Part 2: Antennas

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

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

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

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

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

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

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

(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

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

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

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

(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

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

P

r

t

c

fd4log20 10 (dB)

(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

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

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

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

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

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

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

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

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

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

(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

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%

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)

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

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

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

Antenna FundamentalsDipole 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

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

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

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

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

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

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

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

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

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

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

• Loop & Patch Antennas are easy to embed in a product (e.g. pager)• Broadband antenna - 500k-1600k Hz bandwidth• Not as efficient as larger antennas

Radiation Pattern• maximum to center axis through loop• very low broadside to the loop• useful for direction finding

- rotate loop until signal null (minimum) observed- transmitter is on either side of loop- intersection with 2nd reading pinpoints transmitter

552.14 dB

                              Dipole

                         

3600 dB

                              Isotropic                          

Beamwidth -3 dB

Gain (over isotropic)

ShapeName Radiation Pattern

                              

20

30

50

200

25

14.7 dB

10.1 dB

-0.86 dB

3.14 dB

7.14 dB

                              

                              

                              

                              

                              

                              

Parabolic Dipole

Helical

Turnstile

Full Wave Loop

Yagi

                         

                         

                         

                         

                         

Biconical Horn

1515 dBHorn                          

                         360x20014 dB

Radiation fundamentals

Recall, that using the Poynting’s theorem, the total power radiated from a source can be found as:

rad

s

P E H ds (10.2.1)

Which suggests that both electric and magnetic energy will be radiated from the region.A stationary charge will NOT radiate EM waves, since a zero current flow will cause no magnetic field.In a case of uniformly moving charge, the static electric field:

2 2

1

4

QE u

x

(10.2.2)

The magnetic field is:

2 2

1

4

QH v u

x

(10.2.3)

Radiation fundamentalsIn this situation, the Poynting vector does not point in the radial direction and represent a flow rate of electrostatic energy – does not contribute to radiation!A charge that is accelerated radiates EM waves. The radiated field is:

0[ ]sin

4t

Q aE

R

Where is the angle between the point of observation and the velocity of the accelerated charge and [a] is the acceleration at the earliest time (retarded acceleration). Assuming that the charge is moving in vacuum, the magnetic field can be found using the wave impedance of the vacuum:

(10.3.1)

[ ]sin

4t

Q aH

cR

And the Poynting vector directed radially outward is:2 2 2

02 2

[ ] sin

16t

Q aS

cR

(10.3.2)

(10.3.3)

Radiation fundamentalsOnly accelerated (or decelerated) charges radiate EM waves. A current with a time-harmonic variation (AC current) satisfies this requirement.

Example 10.1: Assume that an antenna could be described as an ensemble of N oscillating electrons with a frequency in a plane that is orthogonal to the distance R. Find an expression for the electric field E that would be detected at that location.The maximum electric field is when = 900:

0 0

4 4

NQ dv dJE

R dt R dt

Where we introduce the electric current density J = NQv of the oscillating current.Assuming that the direction of oscillation in the orthogonal plane is x, then ( ) sin

( ) cos

m

m

x t x t

dxv t x t

dt

(10.4.1)

(10.4.2)

(10.4.3)

Radiation fundamentals

The current density will become:( ) cosmJ t NQx t

Finally, the transverse electric field is

2 0( , ) sin4

mNQxE R t t

R

(10.5.1)

(10.5.2)

The electric field is proportional to the square of frequency implying that radiation of EM waves is a high-frequency phenomenon.

Infinitesimal electric dipole antenna

We assume the excitation as a time-harmonic signal at the frequency , which results in a time-harmonic radiation.The length of the antenna L is assumed to be much less than the wavelength:L << . Typically: L < /50.The antenna is also assumed as very thin:ra << .The current along the antenna is assumed as uniform:

dQI

dt (10.6.1)

For a time-harmonic excitation: ( ) ( )I r j Q r (10.6.2)

Infinitesimal electric dipole antenna

The vector potential can be computed as: 2

202 2

1 ( , )( , ) ( , )

A r tA r t J r t

c t

With the solution that can be found in the form:

0 ( ', )( , ) '

4 v

J r t R cA r t dv

R

Assuming a time-harmonic current density: ( )', ( ') j t k RJ r t R c J r e

The distance from the center of the dipole R = r and k is the wave number. The volume of the dipole antenna can be approximated as dv’ = Lds’.

