In radio frequency (RF) systems, RF coaxial cables are primarily used for transmitting RF signals and are widely used in internal equipment wiring, test cables, and antenna feeders. In practical applications, engineers often bend cables to very small curvatures to save installation space. While short-term testing may not reveal obvious abnormalities, this can lead to impedance distortion, additional losses, phase drift, and in severe cases, permanent mechanical damage to the cable, creating potential system malfunctions. Therefore, the bending radius is a critical parameter that is often overlooked in cable usage. Today, we'll examine the impact of the bending radius on signal transmission.
Cable bending radii are divided into static minimum bending radius and dynamic minimum bending radius. Static bending refers to situations where the cable remains stationary after installation, while dynamic bending corresponds to scenarios where the cable repeatedly bends. Dynamic bending requires a larger bending radius. The minimum bending radius given in specifications is generally expressed as a multiple of the cable's outer diameter. For flexible cables, the static minimum bending radius is typically 6–10 times the cable's outer diameter, while for dynamic bending scenarios, it needs to be increased to 10–20 times the outer diameter. Semi-rigid cables have a solid metal outer conductor, which undergoes permanent deformation after bending, making their bending radius requirements the most stringent. If the bending is too small, the damage is irreversible.
When the actual bending radius is less than the rated minimum, the outer conductor of the cable experiences localized compression deformation, the dielectric layer is compressed, and the inner conductor shifts. The originally uniform 50Ω impedance of the coaxial cable shifts locally at the bend, creating impedance discontinuities, leading to signal reflection, increased VSWR, and worsened return loss. This phenomenon is more pronounced at higher frequencies; even small deformations in the millimeter-wave band can result in considerable additional losses. Simultaneously, damage to the shielding structure can cause signal leakage, increased external electromagnetic interference, and easier coupling of external interference into the cable. Repeated small-radius bends can also cause fatigue fracture of the inner conductor, leading to premature cable failure.
Besides amplitude loss, bending also introduces phase changes. Phase consistency is crucial in phased array radar and time-delay matching testing systems. Cable bending causes dielectric deformation, altering the electromagnetic wave propagation speed and shifting the phase accordingly. In multi-channel links, if the bending conditions of the cables are inconsistent, the phase difference between channels will exceed the design tolerance, directly causing a degraded system performance.
Several practical principles need to be implemented in engineering cabling. First, read the cable specifications before cabling and strictly adhere to static and dynamic bending radii. Do not rely on experience to forcibly bend cables. Second, the cable connector root is a stress concentration point; avoid sharp bends near connectors and add anti-bending sleeves if necessary. Third, semi-rigid cables cannot be used directly in dynamic motion scenarios; replace them with suitable high-flexibility cables. Fourth, after cabling is completed, perform standing wave ratio (SWR) and loss retests to confirm that the cabling has not introduced additional performance degradation.
During on-site commissioning, if test results change after cable movement, prioritize checking bending and stress conditions. Sometimes the problem is not component damage, but rather cable flattening, excessive bending, or excessive stress on the connector root. Reorganizing the cabling and increasing the bending radius may resolve the issue. Many cable faults are not due to inherent cable defects but rather improper installation and bending. Paying close attention to bending radii and standardizing cabling construction can prevent many intermittent faults that are difficult to locate later, ensuring long-term stable operation of the RF link.
