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. 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 antenna often presents a very good match tot his because the proximity of other objects, like the Earth, antenna mounting, etc. 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 voltage is alternating AC in nature. 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.
Hence, the half-wavelength dipole antenna's length is 0. The half-wave dipole antenna is as you may expect, a simple half-wavelength wire fed at the center as shown in Figure 1: Figure 1. Electric Current on a half-wave dipole antenna.
The fields from the half-wave dipole antenna are given by: The directivity of a half-wave dipole antenna is 1. The HPBW is 78 degrees. In viewing the impedance as a function of the dipole length in the section on dipole antennas , it can be noted that by reducing the length slightly the antenna can become resonant.
If the dipole's length is reduced to 0. This is a desirable property since the antenna will be better matched to the radio transmitter or receiver , and hence is often done in practice for thin dipoles.
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