An aerial with bent legs is commonly called a Yagi antenna. Yagi antennas are directional antennas that can focus a radio signal in a particular direction, making them useful for long-distance wireless communication.
What is a Yagi Antenna?
A Yagi antenna consists of an array of parallel metal rods or wires of specific lengths and spacings. The longest rod is called the driven element and is connected to the receiver or transmitter. Shorter rods on one side of the driven element act as reflectors, while rods on the other side act as directors. This configuration creates a directional radiation pattern.
The reflectors and directors are passive elements that reshape and focus the radio waves emitted by the driven element. The reflectors reflect the rearward radiation forward, while the directors focus the main beam. The more directors added, the more directional the antenna becomes.
The spacing between the elements is critical for proper operation. The rods are usually mounted perpendicular to a metal boom that connects them and keeps them in the correct position.
Key Features of Yagi Antennas
- Highly directional – Can focus signals in a specific direction
- High gain – Can increase signal strength in main direction
- Narrow beamwidth – Angular width of main radiation lobe is narrow
- Compact size – Elements are close together along the boom
- Uses passive elements – Only driven element connected to feedline
- Easy to construct – Simple design using metal rods
These features make Yagi antennas useful for point-to-point communication links and applications where long-range communication is required. The directionality reduces interference from unwanted sources.
History of the Yagi Antenna
The Yagi antenna was invented in 1926 by Japanese engineer Hidetsugu Yagi while he was working at Tohoku Imperial University. Yagi published his first paper on the antenna in 1928.
Yagi developed the antenna to improve the gain of a simple dipole antenna. By adding passive reflector and director elements, he was able to focus the signal in one direction. This increased the antenna’s ability to transmit and receive signals over long distances.
The name “Yagi” comes from the name of the inventor. However, Yagi did not work alone – his research assistants Shintaro Uda and Hidetsugu Yagi contributed significantly to the antenna design. Yagi filed a patent for the antenna in Japan in 1927.
Yagi’s design became popular in the US after it was published in the Proceedings of the IRE in 1938. During World War II, Yagi antennas were used extensively for radar and military communication.
Today, Yagi antennas continue to be widely used in television reception, point-to-point communication links, and wireless networking equipment due to their high gain, directionality, and ease of construction.
How Yagi Antennas Work
Yagi antennas work using the principles of electromagnetic wave propagation and interference. The alternating current applied to the driven element causes it to radiate electromagnetic waves. The passive reflector and director elements reshape the radiation pattern due to interference effects.
The reflector element reflects some of the rearward radiation from the driven element forward, strengthening the radiation in the desired direction. The longer length of the reflector causes a phase reversal of the reflected waves.
The director elements focus the electromagnetic waves in the desired direction due to constructive interference in front of the antenna. The shorter length of the directors causes them to re-radiate energy from the driven element in-phase with the original signal.
The combined effect is that the antenna radiates most strongly in the direction from the driven element past the directors, and has reduced radiation in other directions. The Yagi has high directive gain compared to a simple dipole.
Radiation Pattern
The radiation pattern of a Yagi antenna shows the gain as a function of direction. It has a main lobe showing the high gain direction, and weaker side lobes in other directions:
In the main lobe direction, the phases of the signals received by the Yagi elements from the incoming wave build constructively, leading to high gain. In other directions, the phase relationship is less favorable resulting in destructive interference and weaker signals.
Gain and Beamwidth
Adding more directors increases the antenna gain in the main lobe direction but decreases the beamwidth. More reflectors and directors focus the radiation into an ever narrower beam. Gain is increased from 10-20 dBi for a basic 3-element Yagi up to 20-30 dBi for a large array.
However, the beamwidth becomes narrower, meaning the antenna has to be pointed more precisely. There is a tradeoff between gain and beamwidth.
Typical Yagi Antenna Design
A basic Yagi antenna suitable for UHF TV reception may consist of:
- 1 driven element
- 1 reflector
- 5 directors
This 7-element Yagi provides around 10-15 dBi of gain with a beamwidth of around 30 degrees. The exact lengths and spacings are optimized based on the desired frequency of operation.
The driven element is usually a half-wave dipole. The passive elements are typically solid metal rods. The antenna elements are mounted perpendicular to the boom, with insulators used to isolate the elements from the metal boom.
The entire Yagi antenna is oriented to point in the desired direction. Mounting hardware allows adjustment in both elevation and azimuth.
Element Lengths
The driven element is cut to half-wave resonant length:
Length = 142.5 / Frequency (MHz)
The reflector is 5% longer than the driven element. Directors are 5-15% shorter than the driven element. Optimal lengths depend on the number of directors used.
Element Spacings
Typical element spacings are:
- Reflector to driven element: 0.2-0.5 wavelengths
- Director 1 to driven element: 0.3-0.4 wavelengths
- Director n to director n-1: 0.2-0.3 wavelengths
The exact spacing can be adjusted experimentally to optimize performance.
Practical Applications of Yagi Antennas
Some common applications of Yagi antennas include:
TV Antennas
Yagi antennas are extensively used as outdoor TV antennas. They can be oriented to receive signals from a specific direction to maximize reception from the TV transmitter while minimizing interference.
Wireless Networks
Yagi antennas provide extended range for point-to-point wireless links. They are often used for bridge links between buildings. Higher gain models can span distances up to several miles.
Amateur Radio
Hobbyists use Yagi antennas to make long-distance contacts via frequencies like the HF, VHF and UHF amateur radio bands. Using directional antennas improves signal strength.
RFID and IoT Applications
Compact Yagi antennas are used for RFID readers, sensor nodes, and other IoT devices that need to transmit and receive at longer ranges. The focused signal pattern minimizes interference.
Satellite Communication
Yagi antennas pointed at satellites can improve signal reception in satellite TV and data transmission applications. Circularly polarized versions transmit both vertically and horizontally oriented waves.
Advantages of Yagi Antennas
Some key advantages of Yagi antennas include:
- High forward gain – 10-30 dBi gain allows long range communication.
- Directionality – Focused beam pattern minimizes interference.
- Compact size – Only slightly longer than a dipole antenna.
- Light weight – Made from lightweight metal rods.
- Low cost – Simple and inexpensive to manufacture.
- Easy construction – Can be homemade from common hardware materials.
- No active components – Passive elements have high reliability.
These advantages make Yagi antennas suitable for many applications where long range and directional communication is desired.
Disadvantages of Yagi Antennas
The disadvantages of Yagi antennas include:
- Narrow bandwidth – Only works well over a narrow frequency band
- Fixed beam direction – Must be aimed and has limited scan angle
- Sensitive to element spacing – Optimal performance requires precision
- Large arrays are bulky – Weight and wind loading issues
- Suitable for outdoor use only – Metal elements affected by precipitation
Despite these limitations, Yagi antennas offer high performance and flexibility for many directional antenna needs.
Conclusion
The Yagi antenna is a versatile directional antenna invented in 1926 by Hidetsugu Yagi. It consists of a driven element and additional reflector and director elements that shape the radiation pattern.
Yagi antennas can provide high forward gain of 10-30 dBi with a narrow beamwidth. They are compact in size but offer long-range performance. Yagis are easy to construct using metal rods but require careful tuning and positioning.
Applications include TV reception, wireless networking, amateur radio, RFID systems and satellite communication. The directionality helps reduce interference and maximize signal strength.
Overall, the simple but clever Yagi design provides an inexpensive and effective solution for many long-range wireless communication requirements.