(10.7.1)

(10.7.2)

(10.7.3)

Infinitesimal electric dipole antenna

Considering the mentioned assumptions and simplifications, the vector potential becomes:

0( )4

jkr

z

IL eA r u

r

This infinitesimal antenna with the current element IL is also known as a Herzian dipole.

(10.8.1)

Assuming that the distance from the antenna to the observer is much greater than the wavelength (far filed, radiation field, or Fraunhofer field of antenna), i.e. r >> , let us find the components of the field generated by the antenna.Using the spherical coordinates:

cos sinz ru u u

(10.8.2)

Infinitesimal electric dipole antenna

The components of the vector potential are:0

0

( ) cos cos4

( )sin sin4

0

jkr

r z

jkr

z

IL eA A r

r

IL eA A r

rA

The magnetic field intensity can be computed from the vector potential using the definition of the curl in the SCS:

22

0 0

1 1 ( ) 1 1( ) sin

4jkrr

rA A I zH r A u k e u

r r ikr jkr

(10.9.1)

(10.9.2)

(10.9.3)

(10.9.4)

Infinitesimal electric dipole antenna

Which can be rewritten as0

sin4

0

r

jkr

H

jkIL eH

rH

Note: the equations above are approximates derived for the far field assumptions.The electric field can be computed from Maxwell’s equations:

0 0

sin1 1 1 1( ) ( )

sin r

H rHE r H r u u

j j r r r

(10.10.1)

(10.10.2)

(10.10.3)

(10.10.4)

(10.11.1)

Infinitesimal electric dipole antenna

The components of the electric field in the far field region are:

0

0

sin4

0

r

jkr

E

jZ kIL eE

rE

(10.11.2)

(10.11.3)

where0

00

( )377

( )

E rZ

H r

(10.11.4)

is the wave impedance of vacuum.

Infinitesimal electric dipole antennaThe angular distribution of the radiated fields is called the radiation pattern of the antenna.

Both, electric and magnetic fields depend on the angle and have a maximum when = 900 (the direction perpendicular to the dipole axis) and a minimum when = 00.

The blue contours depicted are called lobes. They represent the antenna’s radiation pattern. The lobe in the direction of the maximum is called the main lobe, while any others are called side lobes.A null is a minimum value that occurs between two lobes.For the radiation pattern shown, the main lobes are at 900 and 2700 and nulls at 00 and 1800.Lobes are due to the constructive and destructive interference.

Infinitesimal electric dipole antennaOne of the goals of antenna design is to place lobes at the desired angles.

Every null introduces a 1800 phase shift.

In the far field region (traditionally, the region of greatest interest) both field components are transverse to the direction of propagation. The radiated power:

22* 2

0

0 0

2 22 20 03 2

0 0

2 2 20

1 1Re ( ) ( ) ( ) sin

2 2

sin 1 sin (cos )16 16

12

rad ava

a

vs

av av

v

P E r H r ds Z H r r d d

Z k I L Z k I L

Z k I L

d d

(10.13.1)

We have replaced the constant current by the averaged current accounting for the fact that it may have slow variations in space.

