The antenna's
power gain (or simply "gain") also takes into account the antenna's efficiency, and is often the primary figure of merit. Antennas are characterized by a number of performance measures which a user would be concerned with in selecting or designing an antenna for a particular application. A plot of the directional characteristics in the space surrounding the antenna is its
radiation pattern.
Bandwidth The frequency range or
bandwidth over which an antenna functions well can be very wide (as in a log-periodic antenna) or narrow (as in a small loop antenna); outside this range the antenna impedance becomes a poor match to the transmission line and transmitter (or receiver). Use of the antenna well away from its design frequency affects its
radiation pattern, reducing its directive gain. Generally an antenna will not have a feed-point impedance that matches that of a transmission line; a matching network between antenna terminals and the transmission line will improve power transfer to the antenna. A non-adjustable matching network will most likely place further limits the usable bandwidth of the antenna system. It may be desirable to use tubular elements, instead of thin wires, to make an antenna; these will allow a greater bandwidth. Or, several thin wires can be grouped in a
cage to simulate a thicker element. This widens the bandwidth of the resonance.
Amateur radio antennas that operate at several frequency bands which are widely separated from each other may connect elements resonant at those different frequencies in parallel. Most of the transmitter's power will flow into the resonant element while the others present a high impedance. Another solution uses
traps, parallel resonant circuits which are strategically placed in breaks created in long antenna elements. When used at the trap's particular resonant frequency the trap presents a very high impedance (parallel resonance) effectively truncating the element at the location of the trap; if positioned correctly, the truncated element makes a proper resonant antenna at the trap frequency. At substantially higher or lower frequencies the trap allows the full length of the broken element to be employed, but with a resonant frequency shifted by the net reactance added by the trap. The bandwidth characteristics of a resonant antenna element can be characterized according to its
' where the resistance involved is the
radiation resistance, which represents the emission of energy from the resonant antenna to free space. The ' of a narrow band antenna can be as high as 15. On the other hand, the reactance at the same off-resonant frequency of one using thick elements is much less, consequently resulting in a '''' as low as 5. These two antennas may perform equivalently at the resonant frequency, but the second antenna will perform over a bandwidth 3 times as wide as the antenna consisting of a thin conductor. Antennas for use over much broader frequency ranges are achieved using further techniques. Adjustment of a matching network can, in principle, allow for any antenna to be matched at any frequency. Thus the
small loop antenna built into most AM broadcast (medium wave) receivers has a very narrow bandwidth, but is tuned using a parallel capacitance which is adjusted according to the receiver tuning. On the other hand,
log-periodic antennas are
not resonant at any single frequency but can (in principle) be built to attain similar characteristics (including feedpoint impedance) over any frequency range. These are therefore commonly used (in the form of directional
log-periodic dipole arrays) as television antennas.
Gain Gain is a parameter which measures the degree of
directivity of the antenna's
radiation pattern. A high-gain antenna will radiate most of its power in a particular direction, while a low-gain antenna will radiate over a wide angle. The
antenna gain, or
power gain of an antenna is defined as the ratio of the
intensity (power per unit surface area) I radiated by the antenna in the direction of its maximum output, at an arbitrary distance, divided by the intensity I_\text{iso} radiated at the same distance by a hypothetical
isotropic antenna which radiates equal power in all directions. This dimensionless ratio is usually expressed
logarithmically in
decibels, these units are called
decibels-isotropic (dBi) : G_\text{dBi} = 10\log{I \over I_\text{iso}}\, A second unit used to measure gain is the ratio of the power radiated by the antenna to the power radiated by a
half-wave dipole antenna I_\text{dipole}; these units are called
decibels-dipole (dBd) : G_\text{dBd} = 10\log{I \over I_\text{dipole}}\, Since the gain of a half-wave dipole is 2.15 dBi and the logarithm of a product is additive, the gain in dBi is just 2.15 decibels greater than the gain in dBd : G_\text{dBi} \approx G_\text{dBd} + 2.15\, High-gain antennas have the advantage of longer range and better signal quality, but must be aimed carefully at the other antenna. An example of a high-gain antenna is a
parabolic dish such as a
satellite television antenna. Low-gain antennas have shorter range, but the orientation of the antenna is relatively unimportant. An example of a low-gain antenna is the
whip antenna found on portable radios and
cordless phones. Antenna gain should not be confused with
amplifier gain, a separate parameter measuring the increase in signal power due to an amplifying device placed at the front-end of the system, such as a
low-noise amplifier.
