Half Wave Dipole Antenna / Aerial
- the half wave dipole antenna is a half wavelength version of the popular dipole antenna.
The half wave dipole is the most widely used version of the dipole antenna or aerial.
As the name implies, the half wave dipole is a half wavelength long. The antenna is the shortest resonant length that can be used for a resonant dipole.
Half wave dipole basics
The half wave dipole is formed from a conducting element which is wire or metal tube which is an electrical half wavelength long. It is typically fed in the centre where the impedance falls to its lowest. In this way, the antenna consists of the feeder connected to two quarter wavelength elements in line with each other.
Half wave dipole antenna
The voltage and current levels vary along the length of the radiating section of the antenna. This occurs because standing waves are set up along the length of the radiating element.
As the ends are open circuit current at these points is zero, but the voltage is at its maximum.
As the point at which these quantities is measured moves away from the ends, it is found that they vary sinusoidally: the voltage falling, but the current rising. The current then reaches a maximum and the voltage a minimum at a length equal to an electrical quarter wavelength from the ends. As it is a half wave dipole, this point occurs in the centre.
Half-wave dipole antenna current & voltage waveforms
As the centre point is where the current is a maximum and the voltage is a minimum, this makes a convenient point to feed the antenna as it present a low impedance. This is much easier to feed as high RF voltages can present many problems for feeders and matching units.
For a dipole antenna that is an electrical half wavelength long, the inductive and capacitive reactances cancel each other and the antenna becomes resonant. With the inductive and capacitive reactance levels cancelling each other out, the load becomes purely resistive and this makes feeding the half wave dipole antenna far easier. Coaxial feeder can easily be used as standing waves are not present, and it is also much easier to match to a transmitter output that may only want to see a resistive load. Loads that include reactances lead to higher voltage of current levels that the transmitter may not be able to tolerate.
The impedance for a half wave dipole antenna in free space is dipole 73 Ω which presents a good match to 70Ω coaxial feeder and this is one of the reasons why coax with this impedance was chosen for many applications.
Half wave dipole length
As the name implies the length of the dipole is a half wavelength. The actual length is slightly shorter than a half wavelength in free space because of a number of effects.
Calculations for the for the length of the half wave dipole antenna take into account elements such as the ratio of the thickness or diameter of the conductor to the length, dielectric constant of the medium around the radiating element and so forth.
Read more about dipole length calculations
It is possible to shorten the length of a half wave dipole antenna, or any antenna radiating element for that matter by adding a loading inductor. This is placed in the radiating element. It works because the dipole antenna can be considered as a resonant circuit consisting of a capacitor and inductor. Adding additional inductance will lower the resonant frequency, i.e. a given antenna length will resonate at a lower frequency than that which would be possible had no inductor be present. In this way it is possible to shorten the length of the antenna.
Half wave dipole field strength
It is possible to plot the field strength for an antenna at a distance from the radiating element to see its radiation pattern. For a complete 3D view of the radiation pattern both φ and θ angels are required. However to simplify the overall maths behind any calculations it is possible to express the field strength levels in the planes of interest. These are generally viewed as cross sections through the overall 3D pattern. The most frequently used one are the horizontal where φ=90° and the vertical planes.
Using the half wave dipole formula given above it is possible to determine the radiation pattern of the half wave dipole antenna from the far field E vector.
Half wave dipole radiation pattern & directivity
Using the half wave dipole formula, it is possible to calculate the radiation pattern and hence determine the directivity.
As expected the maximum half wave dipole directivity shows the maximum radiation at right angles to the main radiator.
At other angles, the angle θ in the half wave dipole formula above can be used to determine the field strength.
Half-wave dipole radiation pattern
It is also possible to view the radiation pattern in terms of the plane looking around the dipole antenna, i.e. in the plane cutting the dipole in its field of maximum radiation.
Pattern of radiation with axis of antenna in / out of screen
As can be seen, with the axis of the antenna in / out of the screen, the level of radiation is the same all around the antenna. This is to be expected as there is nothing to distinguish one direction from another or to affect the radiation in different directions in this plane.
When developing, designing and installing a half wave dipole antenna, there are a number of general hints and tips that can be followed to ensure the optimum performance. These are above the normal ones used for antenna installation, for example ensuring height is optimum, etc.
- Use balanced feeder or balun: The dipole antenna is a balanced antenna. It is therefore necessary to use a balanced feeder, or if coaxial feeder needs to be used, then some form of balun must be used.
- Half wave dipole is not a half wave: A half wave dipole antenna is not the same length as a half wavelength in free space. End effects mean that the actual length required is slightly shorter.
- Voltage maxima at the antenna ends: The points of maximum voltage are at the ends of the antenna. If used for transmitting make sure these cannot be accidentally touched, and also ensure they are adequately insulated. This is important when using wire antennas where the ends are used as anchor points. These should also be away from nearby objects that can act to absorb power and detune the antenna.
The half wave dipole antenna is possibly the most widely used forms of the dipole - even the most widely used form of antenna. It is simple, effective and can be incorporated as the driven element in many other forms of antenna from Yagi antennas to parabolic reflectors and many more.
By Ian Poole
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