Understanding of Coaxial Cable

Catalog

What is Coaxial Cable?

A coaxial cable is a type of wire that transmits signals. Four layers of materials form a coaxial cable: the innermost line is a conductive copper wire, and the outside of the line is surrounded by a layer of plastic (for insulator and dielectric). There is a thin layer of net-like conductors outside the insulator (usually copper or alloy). Outside the conductor is the outermost insulating material as the outer skin.

Coaxial Cable

Analog and digital communications can both be transmitted using coaxial lines. They can be used for a variety of purposes, the most common of which are television broadcasting, long-distance phone transmission, short-distance computer system connections, and local area networks. Coaxial cable has grown in popularity as a means of distributing television signals to thousands of homes. A coaxial cable for TV system can carry dozens, if not hundreds, of TV channels, with a transmission range of tens of kilometers. Long-distance telephone networks have relied on coaxial cable for years. Fiber optics, terrestrial microwaves, and satellites are all posing rising rivalry today.

What are the main types of Coaxial Cable?

Baseband coaxial cables and broadband coaxial cables are the two primary coaxial cable types.

Baseband coaxial cable

The baseband coaxial cable's shielding layer is commonly a copper mesh structure with a characteristic impedance of 50. The most common versions are RG-8 (thick cable) and RG-58 (thin cable) for transmitting digital signals (thin cable). The diameter of a thick cable vs a thin cable is the most obvious distinction. Thick cable is ideal for medium-sized local networks. It has a vast standard range and a high level of dependability. However, the thick cable network requires transceivers and transceiver cables, which are both difficult to install, resulting in a high overall cost. The skinny cable, on the other hand, is simpler and less expensive. During the installation, however, the cable must be cut. As a result, when there are a lot of joints, it's easy to make bad contact.

Broadband coaxial cable

In most cases, the shielding layer of broadband coaxial cable is made of aluminum. It has a 75-ohm characteristic impedance. Analog communications are typically sent using this type of connection. The RG-59 is the most widely used model. It's a type of transmission cable that's commonly used in cable television networks. It may simultaneously broadcast numerous channels of TV signals over a single cable. Some computer networks can also use broadband coaxial cable as a transmission method.

What does a Coaxial Cable look like?

There are two coaxial conductors in a coaxial cable, one is the inner conductor and the other is the outer conductor. And their role can be imagined: one is the data transmission path, and the other is the data transmission loop. As shown below:

Inner and outer conductors make up the coaxial wire. The two conductors are arranged coaxially, and the transmission signal is completely confined in the outer conductor. The grounding of the outer conductor is a shielded transmission line to ensure good shielding performance, low transmission loss, and strong anti-interference. It is often used for the transmission of higher frequency signals. For common coaxial cables with copper conductors, the insulating material is usually Teflon or polyvinyl chloride.

How do Coaxial Cables work?

Four layers from the inside to the outside form the coaxial cable: the central copper wire (single-strand solid wire or multi-stranded wire), plastic insulator, mesh conductive layer, and wire sheath. A current loop is formed by the central copper wire and the mesh conductive layer. The coaxial interaction between the center copper wire and the meshed conductive layer gives it its name.

Coaxial cable material

Instead of conducting direct current, coaxial cables conduct alternating current, which means the current's direction is reversed multiple times every second.

If a normal wire is used to transmit a high-frequency current, this wire will be equivalent to an antenna that emits radio outwards. The signal's power is lost, and the received signal's strength is reduced as a result of this consequence.

The coaxial cable can solve this problem. The radio emitted by the central wire is isolated by the mesh conductive layer, which can be grounded to control the emitted radio.

There is also a problem with coaxial cables. If a certain section of the cable is squeezed or twisted, the distance between the center wire and the mesh conductive layer is not consistent, which will cause the internal radio waves to be affected. In this case, the internal radio waves will be reflected back to the source of the signal. This effect reduces the receivable signal power. In order to overcome this problem, a layer of the plastic insulator is added between the center wire and the mesh conductive layer to ensure that the distance between them is consistent. This also causes the cable to be relatively stiff and not easy to bend.

The outside conductor of a coaxial cable is substantially improved by the shielding material. It has progressed from a tubular outer conductor to a single-layer braided and double-layered metal. Although the tubular outer conductor has excellent shielding properties, it is difficult to bend and utilize. The shielding efficiency of single-layer braiding is the worst, and the transfer impedance of double-layer braiding is 3 times lower than that of one-layer braiding. It can be seen that the shielding effect of double-layer braiding has been greatly improved than that of single-layer braiding. Major coaxial cable manufacturers are constantly improving the outer conductor structure of the cable to maintain its performance.

