A Little Background on Yagi Antennas

I just realized that I've been going on and on about different parts of the Yagi antenna without providing much background. A document describing the Yagi antenna, or more properly a Yagi-Uda antenna, was first published in 1926. Many people are most familiar with this antenna in the form of over-the-air TV antennas that were mounted on the rooftop of nearly every house decades ago. Here's a link to a Google Doodle celebrating Yagi Hidetsugu's birthday that illustrates what I'm talking about.

https://www.google.com/doodles/hidetsugu-yagis-130th-birthday

Each of the metal cross-pieces on the antenna is called an element. There may be as few as three elements, but there can be as many as ten or even more elements. There are three types of elements. The longest element is called the reflector and it's located at the "back" of the antenna, and it redirects signals arriving from the other elements towards the "front" of the antenna. The element next to the reflector, and in the middle of the three element array, is called the driven element. It's slightly shorter than the reflector, and is split into two segments. The two segments are connected to a radio transmitter or receiver. Finally there are one or more directors that focus the radio waves into a narrow beam. The directors are shorter than the driven element, and sometimes get even shorter as more are added.

In the above sketch of a Yagi antenna, the "back" is to the left and the "front" is to the right. If the antenna's connected to a radio transmitter, the direction of greatest signal strength is to the right, or in the direction of the director. If the antenna's connected to a radio receiver, the direction of greatest sensitivity is from the right, or from the direction of the director.

Characteristics of the antenna vary with, among other things, the spacing between the elements and the relative lengths of the elements. In my reference design, the elements are about a quarter wavelength apart. The driven element somewhat less than a half wavelength long. A wavelength is roughly equal to the speed of light divided by frequency of the signal of interest. The reflector is four percent longer than the driven element and the director is four percent shorter than the driven element.

One desirable characteristic of the Yagi is gain - that is the increase in signal strength. This increase is not the same in all directions. In fact, signal strength to the sides and back decrease as forward signal strength increases. You don't get something for nothing! 

This trade-off in signal strength results in other desirable characteristics; for example, directionality. Because the Yagi antenna is most sensitive in one direction it's useful in applications such as wildlife tracking.

The optimizations I'm making in these blog posts are no great new discovery. I'm just starting with a reference design and exploring what happens when I vary certain design parameters in a simulation. The purpose is to give me an opportunity to play with the simulation software, and to get a more intuitive understanding of an antenna design that's always fascinated me.