Understanding Yagi Antennas for Ham Radio: How They Work and How to Build One

Yagi antennas, also known as Yagi-Uda antennas, are among the most popular and effective antennas used in ham radio. Renowned for their high gain and directionality, Yagi antennas are particularly favored for their efficiency in both VHF and UHF bands. This article will delve into the mechanics of Yagi antennas, guide you through the process of building one, and explore the specifications for various bands along with their advantages and disadvantages.

How Yagi Antennas Work

A Yagi antenna consists of a linear array of elements: a driven element, a reflector, and one or more directors. Here’s a breakdown of each component:

Driven Element: This is the main element of the antenna that is connected to the transmission line. It acts as the primary radiator of the signal. Typically, it is a half-wave dipole or a quarter-wave monopole, depending on the design.

Reflector: Positioned behind the driven element, the reflector is slightly longer than the driven element. It reflects the signal back towards the front, enhancing the forward gain of the antenna and helping to reduce the signal behind the antenna.

Directors: These elements are placed in front of the driven element and are shorter than the driven element. They work to focus the signal in the forward direction, increasing the antenna’s directivity and gain.

The configuration of these elements results in a high-gain, directional antenna. The gain of a Yagi antenna is often expressed in dB (decibels) relative to a dipole or isotropic radiator.

Building a Yagi Antenna

Building a Yagi antenna involves several steps:

Determine the Frequency: Decide the operating frequency or band for which you want to build the antenna. The length of each element will depend on this frequency.

Calculate Element Lengths: Use the following formulas to calculate the lengths of the elements:Driven Element: Length L=468fL = \frac{468}{f}L=f468​ (where fff is the frequency in MHz).

Reflector: Typically 5% longer than the driven element.

Directors: Typically 5% shorter than the driven element. The length can decrease progressively for each additional director.

Select Material: Aluminum tubing or pipe is commonly used for Yagi antennas due to its lightweight and corrosion-resistant properties. For high-frequency antennas, you can use smaller diameter tubing.

Construct the Boom: The boom is the supporting structure on which the elements are mounted. It should be made from a non-conductive material or insulated from the elements if it's metallic.

Mount the Elements: Attach the elements to the boom at precise intervals. The distance between the elements is critical and should be calculated based on the frequency and the number of directors.

Feed the Antenna: Connect the driven element to the feedline.

Ensure the connection is secure and matches the impedance of the antenna (typically 50 ohms).

Tune and Test: Use an SWR (Standing Wave Ratio) meter to tune the antenna. Adjust the length of the elements if necessary to achieve a low SWR.

Specifications for Different Bands

2-Meter Band (144-148 MHz)Element Length: Driven Element ~ 19.7 inches, Reflector ~ 20.7 inches, Directors ~ 19.5 inches.
Typical Gain: 7-10 dBi.
Advantages: Compact size, suitable for portable operations.
Disadvantages: Limited gain compared to larger Yagis, shorter range.

70-Centimeter Band (420-450 MHz)Element Length: Driven Element ~ 10.6 inches, Reflector ~ 11.1 inches, Directors ~ 10.5 inches.
Typical Gain: 10-12 dBi.
Advantages: High gain in a small form factor, good for local communications.
Disadvantages: More susceptible to obstructions, limited range compared to lower frequency bands.

20-Meter Band (14.0-14.35 MHz)Element Length: Driven Element ~ 16.5 feet, Reflector ~ 17.3 feet, Directors ~ 16.3 feet.
Typical Gain: 8-12 dBi.
Advantages: Good for long-distance communication, efficient for DX (distance) work.
Disadvantages: Larger and less portable, more affected by physical obstructions.

40-Meter Band (7.0-7.3 MHz)Element Length: Driven Element ~ 33 feet, Reflector ~ 34.5 feet, Directors ~ 32.5 feet.
Typical Gain: 7-9 dBi.
Advantages: Effective for long-range communication, relatively high gain.
Disadvantages: Requires a large structure, more difficult to install.

Advantages and Disadvantages of Yagi Antennas

Advantages:

• High Gain: Yagi antennas offer significant gain compared to simple dipole antennas, making them ideal for long-distance communication.
• Directionality: The antenna’s ability to focus energy in one direction improves signal strength and reduces interference from other directions.
• Versatility: They can be designed for various bands and purposes, from VHF/UHF to HF.

Disadvantages:

• Size: Larger Yagi antennas can be cumbersome and require substantial space for installation.
• Complexity: Building and tuning a Yagi antenna requires precision and can be complex, particularly for higher frequency bands.
• Cost: Quality materials and construction can be expensive, especially for multi-element designs.

In summary, Yagi antennas are a valuable tool for ham radio operators seeking to enhance their signal strength and directionality. Understanding their construction and specifications can help you design an antenna that meets your specific needs. Whether for local communication or long-distance DXing, a well-built Yagi can significantly improve your radio experience.

73' Kerry W4KEJ

Image by Goran Horvat from Pixabay