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STANDARD ANTENNA TERMS

AND RELATED FORMULAS

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The "IEEE Standard Definitions of Terms for Antennas" (IEEE STD-145) represents a consistent andcomprehensive vocabulary suited for the effective communication and understanding of antenna theory. Generaluse of these definitions of terms would eliminate much of the wide-spread inconsistency concerning antennacharacteristics, particularly with regard to the basic parameters of gain, beamwidth, polarization and efficiency.For convenience, IEEE antenna terms of general interest are listed here. Wherever these terms appear in thiscatalog, the definitions given below apply. Other commonly used terms, not covered by the IEEE standard, areshown with an "*."

ANTENNA APERTURE. A surface, near or on anantenna, on which it is convenient to makeassumptions regarding the field values for thepurpose of computing fields at external points.Note: The aperture is often taken as that portion ofa plane surface near the antenna, perpendicular tothe direction of maximum radiation, through which

the major part of the radiation passes.

ANTENNA EFFICIENCY OF APERTURE - TYPE

ANTENNA. For an antenna with a specified planaraperture, the ratio of the maximum effective area ofthe antenna to the aperture area.

* ANTENNA FACTOR. That quantity by which thevoltage developed across the output of an antennais related to the incident field strength in which theantenna is immersed. Note: Applicable to lowfrequency antennas and usually refers to a 50 ohm

output.

AFE (dB m ) E (dB V / m) - V (dB V)-1

=

AFE = Electric Field Antenna FactorE = Electric Field Strength at antennaV = Voltage at terminals of antenna

AFH (dB AV m ) H (dB A / m) V(db V)-1 -1

=

AFH = Magnetic Field Antenna FactorH = Magnetic Field Strength at antennaV = Voltage at terminals of antenna

AFE (dB m ) = AFH (dB AV m ) 51.53-1 -1 -1

+

for a plane wave in free space.

AFB (dBpT

V) = B (dBpT) - V

o(dB V)

AFB = Magnetic flux Antenna FactorB = Magnetic flux at the antennapT: picoTesla unitsV = Voltage at the terminals of the antenna

AFB (dBpT

V) AFH (dB

A

Vm) 2

= +

APERTURE ILLUMINATION. The field over theaperture as described by amplitude, phase, andpolarization distributions.

APERTURE ILLUMINATION EFFICIENCY. For aplanar antenna aperture, the ratio of its directivity tothe directivity obtained when the apertureillumination is uniform.

BEAM. The major lobe of the radiation pattern.

CIRCULAR POLARIZATION. It may be either righthand circular polarization (RHCP) or left handcircular polarization (LHCP). The sense ofpolarization is determined by observation of thedirection of rotation of the electric field vector from apoint behind the source, RHCP and LHCPcorrespond to clockwise and counter-clockwiserespectively. Note: RHCP transmit requires a likepolarization to receive.

CO-POLARIZED. The polarization which theantenna is intended to radiate or receive. Also "likepolarization".

* CROSS POLARIZATION DISCRIMINATION

(XPD). Cross polarization discrimination is themeasure of the antennas ability to differentiatebetween the vertical and the horizontal polarizationof an antenna. This difference, shown in relativesignal level, is indicated on directional patternenvelopes (DPEs).

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STANDARD ANTENNA TERMSAND RELATED FORMULAS


* DIRECTIONAL PATTERN ENVELOPES (DPES).In accordance with standard practice, radiationcharacteristics in any given plane of polarization aremeasured and plotted using 360-degree polarcoordinate systems. The resultant Directional PatternEnvelope is the smoothed composite of all thesemeasurements. The purpose of these DPEs is toemphasize the worst composite condition.

DIRECTIVE GAIN. In a given direction, 4 times theratio of the radiation intensity in that direction to thetotal power radiated by the antenna.

DIRECTIVITY. The value of the directive gain in thedirection of its maximum value.

EFFECTIVE AREA OF AN ANTENNA. In a givendirection, the ratio of power available at the terminalsof a receiving antenna to the power per unit area of aplane wave incident on the antenna from thatdirection, polarized coincident with the polarizationthat the antenna would radiate.