Coaxial cable advantages and disadvantages

The advantage of coaxial cable is that it can enable high-bandwidth communication over a long distance without the use of a repeater, but it also has drawbacks. For starters, it's big, and a thin cable's diameter is 3/8 inch thick, so it takes up a lot of space. In the pipeline, there is plenty of room. The second disadvantage is that it is unable to endure tangles, pressures, or abrupt bends, all of which can destroy the cable structure and inhibit signal transmission. Last but not least, there is the expensive expense, but twisted pair can overcome all of these drawbacks.

What is the difference between the coaxial cable and ordinary cable?

Different materials:

Coaxial cables are made of copper conductors separated by insulating materials.

The constituent materials of ordinary cables are several wires or groups of wires.

Different categories:

Baseband and broadband coaxial cables are the two types of coaxial cables.

DC and AC cables are the two types of ordinary cables.

Different types:

Coaxial cables are divided into thin cable RG-58 and thick cable RG-11.

Ordinary cables are divided into flame-retardant rubber sheathed cables, nuclear-grade cables, bare wires, and bare conductor products, power cables, communication cables, and optical fibers, electromagnetic wires, and flexible fire-resistant cables.

Parameter Index of coaxial cable

Electrical parameters

(1) The characteristic impedance of the coaxial cable: the average characteristic impedance of the coaxial cable is 50±2Ω. The periodic change of the impedance along a single coaxial cable is a sine wave. The center average value is ±3Ω, and its length is less than 2 meters.

(2) Coaxial cable attenuation: It refers to the attenuation value of a 500-meter cable length in general. Its value does not exceed 8.5db (17db/km) when measured with a 10MHz sine wave, and it does not exceed 6.0db (12db/km) when measured with a 5MHz sine wave.

(3) The propagation speed of the coaxial cable: The minimum propagation speed required is 0.77C (C is the speed of light).

(4) Coaxial cable DC loop resistance: the sum of the resistance of the center conductor of the cable and the resistance of the shielding layer does not exceed 10 milliohms/m (measured at 20°C)

Physical parameter

A coaxial cable is composed of a central conductor, an insulating material layer, a shielding layer composed of mesh fabric, and an outer isolation material layer.

The coaxial cable is flexible enough to support a bending radius of 254mm (10 inches). The center conductor is a solid copper wire with a diameter of 2.17mm±0.013mm. The insulating material must meet the coaxial cable's electrical characteristics. A metal strip or sheet that meets the transmission impedance and ECM criteria serve as the shielding layer. The inner diameter of the shielding layer is 6.15mm and the outer diameter is 8.28mm. Polyvinyl chloride (PVC) or related polymers are commonly used for external insulation.

What are the characteristics of different dielectric materials of coaxial cable?

Like other cables, coaxial cables have different wire diameters and core diameters, conductor and insulation materials, and structures, and cable sheath materials. The combination of these parameters determines the electrical performance, mechanical performance, and environmental adaptability of the coaxial cable.

Impact on cable delay and propagation speed

The dielectric insulator (dielectric) in the cable is the main factor that affects the delay and propagation speed of the cable. The dielectric materials mainly include solid polyethylene (PE), polyethylene foam (FE/PF), air-insulated polyethylene (ASP), foamed polystyrene (FS), solid Teflon/solid polytetrafluoroethylene (ST /SPTFE), air-insulated Teflon (AST), and so on. The following table mainly summarizes the influence of various materials on cable delay and propagation speed.

The applicable working temperature of the insulating medium and outer skin

This determines the applicable environment of the selected cable, you can directly look at the following table.

Cable sheath material

1)If you need to wrap a certain cable around the object that needs to be connected, the cable sheath is preferably a highly flexible thermoplastic rubber (TPR). But, remember that it is best not to always move the cable with this cable sheath. That will affect performance.

2) For cables that are used in elevators and have flame-retardant requirements, special upright cables (CMR) must be used instead of polyethylene materials. The cable sheath can prevent the spread of flames to unfired areas.

3)For the cables in the hidden space of the horizontal path in the building, the mezzanine cable (CMP) bundled by Teflon outside can be used to prevent the cables from kinking together and only produce low toxicity when on fire.

4)If you need to route cables outside the building or along with the ceiling of the office, you can use PVC or TPR cable skins to increase flexibility without increasing insertion loss.

5)The sandwich cable with fluorinated ethylene propylene (FEP) material has the characteristics of low loss and high-temperature resistance.