3 basic antenas1 - [Download PPT Powerpoint]

1

Key Points1. Principals of EM Radiation2. Introduction to Propagation & Antennas3. Antenna Characterization

2

1. Principals of Radiated electromagentic (EM) fields

two laws (from Maxwell Equation)

1. A Moving Electric Field Creates a Magnetic (H) field

2. A Moving Magnetic Field Creates an Electric (E) field

3

c 3 108m/sl = /2: wave will complete one cycle from A to B and back to A = distance a wave travels during 1 cycle f = c/ = c/2l

l = /2A B

Assume i(t) applied at A with length l = /2

• EM wave will travel along the wire until it reaches the B

• B is a point of high impedence wave reflects toward A and is reflected back again

• resistance gradually dissipates the energy of the wave• wave is reinforced at A

results in continuous oscillations of energy along the wire and a high voltage at the A end of the wire.

An AC current i(t), flowing in a wire produces an EM field

4

Dipole antenna: 2 wires each with length l = /4• attach ends to terminals of a high frequency AC generator• at time t, the generator’s right side = ‘+’ and the left side = ‘−’

• electrons flow away from the ‘−’ terminal and towards the ‘+’ terminal• most current flows in the center and none flows at the ends• i(t) at any point will vary directly with v(t)

current distribution at time t

− +

i(t) l = /4A B − +

+++++++++++

+++++++++++++++++++++++++++++++++++++++++

----------------

------------------------------------

------------------------

voltage distribution at time t

A B

¼ cycle after electrons have begun to flow max number of electrons will be at A and min number at B

vmax(t) is developed

i(t) = 0

5

EM patterns on Dipole Antenna:

• sinusoidal distribution of charge exists on the antenna that reverses polarity every ½ cycle

• sinusoidal variation in charge magnitude lags the sinusoidal variation in current by ¼ cycle. • Electic field E and magnetic field H 90 out of phase with each other• fields add and produce a single EM field• total energy in the radiated wave is constant, except for some absorption • as the wave advances, the energy density decreases

Standing Wave

• center of the antenna is at a low impedance: v(t) 0, imax(t)

• ends of antenna are at high impedence: i(t) 0, vmax(t)• maximum movement of electrons is in the center of the antenna at all times

Resonance condition in the antenna• waves travel back and forth reinforcin• maximum EM waves are transmitted into at maximum radiation

6

POLARIZATION

• EM field is composed of electric & magnetic lines of force that are orthogonal to each other

• E determines the direction of polarization of the wave

vertical polarization: electric force lines lie in a vertical direction

horizontal polarization : electric force lines lie in a horizontal direction

circular polarization: electric force lines rotate 360 every cycle An antenna extracts maximumenergy from a passing EM wave when it is oriented in the same direction as E

• use vertical antenna for the efficient reception of vertically polarized waves• use horizontal antenna for the reception of horizontally polarized waves

if E rotates as the wave travels through space wave has. horizontal and vertical components

7

Ground wave transmissions missions at lower frequencies use vertical polarization

• horizontal polarization E force lines are parallel to and touch the earth. earth acts as a fairly good conductor at low frequencies shorts out• vertical electric lines of force are bothered very little by the earth.

8

Types of antennas

• simple antennas: dipole, long wire

• complex antennas: additional components to shape radiated fieldprovide high gain for long distances or weak signal receptionsize frequency of operation

• combinations of identical antennas phased arrays electrically shape and steer antenna

2. Introduction to Antennas and Propagation

transmit antenna: radiate maximum energy into surroundings

receive antenna: capture maximum energy from surrounding• radiating transmission line is technically an antenna • good transmission line = poor antenna

9

Major Difference Between Antennas And Transmission Lines

• transmission line uses conductor to carry voltage & current

• radio signal travels through air (insulator)

• antennas are transducers

- convert voltage & current into electric & magnetic field

- bridges transmission line & air

- similar to speaker/microphone with acoustic energy

Transmission Line• voltage & current variations produce EM field around conductor• EM field expands & contracts at same frequency as variations• EM field contractions return energy to the source (conductor) • Nearly all the energy in the transmission line remains in the system

10

Antenna • Designed to Prevent most of the Energy from returning to Conductor

• Specific Dimensions & EM wavelengths cause field to radiate several before the Cycle Reversal

- Cycle Reversal - Field Collapses Energy returns to Conductor

- Produces 3-Dimensional EM field

- Electric Field Magnetic Field

- Wave Energy Propagation Electric Field & Magnetic Field

11

transmit & receive antennas

theoretically are the same (e.g. radiation fields, antenna gain)

practical implementation issue:

transmit antenna handles high power signal (W-MW)

