The performance of a cable is largely determined by its electrical characteristics, including low signal loss, consistent impedance, and high return loss. For leaky cables, the key factor is optimal coupling loss, which allows for controlled signal leakage. The primary role of any cable is to efficiently transmit signals, making it crucial that both the structure and materials maintain excellent transmission properties throughout the cable’s lifetime.
1. Inner Conductor
Copper is typically used as the inner conductor in cables, often in the form of annealed copper wire, copper tubes, or copper-clad aluminum. Smaller cables usually use copper wire or copper-clad aluminum, while larger cables employ copper tubes to reduce weight and cost. Additionally, the outer conductor of large cables is embossed to improve flexibility. The inner conductor significantly affects signal transmission because most of the attenuation comes from resistance losses. To minimize this, the material should have high conductivity, especially on the surface, where current tends to flow due to the skin effect. At high frequencies, the effective depth of current penetration—known as skin depth—is very shallow, so the material's surface quality is critical.
The purity and quality of the copper used in the inner conductor are essential. It must be free of impurities, with a clean and smooth surface. The diameter of the inner conductor also needs to be tightly controlled, as even small variations can affect impedance uniformity and return loss. This requires precise manufacturing processes to ensure consistent performance over time.
2. Outer Conductor
The outer conductor serves two main purposes: acting as a return path for the signal and providing electromagnetic shielding. In leaky cables, the outer conductor also determines how much signal is leaked. These conductors are typically made by wrapping a copper strip around the cable, with slits or holes cut into the surface. In embossed cables, the slots are evenly spaced and much smaller than the wavelength of the transmitted signal, allowing for controlled radiation without compromising shielding.
By modifying a standard non-leaky cable, such as by cutting the embossed peaks at 120-degree angles, a leaky structure can be created. The shape, size, and arrangement of these slots directly influence the cable’s performance. The outer conductor must also be made of high-quality copper, with good conductivity and minimal impurities. Its dimensions need to be tightly controlled to ensure stable impedance and high return loss.
3. Insulating Medium
The insulating material in an RF coaxial cable plays a critical role beyond just insulation. It directly impacts the cable’s overall performance, especially in terms of attenuation, impedance, and return loss. Therefore, the choice of dielectric material and its structure are vital. Key requirements for the insulation include:
- Low relative dielectric constant
- Minimal dielectric loss angle to reduce signal loss
- Consistent structure for uniform impedance and high return loss
- Stable mechanical properties for long-term reliability
- Waterproof and moisture-resistant properties
High-foam insulation, such as physical foaming using nitrogen, meets all these requirements. With more than 80% foam content, the electrical properties approach those of air-insulated cables. This method produces a uniform structure, ensuring consistent impedance and improved return loss. Compared to chemical foaming, which only achieves about 50% foaming and higher dielectric loss, physical foaming offers superior performance.
Our RF cables utilize this advanced foaming technology, resulting in low dielectric loss and excellent high-frequency performance. The multi-layer insulation design (inner layer, foam layer, outer layer) ensures a stable, closed foam structure with strong mechanical properties and good moisture resistance. A thin solid PE layer is added to the outer surface of the foam to prevent moisture ingress, particularly important for leaky cables with openings in the outer conductor. The inner layer also contains stabilizers to enhance compatibility with copper and extend the cable’s service life.
4. Sheath
For outdoor applications, black linear low-density polyethylene (LLDPE) is commonly used as the sheathing material due to its balance of strength and flexibility. However, in some cases, high-density polyethylene (HDPE) is preferred for better mechanical resistance and environmental durability. UV-resistant black HDPE is ideal for harsh climates, offering protection against extreme temperatures and UV exposure.
When fire safety is a concern, low-smoke, halogen-free flame retardant materials are used. In leaky cables, a fire-retardant strip can be placed between the outer conductor and sheath to prevent the spread of fire and protect the insulating layer from melting. This ensures safer operation under adverse conditions.
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