Hello, every one. Welcome to Zhuhai hanqin cable co.,ltd , in this article, we are going to discuss the performance of the leaky cables.
1 The mechanism of leaky cables
The purpose of a common coaxial cable is to transmit radio frequency energy from one end to the other, and it is desirable to have the largest horizontal shielding so that the signal cannot penetrate the cable to avoid the loss of radio frequency energy during transmission. However, the design purpose of a leaky cable is precisely to reduce the lateral shielding so that electromagnetic energy can partially penetrate from the cable to the outside of the cable. Of course, the electromagnetic energy outside the cable will also be induced into the cable.
Single-core wires or twisted pairs are leaky feeders, and coaxial cables or waveguides that allow leakage of outer conductors are also leaky feeders. However, the transmission attenuation of single-core wire or twisted-pair wire is too large for high frequency, and the size of the waveguide is large (<3GHz) and the cost is quite expensive. Therefore, the widely used leaky feeder is the coaxial leaky cable—— Its transmission attenuation, cost and installation are close to those of ordinary cables.
Leaky cables generally use thin copper as the outer conductor, and different forms of slots are cut on the outer conductor. According to different leakage mechanisms, leaky cables can be divided into two types: coupling type and radiating type.
The spacing of the slots on the outer conductor of the coupling leaky cable is much smaller than the working wavelength. The electromagnetic field is diffracted through the small hole to excite the external electromagnetic field of the outer conductor of the cable, so that there is current on the outer conductor, so there is electromagnetic radiation. The electromagnetic energy spreads around the cable in concentric circles. The cable with embossed and milled holes on the outer conductor is a typical coupling leaky cable.
The spacing of the slots on the outer conductor of the radiating leaky cable is equivalent to the wavelength (or half-wavelength), and the slot structure makes the signal superimpose in the same phase at the slot. Only a very precise slot structure and a specific narrow frequency band can produce in-phase superposition. Periodically changing slots on the outer conductor are typical radiating leaky cables.
Coupling leakage is the secondary effect of surface waves on the outer conductor of the leaky cable, while radiation leakage is caused by direct radiation from the slot on the outer conductor. Coupling cables are suitable for wide-spectrum transmission. The electromagnetic energy leaked has no directionality and decreases rapidly with the increase of distance. The radiating leaky cable is closely related to the operating frequency. The leaked electromagnetic energy has directivity. The same leaked energy can be relatively concentrated in the radiation direction, and will not decrease rapidly with the increase of the distance (for a specific frequency and a specified direction, the coupling The loss is relatively small).
Therefore, different types of leaky cables can be selected according to different applications. For example, the tunnel coverage of the private network can be the radiation type, and the indoor distribution system of the public network can be the coupling type. Generally speaking, leaky cables have two leakage modes: coupling and radiation. The so-called coupling type and radiation type mean that the leakage is mainly coupling or radiation.
2 Loss analysis
The material and physical structure of the inner and outer conductor, medium and sheath of the cable and its process determine the electrical and physical properties of the cable, and the slot structure on the outer conductor of the leaky cable (slot shape, slot size, arrangement density, arrangement Frame type) determines the different interaction modes of the electromagnetic energy in the cable and the external environment, which will affect almost all the electrical performance indicators of the cable.
The main electrical performance indicators of leaky cables include: 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, dielectric strength (pressure resistance), flame-retardant and smoke toxicity properties, torsion resistance and bending properties, and airtightness.
As mentioned above, a leaky cable is similar to a combination of a transmission line and an antenna, and its purpose is to enhance the field strength coverage along the line. For wireless systems, the most important thing is the total loss index of the leaky cable (longitudinal transmission attenuation of the cable plus lateral coupling loss). It is the goal of system design to select the smallest system loss to maximize the service distance.
2.1 Transmission attenuation
The attenuation coefficient is an important indicator that describes the degree of electromagnetic energy loss transmitted inside the cable. There are two factors that cause the transmission attenuation of a coaxial cable: conductor loss and dielectric loss. For leaky cables, because part of the electromagnetic energy is radiated, there is still leakage loss. The attenuation coefficient of the leaky cable longitudinal transmission can be expressed by the following formula：
a = a1 ×+ a 2 × f+ a 3——-(1)
Among them, a =Attenuation coefficient for a given frequency （dB/100m）
a 1 =Conductor loss factor
a 2 =Dielectric loss factor
a 3 =Leakage loss factor
f =frequency （MHz）
Conductor loss and frequency relate to a 1 。a 1 depends on the impedance and the size of conductor. Obviously, big cable’s conductor has lower loss. Because of the skin effect, the inner conductor of the thick cable can be made of aluminum and copper coated on the surface or a hollow copper tube can be used. For leaky cables, the conductivity of the outer conductor surface layer should also be as large as possible.
Dielectric loss has relations with frequency and a .
a 2 Determined by the relative dielectric constant and loss factor of the medium, foamed polyethylene (mesh In the past, the physical foaming method of injecting nitrogen was used, and the foaming degree can reach 80%.) When it is used as a dielectric material, its loss coefficient is the smallest. Leakage loss coefficient a 3 It depends on the slot structure of the cable, and will also be affected by the frequency and the surrounding environment of the cable.
2.2 Coupling loss
Coupling loss is a comprehensive index describing the radiation amount and receivable amount of a leaky cable. The coupling loss value is defined as the ratio (dB) of the signal in the cable to the signal received by the λ/2 dipole antenna at a specific distance (usually 2 meters) away from the cable.