Infinitesimal electric dipole antennaExample 10.2: A small antenna that is 1 cm in length and 1 mm in diameter is designed to transmit a signal at 1 GHz inside the human body in a medical experiment. Assuming the dielectric constant of the body is approximately 80 (a value for distilled water) and that the conductivity can be neglected, find the maximum electric field at the surface of the body that is approximately 20 cm away from the antenna. The maximum current that can be applied to the antenna is 10 A. Also, find the distance from the antenna where the signal will be attenuated by 3 dB.The wavelength within the body is:

8

9

3 103.3

10 80r

ccm

f

The characteristic impedance of the body is:

0 37742

80c

r

ZZ

Infinitesimal electric dipole antenna

Since the dimensions of the antenna are significantly less than the wavelength, we can apply the far field approximation for = 900, therefore:

5 21 10 10 2 142 320

4 4 0.033 0.2c

I LE Z k V m

r

An attenuation of 3 dB means that the power will be reduced by a factor of 2. The power is related to the square of the electric field. Therefore, an attenuation of 3 dB would mean that the electric field will be reduced by a square root of 2. The distance will be

1 2 1.41 0.2 0.28r r m

Finite electric dipole antenna

Finite electric dipole consists of two thin metallic rods of the total length L, which may be of the order of the free space wavelength.

Assume that a sinusoidal signal generator working at the frequency is connected to the antenna. Thus, a current I(z) is induced in the rods.

We assume that the current is zero at the antenna’s ends (z = L/2) and that the current is symmetrical about the center (z = 0).The actual current distribution depends on antenna’s length, shape, material, surrounding,…

Finite electric dipole antenna

A reasonable approximation for the current distribution is

( ) sin 2mI z I k L z (10.17.1)

Far field properties, such as the radiated power, power density, and radiation pattern, are not very sensitive to the choice of the current distribution. However, the near field properties are very sensitive to this choice.

Deriving the expressions for the radiation pattern of this antenna, we represent the finite dipole antenna as a linear combination of infinitesimal electric dipoles. Therefore, for a differential current element I(z)dz, the differential electric field in a far zone is

'0 ( ) sin

4 '

jkrjZ k edE I z dz

r

(10.17.2)

Finite electric dipole antenna

The distance can be expressed as:

2 2' 2 cos cosr r z rz r z (10.18.1)

This approximation is valid since r >> z

Replacing r’ by r in the amplitude term will have a very minor effect on the result. However, the phase term would be changed dramatically by such substitution! Therefore, we may use the approximation r’ r in the amplitude term but not in the phase term.

The EM field radiated from the antenna can be calculated by selecting the appropriate current distribution in the antenna and integrating (11.17.2) over z.

2cos0

0

2

sinsin

4 2

Ljkrikzm

L

Z I k e LE Z H j k z e dz

r

(10.18.2)

Finite electric dipole antenna

Since cos cos cos sin cosjkze kz j kz

and the limits of integration are symmetric about the origin, only a “non-odd” term will yield non-zero result:

(10.19.1)

2

0

0

sin2 sin cos cos

4 2

LjkrmZ I k e L

E j k z kz dzr

The integration results in:

60 ( )jkr

m

eE j I F

r

(10.19.2)

(10.19.3)

Where F() is the radiation pattern:

1 2

cos cos cos2 2

cos cos cos2 2

sin

sinsina

kL kL

F F

L k

F

k L

(10.19.4)

Finite electric dipole antenna

The first term, F1() is the radiation characteristics of one of the elements used to make up the complete antenna – the element factor.The second term, Fa() is the array (or space) factor – the result of adding all the radiation contributions of the various elements that form the antenna array as well as their interactions.

L = /2 L = L = 3/2 L = 2

The E-plane radiation patterns for dipoles of different lengths.

infinitesimal dipole

If the dipole length exceeds wavelength, the location of the maximum shifts.

Loop antenna

A loop antenna consists of a small conductive loop with a current circulating through it.

We have previously discussed that a loop carrying a current can generate a magnetic dipole moment. Thus, we may consider this antenna as equivalent to a magnetic dipole antenna.

If the loops circumference C < /10The antenna is called electrically small. If C is in order of or larger, the antenna is electrically large. Commonly, these antennas are used in a frequency band from about 3 MHz to about 3 GHz. Another application of loop antennas is in magnetic field probes.

