How to calculate RF signal attenuation in air transmission

1 RF coaxial cable.

structure and transmission characteristics.

Structure. RF coaxial cable is composed of four parts: inner conductor, insulator, outer conductor and sheath, and the insulator insulates the inner and outer conductors and keeps the axial coincident, which is the coaxial cable. The inner and outer conductors are made of dielectrics.

insulating materials), the dielectric determines the transmission speed and loss characteristics of the coaxial cable to a large extent, and the commonly used insulating materials are dry air and polyethylene.

Polypropylene, polyvinyl chloride.

and other mixtures of materials. Physical foam cable is preferred because of low loss, good frequency characteristics, and not easy to enter water.

Transmission characteristics. 1) Electromagnetic field within the coaxial cable.

Distribution. Electric field strength.

According to the sinusoidal distribution, the radio waves transmitted in the coaxial cable will not leak out of the cable, in the application, the outer conductor is usually grounded, so it has a good shielding effect, the transmitted TV signal is not interfered by external clutter, and the signal inside will not radiate out.

2) Skinattractant effect.

When the current of a high-frequency signal flows through the cable, the phenomenon that the current is concentrated on the surface of the conductor, which reduces the effective cross-sectional area of the conductor and increases the resistance value, is called the skin effect. Because of the skin effect, the current in the coaxial cable only flows along the outside of the inner conductor and the inside of the outer conductor, therefore, many properties of the cable depend on the outer diameter of the inner conductor and the inner diameter of the outer conductor, and the electromagnetic fields inside and outside the cable do not interfere with each other. Skin depth.

with the square root of the frequency f (MHz).

is inversely proportional, therefore, the conductor loss of a coaxial cable is proportional to the square root of the frequency.

Coaxial cable performance.

1) Characteristic impedance.

The characteristic impedance zc is defined as the ratio of voltage to current at any point on the cable in the case of coaxial cable termination matching. The characteristic impedance of a coaxial cable is determined by the diameter of the conductor and the interconductor medium, regardless of the length of the cable. In the CATV system, the characteristic impedance of the coaxial cable is 75.

2) Attenuation constant and temperature coefficient.

The amount of attenuation of the RF signal when it is transmitted in the coaxial cable and the size and dielectric constant of the cable.

operating frequency. The degree of attenuation of the coaxial cable signal, expressed as the attenuation constant ( ) the number of dBs per unit length (e.g., 100 m) of the cable to the signal. The attenuation constant is proportional to the square root of the signal frequency, that is, in the same section of cable, the higher the signal frequency, the greater the attenuation constant; The lower the signal frequency, the smaller the attenuation constant.

The temperature coefficient represents the influence of temperature change on the loss value of the cable, and the temperature rises, and the loss value of the cable increases; As the temperature drops, the loss value of the cable decreases. The temperature coefficient is defined as the percentage by which the cable increases or decreases the attenuation of the signal as the temperature rises or falls1.

Table 1 is the result of measuring the Hanson RF coaxial cable-5 and -7 according to the channel configuration of Heping County Cable TV Station at two normal temperatures of 33 and 13.

Table 1: Attenuation constants ( ) of Hansheng cable-7 and -5 types at room temperature

Channel Image Carrier Frequency (MHz).

33℃db/100 m13℃db/100 m

Type 7 - Type 5 - Type 7 - Type 5. ds

The formula illustrates the loss of radio wave propagation in free space.

LOS = + 20LG D(km) +20LG F(MHz) LOS is the propagation loss in dB

d is the distance and the unit is km

f is the operating frequency, and the unit is MHz

In an ideal world, the attenuation is about 35db

The speed of light indeed! Electromagnetic waves, like light waves, travel at speeds of 300,000 kilometers per second.

The radiation of the mobile phone is extremely low during normal use. Except for the possible impact on people who are new to pregnancy, others can be ignored.

Generally, as long as the call time does not exceed 2 hours, there will be no impact.

It's the speed of light, to be exact, but not all the places you can reach are covered.

The RF interface is a radio frequency interface.

Basic workflow for RF interfaces:

The reader sends a certain frequency of radio frequency signal through the transmitting antenna, and when the RF card enters the working area of the transmitting antenna, it generates an induced current, and the RF card obtains energy to be activated; The RF card sends information such as self-coding through the card's built-in sending antenna;

The system receiving antenna receives the carrier signal sent from the radio frequency card, and transmits it to the reader through the antenna conditioner, and the reader demodulates and decodes the received signal and then sends it to the background main system for related processing; The main system judges the legitimacy of the card according to the logical operation, makes corresponding processing and control for different settings, and sends command signals to control the action of the actuator.

