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Yagi-Uda antennas - putting the theory into practice - tailor-made designs!
Some theory
The Yagi-Uda or Yagi antenna is one of the most popular antenna designs. Despite a comparatively simple construction, it has a high gain, typically greater than 10 dB. Yagi-Uda antennas can operate in the HF to UHF bands (3 MHz to 3 GHz), but often within a limited bandwidth around the center frequency.
The concept of Yagi-Uda antenna was invented in Japan by Shintaro Uda in 1926 and published in Japanese. The work was presented for the first time in English by professor Yagi who went to America and greatly contributed to widespread use of the design.
Geometry of Yagi-Uda antennas
The basic geometry of a Yagi-Uda antenna is shown in the diagram below. The antenna has only a single active (driven) element, typically a half wave dipole or folded dipole. It means that only this member (W) of the structure is excited (fed/driven via a feed line from a generator). The rest of the components are parasitic elements (passive radiators) and they help to transmit the energy in a particular direction. The dipole is almost always the second element from the end (the left side in the diagram), with a length to make it resonant at the center frequency (in the presence of passive elements the optimal length of the dipole is somewhere between 0.45-0.48 of the wavelength).
Geometry of Yagi-Uda antennas
The element behind the dipole is the reflector (R). This element is slightly longer than the active element. Generally, there is no need for adding more reflectors, as they would improve performance very slightly. The reflector is important in determining the front-to-back ratio of the antenna. So, the presence of it reduces the relative level of the back-lobe radiation of the antenna pattern and therefore reduces the amount of power radiated in the direction opposite to the intended, increasing the antenna gain. The increased length of the reflector with respect to the active element provides two benefits. First, the longer element reflects the wave more effectively, so increases the antenna gain. Second, if the reflector is longer than the resonating dipole, its impedance has inductive character (the phase of the voltage along the reflector is ahead of the current). In the case of directors (D1, D2, ... - the parasitic components located to the right of the dipole W), which are shorter than the dipole, the impedance is capacitive (current ahead of voltage). Such a distribution of the impedance of the elements leads to a phase progression along the axis of the antenna, which causes that the antenna radiates axially.
Typically, the number of directors ranges from 1 to 20, depending on the required gain and the allowable size of the antenna. The distances (s) between directors are usually constant, and the lengths of the directors decrease with the increasing distance from the dipole, for the required impedance distribution described above.
The most popular active element of Yagi-Uda antennas is a folded dipole. The outline of the folded dipole (below) is a rectangle wherein one of the sides is much shorter than the other (d << L).
Sketch of a folded dipole
The folded dipole can be analyzed as two parallel short-circuited transmission lines with lengths of L/2 (separated in the midpoint by the feed line). So, the impedance of the folded dipole depends on the impedance of the transmission lines. Also, because the currents reinforce each other instead of cancelling (as in half wave dipole), the input impedance also depends on the impedance of a transmission line with the length L.
Balun
An example balun has been designed in microstrip technology with the use of FR4 copper-clad laminate with the thickness h = 1.52 mm, characterized by the relative permittivity εr = 4.4. The layout of the balun is shown in the figure below. The required short-circuit for DC has been realized by means of two wavy lines with high wave impedance, terminated by a short-circuit (pass through the laminate to the ground). There has been no need for 50-ohm output lines, as the load of the loop antenna will directly guarantee impedance matching. This solution has enabled a reduction of the size of the balun.
View of the DC-shorted balun
Designing a tailor-made antenna
DIPOL enables customers to design Yagi-Uda antennas operating within 1.7...3 GHz range.
Further information can be obtained from the Sales Department.
The example Yagi antenna has been designed with the use of specialized software for modeling and optimizing such antennas. The aim of the project was to develop a Yagi-Uda antenna composed of the active element in the form of PCB folded dipole, thirteen directors, and a single reflector, operating in 2 GHz band. The geometry of the developed antenna is presented in the figure below, and the next figure illustrates the printed circuit board with the folded dipole connected with the balun. The simulations of the radiation pattern at 2 GHz, standing wave ratio and gain of the antenna with the balun as functions of frequency are shown in the next sequence of drawings. It can be noted that at the center frequency (2 GHz) the gain of the antenna is approx. 17 dBi and the standing wave ratio does not exceed 1.15.

3D view of the example Yagi-Uda antenna

View of the PCB with the folded dipole and balun

The radiation pattern at 2 GHz

Standing wave ratio as the function of frequency (simulation) within the 1800-2200 MHz range


Energetic gain as the function of frequency (simulation) within the 1800-2200 MHz range