FAR FIELD REGION. That region of the field of anantenna where the angular field distribution isessentially independent of the distance from aspecified point in the antenna region.

* FRONT-TO-BACK RATIO. The ratio of the

maximum directivity of an antenna to its directivity ina specified rearward direction.

* Gain, dBi. The gain expressed in decibelsrelative to an isotropic radiator that is linearlypolarized.

G(dBi) 10log(G) G 10G(dBi)

10= =

* GAIN, dBic. The gain expressed in decibels relativeto an isotropic radiator that is circularly polarized.

HALF-POWER BEAMWIDTH. In plane containing thedirection of the maximum of a beam, the angle betweenthe directions in which the radiation intensity is one halfthe maximum value of the beam.

HALF-WAVE DIPOLE. A half wavelength antennacenter fed so as to have equal current distribution in bothhalves. Mounted vertically, it has a doughnut shapedpattern, circular in the horizontal plane. It is an antennathat can be constructed. It has some inherent lossesWhen used as a gain reference, the half-wave dipole hasa power gain of about 1.7 dBi.

* ISOLATION. Refers to the ability of one port of a duapolarized feed to discriminate against a signal fed into theother port.

ISOTROPIC RADIATOR. A hypothetical antenna havingequal radiation intensity in all directions. Note: Anisotropic radiator represents a convenient reference forexpressing the directive properties of actual antennas.

NEAR-FIELD REGION. The spherical region of spacebetween the antenna and the far field region.

NULL. The region of a radiation pattern, eithercomputed or measured, where the amplitude goesthrough a minimum value. Note: (1) It represents theangular position where the phase or the far field patterncrosses the zero axis if the pattern is plotted as a phasorinstead of a scalar value. Note: (2) The region outsidethe main beam of a directive antenna pattern consists ofa series of minor lobes separated by nulls.

PARALLEL POLARIZATION. The condition where theelectric vector is parallel to the local conducting surface.Note: Over the earth, this is usually referred to as beinghorizontal polarization.

PHASE CENTER. The location of a point associatedwith an antenna such that, if it is taken as the center of asphere whose radius extends into the far-field, the phaseof a given component over the surface of that radiationsphere is essentially constant, at least over the portion ofthe sphere where the radiation is significant.

0-5-10-15-20-60-100-140-180 5 10 15 20 60 100 140 180-70

0

-10

-20

-30

-40

-50

-60

-70

0

-10

-20

-30

-40

-50

-60

RELATIVE GAIN

Azim uth Degrees from M ain Lobe

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STANDARD ANTENNA TERMS

AND RELATED FORMULAS


determined in the far-field region and isrepresented as a function of directional

coordinates. (2) Radiation properties include powerflux density, field strength, phase, and polarization.

* RADIATION RESISTANCE OF ANELECTRICALLY SMALL LOOP ANTENNA. Theresistive component of an antenna's inputimpedance that results from the coupling of theantenna to its environment. This resistancedissipates the power that is actually radiated fromthe antenna.

Rr

= 20 (2 ) n A ohms4 2 2

/

n = number of turnsA = area of the loop

REALIZED GAIN. The power gain of an antenna inits environment, reduced by the losses due to themismatch of the antenna input impedance to aspecified impedance.

* REALIZED RADIATOR EFFICIENCY. Theefficiency of an antenna in its environment reducedby all losses suffered by it, including: ohmic losses,mismatch losses, feedline transmission losses, andradome losses.

RELATIVE POWER GAIN. The ratio of the powergain in a given direction to the power gain of areference antenna in its reference direction.Note: Common reference antennas are half-wavedipoles, electric dipoles, magnetic dipoles,monopoles, and calibrated horn antennas.

RETURN LOSS. The reflection coefficient of amismatch expressed in decibels. Note: Modernswept VSWR techniques actually sense thereflected component which is normalized to theforward component to yield return loss. A 2:1VSWR is equivalent to 9.5 dB return loss.

VSWR. The voltage standing wave ratio of acomponent such as an antenna. It is referred tothe characteristic impedance of the transmissionline being used. Note: The most commoncharacteristic impedance is 50 ohms, but 75 and300 ohms are frequently used in coaxial or twinlines for VHF, UHF applications.

NOTES:

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015 Standard