If you made a half-wavelength dipole antenna for 28.150 MHz, approximately how long would it be?

To find the practical length of a half-wavelength dipole in metres, use the standard formula: Length (metres) = 143 / Frequency (MHz). Plugging in the given frequency gives 143 / 28.150 = 5.0799 metres, which rounds perfectly to 5.08 metres. It is important to use the constant 143 rather than 150 (which represents a theoretical half-wavelength in a vacuum). Physical wire antennas experience 'end effects' and capacitive coupling that slightly slow down the radio wave, making the physical antenna about 5% shorter than the electrical wavelength in free space. If you used 150 / 28.150, you would get 5.33 metres, which is a common distractor. The value 10.66 metres represents a full free-space wavelength (300 / 28.150), which is entirely wrong for a half-wave dipole.