The performance of a cable depends heavily on its electrical characteristics, such as 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 purpose of any cable is to efficiently transmit signals, making it essential 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, available in forms like annealed copper wire, copper tubes, or copper-clad aluminum. Smaller cables often use copper wire or copper-clad aluminum, while larger cables use copper tubes to reduce weight and cost. To improve flexibility, large cables may feature embossed outer conductors.
The inner conductor significantly affects signal transmission, as most of the attenuation comes from resistance losses. High conductivity, especially at the surface, is crucial. A minimum conductivity of 58 MS/m at 20°C is standard, as at high frequencies, current flows only on the surface due to the skin effect. This thin layer, known as the skin depth, varies with frequency and material type.
The copper used must be pure, clean, and smooth. The diameter must be tightly controlled, as even small variations can affect impedance and return loss. Precise manufacturing is necessary to ensure consistent performance over time.
2. Outer Conductor
The outer conductor serves two main purposes: acting as a return path for current and providing shielding. In leaky cables, it also determines the leakage characteristics. Coaxial and super-flex cables typically use embossed copper tubes, which are welded to form a fully enclosed structure, preventing signal radiation.
In many cases, the outer conductor is made by wrapping a copper strip. Slits or holes are cut into the conductor in either longitudinal or transverse directions. These cuts help control signal leakage and are more common in embossed cables, where they are evenly spaced along the length of the cable.
By modifying a non-leaky cable—cutting the outer conductor at a 120-degree angle—a leaky structure can be created. The shape, width, and arrangement of these slots directly influence the cable’s performance. The outer conductor must be of high-quality copper, with tight dimensional tolerances 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. Key requirements for the dielectric include:
- Low relative permittivity to minimize signal loss
- Minimal dielectric loss tangent for reduced attenuation
- Uniform structure for consistent impedance and high return loss
- Stable mechanical properties for long-term reliability
- Moisture and water resistance for durability
High-foam insulation, such as physical foaming using nitrogen, meets all these needs. With over 80% foaming, it closely mimics air insulation, offering excellent electrical performance. This method produces a uniform foam structure, ensuring stable impedance and high return loss. In contrast, chemical foaming only achieves about 50% foaming, leading to higher dielectric loss and less consistency.
Our cables use this advanced foaming technique, resulting in low dielectric loss and superior high-frequency performance. The multi-layer insulation design (inner foam layer, outer solid layer) ensures mechanical strength, moisture resistance, and long-term stability. A thin solid PE layer is added around the foam to prevent moisture ingress, especially important for leaky cables with open outer conductors.
4. Sheath
For outdoor applications, black linear low-density polyethylene (LLDPE) is commonly used due to its balance of strength and flexibility. However, HDPE may be preferred when enhanced mechanical and environmental resistance is needed. UV-resistant 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 between the outer conductor and sheath helps limit fire spread, protecting the internal components and reducing potential hazards.
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