- large conductors & high power connectors,

receive antenna handles low power signal (mW-uW)

Antenna Performance depends heavily on • Channel Characteristics: obstacles, distances temperature,…• Signal Frequency• Antenna Dimensions

12

Propagation Modes – five types

(1) Ground or Surface wave: follow earths contour• affected by natural and man-made terrain• salt water forms low loss path • several hundred mile range• 2-3 MHz signal

(2) Space Wave• Line of Sight (LOS) wave • Ground Diffraction allows for greater distance• Approximate Maximum Distance, D in miles is

(antenna height in ft)

• No Strict Signal Frequency Limitations

rxtx hh 22 D =

hrxhtx

13

(3) Sky Waves

ionospheretransmitted

wavereflected

wave

refracted wave

skip distance

• reflected off ionosphere (20-250 miles high)• large ranges possible with single hop or multi-hop• transmit angle affects distance, coverage, refracted energy

14

Ionosphere

• is a layer of partially ionized gasses below troposphere

- ionization caused by ultra-violet radiation from the sun

- affected by: available sunlight, season, weather, terrain

- free ions & electrons reflect radiated energy

• consists of several ionized layers with varying ion density- each layer has a central region of dense ionization

Layer altitude (miles)

Frequency

Range

Availability

D 20-25 several MHz day onlyE 55-90 20MHz day, partially

at nightF1 90-140 30MHz 24 hoursF2 200-250 30MHz 24 hours

F1 & F2 separate during daylight, merge at night

15

Usable Frequency and Angles

Critical Frequency: frequency that won’t reflect vertical transmission

- critical frequency is relative to each layer of ionosphere

- as frequency increases eventually signal will not reflect

Maximum Usable Frequency (MUF): highest frequency useful for reflected transmissions

- absorption by ionosphere decreases at higher frequencies

- absorption of signal energy = signal loss

- best results when MUF is used

Frequency Trade-Off • high frequency signals eventually will not reflect back to ground• lower frequency signals are attenuated more in the ionosphere

16

angle of radiation: transmitted energy relative to surface tangent

- smaller angle requires less ionospheric refraction to return to earth - too large an angle results in no reflection

- 3o-60o are common angles

critical angle: maximum angle of radiation that will reflect energy to earth

Determination of minimum skip distance: - critical angle - small critical angle long skip distance- height of ionosphere - higher layers give longer skip distances for a fixed angle

multipath: signal takes different paths to the destination

angle of radiation

ionosphere

Critical Angle

17

(4) Satellite Waves

Designed to pass through ionosphere into space• uplink (ground to space) • down link (space to ground)• LOS link

Frequencies >> critical frequency • penetrates ionosphere without reflection• high frequencies provide bandwidth

Geosynchronous orbit 23k miles (synchronized with earth’s orbit)• long distances result in high path loss• EM energy disperses over distances• intensely focused beam improves efficiency

18

total loss = Gt + Gr – path loss (dB)

Free Space Path Loss equation used to determine signal levels over distance

G = antenna gain: projection of energy in specific direction• can magnify transmit power• increase effective signal level at receiver

24

c

fd

P

r

t

c

fd4log20 10 (dB)

19

(5) radar: requires

• high gain antenna

• sensitive low noise receiver

• requires reflected signal from object – distances are doubled

• only small fraction of transmitted signal reflects back

20

3. Antenna Characterization

antennas generate EM field pattern

• not always possible to model mathematically

• difficult to account for obstacles

• antennas are studied in EM isolated rooms to extract key performance characteristics

absolute value of signal intensity varies for given antenna design

- at the transmitter this is related to power applied at transmitter

- at the receiver this is related to power in surrounding space

antenna design & relative signal intensity determines relative fieldpattern

21

forward gain = 10dBbackward gain = 7dB

+10dB+7dB

+ 4dB

0o

270o

180o

90o

Polar Plot of relative signal strength of radiated field• shows how field strength is shaped• generally 0o aligned with major physical axis of antenna• most plots are relative scale (dB)

- maximum signal strength location is 0 dB reference- closer to center represents weaker signals

22

radiated field shaping lens & visible light

• application determines required direction & focus of signal

• antenna characteristics

(i) radiation field pattern

(ii) gain

(iii) lobes, beamwidth, nulls

(iv) directivity

far-field measurements measured many wavelengths away fromantenna

near-field measurement involves complex interactions of decaying electrical and magnetic fields - many details of antenna construction

