All about Half-Wave Dipole Antennas
The dipole is one of the most simplest and most widely used form of antenna.It was invented by German physicist Heinrich Hertz around 1886 during his investigations of radio waves
The half wave dipole has omni-directional radiation, perpendicular to the length of the wire. A half dipole is most practical dipole antenna since it has a real input impedance. Theoretically, the input impedance of half dipole antenna is
The above equation shows that the impedance of the dipole is slightly inductive. However, there exists a resonance frequency where the imaginary part of the dipole’s input impedance goes to zero. This occurs at a slightly lower frequency and produces
The figure below shows the donut shaped radiation pattern of the dipole antenna simulated with 4nec2.
The above figures shows the 3D Radiation and 2D Radiation pattern of the dipole antenna. The dipole is resonant at 434 MHz frequency. It has a directivity of 2.1 dBi. This is the standard dipole antenna.
4nec2 is used as the simulation software for the simulation of dipole. Here, the length of the dipole is varied and the changes in the SWR, impedance, Directivity are recorded and graphs are made in order to get the optimized values for the different length. The diameter of the dipole is 0.001m and the conductor is supposed to be perfect and the radiation pattern is
The actual length of a half wave dipole is not exactly half of the wavelength but slightly smaller than 0.5 times wavelength about 0.45 times wavelength to 0.29 times wavelength.
The half wavelength is formed by spreading the conductors of two wire transmission line out into a straight line. Each of the conductor is of quarter wavelength length with a small gap between the arms at the center. The voltage and current in this antenna have different patterns. The current is zero at the ends and a maximum in the middle where the antenna is driven by transmission line. The voltage is zero in the middle at the gap with a positive maximum at one end and a negative maximum at the other.
The bandwidth of a half wave dipole is between 5 and 15 percent of the center frequency. The major factor which determines bandwidth is the diameter of the conductor used, smaller conductor’s result in a narrower bandwidth than those from larger diameter conductors. This fact is used to design antennas for differing bandwidth needs.
Lets design a Half-Dipole antenna at frequency 434 MHz.
Here,
The 3D Radiation pattern of the above design in CST Studio Suite can be seen as:
