Introduction to the concept of wireless transmission line


Release time:

2019-09-27

Introduction to the concept of wireless transmission line

The wire connecting the antenna and the output (or input) end of the transmitter (or receiver) is called a transmission line or feeder. The main task of a transmission line is to transmit signal energy efficiently. Therefore, it should be able to transmit the signal received by the antenna to the input of the receiver with minimum loss, or the signal sent by the transmitter to the input of the transmitting antenna with minimum loss, and it should not pick up or generate spurious interference signals. Thus, 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, it is called a long transmission line, or long line for short.

Types of 3.1 Transmission Lines

There are generally two types of ultra-short wave transmission lines: parallel line transmission lines and coaxial cable transmission lines (microwave transmission lines have waveguides and microstrips, etc.).

A parallel line transmission line usually consists of two parallel wires. It is a symmetrical or balanced transmission line. This feeder has a large loss and cannot be used in the UHF band.

The two wires of the coaxial cable transmission line are the core wire and the shielded copper mesh. Because the copper mesh is grounded, the two conductors are asymmetrical to the ground, so they are called asymmetric or unbalanced transmission lines.

3.2 the characteristic impedance of the transmission line: the ratio of voltage to current at each point on the infinite transmission line 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 inner diameter of the copper mesh of the outer conductor of the coaxial cable, d is the outer diameter of its core wire, and the relative dielectric constant of the insulation medium between the conductors.

3.3 feeder attenuation constant

The signal is transmitted in the feeder, in addition to the resistance loss of the conductor, there is also the dielectric loss of the insulating material. Both of these losses increase as the length of the feeder increases and the operating frequency increases. Therefore, the reasonable layout should be as short as possible the length of the feeder. The magnitude of the loss is represented by an attenuation constant. The unit is expressed in decibels (dB)/meter or decibels/hundred meters.

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

γ(dB)= 10 × log(P./P) (dB).

The concept of 3.4 matching

What is matching? We can simply consider that the load impedance Z connected to the feeder terminal is equal to the feeder characteristic impedance Z. When the feeder terminal is called a matching connection.

In practice, the input impedance of the antenna is also affected by the presence of surrounding objects and stray capacitance. In order to strictly match the feeder with the antenna, it is also necessary to properly adjust the structure of the antenna or add a matching device through measurement when the antenna is set up.

3.5 reflection loss

When the feeder and the antenna match, the high-frequency energy is all absorbed by the load, and there is only the incident wave and no reflected wave on the feeder. The traveling wave is transmitted on the feeder, the voltage amplitude is equal everywhere on the feeder, and the impedance at any point on the feeder is equal to its characteristic impedance. When the antenna and the feeder are not matched, that is, the antenna impedance is not equal to the characteristic impedance of the feeder, the load cannot absorb all the high-frequency energy transmitted on the feeder, but can only absorb part of the energy. A portion of the energy of the incident wave is reflected back to form a reflected wave.

VSWR of 3.6 feeder and antenna

In the case of mismatch, both incident and reflected waves exist on the feeder line. The ratio between the amplitudes of the reflected wave and the incident wave is called the reflection coefficient.

Amplitude of the reflected wave (Z-Z).

Reflection coefficient Γ =----------------------------------------------------------------------------------------

Amplitude of incident wave (Z + Z)

The ratio of the standing wave antinode voltage to the node voltage amplitude is called the standing wave coefficient, also known as the voltage standing wave ratio (VSWR).

Maximum standing wave antinode voltage amplitude Vmax |(1 Γ)|

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

Minimum value of standing wave node voltage amplitude Vmin |(1-Γ)|

3.7 balance device

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

Keywords: digital TV antenna, vehicle antenna, RF line/feeder, suction cup antenna