Antenna: Difference between revisions
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== Dipoles and monopoles == | == Dipoles and monopoles == | ||
A typical dipole antenna consists of two identical conductive elements such as metal wires or rods. The driving current from the transmitter is applied, or for receiving antennas the output signal to the receiver is taken, between the two halves of the antenna. Each side of the | A typical [[w:dipole antenna|dipole antenna]] consists of two identical conductive elements such as metal wires or rods. The driving current from the transmitter is applied, or for receiving antennas the output signal to the receiver is taken, between the two halves of the antenna. Each side of the [[feed line]] to the transmitter or receiver is connected to one of the conductors. This contrasts with a monopole antenna, which consists of a single rod or conductor with one side of the feed line connected to it, and the other side connected to some type of ground. Outside ham radio, one well-known example of a dipole is the "rabbit ears" television antenna found on old broadcast television sets. | ||
A dipole is the simplest type of antenna, both from a theoretical and practical standpoint. German physicist Heinrich Hertz first demonstrated the existence of radio waves in 1887 using what we now know as a dipole antenna | A dipole is the simplest type of antenna, both from a theoretical and practical standpoint. German physicist Heinrich Hertz first demonstrated the existence of radio waves in 1887 using what we now know as a dipole antenna. On the other hand, Guglielmo Marconi found that he could just ground the transmitter (or one side of a transmission line, if used), dispensing with one half of the antenna, thus inventing the monopole antenna. | ||
Although the term "vertical antenna" is often used to refer to a monopole antenna, either type can be vertical or horizontal; vertical dipoles are quite common, and horizontal monopoles are rarely used due to typically undesirable radiation patterns but are entirely possible. | Although the term "vertical antenna" is often used to refer to a monopole antenna, either type can be vertical or horizontal; vertical dipoles are quite common, and horizontal monopoles are rarely used due to typically undesirable radiation patterns but are entirely possible. | ||
== Directionality == | |||
All antennas that can be made in reality radiate stronger signals in some directions than in others. A theoretical antenna that radiates equally in all directions is called an [[w:Isotropic radiator|isotropic antenna]]. Such an antenna can be hypothetically modeled, and such models can be useful in theoretical discussion, but it is not possible to actually create one. | |||
An [[w:omnidirectional antenna|omnidirectional antenna]], on the other hand, is entirely possible and quite common. An omnidirectional antenna radiates equally in all directions in a plane. This is an important distinction; consider a vertical dipole as an example. Theoretically, a receiver a mile north of the antenna will receive a signal as strong as one a mile west of it; this means it is omnidirectional. (In practice, the signal strengths will likely differ due to many different factors.) However, because the antenna has "nulls" off its ends, a receiver a mile above it will receive a much weaker signal. If it were isotropic, the receiver a mile above the antenna would receive the signal as strong as the receivers a mile away on the ground would. | |||
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{{Wikipedia|Dipole antenna}} | |||
Latest revision as of 18:50, 25 November 2023
An antenna is a device that converts electrical signals in a cable (such as a coaxial cable or ladder line) into radio waves, and vice versa. An antenna is a critical part of virtually any radio system; even the best radio will not work well with a poor antenna. Many different types of antennas have been designed.
Dipoles and monopoles
A typical dipole antenna consists of two identical conductive elements such as metal wires or rods. The driving current from the transmitter is applied, or for receiving antennas the output signal to the receiver is taken, between the two halves of the antenna. Each side of the feed line to the transmitter or receiver is connected to one of the conductors. This contrasts with a monopole antenna, which consists of a single rod or conductor with one side of the feed line connected to it, and the other side connected to some type of ground. Outside ham radio, one well-known example of a dipole is the "rabbit ears" television antenna found on old broadcast television sets.
A dipole is the simplest type of antenna, both from a theoretical and practical standpoint. German physicist Heinrich Hertz first demonstrated the existence of radio waves in 1887 using what we now know as a dipole antenna. On the other hand, Guglielmo Marconi found that he could just ground the transmitter (or one side of a transmission line, if used), dispensing with one half of the antenna, thus inventing the monopole antenna.
Although the term "vertical antenna" is often used to refer to a monopole antenna, either type can be vertical or horizontal; vertical dipoles are quite common, and horizontal monopoles are rarely used due to typically undesirable radiation patterns but are entirely possible.
Directionality
All antennas that can be made in reality radiate stronger signals in some directions than in others. A theoretical antenna that radiates equally in all directions is called an isotropic antenna. Such an antenna can be hypothetically modeled, and such models can be useful in theoretical discussion, but it is not possible to actually create one.
An omnidirectional antenna, on the other hand, is entirely possible and quite common. An omnidirectional antenna radiates equally in all directions in a plane. This is an important distinction; consider a vertical dipole as an example. Theoretically, a receiver a mile north of the antenna will receive a signal as strong as one a mile west of it; this means it is omnidirectional. (In practice, the signal strengths will likely differ due to many different factors.) However, because the antenna has "nulls" off its ends, a receiver a mile above it will receive a much weaker signal. If it were isotropic, the receiver a mile above the antenna would receive the signal as strong as the receivers a mile away on the ground would.
This page contains content copied from or based on the Wikipedia article "Dipole antenna." Copyright © Wikipedia contributors. Used under CC BY-SA 3.0 Unported.
