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"Idiot's guide" for coaxial cable

Coaxial cable structure

Coaxial cable is an electrical cable consisting of a round, insulated conducting wire surrounded by a round, conducting sheath, usually surrounded by a final insulating layer. The cable is designed to carry a high-frequency or broadband signal, usually at radio frequencies. Coaxial Cabling is a two conductor closed transmission medium that is often used for the transmission of RF energy. It yields excellent performance at high frequencies and superior EMI control/shielding when compared to other types of copper cabling. Coaxial cabling is commonly found in broadcast and networking systems. Listed below are some common terms and definitions that are related to coaxial cabling:

Usual terms, used in conjunction with coaxial cable:

Attenuation (Insertion Loss): Loss of power. Attenuation is usually measured in dB loss per length of cable (ex. 31.0 dB/100Ft.). Attenuation increases as frequency increases.
BALUN: An acronym for BALanced/UNbalanced. A device commonly used to change one cabling media to another (ex. coaxial to twisted pair balun).
Center Conductor: The solid or stranded wire in the middle of the coaxial cable. The conductor diameter is measured by the American Wire Gauge (AWG).
Coaxial Adaptor: A device used to change one connector type to another or one gender to another (ex. BNC to SMA Adaptor).
Coaxial Cable: A two conductor cylindrical transmission line typically comprised of a center conductor, an insulating dielectric material and an outer conductor (shielding). Coaxial cable can be flexible (typical to the assemblies found in this catalog), semi-rigid or rigid in nature.
Coaxial Connector: The interconnection device found at each end of a coaxial cable assembly. There are many common types of coaxial connectors such as: BNC, SMA, SMB, F, etc.
Dielectric: The insulating material that separates the center conductor and the shielding.
Electromagnetic Interference (EMI): Electrical or electro-magnetic energy that disrupts electrical signals.
Frequency: The number of times a periodic action occurs in one second. Measured in Hertz.
Impedance: The opposition to the flow of alternating or varying current. Measured in Ohms.
Jack: The female connector usually containing a center socket.
Plug: The male connector usually containing a center pin.
RG/U: Symbols used to represent coaxial cable that is built to U.S. government specifications (R=Radio Frequency, G=Government, U=Universal Specification)
Shielding: Conductive envelope made of wires or metal foil that covers the dielectric and the center conductor
Twinaxial: An offshoot from coaxial cabling. Two center conductors with one dielectric and braided shielding.
VSWR (Voltage Standing Wave Ratio): Amount of reflected power expressed as a ratio (Ex. 1.25:1) VSWR increases as frequency increases.

 

STANDARD CABLE TYPES

Most coaxial cables have a characteristic impedance of either 50 or 75 ohms. The RF industry uses standard type-names for coaxial cables. The U.S military uses the RG-# or RG-#/U format (probably for "radio grade, universal", but other interpretations exist). For example:

Detailed comparison of typical coaxial cables

Type RG-316 RG-174 RG-58/U RG-59 RG-213/UBX RG-213 FOAM AIRCELL 7 BELDEN
H-155
BELDEN
H-500
__
Impedance 50 50 50 75 50 50 50 50 50 Ohm
Outer diameter 2,6 2,6 5,8 6,2 10,3 10,3 7,3 5,4 9,8 mm
Loss at 30 MHz 18 20 9,0 6,0 1,97 3,7 __ 3,4  1,95 dB/100m
144 MHz 32 34 19 13,5 8,5 4,94 7,9 11,2 4,9 dB/100m
432 MHz 60 70 33 23 15,8 9,3 14,1 19,8 9,3 dB/100m
1296 MHz 100 110 64,5 __ 28 18,77 26,1 34,9 16,8 dB/100m
2320 MHz 140 175 __ __ __ 23,7 39   24,5 dB/100m
Velocity factor 0,7 0,66 0,66 __ 0,66 0,8 0,83 0,79 0,81 __
Max. load at 10 MHz 900 200 __ __ __ 2000 2960 550 6450 W
145 MHz 280 9 __ __ __ 1000 1000 240 1000 W
1000 MHz 120 30 __ __ __ 120 190 49 560 W

 

Additional types of coaxial cable

Type Diam. Bending
radius
Imp. Vel. Kg/100m pF/m 10 14 28 50 100 144 435 1296 2400
Aircell 7
7.3
25
50
0.83
7.2
74

