Antenna (electronics)

Most simply, an antenna is a component designed to send and receive radio waves.

More specifically, an antenna is an arrangement of conductorss designed to radiate (transmit) an electromagnetic field in response to an applied alternating electromotive force (EMF) and the associated alternating electric current.

Alternatively, if an antenna is placed into an electromagnetic field, it will produce an alternating voltage (receive) in response to the field. See radio frequency induction.

There are two fundamental types of antennas. The first couples to the electric field of an electromagnetic wave. It's usually a length of wire in which an electric charge moves back and forth. The second couples to the magnetic field of an electromagnetic wave. It is usually a coil or loop of wire forming an electromagnet.

Adding additional conducting rods or coils (called elements) and varying their length, spacing and orientation, an antenna with specific desired properties can be created. Typically, antennas are designed to operate at a specific frequency and to either radiate or receive energy.

The vast majority of antennas are simple vertical rods, which are inexpensive, and both radiate and receive from all directions (omnidirectional). One limitation of this antenna is that it does not radiate or receive very well in the direction in which the rod points. This is called antenna blind cone or null.

Antennas have practical use for the transmission and reception of radio frequency signals (radio, tv, etc.) which can pass through (nonconducting) walls at the speed of light over great distances.

Table of contents

1 Antenna Effectiveness
2 Theoretical antenna types
3 Practical antenna models
4 See also:

Antenna Effectiveness

There are several critical parameters that affect an antenna's performance and can be tuned. These are resonant frequency, impedance, gain, aperture or radiation pattern, efficiency and bandwidth.

The resonant frequency is related to the electrical length of the antenna. This is usually the physical length of the wire multiplied by the ratio of the speed of wave propagation in the wire. Typically an antenna is tuned for a specific frequency, and is effective for a range of frequencies usually centered on that resonant frequency. However, the other properties of the antenna (especially radiation pattern and impedance) change with frequency, so the antenna's resonant frequency may merely be close to the center frequency of these other properties are more important. Also, antennas can be made resonant on harmonic frequencies and with lengths that are fractions of the target frequency.

Impedance is similar to refractive index in optics. As the electric wave travels through the different parts of the antenna system (radio, feed line, antenna, free space) it may encounter differences in impedance. At each interface, some fraction of the wave's energy will reflect back to the source. This can be measured as SWR. A SWR ratio of 1:1 is ideal, and represents 100% forward energy to 100% return energy. A SWR of 1:1.5 is considered to be marginally acceptable in some applications. Minimizing impedance differences at each interface will reduce SWR and maximize power transfer through each part of the antenna system. Impedance is frequency dependent and related to the electrical length of the antenna. The impedance of an antenna can be matched to the radio by adjusting the impedance of the feedline, using the feedline as an impedance transformer. More commonly, the impedance is adjusted at the load (see below) with an antenna tuner, a balun, or a matching transformer.

Gain, aperature, and radiation pattern are tightly linked. Gain is measured by comparing an antenna to a model antenna, typically the isotropic antenna which radiates equally in all directions. All practical antennas radiate more than the isotropic antenna in some directions and less in others. Gain is inherently directional; the gain of an antenna is usually measured in the direction which it radiates best. Gain is one dimensional. Aperature is the shape of the "beam" cross section in the direction of highest gain, and is two dimensional. (Sometimes aperature is expressed as a radius of the circle that approximates this cross section.) Radiation pattern is the three dimensional plot of the gain, but usually the two dimensional horizontal and vertical cross sections of the radiation pattern are considered. Especially antennas with high Gain show side lobes in the radiation pattern. Side lobes are peaks in gain other than the main lobe (the "beam"). Side lobe have bad impact to the antenna quality when ever the direction of a signal is important e.g. on RADAR-systems.

Efficiency is the ratio of power put into the antenna to power actually radiated. A dummy load may have a SWR of 1:1 but an efficiency of 0, as it absorbs all power and radiates none, showing that SWR alone is not an effective measure of an antenna. Radiation in an antenna is caused by radiative resistance which can only be measured as part of total resistance including loss resistance.

