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# Antenna theory

Antenna theory and antenna design is of interest to anyone who receives or transmits radio signals. An antenna is an arrangement of electrical conductors that can transmit and/or receive electromagnetic energy. Antenna theory is grounded in Maxwell's equations.

Antennas may be omnidirectional, sending or receiving from all directions equally, or they may be directional and favor one direction over others. Antenna theory is concerned with understanding how the size and shape of antennas effects directionality and other properties.

Since radio waves are electromagnetic, there are two basically different types of antennas: electric, and magnetic.

The typical electric antenna is a vertical conductive spike. The electric field goes up and down in the spike, and this causes waves that spread out in all directions from the spike. The spike will be more efficient if it resonates. In that way, a larger electric charge can be moved with relatively less input power. Another common trick is to make half of a vertical resonant spike, and then reflect the spike in a mirror, a "ground plane." This reduces the height of the antenna by half.

The height of a resonant full-wave spike in feet is about 286m (936 feet) divided by the frequency in millions of cycles per second. The length is determined by the speed of light (i.e. radio) in copper wire. It varies slightly depending on the type of wire, and real antennas should always be made slightly long, and tuned by cutting them down. The simplest measure of the antenna tuning is to measure the power going into and coming out of an antenna.

Antennas vary in size and shape depending on their intended use. Low frequency radio waves resonate in large antennas. High frequency radio waves resonate in smaller antennas.

The typical magnetic antenna is a horizontal loop. The electric charge goes around the loop, causing the loop to be an electromagnet. Again, a resonant antenna is more efficient, and the resonance depends on the speed of radio in the wire.

Another major concern of antenna theory is antenna gain. Antennas can be designed to amplify signals coming from some directions and reject them from others. The gain of an antenna expresses how much it amplifies a signal.

Directional antennas use reflectors. The simplest reflector is just a second undriven antenna one wave-length behind the first. At this point, the electric or magnetic component of the wave is again at full strength, and it will reflect from the second antenna element. TV antennas use many reflecting elements to make the antenna more directional.

At very high frequencies and short wave-lengths, it is easier to make pipes and mirrors, than numerous tiny pieces of wire. This is why most satellite receivers have a pipe and mirror arrangement.

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