Physics

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Physics

Antenna Length Calculator.

Calculates the physical length of a half-wave dipole antenna for a given frequency, considering the velocity factor of the conductor.

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Your inputs

How it works

  1. 1

    Enter the operating frequency in MHz.

  2. 2

    Enter the velocity factor of the conductor (typically 0.95 for copper).

  3. 3

    The calculator computes the half-wave dipole length using the formula.

  4. 4

    The result is the physical length in meters.

(299.792458 / frequency_mhz) * 0.5 * velocity_factor

Frequently asked questions

What is a velocity factor?

It is the ratio of the speed of a wave in the conductor to the speed of light in vacuum. It accounts for the slower propagation in real materials.

Why is the length half the wavelength?

A half-wave dipole is resonant when its length is approximately half the electrical wavelength, which gives a low impedance at the feed point.

Can I use this for other antenna types?

This formula is specific to half-wave dipoles. Other types like quarter-wave monopoles or Yagis have different length relationships.

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Antenna Length

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How it works

Calculates the physical length of a half-wave dipole antenna for a given frequency, considering the velocity factor of the conductor.

  1. Enter the operating frequency in MHz.
  2. Enter the velocity factor of the conductor (typically 0.95 for copper).
  3. The calculator computes the half-wave dipole length using the formula.
  4. The result is the physical length in meters.

Formulas

The math behind this calculator, written out so you can verify the result.

Half-wave dipole length

L = (c / f) × 0.5 × Vf

c is the speed of light (299.792458 m/s), f is frequency in Hz, Vf is velocity factor.

Example:

Input: f = 100 MHz, Vf = 0.95

Calculation: L = (299.792458 / 100e6) × 0.5 × 0.95

Result: ≈ 1.42 m

Real-world use cases

Where this calculation shows up in everyday life.

Amateur radio

Build a dipole for a specific band, e.g., 20 meters (14 MHz).

Example: For 14.2 MHz, length ≈ 10.0 m with Vf=0.95.

FM broadcast reception

Construct a simple half-wave dipole for better FM reception.

Example: For 98 MHz, length ≈ 1.45 m.

Wireless experiments

Prototype antennas for IoT or hobby projects.

Example: For 433 MHz, length ≈ 0.33 m.

Tips and common mistakes

Tips

  • Use a velocity factor of 0.95 for bare copper wire, 0.80 for coaxial cable as a radiator.
  • Cut the antenna slightly longer and trim for best SWR.
  • Account for end effect by using a factor of 0.95 to 0.98.
  • For a quarter-wave monopole, divide the dipole length by 2.

Common Mistakes to Avoid

  • Forgetting to convert frequency to Hz when using the formula manually.
  • Using the speed of light in km/s instead of m/s.
  • Ignoring the velocity factor, leading to a too-long antenna.

Assumptions and limitations

  • Use the stated inputs and units.
  • Results are estimates for planning and education.
  • Check measurements and source data before making an important decision.