ANTENNA GLOSSARY
A metallic device used in the transmission and reception of electromagnetic waves . An antenna is a passive or an active device which permits transmission.
The ratio of the antenna’s maximum radiation intensity in a stated direction the maximum radiation intensity of a reference antenna ( dipole isotropic antenna) with identical of the object.
The loss in power of electromagnetic signals between transmission and reception points .
Horizontal direction expressed as the angular distance between the direction of a fixed point (as the observer’s heading ) and the direction of the object.
A range of consecutive frequencies comprised of a band( i.e. the US cellular bandwidth is 72 Mhz wide between the frequencies of 824 Mhz-894 Mhz ) over which an antenna shall perform without the need of any adjustment.
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Gain is the practical value of the directivity of an antenna .
One billion cycles per second.
Global Positional Satellite or Global positioning systems.
A man made system of conductors placed below an antenna to serve as an earth ground
A unit of frequency equal to one cycle per second.
TECHNICAL LIBRARY
An antenna is a device to transmit and /or receive electromagnetic waves .
Electromagnetic waves are often referred to as radio waves . Most antennas are resonant devies, which operate efficiently over a relatively narrow frequency band .An antenna must be tuned (matched ) to the same frequency band as the radio system to which it is connected otherwise reception to which it is connected otherwise reception and/or transmission will be impaired .
We often to antenna size relative to wavelength. For example: a ½ wave dipole is approximately half a Wavelength long .Wavelength is the distance a radio wave travels during one cycle. The formula for wavelength is ^=c/f where ^ is the wavelength expressed in units of length , typically meters , feet or inches.
** Note:
The physical length of a half –wave dipole is slightly less a half –wavelength due to end effect. The speed of propagation in coaxial cable is shorter. The velocity of propagation of electromagnetic waves in coax is usually given as a percentage of free space velocity , and is different for different types of coax.
For efficient transfer of energy ,the impedance of the radio, the antenna and the transmission line connecting the radio to the antenna must be the same. Radios typically are designed for the 50 Ohms impedance, and the coaxial cables (transmission line ) used with them also have a 50 Ohms impedance . efficient antenna configurations often have an impedance matching circuit is then required to transform the antenna impedance to 50 Ohms. Antennas manufacture by antenna experts come with the necessary impedance matching circuitry as part of the antenna . we use low loss-components in our matching circuits to provide the maximum transfer of energy between the transmission line and the antenna.
Voltage standing wave ratio (VSWR) is an indication of the quality of the impedance match.VSWR is often abdreviated as SWR. A high VSWR is an indication the signal is reflected priod to being radiated by the antenna . VSWR and reflected power are different ways of measuring and expressing the same thing.
A VSWR of 2.0:1 or less is often considered acceptable. Most commercial antenna, however, are specified to be 1.5:1 or less over some bandwidth.
Based on a 100watt radio .a 1.5:1 VSWR equates to a forward power of 96 watts and a reflected power of 4 watts , or the reflected power is 4.2% of the forward power.
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Bandwidth can be defined in terms of radiation patterns or VSWR/reflected power.The definition used is based on VSWR. Bandwidth often expressed in terms of percent bandwidth is constant relative to frequency . If bandwidth is expressed in absolute units of frequency, for example MHz.
Decibels (dB) are the accepted method of describing a gain or loss relationship in a communication system. The beauty of dB is they may be added and subtracted .A decibel relationshop Ifor power ) is calculated using the following formula db+ 101og (Pa/Pb).
Pa might be the power applied to the connector on an antenna , the input terminal of an amplifier or one end of a transmission line .Pb might be the power arriving at the opposite end of the transmission line ,the amplifier output or the peak power in the main lobe of radiated energy from an antenna. If Pa is larger than Pb the result will be a positive number or gain. If Pa is smaller than Pb the result will be a negative number or loss.
Directivity is the ability of an antenna to focus energy in a particular direction when transmitting or to receive energy better from a particular direction when received .There is a relationship between gain and directivity . we see the phenomena of increased directivity when comparing a light bulb to a spotlight . A 100 watt spotlight will provide more light in a particular direction than a 100 watt light bulb and less light in other directices .we could say the spotlight has more “directivity “ than the light bulb. The spotlight is comparable to an antenna wiuth increased directivity
Gain is the practical value of the directivity . the relation between gain and directivity includes a new parameter n which describes the efficiency of the antenna as G =n*D. For example , an antenna with 3dB of diectivity and 50% of effcuency will have a gain of 0 dB.
Polarization is defined as the orientation of the electric field of an electromagnetic wave . Two often used special cases of elliptical polarization are antenna launching the waves into space determines initial polarization of a radio wave. The environment through which the radio wave passes on its way from the transmit antenna to the receving antenna may cause a change in polarization.
With linear polarization the electric field vector stays in the same plane. . In circular polarization the electric field vector appears to be rotating with circular motion about the direction of propagation ,making one full turn for each RF cycle. The rotation ,may be right –hand or –left hand.
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