When a television station transmits a signal, it is not just automatically picked up by your television set. Your TV requires an antenna, which is a device specially made to receive certain types of signals being broadcast on certain types of frequencies. The TV antenna receives the signal and then sends it to the TV's tuner so it can be turned into audio and video information that can be understood and displayed by your television. As signals lose quality the farther they have to travel, the closer your antenna is to the source of the transmission, the stronger the signal will be, which will result in a better picture from your television.
A television antenna allows a person to receive signals from any local television stations they're within broadcasting range of. Someone with a cable television service, for example, won't need an antenna because the television signals are being sent directly into their house via the cable. The trade-off of this is a monthly fee. A television antenna receive signals from any station that broadcasts within the area you live at no charge. The signal is simply received by the antenna and passed along to the TV tuner for decoding and displaying on your television screen.
Although it may seem hard to believe, using a television antenna will enable you to receive better image quality when watching TV than if you were getting your channels from a cable service. This is because to handle the amount of channels that have to be sent out over cable television, the cable provider compresses them all for transmitting. A TV antenna allows you to receive local stations uncompressed, which means they are at the same resolution they were when they left the station for transmission. This results in a better picture.