The bandwidth of an antenna is the range of frequencies over which it is effective, usually centered around the resonant frequency. The bandwidth of an antenna may be increased by several techniques, including using thicker wires, replacing wires with cages to simulate a thicker wire, tapering antenna components (like in a feed horn), and combining multiple antennas into a single assembly and allowing the natural impedance to select the correct antenna.

Of these parameters, only SWR can be easily measured. Impedance can be measured with some difficulty, as it relates to the complex SWR. Measuring radiation pattern requires a sophisticated setup including significant clear space (enough to get into the antenna's far field), careful study of experiment geometry, and extremely sensitive instrumentation. Bandwidth depends on the overall effectiveness of the antenna, so all of these parameters must be understood to understand bandwidth. However, typically bandwidth is measured by only looking at SWR, i.e., by finding the frequency range over which the SWR is less than 2 or less than 1.5.

All of these parameters are expressed in terms of a transmission antenna, but are identically applicable to a receiving antenna. Impedance, however, is not applied in an obvious way; for impedance, the impedance at the load (where the power is consumed) is most critical. For a transmitting antenna, this is the antenna itself. For a receiving antenna, this is at the (radio) receiver rather than at the antenna.

Theoretical antenna types

A dielectric resonator is a variation on the conventional antenna in which an insulator with a large dielectric constant is used to modify the electromagnetic field. It is claimed that the dielectric contains the antenna's near field and therefore prevents it from interfering with other nearby antennas or circuits, making it suitable for miniature equipment such as mobile phones.
A feedhorn is an antenna system that handles the incoming waveform from the dish to the focal point. It usually comprises of a series of rings with decreasing radius in order to drive the signal to the polarizer.

Practical antenna models

There are many variations of antennas, but here are a few common models:

The dipole antenna is simply two wires pointed in opposite directions arranged either horizontally or vertically. Variations of the dipole include the folded dipole and the whip antenna which is really just half of a dipole using a ground plane as the image of the second half. The dipole antenna is usually a multiple of a half wavelength long. Generally, the dipole is considered to be omnidirectional in the plane perpendicular to the axis of the antenna, but it has deeps nulls in the directions of the axis. The popular J-pole antenna is a variation of the half dipole with a built in quarter wave transmission line impedance matching section.
The yagi antenna is a directional variation of the dipole with parasidic elements added with functionality similar to adding a reflector and lenses to focus a filament lightbulb.
The (large) loop antenna is similar to a dipole, except that the ends of the dipole are connected to form a circle, triangle (delta loop antenna) or square. Typically a loop is a multiple of a half or full wavelength in circumference. A circular loop gets higher gain (about 10%), as gain of this antenna is directly proportional to the area enclosed by the loop, but circles can be hard to support in a flexible wire, making squares and triangles much more popular. Large loop antennas are more immune to localized noise partly due to lack of a need for a groundplane. The large loop has its strongest signal in the plane of the loop, and nulls in the axis perpendicular to the plane of the loop.
The small loop antenna, also called the magnetic loop antenna is less than a wavelength in circumference. (Typically less than 1/10 for a receiving loop, less than 1/4 for a transmitting loop.) Unlike nearly all other antennas in this list, this antenna detects the magnetic field of the wave instead of the electric field. As such, it is less sensitive to electric noise when properly shielded. The loop can be brought to resonance with a tuning capacitor. Due to the small size of the loop, the radiation pattern is 90 degrees from that of the large loop. The radiation pattern is perpendicular to the plane of the loop, with nulls in the plane of the loop.
The quad antenna is an array of square loops that vary in size. The quad is related to the loop in exactly the same way the yagi is related to the dipole. Typically, the quad needs fewer elements to get the same gain as a yagi. Variations of the quad include the delta loop antenna which uses a triangle instead of a square, requiring fewer supports for large wavelength antennas.
The random wire antenna is a dipole, except that it is folded any way most convenient for the space available. Folding will reduce effectiveness and make theoretical analysis extremely difficult. (The added length helps more than the folding typically hurts.) Typically, a random wire antenna will also require an antenna tuner, as it might have a random impedance that varies nonlinearly with frequency.