Basic Antenna Theory and Concepts Downloaded from www

Basic Antenna Theory and Concepts

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IntroductionAn antenna is an electrical conductor or system of conductors

Transmission - radiates electromagnetic energy into spaceReception - collects electromagnetic energy from space

In two-way communication, the same antenna can be used for transmission and reception

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Antenna Definition

An antenna is a circuit element that provides a transition form a guided wave on a transmission line to a free space wave and it provides for the collection of electromagnetic energy.

Antenna research from Miller & Beasley, 2002




Reciprocity

An antenna ability to transfer energy form the atmosphere to its receiver with the same efficiency with which it transfers energy from the transmitter into the atmosphereAntenna characteristics are essentially the same regardless of whether an antenna is sending or receiving electromagnetic energy




PolarizationPolarization is the direction of the electric field and is the same as the physical attitude of the antenna

A vertical antenna will transmit a vertically polarized wave

The receive and transmit antennas need to possess the same polarization




Types of AntennasIsotropic antenna (idealized)

Radiates power equally in all directionsDipole antennas

Half-wave dipole antenna (or Hertz antenna)Quarter-wave vertical antenna (or Marconi antenna)

Parabolic Reflective Antenna




Antenna Array

Antenna array is a group of antennas or antenna elements arranged to provide the desired directional characteristics. Generally any combination of elements can form an array. However, equal elements in a regular geometry are usually used.




Yagi-Uda Antenna

The Yagi-Uda antenna is a simple form of a directional antenna based off of a reflector placed λ/4 from the dipole antenna’s placement. Complex analysis to define the radiated patterns are experimental rather than theoretical calculations




Yagi-Uda Antenna-cont’d

reflectorλ/2

dipole antenna

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The Antenna Formula

λ = c⁄ƒ

= 186,000 mi⁄sec

•c is the speed of lightλ is the wavelength of the signalλ use 3 x 108 when dealing in meters for the speed of light

frequency of the signalwww.yoursww8.tk ©Admin:N.Rajeev



The Antenna Formula - applied

If a half-wave dipole antenna needed to be constructed for a 60 Hz signal, how large would it need to be?

λ = c⁄ƒ

= 186,000 mi⁄sec

60= 3100 mi

λ⁄2 = 1550 miles!




Radiation & Induction Fields

The mechanics launching radio frequencies from an antenna are not full understood. The RF fields that are created around the antenna have specific properties that affect the signals transmission. The radiated field field is known as the (surprisingly!) radiation field




Radiation Resistance

Radiation Resistance is the portion of the antenna’s impedance that results in power radiated into space (i.e., the effective resistance that is related to the power radiated by the antenna. Radiation resistance varies with antenna length. Resistance increases as the λincreases




Effective Radiated Power (ERP)

ERP is the power input value and the gain of the antenna multiplied together

dBi = isotropic radiator gaindBd = dipole antenna gain

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Radiation PatternRadiation pattern is an indication of radiated field strength around the antenna. Power radiated from a λ/2 dipole occurs at right angles to the antenna with no power emitting from the ends of the antenna. Optimum signal strength occurs at right angles or 180° from opposite the antenna




Radiation PatternsRadiation pattern

Graphical representation of radiation properties of an antennaDepicted as two-dimensional cross section

Beam width (or half-power beam width)Measure of directivity of antenna

Reception patternReceiving antenna’s equivalent to radiation pattern




Antenna GainAntenna gain

Power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna)

Effective areaRelated to physical size and shape of antenna

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Antenna GainRelationship between antenna gain and effective area

G = antenna gainAe = effective areaf = carrier frequencyc = speed of light (» 3 ´ 108 m/s)λ = carrier wavelength

2

2

2

44c

AfAG ee πλπ

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Ground Wave Propagation




Ground Wave PropagationFollows contour of the earthCan Propagate considerable distancesFrequencies up to 2 MHzExample

AM radio




Sky Wave Propagation




Sky Wave PropagationSignal reflected from ionized layer of atmosphere back down to earthSignal can travel a number of hops, back and forth between ionosphere and earth’s surfaceReflection effect caused by refractionExamples

Amateur radioCB radio




Line-of-Sight Propagation




LOS Wireless Transmission Impairments

Attenuation and attenuation distortionFree space lossNoiseAtmospheric absorptionMultipathRefractionThermal noise




Types of FadingFast fadingSlow fadingFlat fadingSelective fadingRayleigh fadingRician fading




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