Points for Discussion
- Use of Yagi uda antenna
- Elements of a typical Yagi Uda antenna
- Yagi uda antenna construction
- Yagi Uda Antenna Design
- Yagi uda antenna radiation pattern
- Few mathematical problems related to Yagi-Uda antenna
To define a Yagi-Uda antenna, we should know the proper definition of the antenna. According to IEEE standard definitions of antennas, “An antenna is a means for radiating or receiving radio waves”.
A yagi-uda antenna is basically an array of rectilinear dipoles with a feed element and other parasitic elements. It can be described as an end-fire array which means the array is set of internally connected antennas and the total unit functions as a single antenna.
Yagi Uda antenna is a very realistic antenna for the high-frequency domain as it operates in the high-frequency field to an ultra-high frequency domain.
Professor S. Uda and professor H. Yagi of Tohoku Imperial University, Japan, first described this type of antenna’s operation. The antenna is often interrupted as ‘Yagi Antenna’.
Use of yagi uda antenna || Applications of yagi uda antenna
Yagi antenna is one of the widely used antennae. It has been used as TV antennas at uncountable homes due to its high directivity. Many readers would recognize it just seeing the picture. It has application in amateur radios, in fields of RADARs, in satellites and RFID applications.
Elements of a typical Yagi Uda antenna
As earlier said, a typical Yagi Uda antenna, is an array of small antennas and it has one element for energy feed and others are parasitic.
The most used feed element of a yagi uda antenna is a folded dipole. The radiator is specially constructed for operation of an end-fire array. Parasitic elements at the forward beam act as directors and the pieces at the rear beam act as reflectors. This completes the antenna.
The thin rods are aligned on a crossbar with their centres. There is one driven element, several parasitic elements, a reflector, and one or more directors. As the name suggests, the parasitic elements are not physically connected with the transceiver and work as passive radiators. They radiate radio waves which further affects the radiation pattern. The distance between the two rods depends on the wavelength of the signal. Typically, the distance changes from one-tenth to one-fourth of the wavelength.
The directors’ size is generally shorter than the driven element, which is also more concise than the reflector.
The gain of a yagi uda antenna depends upon the number of parasitic elements present. Increase in the number of parasitic elements increases the overall gain of the antenna. That is why there are numerous directors in a yagi-uda antenna. As the reflector has a negligible effect on the antenna gain, there is only one reflector in the antenna.
Yagi uda antenna construction
We will discuss the construction of a few parts of the yagi uda antenna. The stakes are – Driven element, Director, & the Reflector.
- Director: It is the shortest element of the yagi uda antenna. This part is directed towards the receiving source. The length of the detectors depends upon the distance between the details and the wavelength of the signals. The gain of a yagi uda antenna has a relation with the length of the antenna. The antenna length also increases by increasing the number of directors.
- Driven element: It is the element which has the feed point for energy. The transmitter is connected with this element through the feed point. The feed point typically lies at the centre of the component. The length of the part is half of the wavelength.
- Reflectors: It is a single unit and constructed at the end of the antenna array just after the driven element. It has the highest length among the parasitic elements. The spacing of reflector depends on the wavelength, beamwidth and gain of the yagi uda antenna. The resonant frequency of reflector is generally lower.
Working of yagi uda antenna
Let us draw some attention towards the operation and working of a yagi uda antenna. Assume a typical yagi uda antenna with a reflector, with a driven element and a single director.
As discussed earlier, the driving element’s length is half of the dipole, and it is connected with electrical energy directly. It supplies power throughout the antenna as it has the feed point, and all other parasitic elements are internally associated with this element.
Now, assume the parasitic elements (both the reflectors and directors) as a general dipole element of a measurable diameter and fed at the middle via a short circuit. Transmission line theory says that a short circuit is enabled to reflect power at 180 degrees.
Thus, the operation can be designed as the mixing up of a power receiver dipole element that receives the power and sends to the matched load and a power transmitter dipole element that transmits the power to the array of the antenna.
