Coaxial cable transfers radio frequency power from one point to another and, in the ideal world, the same amount of power would transfer along the cable to the remote end of the coax cable. However, real world conditions include some power loss along the length of the cable. Loss, or attenuation, is one of the most important features to look for when deciding what type of coaxial cable to use in a design.
Loss is defined by decibels per unit length and at a given frequency. Thus, the longer the coaxial cable, the greater the loss. Loss is also frequency dependent, generally increasing with frequency, but the loss is not necessarily linearly dependent upon the frequency. Power loss occurs in a variety of ways.
Resistive loss
Resistive losses within the coaxial cable occur when the resistance of the conductors and the current flowing in the conductors results in heat being dissipated. Skin effect limits the area through which the current flows, which leads to increased resistive losses as the frequency rises.
To reduce the level of resistive loss, the conductive area is increased, resulting in larger low-loss cables. Also, multi-stranded conductors are often used. Resistive losses generally increase as the square root of frequency.
Dielectric loss
Dielectric loss is signal energy dissipated as heat within the insulating dielectric of a cable, but is independent of the size of the coaxial cable. Dielectric losses increase linearly with frequency, and the resistive losses normally dominate at lower frequencies. As resistive losses increase as the square root of frequency and dielectric losses increase linearly, the dielectric losses dominate at higher frequencies.
Radiated loss
Radiated loss in a coaxial cable is usually much less than resistive or dielectric losses, however a poorly constructed outer braid on some coaxial cables may yield a relatively high radiated loss.
Radiated power, problematic in terms of interference, occurs when signal energy passing through the transmission line is radiated outside of the cable. Leakage from a cable carrying a feed from a high-power transmitter may produce interference in sensitive receivers located close to the coax cable or a cable being used for receiving can pick up interference if it passes through an electrically noisy environment.
To reduce radiated loss or interference, double- or triple-screened coaxial cables are designed to reduce the levels of leakage to very low levels.
Of these forms of loss, radiated loss is generally the less concerning as only a very small amount of power is generally radiated from the cable. Thus, most of the focus on reducing loss is placed onto the conductive and dielectric losses, except in certain applications.
Loss over time
Loss or attenuation of coaxial cables tends to increases over time as a result of flexing and moisture in the cable. Although some coax cables are flexible, the level of loss or attenuation will increase if the RF cable is bent sharply or if there is a disruption to the braid or screen.
Contamination of the braid by the plasticisers in the outer sheath or moisture penetration can affect both the braid where it causes corrosion and the dielectric where the moisture will tend to absorb power. Often, coax cables that use either bare copper braid or tinned copper braid experience more degradation than those with the more expensive silver plated braids.
Although foam polyethylene provides a lower level of loss or attenuation when new, it absorbs moisture more readily than the solid dielectric types. Cables with solid dielectric polyethylene are more suited to environments where the level of loss needs to remain constant or where moisture may be encountered. Even though RF coaxial cables are enclosed in a plastic sheath, many of the plastics used allow some moisture to enter; thus, for applications where moisture may be encountered, specialised cables should be used to avoid performance degradation.
For more information contact Andrew Hutton, RF Design, +27 21 555 8400, andrew@rfdesign.co.za, www.rfdesign.co.za
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