Effective area or aperture The
effective area or effective aperture of a receiving antenna expresses the portion of the power of a passing electromagnetic wave which the antenna delivers to its terminals, expressed in terms of an equivalent area. For instance, if a radio wave passing a given location has a flux of 1 pW / m2 (10−12 Watts per square meter) and an antenna has an effective area of 12 m2, then the antenna would deliver 12 pW of
RF power to the receiver (30 microvolts
RMS at 75 ohms). Since the receiving antenna is not equally sensitive to signals received from all directions, the effective area is a function of the direction to the source. Due to
reciprocity (discussed above) the gain of an antenna used for transmitting must be proportional to its effective area when used for receiving. Consider an antenna with no
loss, that is, one whose
electrical efficiency is 100%. It can be shown that its effective area averaged over all directions must be equal to , the wavelength squared divided by . Gain is defined such that the average gain over all directions for an antenna with 100%
electrical efficiency is equal to 1. Therefore, the effective area in terms of the gain in a given direction is given by: :A_{\mathrm{eff}} = {\lambda^2 \over 4 \pi} \, G For an antenna with an
efficiency of less than 100%, both the effective area and gain are reduced by that same amount. Therefore, the above relationship between gain and effective area still holds. These are thus two different ways of expressing the same quantity. eff is especially convenient when computing the power that would be received by an antenna of a specified gain, as illustrated by the above example.
Radiation pattern The
radiation pattern of an antenna is a plot of the relative field strength of the radio waves emitted by the antenna at different angles in the far field. It is typically represented by a three-dimensional graph, or polar plots of the horizontal and vertical cross sections. The pattern of an ideal
isotropic antenna, which radiates equally in all directions, would look like a
sphere. Many nondirectional antennas, such as
monopoles and
dipoles, emit equal power in all horizontal directions, with the power dropping off at higher and lower angles; this is called an
omnidirectional pattern and when plotted looks like a
torus or donut. The radiation of many antennas shows a pattern of maxima or "
lobes" at various angles, separated by "
nulls", angles where the radiation falls to zero. This is because the radio waves emitted by different parts of the antenna typically
interfere, causing maxima at angles where the radio waves arrive at distant points
in phase, and zero radiation at other angles where the radio waves arrive
out of phase. In a
directional antenna designed to project radio waves in a particular direction, the lobe in that direction is designed larger than the others and is called the "
main lobe". The other lobes usually represent unwanted radiation and are called "
sidelobes". The axis through the main lobe is called the "
principal axis" or "
boresight axis". The polar radiation patterns of Yagi antennas become narrower, and their directivity (and thus gain) increases, when they are designed for a relatively narrow frequency range, as compared with wideband designs.
Field regions The space surrounding an antenna can be divided into three concentric regions: The reactive near-field (also called the inductive near-field), the radiating near-field (Fresnel region) and the far-field (Fraunhofer) regions. These regions are useful to identify the field structure in each, although the transitions between them are gradual; there are no clear boundaries.
Efficiency Efficiency of a transmitting antenna is the ratio of power actually radiated (in all directions) to the power absorbed by the antenna terminals. The power supplied to the antenna terminals which is not radiated is converted into heat. This is usually through
loss resistance in the antenna's conductors, or loss between the reflector and feed horn of a parabolic antenna.
Polarization The orientation and physical structure of an antenna determine the
polarization of the electric field of the radio wave transmitted by it. For instance, an antenna composed of a linear conductor (such as a
dipole or
whip antenna) oriented vertically will result in vertical polarization; if turned on its side the same antenna's polarization will be horizontal. In the most general case, polarization is
elliptical, meaning that over each cycle the electric field vector traces out an
ellipse. Two special cases are
linear polarization (the ellipse collapses into a line) as discussed above, and
circular polarization (in which the two axes of the ellipse are equal). In linear polarization the electric field of the radio wave oscillates along one direction. In circular polarization, the electric field of the radio wave rotates around the axis of propagation. Circular or elliptically polarized radio waves are
designated as right-handed or left-handed using the "thumb in the direction of the propagation" rule. Note that for circular polarization, optical researchers use the opposite
right-hand rule from the one used by radio engineers. It is best for the receiving antenna to match the polarization of the transmitted wave for optimum reception. Otherwise there will be a loss of signal strength: when a linearly polarized antenna receives linearly polarized radiation at a relative angle of θ, then there will be a power loss of cos2θ . A circularly polarized antenna can be used to equally well match vertical or horizontal linear polarizations, suffering a 3
dB signal reduction. However it will be blind to a circularly polarized signal of the opposite orientation.
