Micro Coaxial Cable factory

How to Calculate Propagation Delay in Micro Coaxial Cables

Ever wondered how long it takes for your high-speed digital signal or precise RF pulse to travel down a tiny micro coaxial cable? That travel time is called ​propagation delay, and it’s a critical factor in timing-sensitive applications like high-speed digital circuits, radar systems, telecommunications, and high-frequency test equipment. Knowing how to calculate it is essential for designing reliable systems.

Why Propagation Delay Matters

When signals travel along any cable, including micro coaxial cables (like popular types RG178, RG316, or tiny micro-miniature cables), they don’t move instantaneously. They travel at a significant fraction of the speed of light, but less than the speed of light in a vacuum.

• ​Timing Accuracy: In high-speed digital systems (e.g., serial data links like USB, PCIe, HDMI over coax), mismatched delays between different signal paths can cause errors. RF pulses need to arrive at precisely the right time.
• ​Distance Measurement: Systems like Time-Domain Reflectometers (TDRs) use propagation delay to pinpoint the location of faults or impedance changes along the cable.
• ​Phase Relationships: In antenna arrays or RF circuits, the timing (phase) of signals delivered via different lengths of micro coax needs to be carefully controlled.

The Key Ingredient: Velocity Factor (VF)

The fundamental principle driving propagation delay is the ​Velocity Factor (VF), often expressed as a percentage. This number tells you how fast the signal travels relative to the speed of light in a vacuum (c).

• ​Speed of Light in Vacuum (c): Approximately 3 x 10^8 meters per second (300,000,000 m/s) or 11.8 inches per nanosecond (ns).
• ​Why is it slower in a cable? The signal is an electromagnetic wave traveling through the dielectric insulator material (like PTFE/ Teflon or FEP) between the center conductor and the shield. The dielectric constant (εr or Dk) of this material slows down the wave. A higher dielectric constant means a slower signal speed and higher propagation delay.
• ​Formula for Signal Speed:
  Signal Speed in Cable (v) = Speed of Light in Vacuum (c) * Velocity Factor (VF)
  (Where VF is expressed as a decimal, e.g., 0.66 for 66%)
  The Velocity Factor is directly related to the dielectric constant: VF = 1 / √εr

Velocity Factor: The Critical Specification

This is the single most important piece of information you need from the ​micro coaxial cable’s manufacturer datasheet. Typical VF values for common micro coax dielectrics are:

• ​Solid Polyethylene (PE): ~0.66 (66%)
• ​Solid Polytetrafluoroethylene (PTFE – Teflon): ~0.70 (70%)
• ​Foam Polyethylene: ~0.80 (80%) or higher (less common in micro coax)
• ​Foam PTFE: ~0.82 (82%) to 0.86 (86%) (less common in micro coax)

Always double-check the datasheet for the specific cable you are using! VF can vary slightly between manufacturers and specific cable constructions.

Calculating Propagation Delay

Once you have the Velocity Factor (VF), calculating propagation delay is straightforward. It’s simply the ​length of the cable divided by the ​speed of the signal within that cable.

1. ​Calculate Signal Speed (v):
   v = c * VF
   Where:
   • c = Speed of light (3 x 10^8 m/s or 11.8 in/ns)
   • VF = Velocity Factor (as a decimal from the datasheet)
2. ​Calculate Propagation Delay (t_pd):
   t_pd = Cable Length (d) / Signal Speed (v)

Bringing it All Together (Simpler Formulas)

You often see propagation delay expressed in terms of ​delay per unit length. This avoids directly calculating v each time.

The propagation delay per unit length is directly related to the Velocity Factor:

• ​Propagation Delay per Meter:
  Delay per Meter = 1 / v = 1 / (c * VF) = (3.333 ns/m) / VF
  (c is ~3 x 10^8 m/s, so 1/c = ~3.333 nanoseconds per meter)
• ​Propagation Delay per Foot:
  Delay per Foot = (1 ns/ft) / VF * 1.0167 ≈ (1.0167 ns/ft) / VF
  (Since light travels ~11.8 inches/ns, 1 foot/ns is very close: 1 foot / (11.8 inches/ns * 1/12 ft/inch) = 1.0167 ns/ft delay for light. This approximation is excellent for practical calculations).

Practical Calculation Steps

Here’s the simple process using the common “delay per foot” formula:

1. ​Get Datasheet: Find the ​Velocity Factor (VF) for your specific micro coaxial cable from the manufacturer’s datasheet. Express it as a decimal (e.g., 0.66 for 66%).
2. ​Calculate Delay per Foot:
   Delay per Foot ≈ 1.0167 / VF
   (You can often use 1 / VF for a quick estimate good to within ~1.7%)
3. ​Calculate Total Delay: Total Propagation Delay = Delay per Foot * Length in Feet
4. ​**(Alternative – Exact Calculation):** If preferred,
   Total Propagation Delay (ns) = (Length in Feet * 1.0167) / VF
   or Total Propagation Delay (ns) = (Length in Meters * 3.333) / VF

Example Calculation

Let’s say you are using a ​10-foot length of RG316 micro coaxial cable. Its datasheet lists a Velocity Factor of ​0.695 (69.5%).

1. ​Delay per Foot ≈ 1.0167 / 0.695 ≈ 1.463 ns per foot.
   (Quick Estimate: 1 / 0.695 ≈ 1.439 ns/ft – pretty close!)
2. ​Total Propagation Delay = 1.463 ns/ft * 10 ft ≈ 14.63 nanoseconds.

This means a signal entering one end of the 10-foot RG316 cable will emerge from the other end approximately ​14.63 nanoseconds later.

Key Takeaways

1. ​Find the VF: ​Always start with the manufacturer’s datasheet for your specific micro coaxial cable type to get the accurate ​Velocity Factor (VF).
2. ​Basic Formulas:
   • Delay per Foot ≈ 1.0167 / VF (ns/ft) [~1.7% accurate]
   • Total Delay (ns) = (Length in Feet * 1.0167) / VF [Accurate]
   • Total Delay (ns) = (Length in Meters * 3.333) / VF [Accurate]
3. ​Understand the Units: Propagation delay is usually measured in ​nanoseconds (ns) or picoseconds (ps). Pay close attention to the units of cable length (feet or meters) in your formulas.
4. ​Why it Matters: Getting this calculation right ensures precise timing in digital circuits, accurate distance readings in TDRs, and stable phase relationships in RF systems using micro coax.

By using the cable’s Velocity Factor and these simple formulas, you can reliably determine the propagation delay of any micro coaxial cable for your critical designs and measurements.