Yagi Uda Antenna
Yagi-Uda or Yagi antenna is a high gain antenna and it is known as after the name of Professor S. Uda & H. Yagi. The antenna was invented and described in Japanese by the former sometime around 1928 by Professor S. Uda and after wards it was translated or described by H. Yagi in English.
Yagi antenna has one driven element for inserting feed, one reflector for reflecting signals and one or more directors. So, the yagi-uda antenna is an array of one driven element (or active elements where the power from the transmitter is fed or which feeds received power to the receiver) and one or more parasitic elements (which are not connected directly to the transmission line but electrically coupled). The driven element is a resonant half wave dipole generally of metallic rod at the frequency of operation or resonance frequency. The parasitic elements are made my metal in rod shape and they are arranged in one axis, parallel with the driven element and at the same line of sight level.
The parasitic elements receive their excitation from the voltages induced in them by the current flow in the driven elements. The phase of voltage and currents change in the Yagi antenna due to the induced voltage, which is depend on the spacing between the various elements those are used in antenna and also depend upon the reactance of the elements.
The Yagi Uda antenna a directional antenna so its radiation patter is also is directional.
|Radiation Pattern of Yagi Antenna|
Features of antenna
- If three elements array (one reflector, one driven and one director) is used, then such type of Yagi-Uda antenna is generally referred to as beam antenna.
- In its radiation patter, a unidirectional beam of moderate directivity is generated.
- It is a light weight, low cost and simplicity in feed system design antenna.
- The spacing between the elements of antenna is the order of 0.1λ to 0.15λ and the frequency band width of the order of 2% is obtained.
- This antenna has high gain and beam width per unit area, so it is also known as super directive or super gain antenna.
- In antenna the folded dipole is generally used for excitation. In front of the director a straight conductor placed towards transmitter are known as directors. Behind the driven element a straight conductor also placed for reflecting of signals is known as reflector.
- For obtaining high impedance for proper matching between transmitter and free space the folded dipole is used at the place of driven element.
- Directors and reflectors are called parasitic elements.
- The length of the folded dipole about half wavelength and it is at resonance. Length of the director is less than half wavelength and length of reflector is greater than half wavelength.
- It is used as a transmitting antenna at HF and used for TV reception at VHF.
- The effect of parasitic elements on their distance and tuning. In other words, the effect depends on the magnitude and phase of the current induced in them.
- Reflectors are resonated at lower frequency and directors resonated at high frequency compared to that of a driven element.
- More directors can be used to increase the gain. In this case, directors can be equal length or decreasing slightly away from the driven elements.
- It is relatively broadband because of the use of the folded dipole.
- It has good front-to-back ratio and bandwidth is limited.