Basic Antenna Theory and Concepts

IntroductionAn antenna is an electrical conductor or system of conductorsTransmission - radiates electromagnetic energy into spaceReception - collects electromagnetic energy from spaceIn two-way communication, the same antenna can be used for transmission and reception

Antenna DefinitionAn 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

Antenna Definition-contdIn transmit systems the RF signal is generated, amplified, modulated and applied to the antennaIn receive systems the antenna collects electromagnetic waves that are cutting through the antenna and induce alternating currents that are used by the receiver

ReciprocityAn 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 antennaA vertical antenna will transmit a vertically polarized waveThe receive and transmit antennas need to possess the same polarization

Types of AntennasIsotropic antenna (idealized)Radiates power equally in all directionsDipole antennasHalf-wave dipole antenna (or Hertz antenna)Quarter-wave vertical antenna (or Marconi antenna)Parabolic Reflective Antenna

beamwidthantennaAPower 3dB down from maximum point AMax power2 dipoleDirectional AntennaRadiated energy is focused in a specific direction

BeamwidthBeamwidth is the angular separation of the half-power points of the radiated pattern

Half-wave Dipole (Hertz) AntennaAn antenna having a physical length that is one-half wavelength of the applied frequency is called a Hertz antenna or a half-wave dipole antenna. Hertz antennas are not found at frequencies below 2MHz because of the physical size needed of the antenna to represent a half-wave

Vertical (Marconi) AntennaVertical Antennas are used for frequencies under 2 MHz. It uses a conducting path to ground that acts as wavelength portion the antenna above the ground. The above ground structure represents a /4 wavelength

Vertical (Marconi) Antenna contdPoor grounding conditions of the earth/soil surrounding the antenna can result in serious signal attenuation. This problem is alleviated by installing a counterpoise

CounterpoiseCounterpoise is a grounding grid established where the earth grounding cannot satisfy electrical requirements for circuit completion. It is designed to be non-resonant at the operating frequency

Counterpoise-contdsupportsantennaradius =

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

Yagi-Uda Antenna-contdreflector/2dipole antenna/4

antenna2 dipole radiated signal without reflector2 dipole radiated signal with reflectorRadiated Directed Signal

The Antenna Formula c 186,000 misecc is the speed of light is the wavelength of the signal use 3 x 108 when dealing in meters for the speed of lightfrequency of the signal

The Antenna Formula - appliedIf a half-wave dipole antenna needed to be constructed for a 60 Hz signal, how large would it need to be? c 186,000 misec60= 3100 mi2 = 1550 miles!

Radiation & Induction FieldsThe 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 & Induction Fields-contdThere are two induction fields or areas where signals collapse and radiate from the antenna. They are known as the near field and far field. The distance that antenna inductance has on the transmitted signal is directly proportional to antenna height and the dimensions of the waveR 2D2

Radiation & Induction Fields-contdR 2D2Where: R = the distance from the antennaD = dimension of the antenna = wavelength of the transmitted signal

Radiation ResistanceRadiation Resistance is the portion of the antennas 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 increases

Effective Radiated Power (ERP)ERP is the power input value and the gain of the antenna multiplied togetherdBi = isotropic radiator gaindBd = dipole antenna gain

Radiation PatternRadiation pattern is an indication of radiated field strength around the antenna. Power radiated from a /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 patternGraphical representation of radiation properties of an antennaDepicted as two-dimensional cross sectionBeam width (or half-power beam width) Measure of directivity of antennaReception patternReceiving antennas equivalent to radiation pattern

Radiation Pattern for Vertical Antennasantenna/4/2

Antenna GainAntenna gainPower 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

Antenna GainAntenna gain is the measure in dB how much more power an antenna will radiate in a certain direction with respect to that which would be radiated by a reference antenna

Antenna GainRelationship between antenna gain and effective area

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

Propagation ModesGround-wave propagationSky-wave propagationLine-of-sight propagation

Ground Wave Propagation

Ground Wave PropagationFollows contour of the earthCan Propagate considerable distancesFrequencies up to 2 MHzExampleAM 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 earths surfaceReflection effect caused by refractionExamplesAmateur radioCB radio

Line-of-Sight Propagation

Line-of-Sight PropagationTransmitting and receiving antennas must be within line of sightSatellite communication signal above 30 MHz not reflected by ionosphereGround communication antennas within effective line of site due to refractionRefraction bending of microwaves by the atmosphereVelocity of electromagnetic wave is a function of the density of the mediumWhen wave changes medium, speed changesWave bends at the boundary between mediums

Line-of-Sight EquationsOptical line of sight

Effective, or radio, line of sight

d = distance between antenna and horizon (km)h = antenna height (m)K = adjustment factor to account for refraction, rule of thumb K = 4/3

Line-of-Sight EquationsMaximum distance between two antennas for LOS propagation:

h1 = height of antenna oneh2 = height of antenna two

LOS Wireless Transmission ImpairmentsAttenuation and attenuation distortionFree space lossNoiseAtmospheric absorptionMultipathRefractionThermal noise

Thermal NoiseThermal noise due to agitation of electronsPresent in all electronic devices and transmission mediaCannot be eliminatedFunction of temperatureParticularly significant for satellite communication

