An antenna (or aerial) is a transducer designed to transmit or receive electromagnetic waves. In other words, antennas convert electromagnetic waves into electrical currents and vice versa. Antennas are used in systems such as radio and television broadcasting, point-to-point radio communication, wireless LAN, radar, and space exploration. Antennas are most commonly employed in air or outer space, but can also be operated under water or even through soil and rock at certain frequencies for short distances.
Physically, an antenna is an arrangement of conductors that generate a radiating electromagnetic field in response to an applied alternating voltage and the associated alternating electric current, or can be placed in an electromagnetic field so that the field will induce an alternating current in the antenna and a voltage between its terminals. Some antenna devices (parabolic antenna, Horn Antenna) just adapt the free space to another type of antenna.
Thomas Edison used antennas by 1885. Edison patented his system in U.S. Patent 465,971. Antennas were also used in 1888 by Heinrich Hertz (1857-1894) to prove the existence of electromagnetic waves predicted by the theory of James Clerk Maxwell. Hertz placed the emitter dipole in the focal point of a parabolic reflector.
What is ghosting?
Many structures can reflect TV signals much like they reflect light. This can lead to the TV receiver "seeing" more than one version of the TV signal. Ghosting results when a TV signal arrives at the receiver via more than one path. Low areas in a neighborhood are especially susceptible to ghosting.
If ghosting is caused by a single structure that creates one distinct ghost image, a medium or large multi-directional antenna may be useful with careful positioning to eliminate the reflected signal. However, the antenna may require different positioning for each channel.
Directional antennas are the most ghost-resistant antennas since they "see" in only one direction and have a tendency not to see the reflected ghost signal. The further away from structures the antenna is located the less likely a problem will occur. Many factors, however, such as the structure's total surface area, the direction it faces, and neighborhood terrain, will influence how much effect the structure will have on TV reception.
Which structures cause ghosting?
Nearby buildings that are higher than the TV antenna or block the antenna's view in the direction of the TV station's transmitter such as:
- Church steeples
- Apartment/condominium buildings
- School buildings
- Water towers
- Industrial buildings
- Office buildings
- Large communications/radio/TV towers
- Athletic field lighting towers
- High tension power lines and towers
Which structures do not cause ghosting?
Trees and foliage aren't usually a problem, but in some cases foliage can absorb TV signals and reduce their strength. Therefore, the best antenna locations should avoid foliage in the direction of the TV transmitter. The following usually do not cause ghosting:
- Other homes and wooden buildings that aren't taller than antenna location
- Streetlight or utility poles
- Cellular or PCS poles consisting of a single pole no higher than treetops
- Ham radio antennas
- Other TV antennas