Answered
In HDTV
What is the need for antennas what are transmitting antennas what are receiving antennas?
Antennas transfer energy from a cable into the airwaves efficiently. Any transmit antenna can also be a receive antenna and viceversa. If you wish to transmit high power from …an antenna, usually heat generation or high voltage is a concern that can destroy the antenna. To prevent problems, high power antennas require thicker wires and sometimes insulators to prevent overheating/burning and arcing. Many antennas can simply be made of wire of a simple length and they'll be very efficient. The length of an antenna changes it's efficiency to the cable. If you want the antenna to be efficient, you must size it to match the cable impedance. Usually the length of a single wire exiting above a metal plate is one quarter wavelength or 1/4 x 11,803/frequency in megahertz. For FM radio on an automobile, some antennas have motors that extend out of the body of the car. The height (inches) should be 1/4 x 11,803/100 megahertz or just under 30 inches. TV antennas above a house are most often a set of half wavelength wires, so change the 1/4 to 1/2 in the formula above. They use many different wire lengths to cover all the frequencies in the television band, which extends from about 50 megahertz to around 600 megahertz. Cell phone frequencies were picked partly to make the antennas small and partly so that cell phone reception can enter buildings with rebar reinforcement. Cell phone frequencies are roughly 800 megahertz and 1800 megahertz. Receiver antennas are often made smaller than either transmit or transceiver antennas. If you open up the back of an older transistor radio, you are likely to find a bar made of ferrite material with an insulator and wire wrapped around that. The wire is wrapped a number of turns to match the impedance required by the circuit. The ferrite bar is able to focus signals to itself which in turn induces a current in the coil. While impedance is an important issue, simple dipole antennas can be sized by the 468/Frequency in mHz formula. It will give the the length of an antenna in feet. You divide the outcome by 2 to find out what each side length should be. The impedance of an antenna can ofter be adjusted by its shape. The 90 degree V shape yields a near perfect 50 ohms impedance which is commonly used for transmitters and transceivers where as the flat top diode has a higher impedance. The ARRL has good books on antennas. You might want to go to the library and look at a few of them. (MORE)