1. ATTENUATION – Attenuation is loss of power or signal expressed in decibels; it is commonly written and spoken of as dB/100 ft. at a specific frequency. An example is RG 8A/U which has a loss of 5.5 dB/100 ft. at 400 MHz.
2. FREQUENCY – Frequency is the term designating the number of reverses or cycles in the flow of alternating current (AC) in one second. For example, the frequency of AC commonly used in the U.S. is 60 hertz and is usually shown as 60 Hz. Broadcast stations operate at frequencies of thousands of cycles per second and their frequencies are called kilohertz (kHz). Your AM radio dial represents frequencies in kilohertz (kHz). High frequencies are in millions of cycles per second and are called megahertz (MHz). TV is broadcast in the MHz range.
3. IMPEDANCE – Impedance is a term expressing the ratio of voltage to current in a cable of infinite length. In the case of coaxial cables, impedance is expressed in terms of “ohms impedance”.The coaxial cables generally fall into three main classes; 50 ohms, 75 ohms, and 95 ohms.
An example of each class is:
RG 11A/U 75 ohms impedance
RG 22B/U 95 ohms impedance
4. CAPACITANCE (CAPACITY) – Capacitance or capacity is the property of a system of conductors and dielectrics which permits the storage of electricity when a potential or voltage difference exists between the two conductors. A capacity value is expressed in farads.When we deal with coaxial cable, the capacity ranges we have are very small and are expressed in picofarads (pF)
Capacity is the major factor governing impedance. Examples of cables with typical impedances have capacity as follows:
RG or M17 |
Cable Impedance (ohms) |
Dielectric Type |
Capacitance (pF/ft) |
RG 8A/U |
50 |
PE |
29.5 |
RG 231A/U |
50 |
Foam PE |
25.0 |
RG 188A/U |
50 |
Solid TFE |
29.0 |
M17/6 |
75 |
PE |
20.6 |
RG 306A/U |
75 |
FoamPE |
16.5 |
RG 140 |
75 |
Solid TFE |
21.0 |
M17/90 |
93 |
Air space PE |
13.5 |
M17/56 |
95 |
PE |
17.0 |
M17/95 |
95 |
Solid TFE |
15.4 |
RG 24A/U |
125 |
PE |
12.0 |
RG 114A/U |
185 |
Air space PE |
6.5 |
5. VELOCITY OF PROPAGATION – Velocity of propagation, commonly called velocity, is the ratio of the speed of the flow of an electric current in an insulated cable to the speed of light. All insulated cables have this ratio and it is expressed in a percent- age. In the case of coaxial cables with polyethylene dielectric, this ratio is in the range of 65% – 66%.
In selecting coaxial cable, we must carefully consider not only design criteria, but use and application. Selection of materials in relation to overall design considerations is tabulated in folowing tables
INNER CONDUCTORS
INNER CONDUCTORS | ||||||
SOFT BARE COPPER |
TINNED SOFT COPPER |
SILVER – PLATED COPPER |
NICKEL – PLATED COPPER |
TINNED – CADIMUM BRONZE |
COPPER WELD® |
|
Maximum operating temperature °C | 200 | 150 | 200 | 250 | 150 | 200 |
Resistivity at 20°C,ohms – circular mil / ft. | 10.371 | 11.133 | 10.371 | 12.5 | 11.92 | 25.928 |
Average tensile strength psi (1,000) | 37 | 37 | 37.5 | 37.5 | 45 | 130 |
Flexibility | excellent | excellent | excellent | excellent | good | good |
Remarks | most popular – for extra flexibility use stranded | for added resistance to oxidation and easy solderability, best for low frequency application | elevated temperature usein aircraft, missile, and electronics, easy solderability | extra high temperature use | high tensilestrength with flexibility | extra high tensile strength |
Table1-Inner Conductors
INNER CONDUCTORS | |||||
SOFT BARE COPPER |
TINNED SOFT COPPER |
SILVER – PLATED COPPER |
ALUMINUM TUBE | COPPER TUBE | |
Maximum operating temperature °C | 200 | 150 | 200 | – | – |
Flexibility | excellent | excellent | excellent | poor | poor |
Remarks | most popular in braid, minimum .004″ to .010″, add second shield to improve flexibility | most popular in braid, minimum .004″ to .010″, add second shield to improve flexibility, better for low frequency | most popular in braid, minimum .004″ to .010″, add second shield to improve flexibility, for high temperature | for high tensile and crushing loads and lower attenuation | for high tensile strength and crushing loads |
Table2-Outer Jackets
Primary Dielectrics | |||||
POLYETHYLENE (PE) | FOAMED POLYETHYLENE (PE) | Fluorinated Ethylene Propylene (FEP) | Poly Tetrafluoroethylene (PTFE) | BUTYL RUBBER | |
Maximum operating temperature °C | -65 to 80 | -65 to 80 | -65 to 200 | -65 to 260 | -40 to 80 |
Average tensile strength psi (1,000) | 1.9 | 2.2 | 3.6 | 2.7 | 1.1 |
Flexibility | good | good | excellent | good | excellent |
Cut-thru resistance | good | poor | good | fair | excellent |
Water Resistance | excellent | poor | excellent | excellent | good |
Resistance to organic solvents | poor | poor | excellent | excellent | good |
Resistance to acids and alkalies | excellent | excellent | excellent | excellent | good |
Remarks | for use under 80°C maximum | for use under 80°C maximum | for high temperature use to 200°C | for high temperature use to 260°C | for pulse cables and extreme flexibility |
Table 3 – Primary Dielectrics
JACKETS | ||||||
POLYETHYLENE | Tetrafluoroe- thylene (TFE) |
Fluorinated Ethylene Propylene (FEP) |
PVC | NEOPRENE® | GLASS BRAID | |
Maximum operating temperature °C |
80 | 260 | 200 | 105 | 90 | 260 |
Average tensile strength psi (1,000) |
1.9 | 3.5 | 2.7 | 2.5 | 3.2 | – |
Flexibility | good | good | good | good | excellent | excellent |
Resistance to organic solvents |
poor | excellent | excellent | poor | good | excellent |
Resistance to acids and alkalies |
excellent | excellent | excellent | fair | good | excellent |
Abrasion resistance | good | excellent | excellent | good | excellent | poor |
Flame resistance | slow burn | nonflammable | nonflammable | self-extinguishing | self-extinguishing | nonflammable |
Remarks | for added resistance to weathering |
to mate with high temperature dielectric |
to mate with high temperature dielectric |
most widely used | to mate with Butyl dielectric |
to mate with high temperature dielectric |