Transmission Line Calculator Formula

Understand the math behind the transmission line calculator. Each variable explained with a worked example.

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Formulas Used

Propagation Delay

delay_ns = physical_length_m / (prop_speed * 1000000) * 1000000000

Electrical Length

electrical_length_deg = 360 * physical_length_m * frequency_mhz / (prop_speed * 1000)

Wavelength in Line

wavelength_in_line_m = prop_speed / frequency_mhz

Variables

VariableDescriptionDefault
physical_length_mPhysical Length(m)10
velocity_factorVelocity Factor0.66
frequency_mhzFrequency(MHz)100
prop_speedDerived value= 299.792 * velocity_factorcalculated

How It Works

Transmission Line Propagation

Formulas

v = c x VF (propagation speed)

Delay = Length / v

Electrical Length = 360 x Length / wavelength_in_line

Transmission line effects matter when the line is a significant fraction of the wavelength (typically > 1/10).

Worked Example

10 m coax (VF = 0.66) at 100 MHz.

physical_length_m = 10velocity_factor = 0.66frequency_mhz = 100
  1. 01Speed = 299.792 x 0.66 = 197.86 Mm/s
  2. 02Delay = 10 / 197860000 = 50.5 ns
  3. 03Wavelength in line = 197.86 / 100 = 1.979 m
  4. 04Electrical length = 360 x 10 / 1.979 = 1819 degrees

Ready to run the numbers?

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