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Introduction to the concept of wireless transmission lines
Jun 07, 2017

The wires that connect the antenna and transmit (or receive) the output (or input) terminals are called transmission lines or feed lines. The main task of the transmission line is to effectively transmit the signal energy. It should be able to transmit the signal received by the antenna to the receiver input with minimal loss or to transmit the signal from the transmitter to the input of the transmit antenna with minimal loss, while it should not pick up or generate spurious interference signal. In this case, the transmission line must be shielded or balanced. When the geometric length of the transmission line is equal to or greater than the wavelength of the transmitted signal is called the long transmission line, referred to as long lines.


3.1 Types of transmission lines


Ultra-short band transmission lines are generally two: parallel line transmission line and coaxial cable transmission line (microwave transmission line with waveguide and microstrip, etc.).


The parallel line transmission line is usually composed of two parallel wires. It is a symmetrical or balanced transmission line. This feeder loss is large and can not be used in UHF bands.


Coaxial cable transmission line of the two wires for the core and shield copper network, due to copper wire grounding, two conductors to the ground asymmetry, so called asymmetric or unbalanced transmission line.


3.2 transmission line characteristics of the impedance: infinite length of the transmission line on the voltage and current ratio is equal to the characteristic impedance, with the symbol Z. Indicates the characteristic impedance of the coaxial cable


Z. = [138 / εr 1/2] × log (D / d) ohms.


Usually Z. = 50 ohms or 75 ohms


In the formula, D is the coaxial cable outer conductor copper mesh diameter; d is its core diameter; εr is the conductor dielectric medium relative dielectric constant.


3.3 Feeder decay constant


The signal is transmitted in the feeder, except for the resistance loss of the conductor, as well as the dielectric loss of the insulating material. These two losses increase as the feeder length increases and the operating frequency increases. Therefore, the layout should be reasonable as far as possible to shorten the feeder length. The size of the loss is expressed as an attenuation constant. The unit is expressed in decibels (dB) per meter or decibels per 100 meters.


Here the way to explain the concept of decibels, when the input power is P. When the output power is P, the transmission loss can be expressed by γ,


Γ (dB) = 10 × log (P./P) (decibels).


3.4 Matching concepts


What is matching? We can simply assume that the load impedance Z of the feeder terminal is equal to the feeder characteristic impedance Z. , The feeder terminal is called the matching connection.


  In actual work, the input impedance of the antenna will be affected by the presence of surrounding objects and stray capacitance. In order to make the feeder line and the antenna strictly match, in the erection of the antenna also need to measure, adjust the antenna structure, or install the matching device.


3.5 reflection loss


    When the feeder and antenna match, the high-frequency energy is all absorbed by the load, the feeder only incident wave, no reflection wave. The feeder line is transmitted on the line, the voltage across the feeder line is equal, the impedance of any point on the feeder is equal to its characteristic impedance.When the antenna and feeder does not match, that is, the antenna impedance is not equal to the feeder characteristic impedance, The load can not be all on the feeder transmission of high-frequency energy absorption, but only part of the energy absorption. A portion of the incident wave is reflected back to form a reflected wave.


3.6 voltage and antenna voltage standing wave ratio


In the case of mismatch, there are incident and reflected waves on the feeder. The ratio of the reflected wave to the incident wave amplitude is called the reflection coefficient.


Reflected wave amplitude (Z-Z.)


Reflection coefficient Γ =


Incident wave amplitude (Z + Z.)


The ratio of the wave-to-wave voltage to the amplitude of the band is called the standing wave coefficient, also called the voltage standing wave ratio (VSWR)


he maximum amplitude of the wave amplitude of the standing wave is Vmax | (1 + Γ) |


Standing wave coefficient S = - - - - - - - - - - - - - - - -


The minimum value of the amplitude of the standing wave is Vmin | (1 - Γ)


3.7 Balancing device


  Power, load and transmission lines, according to their relationship to the ground, can be divided into two categories of balance and imbalance. If the voltage between the two ends of the power supply and the ground are equal, the opposite polarity is called the balanced power supply, otherwise it is called the unbalanced power supply. Similarly, if the impedance between the two ends of the load or the two conductors is the same Called a balanced load or balanced (feeder) transmission line, otherwise it is an unbalanced load or an unbalanced (feeder) transmission line.