Cable Impedance and Attenuation Factors
The RF microwave cable assembly is an important signal transmission component in the microwave system. A full understanding of the characteristics and parameters of the RF coaxial cable can help users to understand, select and employ product better. Therefore, Focusimple has set up a special collection of technical centers to provide you with reference in terms of materials, design, technology, and applications. In this article, we will focus on the factors that influence cable impedance and attenuation.
Choice of Cable Impedance
In order to match the system to get better transmission performance, the choice of cable impedance must match the impedance of other components in the system. Generally, the classification of impedance is mainly a long-term practice result of comprehensive balancing according to system power and attenuation requirements. As shown in Figure 1 below, the 75-ohm impedance transmission system has the lowest attenuation, while the 35-ohm impedance transmission system can transmit highest power. When choosing an RF cable, the most important thing is to select the cable impedance according to the system impedance.
Figure 1 Impedance vs. Attenuation and Power
The choice of impedance is usually to match the entire system. The most commonly used impedance for coaxial cables are 50 and 75 ohms. Others from 35 to 185 ohms are also useful from time to time. 50 ohms are mainly used in microwave and radio transmission. 75 ohms are mainly used in cable TV and video surveillance systems. 85 and 100 ohms are mainly used in data transmission systems.
What are the factors that affect impedance?
The size of the characteristic impedance is determined by the ratio of the diameter of the outer conductor to the inner conductor of the coaxial cable and the dielectric constant of the insulating medium between the two conductors. Because RF energy is always transmitted on the conductor surface of the RF cable, the diameter of the conductor refers to the outer diameter of the inner conductor and the inner diameter of the outer conductor.
In general, the system impedance we use is fixed. According to the impedance calculation formula in Figure 2, it can be seen that the impedance is inversely proportional to the outer diameter and dielectric constant of the central conductor; the impedance is proportional to the inner diameter of the outer conductor. Changes in the outer diameter of the center conductor, the inner diameter of the outer conductor, and the dielectric constant all cause changes in impedance.
Figure 2 Impedance calculation formula
It is also worth noting that for cables of the same structural size, the central conductor is stranded, and the impedance is greater than that of a solid conductor. That is because the stranded conductor is not a complete circular conductor. So when calculating the impedance of the stranded central conductor cable, the outer diameter of the center conductor needs to be multiplied by the conductor coefficient: 0.871 for 3 stranded conductors, 0.93 for 7 stranded conductors, and 0.97 for 19 stranded conductors.
Loss and Attenuation
Loss refers to the power lost during the signal transmission through the cable assembly. When the RF signal is transmitted in the cable assembly, part of the power is converted into heat and consumed, and part of the power leaks out through the outer conductor of the cable. The power difference between the two parts is called loss, or attenuation. For an RF system, the loss usually has strict indicators. After all, the heat consuming leads to critical loss in power, and 3dB attenuation mean 50% of power loss.
Stranded conductors are more flexible than single-core conductors, but for cables of the same size and structure, stranded conductors will sacrifice part of the loss. This is mainly due to the increase in surface resistance caused by the non-roundness of the conductor surface. For cables of the same size, if a solid conductor is switched to a stranded conductor, the increased attenuation is about 10% to 20%. Taking Focusimple’s FSA-460 as an example, the following figure shows the attenuation comparison of stranded conductors and solid conductors under the same structure and size (Figure 3)
Figure 3 Attenuation under FSA-460 solid and stranded conductors
The outer conductor structure has more choices than the inner conductor structure. For flexible cables, there are usually SPC (Silver-plated copper) wire braiding, SPC ribbon braiding and SPC tape wrapping. SPC wire braiding is invented in the 1960s, which is the most traditional RG cable construction and has a very low cost. This structure has relatively low attenuation, good bending stability, and relatively stable high temperature characteristics; while the silver-plated copper tape wrapping is a structure invented in the 1980s. It is characterized by ultra-low loss and excellent mechanical phase, but it is also the structure with the highest cost, and the production process are also relatively strict.
Attenuation Coefficient of different outer conductor: wire braiding >ribbon braiding >aluminum foil> wrapping.
The loss of SPC tape wrapping is about 20% better than that of SPC ribbon braiding, as shown in Figure 4.
Figure 4 Attenuation under three outer conductor structures of FSA-460
Common Dielectric are: solid PTFE, extruded foamed PTFE, and low-density PTFE. According to different material densities, low-density PTFE is classified as 76% and 83% VoP. Replacing the traditional semi-rigid 141 cable with solid PTFE into a cable with low-density PTFE will reduce the loss by about 30% under the same outer diameter. For the comparison of the 76% VoP and the 83% VoP, there is almost no significant difference in terms of attenuation (size decreases while VoP increases). Taking Focusimple’s semi-rigid FSD-141 cable as an example, Figure 5 details the effects of three different dielectric on cable loss.
Figure 5 Attenuation of FSD-141 under different dielectric
Attenuation is usually measured at 25°C room temperature. The change in temperature has an effect on the attenuation, which can be corrected by a correction factor. The effect of temperature on attenuation is mainly due to the increase of the resistance of the conductor with the increase of temperature and the power factor of the dielectric. Figure 6 is the curve of the attenuation of the Focusimple‘s FSB-330-P cable varies with the temperature change. The attenuation change is basically linear with the temperature change. At the beginning of room temperature, a small inflection point is formed due to the fission process of PTFE, which can be almost ignored.
Figure 6 FSB-330-P attenuation curve with temperature
It should be noted that the slope of the attenuation curve of each cable structure is slightly different. If necessary, please contact Focusimple to obtain more comprehensive data of the corresponding product.