How do you calculate the approximate length in metres of a quarter-wavelength antenna for use on frequencies below 30 MHz?

To calculate the physical length of an antenna in metres, we start with the speed of light (approximately 300 million metres per second). A full wavelength in free space is calculated as 300 divided by the frequency in MHz. For a quarter-wavelength, we divide this by 4, which gives 75 divided by the frequency. However, radio waves travel slightly slower through a physical wire than in a vacuum, a phenomenon known as the "velocity factor." Taking this roughly 5% end effect into account, the constant 75 is reduced to approximately 71.3. Therefore, dividing 71.3 by the frequency in MHz yields the correct practical length for a quarter-wave wire antenna.