Coaxial cables are cables where the two conductors share a single center, or axis, an inner conductor directly running the length of the cable in the center and an outer conductor — generally, but not always, a braid — wrapped around the center conductor (sharing the same center), separated from the center conductor by an insulating material.
Coax is designed to be low loss; however, these cables are not lossless. That is, whatever power arrives at the antenna is less than left the back of the radio. How the cable is manufactured (for example, the material used in the manufacture, the quality of the braid and dielectric [the insulator between the braid and the center conductor]) determine the electrical characteristics of the cable.
Coax, and all transmission cables, also have a characteristic "velocity factor," a fractional (or decimal) value representing the speed of transmission within the cable with respect to the speed of light. Velocity factor is not a signal loss per se, but may inadvertently cause losses when an attempt is made to match the length of the transmission line to the wavelength utilized. In effect, there is a linear relationship between the velocity factor and the distance a wave travels in the line, so a 30 meter line in a cable with a velocity factor of 0.6 would only travel 18 meters, and in order to match the line to the wavelength computations would have to be based on 18 meters, not 30 meters: this leads to the somewhat odd result that using lengths of cable with a low velocity factor is less expensive than using lines with a high velocity factor because a smaller length is needed to match the wavelength. (Note that this assumes that the velocity factor in the antenna itself is 1.0 [unity], but if the antenna were to have a velocity factor below unity it would be necessary to adjust that as well.)
In addition to these losses inherent in the nature of the specific cables, other factors, such as connectors in the line and deformities in the uniform roundness of the cable (which would change the impedance of the line), will also cause losses.
The following table provides loss figures for several typical types of coaxial cable.
Typical
Cable: | RG-174 | RG-58 | RG-8X | RG-213 | RG-6 | RG-11 | RF-9914 | RF-9913 |
* Note: Length is a loss multiplier, so a 200 ft length would have twice the loss shown above, while a 50 ft length would have half the loss. Length-related loss is also related to the wavelength, with the lowest losses when the line is an exact multiple of the wavelength. However, the electrical wavelength of the line is not identical to the physical wavelength of the wave; electrical wavelength is based on the transmission line's "velocity factor," based on the characteristic of the line, and is computed by multiplying the physical wavelength by the velocity factor (e.g., for a transmission line with a velocity factor of 0.8 the physical wavelength of a 1/2 wave transmission line at 20 meters would be 8 meters [about 26 feet, or 26.25 feet to two decimal places]). For an expanded version of this table, see For additional tables, see, for example, For additional details on velocity factor, see, for example, |
||||||||
Impedance: | 50ohm | 50ohm | 50ohm | 50ohm | 75ohm | 75ohm | 50ohm | 50ohm |
Velocity Factor: | 0.66 | 0.66 | 0.75 | 0.66 | 0.75 | 0.66 | 0.82 | 0.84 |
Frequency |
Attenuation in dB per 100 feet * |
|||||||
---|---|---|---|---|---|---|---|---|
1MHz | 1.9dB | 0.4dB | 0.5dB | 0.2dB | 0.2dB | 0.2dB | 0.3dB | 0.2dB |
10MHz | 3.3dB | 1.4dB | 1.0dB | 0.6dB | 0.6dB | 0.4dB | 0.5dB | 0.4dB |
50MHz | 6.6dB | 3.3dB | 2.5dB | 1.6dB | 1.4dB | 1.0db | 1.1dB | 0.9dB |
100MHz | 8.9dB | 4.9dB | 3.6dB | 2.2dB | 2.0dB | 1.6dB | 1.5dB | 1.4dB |
200MHz | 11.9dB | 7.3dB | 5.4dB | 3.3dB | 2.8dB | 2.3dB | 2.0dB | 1.8dB |
400MHz | 17.3dB | 11.2dB | 7.9dB | 4.8dB | 4.3dB | 3.5dB | 2.9dB | 2.6dB |
700MHz | 26.0dB | 16.9dB | 11.0dB | 6.6dB | 5.6dB | 4.7dB | 3.8dB | 3.6dB |
900MHz | 27.9dB | 20.1dB | 12.6dB | 7.7dB | 6.0dB | 5.4dB | 4.9dB | 4.2dB |
1GHz | 32.0dB | 21.5dB | 13.5dB | 8.3dB | 6.1dB | 5.6dB | 5.3dB | 4.5dB |
In UHF and above applications, the cost-to-loss ratio (that is, the high expense of low-loss cable) becomes prohibitive, and wave guides (essentially containers for a transmitted wave) are used. In the terahertz range, (THF), wave guides incur enough loss to incur prohibitive losses, and alternative methods (such as lenses and mirrors) are appropriate.
[ TOP ]