The Structure and Uses of Yagi Antennas!

　　Detailed Explanation of the Yagi Antenna

　　The Yagi antenna is a classic directional antenna invented by Japanese scientists Hidetsugu Yagi and Shintaro Uda in 1926. It is widely used in television reception, radio communications, radar, and radio astronomy. Its high gain and strong directivity make it suitable for long-distance signal transmission and reception.

　　1. Structure and Principle of the Yagi Antenna

　　A Yagi antenna consists of multiple parallel metal elements, primarily including the following components:

　　Component Function

　　The exciter (driven element) is directly connected to the feed line (such as a 50Ω coaxial cable) and is responsible for radiating/receiving electromagnetic waves. It is typically a half-wavelength dipole.

　　The reflector is located behind the exciter and is slightly longer (~5%) than the exciter. It enhances forward radiation and suppresses backward interference.

　　Directors are located in front of the driver and are slightly shorter (~5%-10%) than the driver. They enhance forward gain and directivity.

　　Operating Principle:

　　The reflector and director adjust the phase of the electromagnetic wave through parasitic coupling, concentrating the energy in the forward direction. The greater the number of directors, the higher the antenna gain, but the bandwidth decreases.

　　2. Key Parameters of Yagi Antennas

　　Parameter Typical Value Impact

　　Frequency Range: Depends on the length of the driver (e.g., UHF 470-860 MHz). Driver length ≈ wavelength/2. The higher the frequency, the smaller the antenna size.

　　Gain: 10-20 dBi (depending on the number of directors). The greater the number of directors, the higher the gain (but the bandwidth decreases).

　　Directivity: Main lobe width is approximately 30°-60°. Strong directivity, suitable for point-to-point communication.

　　Impedance: Typically 50Ω or 75Ω. Feeder impedance matching is required (gamma matching or balun transformers are commonly used). Front-to-back ratio (F/B Ratio) 15-25 dB. Optimized reflectors can reduce backscatter interference.

　　3. Advantages and Disadvantages of Yagi Antennas

　　✅ Advantages

　　High gain (3-10 dB higher than dipole antennas), suitable for long-distance communications.

　　Strong directivity reduces interference and improves signal-to-noise ratio.

　　Simple structure, low cost, and easy to DIY.

　　❌ Disadvantages

　　Narrow bandwidth (typically 5%-10%), unsuitable for broadband applications.

　　Large size (especially in low-frequency bands), unsuitable for portable devices.

　　Precise adjustment is required, and the spacing and length of the oscillators affect performance.

　　4. Typical Applications of Yagi Antennas

　　Applications

　　TV reception: Traditional UHF/VHF TV antennas (e.g., fishbone antennas are a variant of the Yagi).

　　Amateur radio: Used for long-distance communications in the HF/VHF/UHF bands (e.g., the 2m/70cm bands). Radar Systems: Early radars used Yagi antenna arrays (modern radars often use parabolic or phased array antennas).

　　Radio Astronomy: Used to receive cosmic radio signals (e.g., small radio telescopes).

　　Wi-Fi Directional Communications: 2.4GHz/5GHz Yagi antennas are used for point-to-point wireless bridging (e.g., Ubiquiti's directional antennas).

　　6. Improved Yagi Antenna Variants

　　Variant Type: Features

　　Log-Periodic Antenna (LPDA): A wideband version suitable for multi-band applications (e.g., TV signal reception).

　　Cross-Yagi Antenna: A combination of two orthogonal Yagi antennas achieves dual polarization (±45°), used for base station communications.

　　Folded-Dip Yagi: A folded-dip Yagi antenna replaces the excitation dipole to improve impedance matching (commonly used for 300Ω → 75Ω conversion).