3.4
3.7
4.8
6.6
7.9
14.0
26.1
38.0
Aircom Plus
10.8
55
50
0.85
15.0
84
0.9

__

__

3.3
4.5
8.2
14.5
23.0
H-2000 Flex
10.3
50
50
0.83
14.0
80

1.4
2.0
2.7
3.9
4.8
8.5
15.7
23.0
H-1000
10.3
75
50
0.83
14.0
80

1.4
2.0
2.7
3.9
4.8
8.5
15.7
23.0
H-500
9.8
75
50
0.81
13.5
82
1.3

__
__
2.9
4.1

__
9.3
16.8
24.5
H-100
9.8
__
50
0.84
__
80

__
__

__
4.5

__

__

__
__
H-43
9.8
100
75
0.85
9.1
52
1.2
__

__
2.5
3.7
__
8.0
14.3
23.7
LCF 12-50
16.2
70
50
?
22
?
0.67
__
< 1.17

__
2.16
< 3
< 4.7
< 9
< 13
LCF 58-50
21.4
90
50
?
37
?
0.5

__
< 0.88

__
1.64
< 2.2
< 3.5
< 7
< 10
LCF 78-50
28
120
50
?
53
?
0.35

< 0.62

__
1.15
< 1.6
< 2.5
< 5
< 7
RG-223
5.4
25
50
0.66
6.0
101

6.1
7.9
11.0
15.0
17.6

__

__

__
RG-213U
10.3
110
50
0.66
15.5
101
2.2

3.1
4.4
6.2
7.9
15.0
27.5
47.0
RG-174U
2.8
15
50
0.66
__
101

__

__


30.9
__
__
__
__
RG-59
6.15
30
75
0.66
5.7
67

__
__


12.0

__
25.0
33.6
__
RG-58CU
5.0
30
50
0.66
4.0
101

__
6.2
8.0
11.0
15.6
17.8
33.0
65.0
100.0
RG-58 others
4.9
32
50
0.78
3.2
82

__

__

__
8.3
11.0
__
23.0
44.8
__
RG-11
10.3
50
75
0.66
13.9
67

__
__

__
4.6
6.9

__
18.0
30.0
__

 

The common RG-58 from Radio Shack is NOT the best you can do and will lower your effective power out! Use it only for short runs. BELDEN makes terrific coax in various qualities and with very low loss (measured in dB’s…decibels). 3 dB loss = 1/4 of your signal strength - either lost or gained. Watch out for the correct impedance…RG-8 and RG-58 have 50 Ohms. RG-59 and RG-6 (Low Loss Version of RG-59) have 75 Ohms. Most antennas are 50 ohm. Check our website for good coax. Don't buy more than you need to make the long run to your antenna and don't make up a few "jumpers" to go between your exciter, VSWR meter and your antenna as all you'll do is create higher SWR and more line losses. Finally, don't use cheap TV cable!

Additional reading, an extensive guide with in-depth information

 

SO WHAT IS THIS SWR (VSWR) EVERYONE TALKS ABOUT?

VSWR is a measure of how well two devices are impedance matched to each other. Typical radio equipment is designed for 50 ohm load impedance, so we usually use 50 ohm cables and build or buy antennas that are specified for 50 ohm. While most cables have a flat impedance over frequency (they measure 50 ohm at all frequencies you are likely to use), the same is not true of the antennas.
A 1.0:1 VSWR is a perfect match. That means the load impedance is exactly 50 ohms. A 2.0:1 VSWR is obtained when the load impedance is either 25 ohms or 100 ohms.

Because most transmitters will deliver full power with a load VSWR of up to 2.0:1, this value is usually considered the limit for acceptable operation. Many prefer to keep their VSWR below that however, but for all practical purposes, it is unnecessary to spend time or money trying to get much below a VSWR of 1.5:1. The benefits will be hard to measure and even harder to notice.

On the other hand, coaxial cable losses increase rapidly, for a given frequency of operation, when the antenna VSWR exceeds 2.0:1. This can even, in some extreme cases, result in the coaxial cable burning, even when running 100 W. Using a higher grade of cable will definitely improve things, but even high quality coaxial cable becomes very lossy when VSWR exceeds 3.0:1 at higher HF frequencies (or VHF and higher).

COMMON CONNECTOR TYPES

"UHF" connector: The "UHF" connector is the old industry standby for frequencies above 50 MHz (during World War II, 100 MHz was considered UHF). The UHF connector is primarily an inexpensive all purpose screw on type that is not truly 50 Ohms. Therefore, it's primarily used below 300 MHz. Power handling of this connector is 500 Watts through 300 MHz. The frequency range is 0-300 MHz.

"N" connectors: "N" connectors were developed at Bell Labs soon after World War II so it is one of the oldest high performance coax connectors. It has good VSWR and low loss through 11 GHz. Power handling of this connector is 300 Watts through 1 GHz. The frequency range is 0-11 GHz.

"BNC" connctor: "BNC" connectors have a bayonet-lock interface which is suitable for uses where where numerous quick connect/disconnect insertions are required. BNC connector are for exampel used in various laboratory instruments and radio equipment. BNC connector has much lower cutoff frequency and higher loss than the N connector. BNC connectors are commonly available at 50 ohms and 75 ohms versions. Power handling of this connector is 80 Watts at 1 GHz. The frequency range is 0-4 GHz.

"TNC" connectors are an improved version of the BNC with a threaded interface. Power handling of this connector is 100 Watts at 1 GHz. The frequency range is 0-11 GHz.

"SMA" connector: "SMA" or miniature connectors became available in the mid 1960's. They are primarily designed for semi-rigid small diameter (0.141" OD and less) metal jacketed cable. Power handling of this connector is 100 Watts at 1 GHz. The frequency range is 0-18 GHz.

"7-16 DIN" connector: "7-16 DIN" connectors are recently developed in Europe. The part number represents the size in metric millimeters and DIN specifications. This quite expensive connector series was primarily designed for high power applications where many devices are co-located (like cellular poles). Power handling of this connector is 2500 Watts at 1 GHz. The frequency range is 0-7.5 GHz.

"F" connector: "F" connectors were primarily designed for very low cost high volume 75 Ohm applications much as TV and CATV. In this connector the center wire of the coax becomes the center conductor.

"IEC antenna connector": This is a very low-cost high volume 75 ohm connector used for TV and radio antenna connections around Europe.

Additional reading here.

 

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