Chapter 1 Technical Principles of Leaky Wave Cable

1.1 Technical Principles of Leaky Wave Cable

Leaky cable is the abbreviation of leaky coaxial cable (Leaky Coaxial Cable), also commonly referred to as leakage cable or leaky cable. Its structure is basically the same as ordinary coaxial cable, consisting of three parts: inner conductor, insulating medium, and outer conductor with periodic slots.
While electromagnetic waves propagate longitudinally in the leaky cable, they also radiate electromagnetic waves to the outside through the slots; external electromagnetic fields can also be induced into the leaky cable through the slots and transmitted to the receiving end. Leaky cables are divided into radiating type and coupling type.
In essence, leaky cable is also a type of coaxial cable that serves both as transmission line and transceiver antenna. In some industries, it’s also called leakage cable or leaky wave cable. Based on different principles, it’s generally classified into two types: radiating leaky cable and coupling leaky cable.

1）Radiation-type Leaky Cable:

The spacing of the slots opened on the outer conductor of the radiation-type leaky cable is equivalent to the wavelength (or half-wavelength). Its slot structure makes the signals at the slots generate in-phase superposition. The outer conductor with periodically changing slots is a typical radiation-type leaky cable. The radiation of the radiation-type leaky cable is directly generated by the slots opened on the outer conductor. The shape and size of the slots are strictly designed according to the applied frequency range. Therefore, the radiation-type leaky cable is a leaky coaxial cable optimized based on the frequency band, and it has very superior frequency characteristics within the relevant frequency band. Moreover, the leaked electromagnetic energy has directivity. The same leaked energy can be relatively concentrated in the radiation direction, and it will not decrease rapidly with the increase of the distance (for the optimized frequency band and direction, the coupling loss is very small).

2) Coupling-type Leaky Cable:

Coupling-type leaky cables come in many different structural forms. For example, a long strip-shaped slot can be cut on the outer conductor, or a set of small holes with a spacing much smaller than the wavelength can be drilled, or slits can be made on both sides of the leaky cable.

The electromagnetic field diffracts through the small holes, exciting the external electromagnetic field of the outer conductor of the leaky cable. The current flows on the outer surface of the outer conductor, and the leaky cable acts like a movable long antenna, radiating electromagnetic waves outward.

The current corresponding to the coupling mode is parallel to the axis of the leaky cable. The electromagnetic energy spreads around the leaky cable in a concentric – circle pattern and decreases rapidly as the transmission distance increases. Therefore, this type of electromagnetic wave is also called a “surface electromagnetic wave”. This kind of electromagnetic wave is mainly distributed around the leaky cable, but there is also a small amount present near obstacles and discontinuities (such as absorption clamps and walls), which then causes diffraction.

The communication principle of the leaky wave cable has similar characteristics to that of the waveguide. However, due to the relatively small diameter and other dimensions of the leaky wave cable, the higher the wireless frequency, the more inappropriate it is to use the leaky wave cable, and its attenuation is relatively large. Therefore, the operating frequency of the leaky wave cable is usually relatively low. At present, in the application scenarios of the leaky wave cable, the frequency of the electromagnetic waves is in the range of several hundred megahertz, generally not higher than 3.6 GHz.

A lower frequency means less available spectrum resources, so the modulation bandwidth is low, which affects the deployment of some high-bandwidth services (such as high-definition video). At the same time, the lower the frequency, the more traditional devices there are, and there are more interference sources, resulting in weak anti-interference ability. And technologies relying on high frequencies can enhance robustness and the anti-interference ability is also stronger. Therefore, the highest frequency of the customized leaky wave cable of Guoxin Loongson supporting up to the 6 GHz frequency band is also a necessity for anti-interference.