(i) antenna field pattern = general shape of signal intensity in far-field

23

Measuring Antenna Field Pattern

field strength meter used to measure field pattern• indicates amplitude of received signal• calibrated to receiving antenna• relationship between meter and receive antenna known

measured strength in uV/meter

received power is in uW/meter

• directly indicates EM field strength

24

0o

270o

180o

90o

Determination of overall Antenna Field Pattern

form Radiation Polar Plot Pattern• use nominal field strength value (e.g. 100uV/m) • measure points for 360o around antenna • record distance & angle from antenna• connect points of equal field strength

100 uV/m

practically • distance between meter & antenna kept constant• antenna is rotated • plot of field strength versus angle is made

25

Why Shape the Antenna Field Pattern ?

• transmit antennas: produce higher effective power in direction of intended receiver

• receive antennas: concentrate energy collecting ability in direction of transmitter

- reduced noise levels - receiver only picks up intended signal

• avoid unwanted receivers (multiple access interference = MAI): - security- multi-access systems

• locate target direction & distance – e.g. radar

not always necessary to shape field pattern, standard broadcast is

often omnidirectional - 360o

26

Gain is Measured Specific to a Reference Antenna

• isotropic antenna often used - gain over isotropic

- isotropic antenna – radiates power ideally in all directions

- gain measured in dBi- test antenna’s field strength relative to reference isotropic antenna

- at same power, distance, and angle

- isotropic antenna cannot be practically realized

• ½ wave dipole often used as reference antenna- easy to build- simple field pattern

(ii) Antenna Gain

27

Antenna Gain Amplifier Gain• antenna power output = power input – transmission line loss• antenna shapes radiated field pattern • power measured at a point is greater/less than that using reference antenna

• total power output doesn’t increase

• power output in given direction increases/decreases relative to reference antenna

e.g. a lamp is similar to an isotropic antenna

a lens is similar to a directional antenna

- provides a gain/loss of visible light in a specific direction- doesn’t change actual power radiated by lamp

28

Rotational Antennas can vary direction of antenna gain

Directional Antennas focus antenna gain in primary direction

• transmit antenna with 6dB gain in specific direction over isotropic antenna 4 transmit power in that direction

• receive antenna with 3dB gain is some direction receives 2 as much power than reference antenna

Antenna Gain

often a cost effective means to(i) increase effective transmit power(ii) effectively improve receiver sensitivity

may be only technically viable means• more power may not be available (batteries)• front end noise determines maximum receiver sensitivity

29

(iii) Beamwidth, Lobes & Nulls

Lobe: area of high signal strength- main lobe - secondary lobes

Nulls: area of very low signal strength

Beamwidth: total angle where relative signal power is 3dB below peak value of main lobe

- can range from 1o to 360o

Beamwidth & Lobes indicate sharpness of pattern focus

0o

270o

180o

90o

beamwidth

null

30

Center Frequency = optimum operating frequency

Antenna Bandwidth -3dB points of antenna performance

Bandwidth Ratio: Bandwidth/Center Frequency

e.g. fc = 100MHz with 10MHz bandwidth

- radiated power at 95MHz & 105MHz = ½ radiated power at fc

- bandwidth ratio = 10/100 = 10%

31

Main Trade-offs for Antenna Design• directivity & beam width• acceptable lobes• maximum gain• bandwidth• radiation angle

Bandwidth Issues

High Bandwidth Antennas tend to have less gain than narrowband antennas

Narrowband Receive Antenna reduces interference from adjacent signals & reduce received noise power

Antenna Design Basics

Antenna Dimensions• operating frequencies determine physical size of antenna elements• design often uses as a variable (e.g. 1.5 length, 0.25 spacing)

32

Testing & Adjusting Transmitter use antenna’s electrical load

• Testing required for- proper modulation- amplifier operation- frequency accuracy

• using actual antenna may cause significant interference

• dummy antenna used for transmitter design (not antenna design)

- same impedance & electrical characteristics

- dissipates energy vs radiate energy

- isolates antenna from problem of testing transmitter

33

Testing Receiver

• test & adjust receiver and transmission line without antenna• use single known signal from RF generator• follow on test with several signals present• verify receiver operation first then connect antenna to

verify antenna operation

Polarization

• EM field has specific orientation of E-field & M field

• Polarization Direction determined by antenna & physical orientation

• Classification of E-field polarization

- horizontal polarization : E-field parallel to horizon

- vertical polarization: E-field vertical to horizon

- circular polarization: constantly rotating

34

Transmit & Receive Antenna must have same Polarization for maximum signal energy induction