Loop antenna

Assuming that the antenna carries a harmonic current:( ) cosi t I t

and that2

1ka a

The retarded vector potential can be found as:'

0( ) '4 '

jkr

L

Iu eA r dl

r

(10.22.1)

(10.22.2)

(10.22.3)

If we rewrite the exponent as:

' ( ' ) 1 ( ' )jkr jkr jk r r jkre e e e jk r r (10.22.4)

where we assumed that the loop is small: i.e. a << r, we arrive at

0 '( ) 1 '

4 'jkr

L L

I dlA r e jkr ik dl u

r

(10.22.5)

Loop antenna

Evaluating the integrals, we arrive at the following expression:

20

20

2

1( ) sin

4sin

4

jkr jkrjkr eI aA r u

I a k euj

r r

Recalling the magnetic dipole moment:

2zm I a u

Therefore, the electric and magnetic fields are found as

0

0

sin4

jkrmkeH

Z r

00 sin

4

jkrmkeE Z H

r

(10.23.1)

(10.23.2)

(10.23.3)

(10.23.4)

We observe that the fields are similar to the fields of short electric dipole. Therefore, the radiation patterns will be the same.

Antenna parameters

In addition to the radiation pattern, other parameters can be used to characterize antennas. Antenna connected to a transmission line can be considered as its load, leading to:

1. Radiation resistance.

We consider the antenna to be a load impedance ZL of a transmission line of length L with the characteristic impedance Zc. To compute the load impedance, we use the Poynting vector…

If we construct a large imaginary sphere of radius r (corresponding to the far region) surrounding the radiating antenna, the power that radiates from the antenna will pass trough the sphere. The sphere’s radius can be approximated as r L2/2.

Antenna parameters

The total radiated power is computed by integrating the time-average Poynting vector over the closed spherical surface:

2

* 2 *

0 0

1 1Re ( ) ( ) Re sin

2 2rad

s

P E r H r ds d r E H d

(10.25.1)

Notice that the factor ½ appears since we are considering power averaged over time. This power can be viewed as a “lost power” from the source’s concern. Therefore, the antenna is “similar” to a resistor connected to the source:

20

2 radrad

PR

I

where I0 is the maximum amplitude of the current at the input of the antenna.

(10.25.2)

Antenna parameters: Example

Example 10.3: Find the radiation resistance of an infinitesimal dipole.

The radiated power from the Hertzian dipole is computed as:

2 2 20

12av

rad

Z k I LP

Using the free space impedance and assuming a uniform current distribution:

22 22 2

0

80 80avrad

IL LR

I

Assuming a triangular current distribution, the radiation resistance will be: 2

220rad

LR

Small values of radiation resistance suggest that this antenna is not very efficient.

(10.26.1)

(10.26.2)

(10.26.3)

Antenna parameters

For the small loop antennas, the antennas radiation resistance, assuming a uniform current distribution, will be:

4220radR ka

For the large loop antennas (ka >> 1), no simple general expression exists for antennas radiation resistance.

Example 10.4: Find the current required to radiate 10 W from a loop, whose circumference is /5.

(10.27.1)

We can use the small loop approximation since ka = 2a/ = 0.2. The resistance:

2 420 0.2 0.316radR

The radiated power is:21

( )2rad radP R I

2 2 10( ) 7.95

0.316rad

rad

PI A

R

Antenna parameters

2. Directivity.

The equation (10.25.1) for a radiated power can also be written as an integral over a solid angle. Therefore, we define the radiation intensity as

2( , ) ( , )rI r S

The power radiated is then:

4

( , )radP I d

Introducing the power radiation pattern as

max

( , )( , )

( , )n

II

I

The beam solid angle of the antenna is2

4 0 0

( , ) ( , )sinA n nI d d I d

(10.28.1)

(10.28.2)

(10.28.3)

(10.28.4)

Time-averaged radial component of a Poynting vector

Antenna parameters

(10.29.1)

It follows from the definition that for an isotropic (directionless – radiating the equal amount of power in any direction) antenna, In(,) = 1 and the beam solid angle is A = 4.We introduce the directivity of the antenna:

max

4

( , ) 4

4 ( ,

4

)rad n A

I

P ID

d

Note: since the denominator in (10.29.1) is always less than 4, the directivity D > 1.

