What's the characteristic of rg 174 coaxial cable
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What's the characteristic of rg 174 coaxial cable

Characteristic impedance of RG 174

RG 174 coaxial cable consists of an inner conductor and an outer conductor. The main characteristic of RG 174 coaxial cable is the insulation medium with a certain capacitance between the inner and outer conductors. When a certain potential difference is applied between the inner and outer conductors, an electric field will exist between the two layers of conductors, forming a certain capacitance in the coaxial transmission line. When a high-frequency signal is transmitted on the coaxial transmission line, a certain inductance will be formed on any length of the coaxial transmission line. These capacitance and inductance exist in a distributed state in the coaxial cable, and the combination of parallel capacitance and series inductance determined by the capacitance per unit length and inductance per unit length of the coaxial transmission line forms the characteristic impedance. The characteristic impedance of coaxial cable refers to the average characteristic impedance of the cable around 200 MHz frequency. Due to the limitations of materials and manufacturing processes, it is not possible to absolutely guarantee that the characteristic impedance of different parts of the same coaxial cable is exactly the same, but only the arithmetic average of all local characteristic impedances along the line (commonly 75 ohms).

Hongsen RG 174 1Hongsen RG 174 2

Reflection loss of RG 174

Reflection loss, also known as return loss, is represented by RL. The current industry standard in China stipulates that the reflection loss is 18 dB. The reflection loss of most domestic physical foamed polyethylene coaxial cables is between 18 dB and 22 dB. Generally, the higher the working frequency, the smaller the reflection loss. For example, the RL of the SYWV-75-5 cable is greater than 21 dB when f < 300 MHz, and RL is greater than or equal to 18 dB when f > 300 MHz. From the perspective of signal transmission quality, the larger the reflection loss, the better. Because a larger RL value indicates a more uniform internal structure of the cable, which is less likely to form reflected waves and thus more difficult to form standing waves. A cable with low reflection loss can easily cause adverse phenomena such as poor TV image clarity, ghosting, or moiré interference.

Humidity characteristics

Humidity characteristics refer to the characteristics of the attenuation of RG 174 coaxial cable changing with the humidity of the insulation material. The key to this characteristic lies in the physical structure of the insulation material, the performance of the material used, and the bonding process of the inner and outer conductors. At present, a large number of physical foamed and skin-core insulation polyethylene materials are used, and the physical foamed insulation type has much better moisture resistance, waterproof performance, and service life than the skin-core insulation type cable.

After the cable is damp or water enters, the capacitance and inductance inside the cable will increase to varying degrees. Generally, the former will increase more than the latter, resulting in a decrease in the total capacitance reactance, but an increase in the inductance reactance, thereby enhancing the bypass effect of the signal between the inner and outer conductors (that is, the degree of signal leakage), increasing the relative transmission resistance, and causing an increase in the signal attenuation amount.

Temperature characteristics of RG 174

Temperature characteristics refer to the characteristics of the attenuation of RG 174 coaxial cable changing with the environmental temperature. Generally speaking, when the environmental temperature changes, both the metal loss and dielectric loss of the coaxial cable will change to a certain extent, leading to fluctuations in the signal attenuation. Usually, when the temperature changes by 1℃, the attenuation will change by about 0.2%. The smaller the change, the better. Otherwise, due to the large temperature fluctuations caused by seasonal changes, day-night temperature differences, and certain special places, the change in signal attenuation of the coaxial cable will cause large fluctuations in the amplifier output level. For multi-stage amplifiers, the accumulation of this influencing factor is even more serious, and it may even make the amplifier unable to work.

Frequency characteristics

Frequency characteristics refer to the characteristic curve that the attenuation of RG 174 coaxial cable is proportional to the square root of the frequency. Since the signal frequency band transmitted by coaxial cable is very wide, as a centralized parameter and distributed parameter inside the coaxial transmission line, the performance and response of various signal components with vastly different frequencies are also different, which will cause different attenuation of different frequency band signals. Coaxial cables with excellent frequency characteristics have relatively smooth characteristic curves over the entire frequency band. On the contrary, if a dip occurs, cross-stripe interference and distortion may occur, and if a peak occurs, the signal transmission may be greatly blocked or even interrupted.

Aging characteristics of RG 174

Aging characteristics refer to the characteristics of the relationship between the resistance of the inner and outer conductors, the leakage of the insulation material, and the attenuation index of the cable with the use time of the coaxial cable. According to statistics, for cables with poor materials and processes, after 3 to 6 years of use, the attenuation of transmitted signals increases by about 1.2-1.5 times. It is no wonder that coaxial cables used in harsh environments require updates every 5 years.

Attenuation constant

The attenuation constant is mainly determined by the metal attenuation and dielectric attenuation of the cable. Factors from the metal side refer to the size, material quality, and shape of the inner and outer conductors, while factors from the dielectric side mainly refer to the material quality, physical structure, and relative dielectric constant of the insulation layer between the inner and outer conductors. The attenuation constant is closely related to the characteristic impedance of the cable and is inversely proportional to it. It is also related to the working frequency and presents different attenuation at different frequencies. The general rule is that the higher the working frequency, the greater the attenuation constant. For example, the attenuation constant (dB/100m) of the SYWV-75-5-1L coaxial cable is ≤ 2.0 when the working frequency is f = 5MHz, and ≤ 4.7 when f = 50MHz, ≤ 15.8 when f = 550MHz, and ≤ 22.0 when f = 1000MHz. It can be seen that the dB values of attenuation in the frequency range of 5MHz to 1000MHz differ by as much as 10 times. Coaxial cables with good quality have very smooth frequency characteristic curves of attenuation, without absorption points, and the attenuation values at each frequency point on the curve can meet the requirements of the specified values.

Shielding attenuation of RG 174

Shielding attenuation is a technical parameter that measures the shielding performance of RG 174 coaxial cable. If the shielding performance of the cable is not good, external electromagnetic noise interference will invade, and the signal transmitted internally will also radiate outward and affect its characteristic impedance. The transmitted signal will produce reflection in the cable, forming a composite wave of the incident wave and the reflected wave-mixed wave, which is called standing wave. Reflection will cause a decrease in the efficiency of signal transmission, a decrease in the quality of image and accompanying sound, data signal jitter, and a sudden increase in the data error rate, causing chaos in the system.

The shielding layer of ordinary braided mesh coaxial cables is composed of a layer of aluminum foil and a layer of metal braided mesh. The denser the braided mesh, the more favorable the shielding. When copper foil is used instead of aluminum foil, the shielding performance is better. Adding a layer of metal foil outside the braided mesh will constitute a triple-shielded braided mesh coaxial cable, which will further improve the shielding performance. If another layer of metal braided mesh is added outside the triple-shielded braided mesh coaxial cable, a quadruple-shielded braided mesh coaxial cable with better shielding performance can be formed, such as RG6, RG11, SYWV-75-7-2P, SYWV-75-5-2P, etc. Although the shielding attenuation of the quadruple-shielded braided mesh cable can reach as high as 100 dB, the shielding attenuation of coaxial cables with aluminum or copper tubes can reach 120 dB.

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