The RF interface is an antenna interface, and it is also a must-have interface for old-fashioned cable TV, and it is also a very antique standard. Its development in our country can even be traced back to the 80s of the last century.

RF interfaceWith the upgrade of the digital set-top box provided by broadband providers, this interface is becoming more and more useless, and even some Internet TVs no longer provide RF input interface.

RF is an abbreviation for Radio Frequency, which is Radio Frequency.

RF interface,(also called RF interface,Coaxial cable interface,Closed line interface)It belongs to the analog signal interface,All TVs support this interface,Closed-circuit signal is transmitted into the TV through this interface。

In the theory of electronics, an electric current flows through a conductor and a magnetic field is formed around the conductor; An alternating current passes through a conductor, and an alternating electromagnetic field, called an electromagnetic wave, is formed around the conductor.

When the frequency of electromagnetic waves is lower than 100kHz, electromagnetic waves will be absorbed by the surface and cannot form an effective transmission, but when the frequency of electromagnetic waves is higher than 100kHz, electromagnetic waves can propagate in the air. RF refers to high-frequency electromagnetic waves with long-distance transmission capabilities, and RF technology is widely used in the field of wireless communication.

The communication distance is related to the transmit power, the receiving sensitivity, and the operating frequency.

lfs](db)=

where lfs is the transmission loss, d is the transmission distance, and the unit of frequency is calculated in MHz.

As can be seen from the above equation, the propagation loss (also known as attenuation) of radio waves in free space is only related to the operating frequency f and the propagation distance d, and when f or d is doubled, lfs will increase by 6dB. respectively

Path loss increases with the transmission distance.

Wireless transmission distance calculation.

pr(dbm) = pt(dbm) -ct(db) +gt(db) -lfs(db) +gr(db) -cr(db)

PR: Receiver sensitivity.

pt: Sender power.

CR: Receiver connector and cable loss.

ct: Sender connector and cable loss.

gr: Antenna gain at the receiving end.

gt: Sender antenna gain.

LFS: Free Space Loss.

Substituting the path loss equation and determining other fixed parameters can give a relationship between transmit power and transmission distance, but this is ideal and cannot be achieved in practice.

It depends on what band.

Long waves... Mainly rely on ground transmission, I guess you are not, generally thousands of W.

Mw... If you want to transmit 50km, build a wireless transmitter tower. Just like the local station, about 1 thousand w or more.

Shortwave... The power is very small, it is reflected by the ionosphere of the sky, starting from a few tens of w. The application is relatively common, and many radio enthusiasts use this to communicate.

Ultra-short wave (microwave). It is generally used for directional transmission. A few watts - tens of w.

db（m）＝10lgp（w）

From the above formula, it can be calculated that p 125w corresponds to subtracting the attenuation of 20db

The remainder is brought back to the formula, and p can be obtained

Or 20 10lg (125 x).

Finding x is actually an attenuation of 20db is an attenuation of 100 times.

The electrical characteristics of the RF cable are like a four-terminal network, and the equivalent circuit is shown in the attached diagram, and a cable can be seen as being connected in series by an infinite number of such circuits.

The inner and outer conductors of a coaxial cable (a common structure of RF cables) are placed concentrically, and the electromagnetic energy is confined to the medium between the inner and outer conductors.

RF currents are transmitted across the copper surface, and the higher the frequency, the more concentrated it is on the surface.

Professionally called the skin effect, it is caused by the mutual repulsion between the conducted electrons, the higher the frequency, the greater the repulsion force, the more the current is transmitted on the surface of the conductor, the RF cable usually carries a high voltage of a few GHz, and the current is basically only transmitted within a few mm of the surface.

The network cable cannot transmit RF signals.

Because network cables and RF lines have their own impedance values.

The RF cable is generally between 50 ohms and 75 ohms, and the network cable is between 200 ohms and 300 ohms, so there will be a large loss when the network cable is transmitted.

It can't be transmitted, and the RF signal needs to be transmitted with a coaxial cable.

What happens when an RF signal is reflected on a metal surface, not refracted by it. Theoretically, it does not decay.

In the actual operation of the user, the change of optical power caused by the change of RF signal can only be reflected from the RF output port of the optical receiver, if it is a simple optical receiver without AGC control, when the input optical signal changes by 5dB, the RF output of the simple optical receiver is reduced by no less than 20dB.

In addition, the level of the RF signal (dB) is not directly related to the power of the RF signal. Examples:

At the same point, a 5W amplifier can output a signal level of 118dB, and a 40W bridge Jumbo amplifier can output a signal level of only 108dB. You can't say that the 118db signal is more powerful than the 108db.

In engineering, the optical attenuation is 1dB, and the RF attenuation is 2dB, so if the light changes from 0dB to -5dB, the RF signal power will be reduced by 10dB.