The leaky wave cables currently selected by Guoxin Loongson mainly refer to the radiation-type leaky cables. Compared with “air interface wireless communication”, the leaky cables enhance the anti-interference ability of the wireless system. And as long as the leaky cables are not broken or damaged, it can effectively avoid the problems faced by traditional wireless communication, such as obstruction by obstacles, blockage due to geological disaster collapses, etc. Therefore, in order to meet the high-reliability wireless applications in some special scenarios (such as tunnels, mines, automation control in electromagnetic interference environments, etc.), Guoxin Loongson has specially launched a wireless private network solution based on leaky wave cable technology.

1.2 Structural Characteristics and Parameters

Typical leaky wave cables generally adopt a three-layer composite structure:

• Inner Conductor: 99.99% oxygen-free copper core (or copper-clad aluminum), with a diameter of 2.8-6.8mm.
• Grooved Dielectric Layer: Fluoroplastic base material, with a grooving spacing of λ/4~λ/2 (λ is the operating wavelength).
• Outer Shielding Layer: Corrugated copper tube + polymer sheath.

The key technical parameters include: operating frequency band (e.g., 400MHz-6GHz), coupling loss (typical: 65±5dB), voltage standing wave ratio (≤1.5), etc. By adjusting the shape of the slots (figure-eight shape/elliptical shape), the polarization direction of the electromagnetic wave can be changed to meet the requirements of different scenarios.

1.3 Analysis of Transmission Modes

The mixed-mode transmission theory is adopted, supporting both TEM wave transmission and TE/TM mode radiation. In the 2GHz frequency band, each 100-meter cable can form about 20-30 effective radiation points, and a continuous electromagnetic field is constructed through phase superposition.

1.4 Technical Innovations of Guoxin Loongson’s Leaky Wave Cable Solution

In order to effectively improve the anti-interference ability and increase the bandwidth, Guoxin Loongson’s leaky wave cable solution selects two frequency bands, 2.4GHz or 5GHz (typical: 5.8GHz&5.2GHz). Choosing the medium and high frequency bands, which are higher than the application frequency bands of traditional leaky wave cables, can not only effectively increase the access bandwidth but also obtain more available frequency points, further enhancing the anti-interference ability of the system. Especially for the wireless access requirements of intelligent control with lower bandwidth demands (such as corridor robots, underground operation machinery, etc.), with the deployment of Guoxin Loongson’s unique “multi-link data mirroring” technology of the iMAX wireless system, its robustness can be improved, making the system more user-friendly.

Of course, there is a performance loss when using leaky wave cables in medium and high frequencies. Due to the relatively high frequency band, the working length of the leaky wave cable is limited, and the typical communication distance is 100-150 meters. Therefore, to extend the coverage distance and area of the base station, in the actual design, we will choose a dual-mode wireless solution (a single device can access 4 leaky wave cables to meet the wireless access requirements of more areas). This solution will have better economic efficiency in scenarios such as enclosed spaces and industrial Internet of Things.

Chapter 2 Application Scenarios of the Medium and High Frequency Leaky Wave Cable Solution

The leaky wave cable has relatively excellent anti-interference ability. Compared with the waveguide solution, which has even better anti-interference ability, it has the characteristics of lower cost, the ability to make turns according to the shape of the site, and easier installation and fixation. Therefore, it is one of the best choices for deploying wireless systems in enclosed spaces, intelligent control in strong electromagnetic interference environments, disaster backup in special scenarios, etc.

2.1 Wireless Coverage in Enclosed Spaces

In long and narrow enclosed scenarios such as tunnels, subways, and mines, traditional base stations have the problem of multiple reflection interference. The electromagnetic radiation of the operating machinery in the environment, as well as the signal obstruction and attenuation caused by the turns of the lines, also reduce the link stability and even the availability of the wireless system. Therefore, the leaky wave cable is particularly suitable for the following scenarios:

• Wireless access in tunnels of expressways and railways
• Construction of wireless systems in underground mines (especially in scenarios of emergency rescue, broadcasting for help, and disaster backup)
• Deployment of wireless systems in enclosed compartments such as ships and drilling platforms
• Wireless signal coverage in long and narrow enclosed spaces such as utility corridors and culverts to ensure the communication continuity in scenarios such as inspection and monitoring

2.2 Wireless Access for Automation Control

It supports the continuous data transmission of mobile platforms (such as overhead traveling cranes and rail transportation systems) in complex industrial environments, and improves the reliability and production efficiency of the automation system.

