What is a Coaxial Amplifier?

The same axis is a guiding system consisting of two coaxial cylindrical conductors, and a wideband microwave transmission line filled with air or high-frequency medium between the inner and outer conductors.

The coaxial line is a shielded and non-dispersive structure, and the dominant mode of the guided wave in the coaxial line is a TEM wave, but it can also transmit

The structure of the same axis is composed of four parts: the sheath, the outer conductor layer, the insulating dielectric layer, and the inner conductor in order from the outside to the inside. The role of each part is described below.

The sheath, that is, the outermost layer is an insulating layer for protection.

The outer conductor has a dual role as a shielding layer. It can both conduct low levels through the transmission loop and also has a shielding effect. The outer conductor usually has 3 structures.

1. Metal tube. This structure is made of copper or aluminum tape longitudinal package welding, or seamless copper pipe extrusion and drawing. This structure has the best shielding performance, but poor flexibility, and is often used in trunk cables.

2. Longitudinal overlapping of aluminum-plastic composite tape. This structure has a good shielding effect and low manufacturing cost, but because the outer conductor is a circular tube with a longitudinal slit, electromagnetic waves will pass through the slit and leak.

3. Longitudinal combination of woven mesh and aluminum-plastic composite belt. It is developed from a single woven mesh structure. It has the characteristics of good flexibility, light weight, and reliable joints. The reasonable composite structure has greatly improved the shielding performance. At present, this structure is widely used.

Insulation medium, PE material, is mainly to improve anti-interference performance and prevent water and oxygen erosion.

Internal conductor, copper is the main material of the internal conductor, and can be in the following forms: annealed copper wire, annealed copper pipe, copper-clad aluminum wire. Generally, the inner conductor of a small cable is a copper wire or a copper-clad aluminum wire, while a large cable uses a copper tube to reduce the weight and cost of the cable. Tie the outer conductor of a large cable, so that you can get enough good bending performance.

Coaxial structure is a two-conductor transmission line, and a static field-like electromagnetic field distribution can be established on its cross section. One of the characteristics of coaxial lines is that they can be applied from the DC section to the millimeter wave band.

The same axis is widely used in the design of wide-band feeders and components mainly in the TEM mode. When the wavelength of the transmission signal is much longer than the length of the transmission line, the magnitude and phase of the current (or voltage) at each point on the transmission line can be approximately the same. At this time, it is not necessary to consider the distribution parameter effect. However, when the wavelength of the transmission signal and the length of the transmission line can be compared with each other, the magnitude and phase of the current (or voltage) at each point on the transmission line are different, showing the distribution parameter effect. At this time, the transmission line must be treated as a distribution parameter circuit. It means that TE and TM modes will appear in the coaxial line, that is, higher-order modes of the coaxial line. ^[1]

According to the application location of the coaxial line, it can be roughly divided into 3 types.

1. Trunk cable: Thick cable with an insulation outer diameter of generally 9mm or more, which requires low loss and low flexibility.

2. Branch line cables: medium and thick cables with an insulation outer diameter of generally 7mm or more, which require small losses and also have a certain degree of flexibility.

3, user distribution network cable: its insulation outer diameter is generally 5mm, loss requirements are not major, but require good flexibility and indoor uniform straight coordination.

1. Characteristic impedance. The main body of a coaxial cable is composed of inner and outer conductors. There is resistance and inductance for the current flowing in the conductor, and conductance and capacitance for the voltage between the conductors. These characteristics are distributed along the line. , Called the distribution constant, due to the dimensional accuracy and the non-uniformity of the purity of the dielectric material in manufacturing. Although the required impedance of a coaxial cable in a cable television system is 75, the characteristic impedance of a coaxial cable actually used is usually (75 ± 5) . Therefore, in order to prevent signal energy reflection and achieve the best transmission effect, the terminal load impedance should also be as equal to the characteristic impedance of the cable as much as possible.

2. Attenuation characteristics. The attenuation characteristics of coaxial cables are usually expressed by attenuation constants. The attenuation constant is proportional to the average square root of the operating frequency of the signal, that is, the higher the frequency, the larger the attenuation constant, the lower the frequency, and the smaller the attenuation constant.

3. The use period of the cable, any cable has a certain life. After using the cable for a period of time, due to the aging of the material, the conductor resistance becomes larger, and the leakage current of the insulating medium increases. The cable should be renewed, and the life of the general cable is 7-20 years depending on the quality and the occasion of use.

4. Temperature coefficient. The temperature coefficient indicates the degree of influence of temperature changes on the characteristics of the cable. As the temperature increases, the cable loss increases, the temperature decreases, and the cable loss decreases. In actual work, the measures to eliminate the influence of temperature changes on the system are the use of temperature-compensated amplifiers, automatic gain control amplifiers, and automatic slope control amplifiers.

5. Shielding characteristics. Shielding characteristics are a parameter for measuring the anti-interference ability of coaxial cables and an important parameter for measuring the leakage prevention of coaxial cables. If the cable is not well shielded, the transmission signal will not only be subject to crosstalk from external clutter, which will affect the quality of the cable TV signal, but also leak out to interfere with other signals.

1. Observe the roundness of the insulating medium. The cross section of the standard coaxial cable is very round. The outer conductor of the cable and the aluminum foil are attached to the outer surface of the insulating medium. The more round the outer surface of the medium, the smaller the gap between the aluminum foil and its surface. , The less round the gap, the larger the gap. Practice has proved that the smaller the gap, the better the performance of the cable. In addition, the large gap air easily penetrates into the shield and affects the service life of the cable.

2. Detect the consistency of the insulation medium of the coaxial cable. The diameter fluctuation of the insulation medium of the coaxial cable mainly affects the echo coefficient of the cable. This inspection can cut out a section of the insulation medium of the cable. Carefully check the outer diameter of each point with a micrometer to see its Are they consistent?

3. Test the braided mesh of the coaxial cable and check whether the braided mesh is tight and flat. The method is to cut the outer sheath of the coaxial cable and cut a short section of the braided mesh of the coaxial cable to identify the number of braided mesh and the given index. If the values match, it is qualified. In addition, a single micro braided network cable is measured with a spiral micrometer. At the same price, the thicker the wire diameter, the better the quality.

4. Check the quality of aluminum foil. Aluminum foil plays an important role in shielding coaxial cables. It plays an important role in preventing external open-circuit interference and cable TV signal leakage. Therefore, the quality of aluminum foil should be checked for new coaxial cables. Cut the sheath layer and observe whether the surface of the braided network cable and the aluminum foil layer maintains good gloss. Then take a section of cable and wind it tightly on the small metal shaft, straighten it and twist it in the opposite direction. Repeat several times, and then cut the cable sheath Check the aluminum foil for cracks. You can also cut out a small section of aluminum foil repeatedly rubbed and stretched in your hand. After repeated rubbing and stretching, it does not break, and those with certain toughness are qualified.

5. Check the tightness of the outer sheath. The outer sheath of high-quality coaxial cables is tightly packed. This can reduce the gap in the shield layer, prevent air from entering and causing oxidation, and prevent the relative sliding of the shield layer from causing electrical performance. Drifting, but squeezing the package too tightly will cause the protective layer to peel off, and it is appropriate that the core cannot be pulled out by force.

6. Observe the shape of the loop of the cable. The loop of the cable is not only an aesthetic issue, but also a quality issue. The cables are flat in a circle, each cable is kept on the same concentric plane, and the cables and the cables form an arc in parallel to make overall contact, which can reduce the mutual stress of the cables, and it is not easy to deform and damage when stacked.