Power from a transmitter may be applied to be radiated, or power picked up by the antenna may be connected to a receiver. Normally the receiver or transmitter is connected to the dipole antenna via an intermediate feeder which enables the power to be transferred from one point to another. The length of the radiating elements determine many of the properties of the dipole antenna: its feed impedance, centre operating frequency, whether it is a resonant antenna, etc..
As such the dipole length is an important aspect of the dipole antenna parameters. The most common form of the dipole antenna is the half wave dipole which gains its name because its length corresponds to an electrical half wavelength.
However many other types of dipole antenna are also available. The various different types or variants of the dipole antenna tend to be used in different applications - accordingly it can be seen that the dipole is a very flexible and useful antenna. Half wave dipole antenna: The half wave dipole antenna is the one that is most widely used. This type of dipole antenna is resonant, operating at a point where it is an electrical half wavelength long.
The electrical half wavelength n the antenna is slightly shorter than that of a half wavelength in free space because of the effect of the wire in which the wave is travelling. The half wave dipole is typically centre fed. This gives a low impedance feed point which is easy to manage. As the antenna is balanced, i. The radiation pattern of the basic half wave dipole is relatively straightforward. It is at a maximum at right angles to the axis of the wire, and a minimum along the axis of the wire.
If plotted, it forms a figure of eight type of plot. Again, this type of antenna is generally centre fed and again this provides a low feed impedance. It is worth noting that only by having an odd number of half wavelengths does a point of current maximum and voltage minimum occur at the centre to provide a low impedance feed point which enables easy feeding of the antenna.
This type of antenna can be useful when wanting to use the antenna at its fundamental frequency of resonance and then at three or more times this figure, making a multi-band dipole option.
A short dipole will need special feed arrangements to enable the much higher feed impedance to be accommodated. However, the antenna will tend to have similar characteristics over a wide bandwidth. Against this the efficiency level can be much lower than that of a dipole antenna which is larger in terms of its length in wavelengths.
At the positive peak of the voltage, the electrons tend to move in one direction and at the negative peak, the electrons move in the other direction. This can be explained by the figures given below. Fig 1 shows the dipole when the charges induced are in positive half cycle.
Now the electrons tend to move towards the charge. Fig 2 shows the dipole with negative charges induced. The electrons here tend to move away from the dipole. Fig 3 shows the dipole with next positive half cycle. Hence, the electrons again move towards the charge. The cumulative effect of this produces a varying field effect which gets radiated in the same pattern produced on it.
Hence, the output would be an effective radiation following the cycles of the output voltage pattern. Thus, a half-wave dipole radiates effectively. Hence, the dipole antenna is an example of an omnidirectional antenna. Further, the E-field only has one vector component and consequently the fields are linearly polarized. When viewed in the x-y plane for a dipole oriented along the z-axis , the E-field is in the -y direction, and consequently the dipole antenna is vertically polarized.
The 3D pattern for the 1-wavelength dipole antenna is shown in Figure 4. This pattern is similar to the pattern for the quarter- and half-wave dipole antenna. Figure 4. Normalized 3d radiation pattern for the 1-wavelength dipole antenna.
The 3D radiation pattern for the 1. Figure 5. Normalized 3d radiation pattern for the 1. The peak directivity of the dipole antenna varies as shown in Figure 6. Figure 6. Dipole Antenna directivity as a function of dipole length.
However, for longer lengths the directivity has an upward trend but is no longer monotonic.
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