Noise TerminologyIntermodulation noise occurs if signals with different frequencies share the same mediumInterference caused by a signal produced at a frequency that is the sum or difference of original frequenciesCrosstalk unwanted coupling between signal pathsImpulse noise irregular pulses or noise spikesShort duration and of relatively high amplitudeCaused by external electromagnetic disturbances, or faults and flaws in the communications system

Other ImpairmentsAtmospheric absorption water vapor and oxygen contribute to attenuationMultipath obstacles reflect signals so that multiple copies with varying delays are receivedRefraction bending of radio waves as they propagate through the atmosphere

Multipath Propagation

Multipath PropagationReflection - occurs when signal encounters a surface that is large relative to the wavelength of the signalDiffraction - occurs at the edge of an impenetrable body that is large compared to wavelength of radio waveScattering occurs when incoming signal hits an object whose size in the order of the wavelength of the signal or less

The Effects of Multipath PropagationMultiple copies of a signal may arrive at different phasesIf phases add destructively, the signal level relative to noise declines, making detection more difficultIntersymbol interference (ISI)One or more delayed copies of a pulse may arrive at the same time as the primary pulse for a subsequent bit

Types of FadingFast fadingSlow fadingFlat fadingSelective fadingRayleigh fadingRician fading

Error Compensation MechanismsForward error correctionAdaptive equalizationDiversity techniques

Forward Error CorrectionTransmitter adds error-correcting code to data blockCode is a function of the data bitsReceiver calculates error-correcting code from incoming data bitsIf calculated code matches incoming code, no error occurredIf error-correcting codes dont match, receiver attempts to determine bits in error and correct

Adaptive EqualizationCan be applied to transmissions that carry analog or digital informationAnalog voice or videoDigital data, digitized voice or videoUsed to combat intersymbol interferenceInvolves gathering dispersed symbol energy back into its original time intervalTechniquesLumped analog circuitsSophisticated digital signal processing algorithms

Antenna HeightAntenna height above the ground is directly related to radiation resistance. Ground reflections causing out-of-phase signals to be radiated to receiving antennas will degrade the transmission. Physical length and electrical length of most antennas are approximately 95% of the physical length. Ideal antenna height is usually based on trial and error procedures

Smart Antennas

Smart Antennassmart antennas are base station antennas with a pattern that is not fixed, but adapts to the current radio conditionssmart antennas have the possibility for a large increase in capacity: an increase of three times for TDMA systems and five times for CDMA systems has been reported.

Smart Antennas-contdMajor drawbacks and cost factors include increased transceiver complexity and more complex radio resource management

Smart Antennas-contdThe idea of smart antennas is to use base station antenna patterns that are not fixed, but adapt to the current radio conditions. This can be visualized as the antenna directing a beam toward the communication partner only

Smart Antennas-contdSmart antennas add a new way of separating users, namely by space, through SDMA (space division multiple access)By maximizing the antenna gain in the desired direction and simultaneously placing minimal radiation pattern in the directions of the interferers, the quality of the communication link can be significantly improved

Elements of a Smart AntennaSmart antennas consists of a number of radiating elements, a combining/dividing network and a control unit

Phased Array AntennaPhased Array antennas are a combination of antennas in which there is a control of the phase and power of the signal applied at each antenna resulting in a wide variety of possible radiation patterns

Types of Intelligent AntennasSwitched lobe (SL): This is also called switched beam. It is the simplest technique, and comprises only a basic switching function between separate directive antennas or predefined beams of an array. The setting that gives the best performance, usually in terms of received power, is chosen

Intelligent Antennas-contdDynamically phased array (PA): By including a direction of arrival (DoA) algorithm for the signal received from the user, continuous tracking can be achieved and it can be viewed as a generalization of the switched lobe concept

Intelligent Antennas-contdAdaptive array (AA): In this case, a DoA algorithm for determining the direction toward interference sources (e.g., other users) is added. The radiation pattern can then be adjusted to null out the interferers. In addition, by using special algorithms and space diversity techniques, the radiation pattern can be adapted to receive multipath signals which can be combined. These techniques will maximize the signal to interference ratio (SIR)

SMDASpace Division Multiple Access (SDMA) implies that more than one user can be allocated to the same physical communications channel simultaneously in the same cell, only separated by angle. In a TDMA system, two users will be allocated to the same time slot and carrier frequency at the same time and in the same cell

SMDA-contdIn systems providing full SDMA, there will be much more intracell handovers than in conventional TDMA or CDMA systems, and more monitoring by the network is necessary

Antenna Installation ConsiderationsSafetystandard operating procedure priorityGroundinglightning strikesstatic chargesSurge protectionlightning searches for a second path to ground

Antenna Installation Considerations-contdAdaptive array antenna placement needs to be considered differently than current technologies serving the mobile environment. They need to be place so they have a greater angular approach to the receiving units. Existing tower placement with close proximity to roads and highways would need to be reconsidered.

Antenna Installation ConsiderationsBase, mast, and supporting structure needs clearance, serviceability (access), and complies with state, federal, and municipal guidelines