Where,
Frequency  -  Frequency of wave
C  - Speed of Light (3 x 10⁸)

Far-field Distance - The far-field distance is the distance from the transmitting antenna to the beginning of the Fraunhofer region or far-field. In the far-field, the radiation pattern does not change shape as the distance increases. 

A dipole antenna is the most basic antenna used in broadcasting and receiving radio frequency signals. It consists of two conductors of equal length oriented end-to-end with the feedline connected between them. The length of the radiating elements determines many of the properties of the dipole antenna: its feed impedance, center operating frequency, whether it is a resonant antenna, etc. As such the dipole length is an important aspect of the dipole antenna parameters.
Log-periodic antennas are multi-element directional antennas used for operating in a wide range of frequencies. The most common log-periodic antenna used are LPDA(Log-Periodic Dipole Array) antennas which is a combination of a dipole antenna and log-periodic antenna. The construction and operation of a log-periodic antenna are similar to that of a Yagi-Uda antenna. The main advantage of this antenna is that it exhibits constant characteristics over a desired frequency range of operation.