Obviously, the smaller the coupling loss (the more leakage), the greater the transmission attenuation, but the slot structure can be selected to maximize the radiated energy and minimize the additional transmission attenuation due to leakage (the above-mentioned radiating leaky cable is an example). In the process of design or selection, the factors that can be controlled are: slot size, shape, frame format, and spacing.
For coupled leaky cables, the increase in frequency will reduce the coupling loss, so to a certain extent, it will compensate for the increased transmission attenuation due to the increase in frequency.
Since the influence is mutual, a similar method can also be used to analyze the coupling of the signal from the external antenna to the cable.
2.3 Total loss
The total loss of the leaky cable is the basis of the link design, which is defined as the sum of the cable transmission attenuation and the coupling loss. The total loss of the leaky cable shall not exceed the allowable system loss (transmitting power-receiving sensitivity). Taking the honeycomb system as an example, it may
The typical value of the acceptable system loss is 130dB, and the attenuation caused by the duplexer, shielding and other factors will be about 15dB. Therefore, considering the system margin, the total loss of the leaky cable should not exceed 105dB.
The above coupling loss is based on the premise that the antenna is 2 meters away from the leaky cable. Assuming that the antenna distance is 6 meters instead of 2 meters, the measured coupling loss will be about 5dB greater.
Figure 1 shows the total loss of two leaky cables with the same size but different leakage. The radiation of the leaky cable ② is greater than that of the leaky cable.
①, the transmission attenuation is therefore greater than ①. It can be seen that as the length increases, the total loss of the leaky cable ② with larger radiation will exceed that of the leaky cable ① and its dynamic change is relatively large.
Figure 1 Transmission attenuation + coupling loss = total loss of leaky cable
2.4 Dynamic range of total loss
The change in the loop loss between the mobile station and the base station caused by the change in the position of the mobile station relative to the leaky cable is quite large. When there are only base stations and mobile stations, fluctuations in total loss are not a problem, because both base stations and mobile stations have a large dynamic adaptation range: automatic gain control (AGC) can compensate for far-near effects and the damage caused by barriers and multipaths. Profit decline.
But once a repeater (especially an optical repeater) is introduced, the dynamic range of the uplink signal becomes a problem: if the signal level is too low, it may be overwhelmed by noise; if the signal is too strong, it will cause intermodulation in the waveband. Efforts can be made on the relay equipment side: such as using low-noise amplifiers to increase sensitivity; using frequency-selective repeaters to suppress intermodulation interference; or using feedforward technology in broadband repeaters to increase linearity.
However, it is most beneficial to make the total loss curve of the leaky cable flat. Two factors affect the dynamic range of the total loss: the gradual increase in transmission attenuation; the jitter of the coupling loss (Rayleigh fading).
- Reducing the dynamic range The dynamic range of the loss is very important to the system design, and each transceiver has its requirements. Along the leaky cable, the total loss (transmission attenuation plus coupling loss) increases. Therefore, the coupling loss is gradually reduced along the direction
- To compensate for the longitudinal transmission attenuation, the usable length of the cable will increase significantly. According to the principle of gradual decrease in coupling loss (increase in relative leakage), the segmented design of the slot structure (for example, the slot is changed from sparse to dense) can reduce the total loss dynamic range of the entire leaky cable-that is, the actual field strength distribution along the line is more The conventional leaky cable will be more uniform, as shown in Figure 2.
Coupling loss and “reception probability
Due to the superposition of different phases, the sampling value of the coupling loss varies along the longitudinal direction of the cable with jitter, as shown in Figure 3.Commonly referred to as coupling
Figure 2 It is necessary to reduce the coupling loss in sections to reduce the dynamic range to standardize the expression of this jitter. It is also conducive to the reliable design of the system. We get a specific coupling loss value based on the reception probability.
Figure 3 The receiving probability curve derived from the jitter of the coupling loss
Commonly used are 50% and 95% probability values. 50% reception probability value-50% of the measured data of coupling loss is better (less than) this value.
95% reception probability value-95% of the measured data of coupling loss is better (less than) this value.
Using the 95% reception probability value as the coupling loss value is closer to actual needs, and the determined loss value is greater than the 50% reception probability loss value. Similarly, a 5% probability value can be calculated. The 95% probability loss value and the difference between the 95% probability and 5% probability loss value (a measure of jitter) are a basis for link design.
2.5 Factors to be considered when designing the system margin
From base station to mobile station From mobile station base station
——————————————————————————————————————————————————-
Base station transmitter output power Mobile station transmitter output power
Jumper cable loss Mobile station antenna loss or gain
Power splitter loss Coupling loss
Leaky cable transmission attenuation Required system margin
Repeater gain Leaky cable transmission attenuation
Coupling loss Repeater gain
Required system margin Power splitter loss
Mobile station antenna loss or gain Jumper cable loss
Mobile station receiving sensitivity Base station receiving sensitivity
3 environment affection
Installation location
The installation position of the leaky cable has a great influence on the coupling loss. During installation, the cable axis should be kept at a distance of more than 20cm from the wall, and the cable should be kept at least 1 meter away from the corner of the wall.
Metal bracket
Non-metallic brackets should be used to install leaky cables. Metal brackets with periodic outlets will affect the standing waves in the leaky cables to a certain extent.
Oil & Water
The dust and oil stains containing salt or metal particles accumulated on the sheath of the leaky cable are potential loss factors and can increase the transmission attenuation.
Opening direction of radiating leaky cable
There are a series of openings on the outer conductor of the leaky cable. In order to obtain the smallest coupling loss and the smallest field strength fluctuation, the direction of the opening of the leaky cable must be directed toward the mobile device.