Antenna parameters: Example

Example 10.5: Find the directivity of an infinitesimal (Hertzian) dipole.

Assuming that the normalized radiation pattern is2( , ) sinnI

the directivity will be

2 2

0 0

4 2 21.5

2 3 22 sin sin cos 1 (cos )

D

d d

Note: this value for the directivity is approximate. We conclude that for the short dipole, the directivity is D = 1.5 = 10lg(1.5) = 1.76 dB.

Antenna parameters

3. Antenna gain.

The antenna gain is related to directivity and is defined as

G D

Here is the antenna efficiency. For the lossless antennas, = 1, and gain equals directivity. However, real antennas always have losses, among which the main types of loss are losses due to energy dissipated in the dielectrics and conductors, and reflection losses due to impedance mismatch between transmission lines and antennas.

(10.31.1)

Antenna parameters

4. Beamwidth.

Beamwidth is associated with the lobes in the antenna pattern. It is defined as the angular separation between two identical points on the opposite sides of the main lobe.

The most common type of beamwidth is the half-power (3 dB) beamwidth (HPBW). To find HPBW, in the equation, defining the radiation pattern, we set power equal to 0.5 and solve it for angles.

Another frequently used measure of beamwidth is the first-null beamwidth (FNBW), which is the angular separation between the first nulls on either sides of the main lobe.

Beamwidth defines the resolution capability of the antenna: i.e., the ability of the system to separate two adjacent targets.

For antennas with rotationally symmetric lobes, the directivity can be approximated:

4

HP HP

D

(10.32.1)

Antenna parameters: Example

Example 10.6: Find the HPBW of an infinitesimal (Hertzian) dipole.

Assuming that the normalized radiation pattern is2( , ) sinnI

and its maximum is 1 at = /2. The value In = 0.5 is found at the angles = /4 and = 3/4. Therefore, the HPBW is HP = /2.

Antenna parameters

5. Effective aperture.

Antennas exhibit a property of reciprocity: the properties of an antenna are the same whether it is used as a transmitting antenna or receiving antenna.

For the receiving antennas, the effective aperture can be loosely defined as a ratio of the power absorbed by the antenna to the power incident on it.

More accurate definition: “in a given direction, the ratio of the power at the antenna terminals to the power flux density of a plane wave incident on the antenna from that direction. Provided the polarization of the incident wave is identical to the polarization of the antenna.”

The incident power density can be found as:2 2

02 240av

E ES

Z (10.34.1)

Antenna parameters

(10.35.1)

Assuming that the antenna is matched with the transmission line, the power received by the antenna is

L av eP S A

where Ae is the effective aperture of the antenna.

Maximum power can be delivered to a load impedance, if it has a value that is complex conjugate of the antenna impedance: ZL = ZA

*. Replacing the antenna with an equivalent generator with the same voltage V and impedance ZA, the current at the antenna terminals will be:

0A L

VI

Z Z

Since ZA + ZA

* = 2RA, the maximum power dissipated in the load is

(10.35.2)

2 220 *

1 1

2 2 8L L LA A A

V VP I R R

Z Z R

(10.35.3)

Antenna parameters

For the Hertzian dipole, the maximum voltage was found as EL and the antenna resistance was calculated as 202(L/)2. Therefore, for the Hertzian dipole:

2 2 2

2 22 6408 80L

EL EP

L

(10.36.1)

Therefore, for the Hertzian dipole:2 23 3

4 2 8eA

(10.36.2)

In general, the effective area of the antenna is:2

2

4

4

e

e

A D

A G

(10.36.3)

(10.36.4)

