaxial cable impedances in use in various applications are 50 ohms and 75 ohms. 50 ohms cable is used in radio transmitter antenna connections, many measurement devices and in data communications (Ethernet). 75 ohms 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 ohms). 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 maximum power handling, somewhere between 30 and 44 Ohms is the optimum. Impedance somewhere around 77 Ohms gives the lowest loss in a dielectric filled line. 93 Ohms cable gives low capacitance per foot. It is practically very hard to find any coaxial cables with impedance much higher than that.

Here is a quick overview of common coaxial cable impedances and their main uses:

The characteristic impedance of a coaxial cable is determined by the relation of outer conductor diameter to inner conductor diameter and by the dielectric constant of the insulation. The impednage of the coaxial cable chanes soemwhat with the frequency. Impedance changes with frequency until resitance is a minor effect and until dielectric dielectric constant is table. Where it levels out is the "characteristic impedance". The freqnency where the impedance matches to the characteristic impedance varies somwehat between different cables, but this generally happens at frequency range of around 100 kHz (can vary).

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 on 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 number 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 same thing as VSWR term used in radio world, 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 (most data from http://dct.draka.com.sg/coaxial_cables.htm, http://www.drakausa.com/pdfsDSC/pCOAX.pdf and http://users.viawest.net/~aloomis/coaxdat.htm):

Cable type            RG-6        RG-59 B/U   RG-11       RG-11 A/U    RG-12 A/U  RG-58 C/U  RG-213U  RG-62 A/U 
Impedance (ohms)      75          75          75          75           75         50         50       93
Conductor material    Bare        Copper      Bare        Tinned       Tinned     Tinned     Bare     Copper
                      Copper      Planted     Copper      Copper       Copper     Copper     Copper   Planted
                                  Steel                                                               Steel
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"      0.023"                  0.048"                  0.035"     0.089"   0.025"
                      21AWG       23AWG                   18AWG                   20AWG      13AWG    22AWG
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       7.25     3.7 mm
Outer conductor       Aluminium   Bare        Aluminium   Bare         Base       Tinned     Bare     Bare
                      polyester   copper      polyester   copper       copper     copper     copper   copper
                      tape and    wire        tape and    wire         wire       wire       wire     wire
                      tin copper  braid       tin copper  braid        braid      braid      braid    braid
                      braid                   braid      
Coverage              Foil 100%   95 %        Foil 100%   95%          95%        95%        97%      95%
                      braid 61%               Braid 61%
Outer sheath          PVC         PVC         PVC         PVC          PE         PVC        PVC      PVC
Outside diameter      6.90 mm     6.15 mm     10.3 mm     10.3 mm      14.1 mm    4.95 mm    10.3     6.15 mm
Capacitance per meter 67 pF       67 pF       57 pF       67 pF        67 pF      100 pF     100 pF   
Capacitance per feet  18.6        20.5        16.9        20.6         20.6 pF    28.3 pF    30.8     13.5 pF
Velocity              78%         66%         78%         66%          66%        66%        66%      83%
Weight (g/m)          59          56          108         140          220        38
Attenuation db/100m
50 MHz                5.3         8           3.3         4.6          4.6                            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                             20
900 MHz		      21          39.5	      17.1	  24	       24                             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 manifacturer. There can be some differences on the performance and specifications of different cables 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 cna 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 0.80).

General data on some other 75 ohm coaxial cables compared to RG-59 (most data from http://dct.draka.com.sg/coaxial_cables.htm and http://users.viawest.net/~aloomis/coaxdat.htm and Tasker catalogue):

Cable type            RG-6        RG-59 B/U   RG-11       RG-11 A/U    RG-12 A/U  TELLU 13      Tasker RGB-75
Impedance (ohms)      75          75          75          75           75         75            75
Impedance accuracy                +-2 ohms    +-3 ohms                            +-2 ohms      +-3%
Conductor material    Bare        Copper      Bare        Tinned       Tinned     Bare          Bare
                      Copper      Planted     Copper      Copper       Copper     Copper        Copper
                                  Steel     
Conductor strands     1           1           1           7            7          1             10
Conductor strand(mm2) 0.95        0.58        1.63        0.40         0.40       1mm diameter  0.10mm diameter
Resistance (ohm/km)   44          159         21          21                      22            210
Insulation material   Foam PE     PE          Foam PE     PE           PE         Foam PE
Insulation diameter   4.6 mm      3.7 mm      7.24 mm     7.25 mm      9.25 mm    
Outer conductor       Aluminium   Bare        Aluminium   Bare         Base       Copper        Tinned
                      polyester   copper      polyester   copper       copper     foil under    copper
                      tape and    wire        tape and    wire         wire       bare copper
                      tin copper  braid       tin copper  braid        braid      braid 
                      braid                   braid
Coverage              Foil 100%   95 %        Foil 100%   95%          95%        Foil          ~95%       
                      braid 61%               Braid 61%                           Braid 66%     
Resistance (ohm/km)   6.5         8.5         4           4                       12            ~40
Outer sheath          PVC         PVC         PVC         PVC          PE         PVC (white)    PVC
Outside diameter      6.90 mm     6.15 mm     10.3 mm     10.3 mm      14.1 mm    7.0 mm        2.8 mm
Capacitance per meter 67 pF       67 pF       57 pF       67 pF        67 pF      55 pF         ~85 pF
Capacitance per feet  18.6        20.5        16.9        20.6         20.6 pF
Velocity              78%         66%         78%         66%          66%        80%           66%
Screening factor                                                                  80 dB
Typical voltage (max)             2000V                   5000V                                 1500V
Weight (g/m)          59          56          108         140          220        58
Attenuation db/100m
5 MHz                             2.5                                             1.5          
50 MHz                5.3         8           3.3         4.6          4.6        4.7           19.5
100 MHz               8.5         12          4.9         7            7          6.2           28.5
200 MHz               10          18          7.2         10           10         8.6           35.6
400 MHz               12.5        24          10.5        14           14         12.6          60.0
500 MHz               16.2        27.5        12.1        16           16         ~14           ~70 
900 MHz               21          39.5        17.1        24           24         19.2          90.0
2150 MHz                                                                          31.6
3000 MHz                                                                          37.4
NOTE: The numbers with ~ mark in front of them are approximations calculated and/or measured from cables or cable data. Those numbera are not from manufacturer literature. NOTE2: Several of cables mentioned above are available in sepcial versionswith foam insulation material. This changes the capacitances to somewhat lower value and gives higher velocity (typically around 0.80).

