Dipole antenna

A dipole antenna (from latin di two '; German dipole antenna, and antenna dipole ) antenna is a stretched, straight metal rod or wire is composed of a (possibly folded ), which can also be shared. It converts the high frequency alternating current, and the electromagnetic wave to one another, so it can be used both for transmitting and for receiving.

The optimum length of a λ/2-Dipolantenne is about half the wavelength λ of the supplying high frequency alternating current. Shortening or lengthening of the rods has a change in the resonant frequency result.

λ/2-Dipolantennen

The principle of the dipole antenna goes back to the German physicist Heinrich Hertz, who was able to demonstrate experimentally first electromagnetic waves. His dipole ( Hertzian dipole ), which was significantly smaller than λ / 8, has only theoretical significance. The extension to approximately λ / 2 results in a response that facilitates the matching of the antenna to the feed line, and increases the efficiency. Alexander Stepanovich Popov, Russian physicist used in 1895 for the first time a dipole antenna for receiving electromagnetic waves.

The animation on the left shows how you can imagine the emergence of a resonant dipole of an oscillating circuit.

The picture on the right shows the phases of a vibration of the λ/2-Dipols. The electrical stimulation may start at time zero, when the left end, there is the largest surplus of electrons. At the same time the potential is at the right end particularly positive, there exists electron deficiency. There still is no current flowing.

Opposite charges attract each other, so many electrons move to the right. A quarter period later, at time T / 4, is measured in the middle of the dipole, a current maximum, then there is also formed the strongest magnetic field. The voltage across the dipole is balanced at these time points.

The magnetic field prevents the current ceases abruptly. It drives the electrons continue to the other side. Just half an oscillation period after the start ( T / 2), the electron densities have interchanged and the maximum negative voltage is measured now at the right end of the dipole. The power has come to a standstill. Now starts the opposite balancing process. After a long period of time T, the initial state is restored.

There are two types of the dipole:

• It can be divided in the middle, there to connect a balanced cable. Its impedance must be for performance adjustment is relatively low ( about 70 Ω ). If an unbalanced coaxial cable to be used, a balun ( balun ) is required at this point.
• At one end of the dipole an unbalanced cable can be connected, the impedance is relatively high ( 2200 Ω possible ). If a low impedance coaxial cable to be used, a resonant transformer for power adjustment is necessary at this point (see also wave impedance # antenna rod ).

The attenuation of the resonance of the radiation in the space causes a shift of the resonance to a slightly lower frequency, and a complex portion of the feedpoint impedance. With an open dipole of length λ / 2 these ( 73.1 j 42.5 ) Ω.

To compensate for the frequency offset and to eliminate the imaginary part of the dipole is reduced by a factor of 0.96, that is, a λ/2-Dipol must be 0.48 λ in length. This applies to a dipole which is infinitely thin. However, since in reality the diameter of the dipole elements > 0, the shortening factor, depending on the diameter decreases further. Also, objects near the antenna increase the velocity factor. The primary cause for the additional shortening is the capacity of the conductor to its surroundings.

Also, the folded dipole is a kind of λ/2-Dipols. With it the supply in the middle of two parallel conductors which are connected to each other at the ends takes place. Its impedance is four times higher than that of the elongated λ/2-Dipols, since only half the current flowing through the feed points. This fits very well with the balun from a λ/2-Umwegleitung, which reduces the impedance at 1 /4 and allows the connection of a standard coaxial cable.

The radiation pattern of an antenna, which is the angular dependence of the radiation intensity can, calculated from the current distribution over the length of the conductor. From the approximately sinusoidal current distribution on the λ/2-Dipol ( in the right figure above left) results in the expression

Where h is the vertical antenna in the elevation angle. By comparison, the corresponding expression of the Hertzian dipole is simply cos ( h), next in the right figure. One can see that the emission of slightly directed λ/2-Dipols The emitted power is decreased even at an elevation angle of 39 ° instead of 45 ° to a half.

The integration of the angular dependence of all the directions provides the overall performance and the uniform distribution of a reference value for the radiation intensity ( isotropic radiator ). Λ/2-Dipol for the radiation intensity (h = 0) by a factor G = 1.64 (2.15 dBi ) is greater than the reference value in the main direction. When Hertzian dipole this antenna gain G is only 1.5 (1.76 dBi ).

A performance- matched antenna takes an incident plane wave from the main direction of a performance that corresponds to its effective area AW. For the λ/2-Dipol is this:

The directivity of a dipole antenna can be increased by adding further elements, see Yagi antenna.

In cases where the directivity is not desired, eg Rotating angestrebtem reception or transmission, can take up a Knickdipol, in which the two metal rods at an angle of 90 ° to each other are arranged.

Ganzwellendipole

Is one, two half-wave dipoles unpowered along one another and feeds the mutually facing ends, results in a Ganzwellendipol. Are located at the ends of the λ/2-Elemente voltage maxima, Ganzwellendipole therefore have a very high supply impedance ( > 1 k ). Advantageously, compared to the half-wave dipole is the slightly improved directivity and thus an increased antenna gain. The directional characteristic of the λ - dipole corresponds to the λ / 2 long monopole antenna.

To assemble antennas of two half-wave dipoles and achieve common cable impedances ( about 50 Ω ), there are several options:

• It switches the half-wave dipoles together to form an array and feeds it the correct phase in parallel.
• It changes the feed impedance using a resonant transformer.
• One can reduce the impedance and at the same time increase the bandwidth, if one chooses Flächendipole instead of thin rods. Such Ganzwellendipole of two triangular faces ( or the same X-shape of rods ) is used as a radiator in the Yagi antennas. The passive elements of these antennas are sized for the upper part of the frequency band width of the flat dipole, so that the gain for higher frequencies increases towards.

Another joining of dipoles is rarely done because, this results in the radiation pattern undesirable " side lobes ". The antenna " squinting " then.