Editing Directional antenna (section)

== Principle of operation ==

When transmitting, a ''high-gain antenna'' allows more of the transmitted power to be sent in the direction of the receiver, increasing the received signal strength. When receiving, a high gain antenna captures more of the signal, again increasing signal strength. Due to [[reciprocity (electromagnetism)|reciprocity]], these two effects are equal—an antenna that makes a transmitted signal 100&nbsp;times stronger (compared to an [[isotropic radiator]]) will also capture 100&nbsp;times as much energy as the isotropic antenna when used as a receiving antenna. As a consequence of their directivity, directional antennas also send less (and receive less) signal from directions other than the main beam. This property may avoid interference from other out-of-beam transmitters, and always reduces antenna noise. (Noise comes from every direction, but a desired signal will only come from one approximate direction, so the narrower the antenna's beam, the better the crucial [[signal-to-noise ratio]].)

There are many ways to make a high-gain antenna; the most common are [[parabolic antenna]]s, [[helical antenna]]s, [[Yagi-Uda antenna]]s, and [[phased array]]s of smaller antennas of any kind. [[Horn antenna]]s can also be constructed with high gain, but are less commonly seen. Still other configurations are possible—the [[Arecibo Observatory]] used a combination of a ''line feed'' with an enormous spherical reflector (as opposed to a more usual parabolic reflector), to achieve extremely high gains at specific frequencies.