A Yagi antenna is a directional antenna consisting of parasitic elements-
With a reflector and one or more directors
Understanding the Yagi Antenna: Parasitic Elements
A Yagi-Uda antenna, commonly known as a Yagi antenna, is a highly directional antenna widely used for television reception and amateur radio communication. It consists of a driven element (usually a half-wave dipole) and several parasitic elements.
The parasitic elements in a Yagi antenna are not directly connected to the transmission line. Instead, they are positioned near the driven element and interact with the electromagnetic field radiated by it. This interaction causes them to re-radiate energy in specific phases, shaping the overall radiation pattern of the antenna and increasing its gain in a particular direction.
Key Components of a Yagi Antenna
A typical Yagi antenna includes:
- Driven Element: This is the only element directly connected to the transmitter or receiver. It's usually a half-wave dipole or a folded dipole.
- Reflector: This is a parasitic element placed behind the driven element (away from the intended direction of radiation). It is typically slightly longer than the driven element. Its purpose is to reflect the energy radiated by the driven element forward, contributing to the antenna's directionality and gain.
- Directors: These are parasitic elements placed in front of the driven element (in the intended direction of radiation). They are typically slightly shorter than the driven element. Directors help to concentrate the radiated energy into a narrow beam and increase the gain in the forward direction. A Yagi antenna can have one or more directors; adding more directors generally increases the gain and directionality, up to a point.
Analyzing the Options for Parasitic Elements
The question asks about the parasitic elements in a Yagi antenna.
- Option 1: With one or more directors
This is incomplete. While directors are parasitic elements, they are not the only type of parasitic element in a standard directional Yagi antenna. - Option 2: With a reflector and one or more directors
This option correctly identifies both types of parasitic elements typically used in a directional Yagi antenna: a reflector placed behind the driven element and one or more directors placed in front of the driven element. - Option 3: Only director
This is incorrect. A standard directional Yagi antenna uses both directors and a reflector to achieve its characteristic radiation pattern and gain. An antenna with only a driven element and directors would still have some directionality but would lack the forward gain enhancement provided by the reflector. - Option 4: With a reflector
This is also incomplete. While a reflector is a parasitic element, a standard directional Yagi antenna also includes one or more directors to achieve significant forward gain and directivity. An antenna with just a driven element and a reflector is sometimes called a two-element Yagi or a Yagi-Uda with only a reflector, but the typical directional Yagi includes both.
Based on the structure and function of a standard directional Yagi antenna, the parasitic elements include both a reflector and one or more directors.
| Element Type | Connection | Relative Length | Position (Relative to Driven Element) | Primary Function |
|---|---|---|---|---|
| Driven | Connected to feed line | ≈ λ/2 (half-wave) | Center | Receives/Radiates energy directly |
| Reflector | Parasitic (not connected) | Slightly longer than driven element | Behind driven element | Reflects energy forward |
| Director(s) | Parasitic (not connected) | Slightly shorter than driven element | In front of driven element | Directs energy forward, increases gain |
Conclusion
A Yagi antenna is a directional antenna that achieves its performance characteristics through the interaction of a driven element with parasitic elements. These parasitic elements are typically a reflector and one or more directors.
Revision Table: Yagi Antenna Components
| Component | Description |
|---|---|
| Driven Element | The active part, connected to the radio. |
| Reflector | Parasitic element behind the driven element, longer than it. |
| Director(s) | Parasitic element(s) in front of the driven element, shorter than it. |
Additional Information: Yagi Antenna Characteristics
Yagi antennas are popular due to their good gain and directivity relative to their size and complexity. Some key characteristics include:
- Gain: Yagi antennas provide significant forward gain compared to a simple dipole antenna. Gain typically increases with the number of directors, up to a certain point.
- Directivity: They have a highly directional radiation pattern, meaning they are much more sensitive to signals arriving from the front and much less sensitive to signals from the sides or back. This helps in reducing interference.
- Bandwidth: Yagi antennas are generally narrow-band antennas, meaning they work effectively over a limited range of frequencies. Bandwidth tends to decrease as the number of elements increases.
- Applications: Commonly used for terrestrial television reception (especially in fringe areas), amateur radio communication (HF, VHF, UHF bands), and other point-to-point communication links.
The careful tuning and spacing of the parasitic elements relative to the driven element and the operating wavelength (λ) are crucial for optimizing the antenna's performance in terms of gain, directivity, and impedance matching.
Important Questions from Basics of Antenna
The antenna which provides circularly polarized waves is
A dipole carries RMS current of about 300 A across the radiation resistance 2 Ω. What would be the power radiated by an antenna?
Arrange the following antennas in ascending order of their radiation resistance.
A. Short dipole (L = \(\frac{\lambda}{10}\) )(I av = l o)
B. Short dipole (L= \(\frac{\lambda}{10}\) )( I av = l o/2)
C. Linear \(\frac{\lambda}{2}\) dipole (sinusoidal current distribution)
D. Small Loop (square loop) single turn of(L = \(\frac{\lambda}{10}\) )
Choose the correct answer from the options given below
If the frequency of the signal is 1 MHz, the minimum height of the transmitting antenna should be:
Which of the following is NOT true with respect to antennas?
