1/4 Wave Antenna Length Calculator

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This calculator gives the lengths of the radiating element of a quarter wave (λ/4) ground plane monopole and the radials.

Calculator

Enter

  • Frequency or wavelength of operation (enter either and the other will be calculated from that)
  • Velocity factor
  • % increase of radial length over the main vertical radiating element (0% means the radial length is the same as the monopole)

and the tool will provide the lengths of:

  • Vertical radiating element
  • Radials

Example Calculations

A quarter wave (λ/4) antenna designed for 20 meter applications (15 MHz) has a length of 3.3 meters. The length of radials, assuming a 10% increase, is 3.6 meters. This assumes a velocity factor of 0.66.

Background

A quarter-wave antenna, often used as a monopole antenna is mounted vertically and operates effectively over a ground plane. The radials in such an antenna system serve as the “mirror” or counterpoise to the vertical radiating element, simulating a ground plane and thus completing the antenna system.

Antenne gp vhf 3

Length of Radials

For elevated antennas (such as the one shown in the picture above), it’s beneficial for the radials to be resonant, meaning they should be at least a quarter wavelength long at the frequency of operation. This enhances the antenna’s performance by reducing ground losses and improving the radiation pattern. Non-resonant radials can still be used but might not be as effective. Sometimes it is beneficial to increase the length of the radial relative to the main radiating element by a certain % amount.

How many radials should be used?

Ground mounted antennas: Even with as few as 4 radials, the antenna can perform significantly better than with no radials at all. This setup can work for temporary installations or when space and resources are limited.

Elevated Verticals: These antennas require at least 4. However, going up to 8 or more can further enhance the antenna’s efficiency and radiation pattern. While adding more radials improves performance, the improvement per additional radial decreases beyond a certain point. For example, the difference in performance between 4 and 16 radials is more significant than between 32 and 64 radials.

Velocity Factor

The velocity factor (VF) of a transmission medium significantly impacts the physical dimensions of a monopole antenna, particularly its length. It represents the ratio of the speed of an electromagnetic wave in a medium compared to the speed of light in a vacuum. This number affects how the antenna resonates at the desired frequency. A common value for VF is 0.66. This is the default number used in the calculator above.

Velocity factors for different cables are provided here.

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