2.3 Wireless Coverage for Industrial Internet of Things

In the intelligent manufacturing workshop, the leaky wave cable demonstrates unique advantages:

• For the stacker cranes in the stereoscopic warehouse and intelligent logistics (to solve the signal attenuation caused by the metal framework)
• Communication for robot clusters such as AGVs, RGVs, and IGVs in the factory (with a time delay of < 10ms)
• Wireless coverage in the explosion-proof areas of chemical plants

Chapter 3 Wireless Private Network Solution of Guoxin Loongson’s Leaky Wave Cable

3.1 System Architecture Design of the Medium and High Frequency Leaky Cable Private Network

The leaky cable private network solution consists of three major subsystems:

The Guoxin Loongson leaky cable solution supports multi – mode single – polarization deployment and also supports MIMO 2×2 configuration. A single base station can support a maximum of 256 terminal accesses, with a throughput of up to 100Mbps×2 per single base station, and the total system bandwidth can reach over 10Gbps.

3.2 Medium and High – Frequency Leaky Cable Solutions

As the application scenarios of leaky cables vary, the system functions and focuses also differ, and thus the deployment methods are bound to be different. This article will take three different scenarios as examples to respectively introduce the medium and high – frequency leaky wave cable solutions of Guoxin Loongson.

1) Intelligent Control Scenario for Overhead Traveling Cranes (Crane, Overhead Crane)

In the remote and intelligent control scenarios for overhead traveling cranes (overhead cranes), in some factory environments, due to strong electromagnetic interference, the use of traditional “air – interface wireless” hardly meets the ultra – low packet loss rate and low latency essential for automation control, especially when it comes to the transmission of control signals, where the effectiveness, reliability, and stability must be ensured. The networking schematic diagram of the Guoxin Loongson medium and high – frequency leaky wave cable solution is as follows:

In this solution, we use the medium and high frequency leaky wave cable system to transmit sensor, data collection, and control information, ensuring a low packet loss rate and low latency for the control system. The air interface wireless technology is adopted to transmit high-bandwidth and low-sensitivity information such as videos. The two links can operate independently, and the overall high reliability of the system is achieved through the dual-link VRRP hot standby redundancy.

Explanation: Due to the relatively large signal attenuation and low bandwidth, the leaky wave cable solution has limited bandwidth carrying capacity, and its anti-interference ability is also difficult to compare with that of the waveguide wireless system. Therefore, it is difficult to meet the needs of intelligent control or unmanned operation of large cranes such as gantry cranes and quay cranes. However, it serves as a low-cost alternative to the “waveguide wireless system” for the intelligent control of overhead traveling cranes (especially for the transmission of control information).

For cases where Guoxin Loongson’s conventional “air interface wireless” and waveguide systems are used for the remote control of overhead traveling cranes, please refer to the “Collection of High-Reliability Private Network Cases for the Intelligent Control of Overhead Traveling Cranes”. The leaky cable solution provides users with a compromise option.

2) Wireless Coverage Solution for Stereoscopic Warehouse

At the site of intelligent warehousing projects such as stereoscopic warehouses, there are a large number of mechanical equipment that need to be connected to the network, such as stacker cranes, AGVs, unmanned forklifts, etc. Some of them work on the ground, while others work at high altitudes. The information that needs to be transmitted through the network mainly includes control information, positioning information, and sensor data. Occasionally, there are videos and other information used for AI analysis. Generally, the bandwidth requirement is not high, but there are high requirements for the packet loss rate, latency, and the effectiveness and stability of network coverage.

