Understanding Velocity Factor (VF) is crucial for designing and implementing RF systems using micro coaxial cables. Also known as the velocity of propagation (VP), it represents the speed at which an electrical signal travels along a cable compared to the speed of light in a vacuum (c), expressed as a decimal or percentage. A VF of 0.75 means the signal travels at 75% of the speed of light. Getting this calculation right ensures precise timing, impedance matching, and optimal performance in high-frequency applications like antennas, high-speed data links, and medical devices.
Micro coaxial cables carry critical signals in compact electronics. An incorrect VF can cause:
The velocity factor depends primarily on the cable’s dielectric constant (εᵣ or Dk). The fundamental calculation is straightforward:
VF = 1 / √εᵣ
Where:
VF
** = Velocity Factor (a value between 0 and 1).εᵣ
** = Relative permittivity (dielectric constant) of the insulating material separating the inner conductor and shield.VF = 1 / √2.1 ≈ 1 / 1.449 ≈ 0.69
If εᵣ is unknown, measure VF using a Time-Domain Reflectometer (TDR) or Vector Network Analyzer (VNA):
t = Δt / 2
.VF = (2L) / (c * Δt)
Where c
= speed of light in vacuum (3 × 10⁸ m/s).VF = (360 ° * L * f) / (Δφ * c)
(Ensure units are consistent: L
in meters, f
in Hz).Electrical Length = Physical Length / VF
. Essential for antenna feed lines or resonant circuits where λ/4 or λ/2 lengths are critical.Delay (ns/m) = 3.33 / VF
. A VF of 0.7 gives ≈4.76 ns/m delay—crucial for high-speed data synchronization.λ_cable = λ_air * VF
. Determines how signals fit physically within devices.Calculating the velocity factor (VF = 1 / √εᵣ
) is essential for precision in RF systems using micro coaxial cables. Always start with the dielectric constant (εᵣ) from your cable datasheet for the most accurate result. For unknown cables, experimental methods like TDR provide reliable measurements. Understanding VF ensures your designs operate at peak efficiency, avoiding timing errors and mismatches in sensitive applications. Keep this formula central to your micro coax implementation workflow for predictable, high-performance outcomes.
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