Transmission Line Calculator Formula
Understand the math behind the transmission line calculator. Each variable explained with a worked example.
Formulas Used
Propagation Delay
delay_ns = physical_length_m / (prop_speed * 1000000) * 1000000000Electrical Length
electrical_length_deg = 360 * physical_length_m * frequency_mhz / (prop_speed * 1000)Wavelength in Line
wavelength_in_line_m = prop_speed / frequency_mhzVariables
| Variable | Description | Default |
|---|---|---|
physical_length_m | Physical Length(m) | 10 |
velocity_factor | Velocity Factor | 0.66 |
frequency_mhz | Frequency(MHz) | 100 |
prop_speed | Derived value= 299.792 * velocity_factor | calculated |
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
- 01Speed = 299.792 x 0.66 = 197.86 Mm/s
- 02Delay = 10 / 197860000 = 50.5 ns
- 03Wavelength in line = 197.86 / 100 = 1.979 m
- 04Electrical length = 360 x 10 / 1.979 = 1819 degrees
Frequently Asked Questions
When do transmission line effects matter?
When the line length exceeds about 1/10 of the wavelength. At 100 MHz with VF=0.66, this is about 20 cm.
What happens at quarter-wavelength?
A quarter-wave line transforms impedance: Z_in = Z0^2 / Z_load. This is used for impedance matching.
What is velocity factor?
The ratio of signal speed to the speed of light. Determined by the dielectric material surrounding the conductors.
Ready to run the numbers?
Open Transmission Line Calculator