A Yagi antenna, radiates electromagnetic waves primarily through a combination of constructive interference and directional focusing. Oh boy that’s a mouthful. A Yagi antenna is a highly directional antenna commonly used in television reception, radio communication, and other applications where directional gain and extended range are desired. Lets look at the radiation factors for a Yagi antenna:
Driven Element
The Yagi antenna consists of several elements, with one of them being the “driven element.” The driven element is connected directly to the feedline, which carries the RF signal from a transmitter or receiver. When an RF signal is applied to the driven element, it generates an oscillating electric current along its length.
Director Elements
In front of the driven element, there are one or more shorter elements called “director elements.” These director elements are positioned at precise distances and spacings relative to the driven element. They do not connect to the feedline but serve to focus and direct the radiated energy in a specific direction.
Reflector Element
Behind the driven element, there is a longer element called the “reflector element.” Like the directors, the reflector does not connect to the feedline. Instead, it reflects RF energy back towards the driven element. The reflector is positioned at a specific distance behind the driven element to create constructive interference with the waves emitted by the driven element.
Principle of Phased Array
The key to the Yagi antenna’s operation lies in the principle of phased array. By carefully selecting the lengths and positions of the director and reflector elements, the Yagi antenna creates a specific phase relationship between the waves emitted by the driven element and the waves reflected by the reflector and directed by the directors. This phase relationship results in constructive interference in one direction, while destructive interference occurs in other directions.
Directional Gain
The constructive interference in the forward direction enhances the radiated signal’s strength, resulting in directional gain. This means that the Yagi antenna is highly directional, with the majority of its radiated energy focused in one main beam direction. This beam direction is determined by the design and positioning of the director and reflector elements.
Radiation Pattern
The Yagi antenna’s radiation pattern is characterized by a narrow, focused main lobe in the direction of maximum gain and one or more smaller, weaker side lobes in other directions. The radiation pattern can be adjusted by changing the number and spacing of director and reflector elements.
Gain and Directivity
The Yagi antenna’s gain is determined by the number of director elements, the spacing between elements, and their lengths. A well-designed Yagi can provide significant gain compared to a simple dipole antenna, allowing for longer-range communication or better reception in a specific direction. How much gain? Well, with a 3 element yagi you might get 5 dBd of gain (dBd is in reference to a dipole). Hmm, what does that mean? Well if your transmitting 10 watts, then using an antenna with 5 dBd gain you would really be transmitting 31.5 watts (roughly) out the antenna.
When would you want to use a Yagi antenna? Well, when you need directionality. When you need your antenna to look in a particular direction. Or, if you are working with really weak signals. Here is an example. Lets say you primarily operate on 2 meters with a particular repeater. That repeater happens to be far from your QTH, as an example lets say 20 miles. A yagi would help focus your electromagnetic energy in a particular direction.
Summary
A Yagi antenna radiates electromagnetic waves by using a combination of a driven element, director elements, and a reflector element to create constructive interference in one direction, resulting in a highly directional radiation pattern with significant gain. The precise design of the Yagi’s elements determines its performance characteristics. You would want to use this type of antenna when you are trying to pick up weak signals, like something from space, or a repeater that is just too far away for your vertical antenna to reach. 73.