Telecom ABC - Y
A Yagi antenna, or actually a Yagi-Uda antenna, is basically a standard 1/2 wavelength dipole-antenna, but with additional "elements" placed in front and behind of it to focus the energy for transmission in one direction.
A typical Yagi antenna has one element behind the dipole. This element is called a reflector. The reflector is slightly longer than 1/2 wavelength. The elements in front of the dipole are called director. A typical Yagi antenna has more than one of them. The first director is slightly shorter than 1/2 wavelength. Each following director is slightly shorter than the one behind it. The "reflector" and "director" elements increase the strength and narrow the antenna beam in the direction in front of the antenna. The more directors the Yagi has, the more forward antenna gain the antenna has. The practical gain of a Yagi is 6 to 20 dBi, depending on the number of elements.
As a consequence of its ability to provide antenna gain in a specific direction, the Yagi antenna has a narrow frequency range. Typically the bandwidth is about 10% from the operating frequency. This means that at a operating frequency of 400 MHz, the bandwidth is from 380 - 420 MHz.
Another point is that a Yagi antenna has a power limitation which prevents the radiation of many kilowatts. Hence the antenna is not suitable for e.g. short wave high power broadcasting transmission (it can be used for reception though).
Yagi antennas can be used in the frequency range from about 3 - 3000 MHz, with the best operating range below about 1500 MHz.
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