Every radio frequency has a given length (wavelength). The early MW radio receivers used to have the wavelength on the tuning dial rather than the frequency. For example Radio Luxembourg was marked as 208m which equates to 1439 KHz, therefore every antenna has a given length at which it operates at it’s most efficient.
To calculate the length of a dipole the standard formula of 468/f is used, where f is the frequency in MHz. For example, a 14.100 MHz radio wave in free space is approximately 68.6 feet long. It therefore follows that half a wavelength is 34.3 feet in length. Because the RF energy travels about 5% slower in copper wire our formula gives us 33.19 feet for the total length of our dipole.