Dipole Antennas Include:
Dipole antenna basics Current & voltage Half wave dipole Folded dipole Short dipole Doublet Dipole length Dipole feeds Radiation pattern Build HF ham dipole Inverted V dipole FM dipole design
The half wave dipole is the most popular version of the dipole antenna or aerial.
As the name implies, the half wave dipole is a half wavelength long. This is the shortest resonant length that can be used for a resonant dipole. It also has a very convenient radiation pattern.
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. The half wave dipole is normally fed in the middle 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.
It should be remembered that the length of the half wave dipole is an electrical half wavelength for the wave travelling in the antenna conductors. This is slightly shorter than the equivalent length of a wave travelling in free space as the antenna conductors affect the wavelength.
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 feed impedance
One of the major considerations with any antenna is the feed arrangements – how to transfer the power from the feeder / transmission line intoth e antenna itself. Impedance matching, balanced or unbalanced and many other aspects need to be considered.
In many aspects the half wave dipole is very easy to feed. The feeder is normally connected to the centre point is where there is a current maximum and a voltage minimum. This results in the antenna presenting a low impedance to the feeder. This is much easier to feed because the high RF voltages associated with high impedance feed arrangements 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 at the resonant frequency. 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.
The dipole is a balanced antenna and therefore a balanced feed arrangement is required. This would normally need a form of twin or balanced feeder to be used. However it is possible to use coaxial feeder if a balun (balanced to unbalanced transformer) is used.
Coaxial feeder presents a very attractive option when the impedance match is good and 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.
A half wave dipole is often fed with a 50Ω feeder. The antenna often presents a very good match tot his because the proximity of other objects, like the Earth, antenna mounting, etc. means that the impedance is lowered below the 73Ω it presents in free space.
Half wave dipole length
Although the name of the dipole gives away its approximate length, when designing and building a real dipole, a more exact length is needed.
The actual length of the half wave dipole is slightly shorter than a half wavelength in free space because of a number of effects associated with the fact that the RF waveform is carried within a wire and also most likely not in a vacuum.
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
In some instances it is necessary to shorten the length of a half wave dipole antenna. This can be achieved 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.
This principle can be used for any form of antenna, and is often used where space is a major consideration.
Half wave dipole radiation pattern & directivity
It is possible to calculate the radiation pattern and hence determine the directivity.
As might be 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.
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.
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 a balun must be used – there are several types that can easily be constructed.
- 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.
- Current maximum sections: It can be shown that the areas of the antenna where the current is a maximum contribute most to the radiation / reception. To ensure the most effective operation, these areas should be free from obstacles and be given the optimum position for radiation. This is most applicable for antennas used for lower frequencies where lengths are much long. For VHF and UHF antennas, where antennas are very much shorter, the whole length of the dipole is likely to have a similar ‘view’. For HF antennas, sometimes the centre of the antenna may be kept higher than the ends and therefore have a better chance of radiating a signal better.
- 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.
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