First, the characteristic impedance
The coaxial cable consists of coaxial inner and outer conductors. Between the inner and outer conductors filled with coaxial cable main has a certain dielectric rate of insulating medium. On the inner and outer conductors plus a certain value of the potential difference between the two conductors that will exist between the electric field, coaxial transmission line will form a certain capacitance. When the coaxial transmission line in the discussion of high-frequency signals, any length of the coaxial transmission line will form a certain amount of inductance. These capacitors and inductors in the coaxial cable is distributed in the state, the coaxial transmission line per unit length of the capacitance and inductance of the length of the parallel to determine the capacitance and the combination of inductors in series
State, it formed a characteristic impedance. The characteristic impedance of a coaxial cable is the average characteristic impedance of the cable in the vicinity of the 200 MHz frequency. This is due to material and manufacturing processes and other factors, and can not absolutely guarantee the same coaxial cable throughout the characteristic impedance is identical, but can only take along all the local characteristic impedance of the arithmetic average (common 75 ohms cable).
Second, the reflection loss
Return loss, also known as return loss, is denoted as RL. China's current industry standard for the reflection loss requirements of 18dB. The domestic most of the physical foam polyethylene coaxial cable return loss of about 18dB a 22dB between the usually higher operating frequency, the reflection loss is also smaller, such as RG6 cable in the f ~ <300MHz RL & gt; 21) dB, f & gt; 300 MHz, RL & gt; From the perspective of the quality of signal transmission, the reflection loss should be the better, because the greater the RL value, indicating that the more uniform the internal structure of the cable, the more difficult to form a reflected wave. The more difficult it is to form standing waves. Low reflection loss cable can cause poor TV image clarity, ghosting or moire interference and other undesirable phenomena.
Third, the humidity characteristics
Humidity characteristics are the characteristics of coaxial cable attenuation with the insulator humidity changes. The key to this feature is the physical structure of the insulator and the properties of the materials used, as well as internal and external conductor bonding process level. At present, extensive use of physical foam insulation and lotus core insulation of polyethylene materials, and physical foam insulation moisture, water and life than the lotus core insulation cable is much stronger.
Cable damp or water, the internal capacitance and inductance have varying degrees of increase, under normal circumstances the former increase is larger than the latter, the result is the total capacitance decreased, while the inductive reactance is increased , So that the signal between the internal and external conductors to enhance the role of (ie, increased leakage of the letter bow), the relative increase in transmission resistance, resulting in an increase in the amount of signal attenuation.
Fourth, the temperature characteristics
Temperature characteristics is the coaxial cable attenuation with the ambient temperature change characteristics. In general, when the ambient temperature changes, the coaxial cable metal loss and dielectric loss will occur to a certain degree of change. And causes a fluctuation of the signal attenuation amount. Normally, the attenuation will vary by about 0.2% for every 1 ° C change in temperature. The smaller the better, otherwise, due to the replacement of the season, when the temperature difference between day and night, and some special occasions, large temperature fluctuations due to coaxial cable attenuation of the transmission signal changes will cause the amplifier output level Resulting in greater volatility. For multi-level amplification, the accumulation of such factors is more serious. Or even make the amplifier can not work.
Fifth, the frequency characteristics
Frequency characteristic refers to the coaxial cable attenuation and the square root of the frequency is proportional to the characteristic curve. Because coaxial cable transmission signal band is very wide, as a coaxial transmission line inside the concentration parameters and distribution parameters, for the frequency difference between the various signal components of the performance and response are not the same, so the signal will have different frequency bands Of the attenuation. Frequency characteristics of the coaxial cable, the characteristic curve in the entire band are relatively smooth. On the other hand, when the valley point is formed, the phenomenon of striped interference and distortion is liable to occur, and the peak point may largely prevent the signal from being transmitted or even blocked.
Sixth, aging characteristics
Aging characteristics refers to the coaxial cable, the outer conductor of the resistivity, insulator leakage and cable attenuation and the relationship between the characteristics of the use of time. According to the statistics. The cable and cable with poor material and technology can increase the attenuation of transmission signal about 1.2-1.5 times after 3 ~ 6 years. It is no wonder that coaxial cable used in harsh environment will be updated every 5 years. .
Seventh Attenuation constant
one. This parameter is mainly determined by the metal attenuation of the cable and the attenuation of the insulation medium. Metal aspects of the factors
Is the internal and external conductor size, material texture and shape; insulation medium factors are mainly between the inner and outer conductors of the insulating layer of material quality, physical structure and the relative permittivity and so on. The decay constant is closely related to the characteristic impedance of the cable and has an inverse relationship with the operating frequency. At different frequencies, the decay constant is different from that of the cable. The general rule is that the higher the operating frequency, the greater the attenuation constant. Such as the RG6 cable attenuation constant (dB / 100m), at the operating frequency f = 5MHz. When B = 2.0: f = 50 MHz, B? 4.7; f = 550 MHz, B? 15.8; f = 1000 MHz. Can be seen in the frequency range of 5MHz ~ 1000MHz, the decibel decay even a difference of 10 times.The quality of the coaxial cable, the attenuation of the frequency characteristic curve is very smooth, does not appear absorption point, and the curve of the attenuation of the frequency point can meet the requirements of the specified value.
Eight, shielding attenuation
Shield attenuation is a measure of coaxial cable shielding performance of the technical parameters. If the cable shielding performance is poor, its external electromagnetic noise interference will invade, and internal transmission of the signal will be radiated outward, and affect its characteristic impedance. The transmitted signal will be reflected in the cable, resulting in a mixed wave of incident and reflected waves - standing waves. Reflection will lead to reduced signal transmission efficiency, image and sound quality degradation, data signal jitter, severe data bit error rate, the system will appear chaotic.
Ordinary braided mesh coaxial cable shielding layer is composed of a layer of aluminum foil and a layer of metal braided mesh composition, the greater the density of the braid more conducive to shielding; and the use of copper foil instead of aluminum foil, the shielding performance is better. In addition to a layer of metal foil in the braid, which constitute triple shielded woven coaxial cable, the shielding performance will be further improved. If the triple shielded woven coaxial cable outer layer plus a layer of metal braid, can constitute a more excellent shielding performance of the four-Shielded Braided Coaxial Cable.