• if polarizations aren’t same E-field of radiated signal will try to induce E-field into wire to correct orientation

- theoretically no induced voltage

- practically – small amount of induced voltage

Circular Polarization

• compatible with any polarization field from horizontal to vertical

• maximum gain is 3dB less than correctly oriented horizontal or vertically polarized antenna

35

Antenna Fundamentals

Dipole Antennas (Hertz): simple, old, widely used- root of many advance antennas

• consists of 2 spread conductors of 2 wire transmission lines• each conductor is ¼ in length• total span = ½ + small center gap

Distinct voltage & current patternsdriven by transmission line at midpoint

• i = 0 at end, maximum at midpoint

• v = 0 at midpoint, vmax at ends

• purely resistive impedance = 73• easily matched to many transmission lines

gap

¼ ¼ ½

Transmission Line

+v

-v

i

High Impedance 2k-3k

Low Impedance 73

36

E-field (E) & M-field (B) used to determine radiation pattern

• E goes through antenna ends & spreads out in increasing loops• B is a series of concentric circles centered at midpoint gap

E B

37

Azimuth Pattern

Elevation Pattern

Polar Radiation Pattern

3-dimensional field pattern is donut shaped antenna is shaft through donut center radiation pattern determined by taking slice of donut

- if antenna is horizontal slice reveals figure 8- maximum radiation is broadside to antenna’s arms

38

½ dipole performance – isotropic reference antenna • in free space beamwidth = 78o

• maximum gain = 2.1dB• dipole often used as reference antenna

- feed same signal power through ½ dipole & test antenna- compare field strength in all directions

Actual Construction

(i) propagation velocity in wire < propagation velocity in air

(ii) fields have ‘fringe effects’ at end of antenna arms- affected by capacitance of antenna elements

1st estimate: make real length 5% less than ideal - otherwise introduce reactive parameter

Useful Bandwidth: 5%-15% of fc

• major factor for determining bandwidth is diameter of conductor• smaller diameter narrow bandwidth

39

Multi-Band Dipole Antennas

Transmission Line

1/4C

L

C

L

1/4

2/42/4

use 1 antenna support several widely separated frequency bandse.g. HAM Radio - 3.75MHz-29MHz

Traps: L,C elements inserted into dipole arms• arms appear to have different lengths at different frequencies• traps must be suitable for outdoor use• 2ndry affects of trap impact effective dipole arm length-adjustable• not useful over 30MHz

40

Transmit Receive Switches• allows use of single antenna for transmit & receive• alternately connects antenna to transmitter & receiver• high transmit power must be isolated from high gain receiver• isolation measured in dB

e.g. 100dB isolation 10W transmit signal 10nW receive signal

41

Elementary Antennas

low cost – flexible solutions

Long Wire Antenna

• effective wideband antenna • length l = several wavelengths

- used for signals with 0.1l < < 0.5l

- frequency span = 5:1

• drawback for band limited systems - unavoidable interference

• near end driven by ungrounded transmitter output• far end terminated by resistor

- typically several hundred - impedance matched to antenna Z0

• transmitter electrical circuit ground connected to earth

Antenna

Transmission Line

earth ground

R=Z0

42

practically - long wire is a lossy transmission line

- terminating resistor prevent standing waves

Polar radiation pattern • 2 main lobes

- on either side of antenna

- pointed towards antenna termination

• smaller lobes on each side of antenna – pointing forward & back

• radiation angle 45o (depending on height) useful for sky waves

angular radiation pattern

horizon

feed

polar ration pattern

43

poor efficiency: transmit power

- 50% of transmit power radiated- 50% dissapated in termination resistor

receive power- 50% captured EM energy converted to signal for reciever- 50% absorbed by terminating resistor

44

Folded Dipole Antenna

- basic ½ dipole folded to form complete circuit

- core to many advanced antennas

- mechanically more rugged than dipole

- 10% more bandwidth than dipole

- input impedance 292

- close match to std 300 twin lead wire transmission line

- use of different diameter upper & lower arms allows variable impedance

/2

45

Loop & Patch Antenna – wire bent into loops

Patch Antenna: rectangular conducting area with || ground plane

Area A

N-turns

V = maximum voltage induced in receiver by EM fieldB = magnetic field strength flux of EM fieldA = area of loopN = number of turnsf = signal frequencyk = physical proportionality factor

V = k(2f)BAN

Antenna Plane