Transmission lines are primarily cables that transmit power, conduct low-frequency electromagnetic waves to the load in a guided manner, minimize reflections and power losses, and use impedance matching to carry power. Therefore, loss is one of the most important parameters for RF transmission lines. As one of the types of RF transmission lines, coaxial cable is the first to lose coaxial cable today.
(1) The type of transmission line
A transmission line is a structure of conductive electromagnetic waves composed of two conductors parallel to each other. In the case of a transmission line, when the physical length of the transmission line is very different from the signal wavelength, the current and voltage of the transmission line are not much different from the source side. However, as the frequency increases, the length of the transmission line can be compared to the signal wavelength, and due to a series of reflections, the terminal voltage and current of the transmission line will change.
For any electromagnetic wave transmission lines are divided into two main categories:
The first type is the two-conductor transmission line developed through the theory of road, including: parallel double wire, coaxial line, microstrip line, strip line and so on.
The other type is the waveguide transmission line developed from the theory of waves, including rectangular waveguides, circular waveguides, dielectric waveguides, ridge waveguides, substrate integrated waveguides, etc.
(2) Coaxial cable loss
Coaxial cables are used to transmit RF energy from one location to another. Ideally, the energy transmitted along the coaxial cable to the far end should be equal to the energy at the starting point. However, under real conditions, losses occur as energy travels along the cable. Loss, also known as attenuation, is one of several important considerations in the design to determine the type of coaxial cable to be used.
Low-loss SPF-250 corrugated super-flexible fire-rated coaxial cable with black FRPE sheath
Losses are defined by the number of decibels per unit length at a given frequency. It can be seen that the longer the coaxial cable, the greater the loss. In general, the higher the frequency, the greater the loss, but the relationship between loss and frequency is not necessarily linear. Energy loss comes in many forms:
1) Resistance loss
Resistance losses occur within the coaxial cable when the resistance of the conductor and the current flowing through the conductor cause heat dissipation. The skin effect limits the area through which the current can flow, resulting in resistive losses that increase with frequency. In order to reduce the degree of resistance loss, the conductive area must be increased, so that the size of the coaxial cable with lower losses is larger than that of the cable with higher losses, and the losses are often reduced by using stranded conductors. Resistance losses usually increase with the square root of the frequency.
75 ohm flexible RG6-CATV coaxial cable with triple shielded black PVC (NC) sheath
2) Dielectric loss
Dielectric loss is the signal energy dissipated in the form of heat within the insulating dielectric of the cable, but is independent of the size of the coaxial cable. Dielectric losses increase linearly with frequency, while resistive losses typically play a dominant role at lower frequencies. Since the resistive loss increases with the square root of the frequency, and the dielectric loss increases linearly with the frequency, the dielectric loss plays a dominant role at higher frequencies.
3) Radiation loss
The radiation losses of coaxial cables are usually much smaller than their resistive or dielectric losses, yet some coaxial cables may have higher radiation losses due to the use of a poorer quality outer braid. When the signal energy carried by the transmission line radiates to the outside of the cable, the radiated power is generated, which causes interference problems. When a leak occurs in the feeder of a high-power transmitter, it can cause interference to the sensitive receiver located near the coaxial cable. Alternatively, when the cable used for the receiving function is routed in an environment with noisy electrical noise, it may be affected by interference. In order to reduce radiation losses or interference, double- or triple-shielded coaxial cables have been designed to minimize leakage.
Semi-rigid coaxial cable, copper outer conductor, diameter 0047 inches
Of the above forms of loss, radiation losses are generally less involved, as only a very small amount of energy is usually radiated from the cable. Therefore, with the exception of certain applications, most of the focus of loss reduction is on conductive and dielectric losses.
4) Wear and tear over time
Because cables are subjected to repeated bending and moisture erosion, loss or attenuation of coaxial cables tends to increase over time. While some coaxial cables are flexible, the degree of loss or attenuation will increase if the RF cable is severely bent, or if the braid or shield is damaged. Contamination of the braid by plasticizers inside the outer sheath or penetration of moisture can have an effect on both the braid (which causes corrosion) and the dielectric (which absorbs energy). In general, coaxial cables that use bare copper braid or tinned copper braid are more likely to experience performance degradation than coaxial cables that use a higher silver braid layer. Although foamed polyethylene has a lower degree of loss or attenuation when first used, it absorbs moisture more easily than solid-like dielectrics.
Low loss 1 4 ultra-flexible spiral corrugated coaxial cable with black PE sheath
Cables using solid dielectric polyethylene are more suitable for environments where the level of loss needs to be constant or humid. Although the RF coaxial cable is encapsulated in a plastic sheath, the plastic used will get some moisture into it to a greater or lesser extent, so for applications where moisture may occur, a dedicated cable should be used to avoid degradation in performance.