Antenna parameters

6. Friis transmission equation.

Assuming that both antennas are in the far field region and that antenna A transmit to antenna B. The gain of the antenna A in the direction of B is Gt, therefore the average power density at the receiving antenna B is

24t

av t

PS G

R (10.37.1)

The power received by the antenna B is:

22

, 224 4 4t t t r

r av e r t r

P PG GP S A G G

R R

The Friis transmission equation (ignoring polarization and impedance mismatch) is:

2, ,

2 2 24

e t e rt rr

t

A AG GP

P RR

(10.37.2)

(10.37.3)

Antenna arrays

It is not always possible to design a single antenna with the radiation pattern needed. However, a proper combination of various types of antennas might yield the required pattern.

An antenna array is a cluster of antennas arranged in a specific physical configuration (line, grid, etc.). Each individual antenna is called an element of the array. We initially assume that all array elements (individual antennas) are identical. However, the excitation (both amplitude and phase) applied to each individual element may differ. The far field radiation from the array in a linear medium can be computed by the superposition of the EM fields generated by the array elements.

We start our discussion from considering a linear array (elements are located in a straight line) consisting of two elements excited by the signals with the same amplitude but with phases shifted by .

Antenna arrays

The individual elements are characterized by their element patterns F1(,).

At an arbitrary point P, taking into account the phase difference due to physical separation and difference in excitation, the total far zone electric field is:

2 21 2( ) ( ) ( )j jE r E r e E r e (10.39.1)

Field due to antenna 1 Field due to antenna 2

Here: coskd (10.39.2)

The phase center is assumed at the array center. Since the elements are identical2 2

1 1( ) 2 ( ) 2 ( )cos2 2

j je eE r E r E r

(10.39.3)

Relocating the phase center point only changes the phase of the result but not its amplitude.

Antenna arrays

The radiation pattern can be written as a product of the radiation pattern of an individual element and the radiation pattern of the array (array pattern):

1( , ) ( , ) ( , )aF F F

where the array factor is:cos

( , ) cos2a

kdF

Here is the phase difference between two antennas. We notice that the array factor depends on the array geometry and amplitude and phase of the excitation of individual antennas.

(10.40.1)

(10.40.2)

Antenna arrays: Example

Example 10.7: Find and plot the array factor for 3 two-element antenna arrays, that differ only by the separation difference between the elements, which are isotropic radiators. Antennas are separated by 5, 10, and 20 cm and each antenna is excited in phase. The signal’s frequency is 1.5 GHz.

The separation between elements is normalized by the wavelength via

2kd d

The free space wavelength: 8

9

3 1020

1.5 10

ccm

f

Normalized separations are /4, /2, and . Since phase difference is zero ( = 0) and the element patterns are uniform (isotropic radiators), the total radiation pattern F() = Fa().

Antenna arrays

Another method of modifying the radiation pattern of the array is to change electronically the phase parameter of the excitation. In this situation, it is possible to change direction of the main lobe in a wide range: the antenna is scanning through certain region of space. Such structure is called a phased-array antenna.We consider next an antenna array with more identical elements.

There is a linearly progressive phase shift in the excitation signal that feeds N elements.

( 1)0( ) ( ) 1 ...j j NE r E r e e

The total field is:

(10.42.1)

Utilizing the following relation:1

0

1

1

NNn

n

qq

q

(10.42.2)

Antenna arrays

(10.43.1)

the total radiated electric field is

0

1

1

jN

j

eE E

e

Considering the magnitude of the electric field only and using 21 2 sin 2sin

2 2j je je

we arrive at0( ) sin sin

2 2

NE E

where coskd

is the progressive phase difference between the elements. When = 0:

max 0( )E E NE

(10.43.2)

(10.43.3)

(10.43.4)

(10.43.5)

Antenna arrays

The normalized array factor:sin

2( )

sin2

a

N

FN

(10.44.1)

The angles where the first null occur in the numerator of (10.43.1) define the beamwidth of the main lobe. This happens when

2 ,k N k is integer (10.44.2)

Similarly, zeros in the denominator will yield maxima in the pattern.