Most stereoscopic warehouses are composed of steel structures (or metal frames). Metals can block electromagnetic waves, and they will also absorb and scatter them, which seriously affects the performance of the wireless system. When the goods are in a relatively static storage state, the goods also pose the problem of blocking the wireless signals. By laying the leaky cables in the Guoxin Loongson medium and high frequency leaky cable solution, the effectiveness of wireless coverage can be greatly improved, the wireless blind areas can be reduced, the packet loss rate and latency of the system can be decreased, and the more efficient collaborative work of various intelligent equipment in the stereoscopic warehouse can be ensured.

The typical medium and high frequency leaky cable solution for stereoscopic warehouses is as follows:

Guoxin Loongson has a dedicated intelligent warehousing solution. Please refer to the “Wireless Network Solution for Intelligent Warehousing”, which mainly focuses on the application of wireless products with various technologies in the warehousing field. The introduction of the leaky cable solution provides users, especially those in intelligent control scenarios with high requirements for the reliability of wireless signals and low bandwidth, with a brand-new option.

3) Wireless Coverage in Tunnels, Pipe Galleries, and Mine Roadways

Scenarios such as tunnels, pipe galleries, and mine roadways have similar commonalities, namely long, narrow, and enclosed passageways. When deploying conventional air interface wireless systems in such areas, a series of practical problems are often encountered. For example: the wireless “Fresnel effect” affects the wireless communication distance and transmission bandwidth; the bends and turns of the road will affect the effectiveness of wireless signal propagation; control systems of pipe gallery robots with high-speed movement and roaming are sensitive to network interruptions and delays. If there is electromagnetic interference in the pipe gallery, the deployment effect of the conventional wireless system is difficult to meet its requirements…

The medium and high frequency leaky cable solution of Guoxin Loongson, through the combination of the BreezeAIR WiFi wireless system, the iMAX wireless private network system, etc. with the leaky wave cable, enables the wireless signal to better meet the deployment requirements of such enclosed scenarios. The following takes the construction of a WiFi coverage system in a certain mine roadway as an example to illustrate the structural topology diagram of the WiFi coverage system in tunnel or roadway scenarios:

Chapter 4 Summary and Outlook

The traditional leaky wave cable solution, due to its use of a relatively low frequency and having a low bandwidth, hinders its ability to play a role in more scenarios. The medium and high frequency leaky cable solution of Guoxin Loongson is an iterative innovation. By combining existing medium and high frequency wireless private network technologies with new leaky wave cables, this “old” leaky cable technology has been rejuvenated.

Compared with the waveguide system (refer to “An Analysis of Waveguide Communication Technology and Its Applications”), the cost of leaky cables is significantly reduced. Through calculations based on sample applications, its deployment cost is only one-third or even lower than that of the waveguide wireless solution. However, the “wireless wired” solution greatly improves the anti-interference ability of the system, reduces the packet loss rate and latency of the network, and enhances the stability and reliability of the system.

Guoxin Loongson will also attempt to promote more technological innovations in the use of leaky cables:

• Multi-frequency integration: Attempt the innovation of multi-service and multi-frequency sharing the same cable (for example: 400MHz trunking intercom + 2.4GHz WiFi + 5GHz private network)
• New materials: Try to use new material media to improve the signal attenuation of leaky wave cables while minimizing costs as much as possible.

Some institutions predict that by 2028, the global market size of leaky cable private networks will reach 34 billion yuan, and a large-scale application ecosystem will be formed in fields such as smart cities, Industry 4.0, and national defense communications. As the most professional supplier of wireless private network products and solutions in China, Guoxin Loongson is bound to become one of the best suppliers for leaky cable application scenarios.

This article systematically expounds on the technical principles, applicable scenarios, and advantages of leaky wave cables, providing an innovative solution for the construction of leaky wave cable wireless private networks. With the evolution of 5G/6G technologies, the innovative transmission method of “wireless wired” will continue to unleash its unique value. For specific requirements, please call Guoxin Loongson for consultation.