Main performance indicators and measurements of leaky coaxial cable
The communication between base stations and mobile stations usually relies on radio transmission. With the continuous development of the communication industry, it is required that the base stations and mobile stations in the tunnel can also be connected anytime and anywhere. However, in underground buildings such as tunnels and subways, the electromagnetic wave propagation effect of mobile communications is not good, and it is usually difficult to transmit using antennas, so the research on leaky coaxial cables has also come into being.
The Introduction of leaky coaxial cables
Leaky coaxial cables can both transmit signals and have antenna functions. This type of cable generally uses thin copper as the outer conductor, and opens a series of slots or gaps on the outer conductor of the coaxial tube to radiate and receive the controlled electromagnetic wave energy evenly along the line, so as to achieve the coverage of the electromagnetic field blind area, so as to achieve the purpose of smooth mobile communication.
Its insulation adopts a uniform, fine and closed micro-bubble structure with high physical foaming. Compared with the traditional air insulation structure, the transmission parameters such as characteristic impedance, standing wave ratio, and attenuation are more uniform and stable, and it can also resist the intrusion of moisture into the cable in a humid environment, which causes the transmission performance to decrease or lose.
Generally speaking, there are two leakage modes of leaky cables: coupling and radiation. The so-called coupling and radiation refer to the leakage being mainly coupling or radiation.
The spacing of the slots on the outer conductor of the coupled leaky cable is much smaller than the working wavelength. Cables with corrugated outer conductors and milled holes on the corrugations are typical coupled leaky cables. Coupled leaky cables are suitable for wide-spectrum transmission. The leaked electromagnetic energy is non-directional and decreases rapidly with increasing distance.
The spacing of the slots on the outer conductor of the radiating leaky cable is equivalent to the wavelength (or half wavelength), and its slot structure makes the signals at the slots produce in-phase superposition. Only very precise slot structures and specific narrow frequency bands can produce in-phase superposition. The slots with periodic changes on the outer conductor are typical radiating leaky cables. The radiation type leaky cable is closely related to the working frequency. The leaked electromagnetic energy is directional. The same leakage energy can be relatively concentrated in the radiation direction and will not decrease rapidly with the increase of distance.
Main performance indicators of leaky coaxial cable
- Main performance indicators of leaky coaxial cable
The materials, physical structure and process of the inner and outer conductors, dielectrics and sheaths of the cable determine the electrical and physical properties of the cable, while the slot structure on the outer conductor of the leaky cable determines the different interaction modes between the electromagnetic energy in the cable and the external environment, which will affect almost all electrical performance indicators of the cable.
The main electrical performance indicators of leaky coaxial cable are: frequency range, characteristic impedance, coupling loss, transmission attenuation, dynamic range of total loss, standing wave ratio, and transmission delay. The main physical performance indicators are: insulation resistance, insulation dielectric strength (voltage resistance), flame retardancy and smoke toxicity, torsion resistance and bending performance, and sealing.
The most important indicator of leaky coaxial cable is its total loss, which is the sum of transmission attenuation and coupling loss.
- Two important indicators of leaky coaxial cable
◆ Transmission attenuation
◆ Coupling loss
2.1 Transmission attenuation
Transmission attenuation is an important indicator for assessing the energy loss of electromagnetic waves transmitted inside the cable.
The attenuation performance of ordinary coaxial cable mainly depends on the type of insulation layer and the size of the cable. For leaky cable, the surrounding environment will also affect the attenuation performance, because a small amount of energy inside the cable propagates in the external environment near the outer conductor. Therefore, the attenuation performance is also subject to the arrangement of the slots of the outer conductor.
There are three factors that cause the transmission attenuation of leaky cable: conductor loss, dielectric loss and leakage loss. The longitudinal transmission attenuation of leaky cable can be expressed by the following formula:
a= a1* f + a2 * f + a3
a = Attenuation coefficient at a given frequency (dB/100m)
a1 = Conductor loss coefficient
a2 = Medium loss coefficient
a3 = Leakage loss coefficient
f = Frequency (MHz)
Conductor losses is depend on frequency, as well as a1, a1 depends on the impedance and size of the conductor, with thicker cables obviously having lower conductor losses.
Dielectric loss is related to frequency, as well as a2 , a2 is determined by the relative dielectric constant and loss factor of the medium. When foamed polyethylene is used as the dielectric material, its loss factor is the smallest.
The leakage loss coefficient a3 depends on the cable slot structure L, and will also be affected by the frequency and the cable surrounding environment.