Now, at an instant, if the received and sent power are in 180 degrees out of phase with each other, then the result will be zero voltage. That signifies the short circuit of the diode at the feed point. That is why the radiated power is in 180 degrees phase out with the incident waves.
The parasitic elements in the antenna are shorter than ½λ. The reflector is longer than ½λ, and it generally lags the phase of open-circuit voltage. The incoming signal generates the voltage. The director is also shorter than ½λ. It lags the voltage that of current.
Yagi Uda antenna design
Unlike the horn antenna, there are no hard and fast rules to design a yagi-uda antenna. There are some critical physical parameters which resist doing so. Some of the parameters are as follow –
- ‘Length of element and distance between them.’
- The measurement of the rods or the diameter of the rods.
- Some critical parameters like – Gain and input resistance.
Though, there are some methods for analysis and calculation to find out the desired results. For an n-element yagi uda antenna, there are 2n-1 numbers of parameters to consider.
The analysis for current distribution is done by solving the ‘Hallen’s integral equation’. The assumption of a classical standing wave and condition of other conductors are also taken into account. The analysis method is complicated and requires accurate results though some vital approximations are necessary to complete it.
The designed antennas go through trial-and-error methods to modify further. Sometimes, the antenna starts with a design and ends up with another after certain modifications in the process. Nowadays, computer simulation helps designers/ engineers to check the result.
Yagi Uda antenna radiation pattern
Radiation Pattern is the angular dependence of the strength of the radio waves from any electromagnetic source. The below image shows the radiation pattern of a yagi uda antenna.
Advantages yagi uda Antenna || Disadvantages of yagi uda antenna
Yagi uda antenna has both its advantages and disadvantages. But there is no doubt that this antenna has made some drastic changes in the field of commercial antennas. It has the highest ever popularity as TV antennas because of its large bandwidth. Let us discuss some of its advantages.
Advantages of yagi uda antenna
- Yagi uda antenna has a decent gain of 7dB, which is sufficient for its applications.
- Yagi uda antenna array is direction type of antenna.
- This type of antennas is suitable for applications in high frequency to the ultra-high frequency range.
- These antennas have adjustable from to ack ratio.
Let us discuss some drawbacks of yagi uda antenna.
Disadvantages of yagi uda antenna
- Though the applications of yagi uda antennas are suitable for the antenna’s gain, the gain is not very high compared to any other types of antenna.
- The designing has a requirement of a large number of elements.
- Any damage to the parasitic elements leads to the dysfunctionality of the whole antenna.
- The size is quite large, that is why nowadays the antennas are not used by peoples.
Few mathematical problems related to Yagi Uda Antenna
1. Design a yagi uda antenna with the following specifications. Directivity: Relative to ½λ dipole and situated at the same level. Magnitude: 9.2 dB. f0 = 50.1 MHz. The desired diameter of the parasitic rods: 2.54 cm. The desired diameter of the metal supporting boom: 5.1 cm. Find out the spacings between elements, lengths and length of the entire array.
The operating frequency is given as 50.1 MHz. The wavelength comes as λ = 5.988m.
The desired diameter of the parasitic rods is given as d = 2.54 cm.
Therefore, d /λ = 2.54/598.8
Or, d /λ = 4.24 x 10-3
The desired diameter of the metal supporting boom is given as D = 5.1cm.
Therefore, D /λ = 5.1 / 598.8
Or, D /λ = 8.52 x 10-3
We need to use a chart that gives us ‘optimized uncompressed lengths of parasitic elements of a yagi-uda antenna’. Using this chart, we can understand that the desired antenna array would have a total of five elements (one driven element, one reflector and three directors).
The second column of the chart gives us the optimum uncompressed length for the value of d/λ = 0.0085.
l1 = 0.482λ
l3 = 0.428λ
l4 = 0.424λ
l5 = 0.428λ
The overall antenna length will be L = (0.6 + 0.2) λ = 0.8λ. The spacing or the distance between the directors parasitic will be 0.2λ and the spacing of the reflector will be same that is 0.2λ.