Impedance matching Maximum power transfer requires matching the impedance of an antenna system (as seen looking into the transmission line) to the
complex conjugate of the impedance of the receiver or transmitter. In the case of a transmitter, however, the desired matching impedance might not exactly correspond to the dynamic output impedance of the transmitter as analyzed as a
source impedance but rather the design value (typically 50 Ohms) required for efficient and safe operation of the transmitting circuitry. The intended impedance is normally resistive, but a transmitter (and some receivers) may have limited additional adjustments to cancel a certain amount of reactance, in order to "tweak" the match. When a transmission line is used in between the antenna and the transmitter (or receiver) one generally would like an antenna system whose impedance is resistive and nearly the same as the
characteristic impedance of that transmission line, in addition to matching the impedance that the transmitter (or receiver) expects. The match is sought to minimize the amplitude of
standing waves (measured via the
standing wave ratio; SWR) that a mismatch raises on the line, and the increase in transmission line losses it entails.
Antenna tuning at the antenna Antenna tuning, in the
strict sense of modifying the antenna itself, generally refers only to cancellation of any reactance seen at the antenna terminals, leaving only a resistive impedance which might or might not be exactly the desired impedance (that of the available transmission line). Although an antenna may be designed to have a purely resistive feedpoint impedance (such as a dipole 97% of a half wavelength long) at just one frequency, this will very likely not be exactly true at other frequencies that the antenna is eventually used for. In most cases, in principle the physical length of the antenna can be "trimmed" to obtain a pure resistance, although this is rarely convenient. On the other hand, the addition of a contrary inductance or capacitance can be used to cancel a residual capacitive or inductive reactance, respectively, and may be more convenient than lowering and trimming or extending the antenna, then hoisting it back. Antenna
reactance may be removed using lumped elements, such as
capacitors or
inductors in the main path of current traversing the antenna, often near the feedpoint, or by incorporating capacitive or inductive structures into the conducting body of the antenna to cancel the feedpoint reactance – such as open-ended "spoke" radial wires, or looped parallel wires – hence
genuinely tune the antenna to resonance. In addition to those reactance-neutralizing add-ons, antennas of any kind may include a
transformer and / or transformer
balun at their feedpoint, to change the resistive part of the impedance to more nearly match the feedline's
characteristic impedance.
Line matching at the radio Antenna tuning
in the loose sense, performed by an
impedance matching device (somewhat inappropriately named an "
antenna tuner", or the older, more appropriate term
transmatch) goes beyond merely removing reactance and includes transforming the remaining resistance to match the feedline and radio. An additional problem is matching the remaining resistive impedance to the
characteristic impedance of the transmission line: A general
impedance matching network (an "
antenna tuner" or ATU) will have at least two adjustable elements to correct both components of impedance. Any
matching network will have both power losses and power restrictions when used for transmitting. Commercial antennas are generally designed to approximately match standard 50
Ohm coaxial cables, at standard frequencies; the design expectation is that a matching network will be merely used to 'tweak' any residual mismatch.
Extreme examples of loaded small antennas In some cases matching is done in a more extreme manner, not simply to cancel a small amount of residual reactance, but to resonate an antenna whose resonance frequency is quite different from the intended frequency of operation. ;Short vertical "whip": For instance, for practical reasons a "
whip antenna" can be made significantly shorter than a quarter-
wavelength and then resonated, using a so-called
loading coil. : The physically large inductor at the base of the antenna has an inductive reactance which is the opposite of the capacitative reactance that the short vertical antenna has at the desired operating frequency. The result is a pure resistance seen at feedpoint of the loading coil; although, without further measures, the resistance will be somewhat lower than would be desired to match commercial
coax. ;Small "magnetic" loop: Another extreme case of impedance matching occurs when using a small
loop antenna (usually, but not always, for receiving) at a relatively low frequency, where it appears almost as a pure inductor. When such an inductor is resonated via a capacitor attached in parallel across its feedpoint, the capacitor not only cancels the reactance but also greatly magnifies the very small
radiation resistance of a
small loop to produce a better-matched feedpoint resistance. : This is the type of antenna used in most portable
AM broadcast receivers (other than car radios): The standard AM antenna is a loop of wire wound around a
ferrite rod (a "
loopstick antenna"). The loop is resonated by a coupled tuning capacitor, which is configured to match the receiver's tuning, in order to keep the antenna resonant at the chosen receive frequency over the AM broadcast band. ==Effect of ground==