Antenna arrays

Field patterns of a four-element (N = 4) phased-array with the physical separation of the isotropic elements d = /2 and various phase shift.

4

4

3

4

2

4

4

0

4

2

4

3

4

4

4

The antenna radiation pattern can be changed considerably by changing the phase of the excitation.

Antenna arrays

Another method to analyze behavior of a phase-array is by considering a non-uniform excitation of its elements.

Let us consider a three-element array shown. The elements are excited in phase ( = 0) but the excitation amplitude for the center element is twice the amplitude of the other elements. This system is called a binomial array.

Because of this type of excitation, we can assume that this three-element array is equivalent to 2 two-element arrays (both with uniform excitation of their elements) displaced by /2 from each other. Each two-element array will have a radiation pattern:

1( ) cos cos2

F

(10.46.1)

Antenna arrays

(10.47.1)

Next, we consider the initial three-element binomial array as an equivalent two-element array consisting of elements displaced by /2 with radiation patterns (10.46.1). The array factor for the new equivalent array is also represented by (10.46.1). Therefore, the magnitude of the radiated field in the far-zone for the considered structure is:

21( ) ( ) ( ) cos cos

2AF F F

Element pattern F1() Array factor FA() Antenna pattern F()

No

side

lobe

s!!

Antenna arrays (Example)

Example 10.8: Using the concept of multiplication of patterns (the one we just used), find the radiation pattern of the array of four elements shown below.

This array can be replaced with an array of two elements containing three sub-elements (with excitation 1:2:1). The initial array will have an excitation 1:3:3:1 and will have a radiation pattern, according to (10.40.1), as:

2 3( ) cos cos cos cos cos cos2 2 2

F

Element pattern

Antenna array pattern

Array factor

Antenna arrays

Continuing the process of adding elements, it is possible to synthesize a radiation pattern with arbitrary high directivity and no sidelobes if the excitation amplitudes of array elements correspond to the coefficients of binomial series. This implies that the amplitude of the kth source in the N-element binomial array is calculated as

!, 0,1,...,

!( )!k

NI k N

k N k

It can be seen that this array will be symmetrically excited:

N k kI I

Therefore, the resulting radiation pattern of the binomial array of N elements separated by a half wavelength is

1( ) cos cos2

NF

(10.49.1)

(10.49.2)

(10.49.3)

Antenna arrays

During the analysis considered so far, the effect of mutual coupling between the elements of the antenna array was ignored. In the reality, however, fields generated by one antenna element will affect currents and, therefore, radiation of other elements.

Let us consider an array of two dipoles with lengths L1 and L2. The first dipole is driven by a voltage V1 while the second dipole is passive. We assume that the currents in both terminals are I1 and I2 and the following circuit relations hold:

11 1 12 2 1

21 1 22 2 0

Z I Z I V

Z I Z I

(10.50.1)

where Z11 and Z22 are the self-impedances of antennas (1) and (2) and Z12 = Z21 are the mutual impedances between the elements. If we further assume that the dipoles are equal, the self-impedances will be equal too.

Antenna arrays

In the case of thin half-wavelength dipoles, the self-impedance is

11 73.1 42.5Z j The dependence of the mutual impedance between two identical thin half-wavelength dipoles is shown. When separation between antennas d 0, mutual impedance approaches the self-impedance.

For the 2M+1 identical array elements separated by /2, the directivity is: 2

2M M

n nn M n M

D I I

(10.51.1)

Antenna arrays: Example

Example 10.9: Compare the directivities of two arrays consisting of three identical elements separated by a half wavelength for the:a) Uniform array: I-1 = I0 = I1 = 1A;b) Binomial array: I-1 = I1 = 1A; I0 = 2A.

We compute from (10.51.1):

Uniform array:

Binomial array:

The directivity of a uniform array is higher than of a binomial array.

21 1 1

3 4.771 1 1

D dB

21 2 1 16

4.261 4 1 6

D dB