Yagi. OK, it’s a silly sounding name. Maybe it conjures up images of the original Karate Kid or a certain picnic-basket-stealing bear. The yagi antenna, however, is one of the most important advances in outdoor antenna technology and when you disrespect the name, you disrespect one of the people who made that breakthrough.
The yagi antenna is properly called a “Yagi-Uda” antenna after Shintaro Uda and Hidetsugu Yagi. They cooperated to invent the antenna in 1926 in Tokyo. Dr. Uda actually seems to have done more work than Dr. Yagi actually although history remembers Dr. Yagi for whatever reason. The two men cooperated to create a set of formulas that would let you place multiple dipole antennas in front of each other with minimal interference between. The result is the well-known antenna design you’ve seen on roofs your whole life.
In a yagi antenna, smaller dipole antennas sit in front of ever larger ones. (The image above shows the antenna from the back.) Each dipole antenna is precisely tuned to pick up a specific frequency range, and the distance between the dipoles is designed specifically to eliminate interference and let every dipole do its job well. For the most part, the dipoles are placed right behind each other and stick out horizontally, to better align with the radio transmissions most commonly used in the US.
Yagi antennas can also have “reflectors” and “directors” which are other elements designed to concentrate the signal on a specific part of the antenna. In the image above, the bars that stick out from the top and bottom of the antenna are reflectors that send more signal to the directors in front of them, which in turn bounce them into a central receiving element. The reflectors are very carefully placed so that UHF signals bounce back and VHF signals continue through to the array behind them.
If you happen to have the time and inclination, a couple of internet searches will help you find far, far more than you will ever want to know about the physics involved in creating a yagi antenna and how they can be best optimized for different frequency ranges. It’s great bedtime reading. What you need to know more than anything, as far as the kind of broadcasts you’ll likely need to worry about, is that multiple-element yagi antennas like the one you see above really work best for lower frequencies like FM and VHF, or for situations like cellular where there are three frequencies that are spaced very far apart.
UHF television and satellite antennas tend not to use yagi antennas unless there is a need for really really long range reception. Those antennas tend to work just as well with a single dipole, and that’s why UHF antennas can be so much more compact than VHF ones.