General coaxial cable details

The dielectric of a coaxial cable serves but one purpose - to maintain physical support and a constant spacing between the inner conductor and the outer shield. In terms of efficiency, there is no better dielectric material than air. In most practical cables cable companies use a variety of hydrocarbon-based materials such as polystyrene, polypropylenes, polyolefins and other synthetics to maintain structural integrity.

Sometimes coaxial cables are used also for carrying low frequency signals, like audio signals or measurement device signals. In audio applications especially the coaxial cable impedance does not matter much (it is a high frequency property of cable). Generally coaxial has a certain amount of capacitance (50 pF/foot is typical) and a certain amount of inductance. But it has very little resistance.

General characteristics of cables:

Please note that these are general statements. A specific 75 ohm cable could be 20pF/ft. Another 75 ohm cable could be 16pF/ft. There is no exact correlation between characteristic impedance and capacitance.

In general, a constant impedance (including connectors) cable, when terminated at both ends with the correct load, represents pure resistive loss. Thus, cale capacitance is immaterial for video and digital applications.

Typical coaxial cable constructions are:

Often you will hear the term shielded cable. This is very similar to coaxial cable except the spacing between center conductor and shield is not carefully controlled during manufacture, resulting in non-constant impedance.

If the cable impedance is critical enough to worry about correctly choosing between 50 and 75 Ohms, then the capacitance will not matter. The reason this is so is that the cable will be either load terminated or source terminated, or both, and the distributed capacitance of the cable combines with its distributed inductance to form its impedance.

A cable with a matched termination resistance at the other end appears in all respects resistive, no matter whether it is an inch long or a mile. The capacitance is not relevant except insofar as it affects the impedance, already accounted for. In fact, there is no electrical measurement you could make, at just the end of the cable, that could distinguish a 75 Ohm (ideal) cable with a 75 Ohm load on the far end from that same load without intervening cable. Given that the line is teminated with a proper 75 ohm load (and if it's not, it damn well should be!), the load is 75 ohms resistive, and the lumped capacitance of the cable is irrelevant. Same applies to other impedance cables also when terminated to their nominal impedance.

There exist an effect that characteristic impedance of a cable if changed with frequency. If this frequency-dependent change in impedance is large enough, the cable will be impedance-matched to the load and source at some frequencies, and mismatched at others. Characteristic impedance is not the only detail in cable. However there is another effect that can cause loss of detail fast-risetime signals. There is such a thing as frequency-dependent losses in the cable. There is also a property of controlled impedance cables known as dispersion, where different frequencies travel at slightly different velocities and with slightly different loss.

In some communications applications a pair of 50 ohm coaxial cables are used to transmit a differential signal on two non-interacting pieces of 50-ohm coax. The total voltage between the two coaxial conductors is double the single-ended voltage, but the net current in each is the same, so the differential impedance between two coax cable used in a differential configuration would be 100 ohms. As long as the signal paths don't interact, the differential impedance is always precisely twice the single-ended impedance of either path.

Coax Connnector Information

RF coax(ial) connectors are a vital link in the system which uses coaxial cables and high frequency signals. Coax connectors are often used to interface two units such as the antenna to a transmission line, a receiver or a transmitter. The proper choice of a coax connector will facilitate this interface.

Coax connectors come in many impedances, sizes, shapes and finishings. There are also female and male versions of each. As a consequence, there are thousands of models and variations, each with its advantages and disadvantages. Coax connectors are usually referred to by series designations. Fortunately there are only about a dozen or so groupings or series designations. Each has its own important characteristics, The most popular RF coax connector series not in any particular order are UHF, N, BNC, TNC , SMA, 7-16 DIN and F. Here is quicl introduction to those connector types:

There are also some special connectors and special variations of connectors used for some special applications. For example FCC has required that suppliers of RF LANs (local area networks) have an RF interface that cannot be matched by the present available RF connector series (idea is to prevent connecting higher gain antennas to those devices). As a result, several so called "reverse polarity connectors" have been designed. The reverse polarity TNC is one of the most popular where the threads are left-hand instead of the conventional right-hand type.


Tomi Engdahl <[email protected]>