Satellite Communication PPT by Chen, Zhi Ning

EE5404 Antenna Basics 1

September 2, 2003

Satellite CommunicationsSatellite Communications

Chen, Chen, Zhi NingZhi Ning

e-mail: [email protected]: http://www1.i2r.a-star.edu.sg/~chenzn


3 Antenna Basics3 Antenna Basics3.1 Introduction3.2 System Parameters

•Field and power radiated by an antenna•Far-field distance•Radiation intensity•Radiation patterns •Directivity•Radiation efficiency•Gain•Aperture efficiency•Effective area•Antenna polarization


Radio Channel

receive & transmit antennas

The propagation of electromagnetic energy is the key physical phenomenon in any wireless communications. It links transmitters to receivers in a wireless communication system.

The antennas are the key devices in the energy transfer. Therefore, the antennas are playing a vital role in any satellite communication systems. With the antennas, it is possible to build up the communications between the satellites in space and the users on ground.

In this chapter we will characterise the antennas in a systems point of view. Our goal is to calculate the received signal and noise power in terms of transmit power, range, antenna gain, and efficiency.

receive & transmit antennas

3.1 Introduction3.1 Introduction


3.1 Introduction3.1 Introduction--antenna and EM theoryantenna and EM theory

However, antenna theory and design are complex topic.

The electromagnetic theory is found on the mathematics and physics. The antenna theory and design are one of electromagnetic applications.

The antenna engineering involves not only electromagnetics, applied mathematics but also material, mechanics, manufacturing and so on.

Usually, we must take some particular courses to study the antennas.

Here, we are not interest in the detailed EM theory of the antenna operation. We are just interested in the system aspect of the antennas such as radiation patterns, directivity, gain, efficiency, and polarization.

antenna theory & design

••••••electromagnetics

mathematics physics


3.1 Introduction3.1 Introduction--what is an antennawhat is an antennaIn an EM point of view, an antenna is a radiator or an inductor of EM fields.

When the antenna radiates EM fields, the electric currents on the antenna are excited by its feed and then excite the electromagnetic fields in space.

When the antenna receives the EM fields, the impressed electromagnetic fields on the surface of the antenna excite the electric currents on the antenna surface, that is, the electric currents are induced on the antenna surface. Then the induced currents are received by load of antennas, usually, RF circuits.

plane wave

Pt

Spherical waveZs

Pr Zl

Vs

Transmit antenna receive antenna

currents EM fields currents


3.1 Introduction3.1 Introduction--what is an antennawhat is an antennaIn a radio systems point of view, an antenna is a device to convert a guided EM wave on a transmission system to a plane wave propagation in free space.

One side of an antenna acts as an electric circuit element. Meanwhile, the other side of the antenna provides an interface with a propagating plane wave.

So, the EM waves with RF signals are transmitted and received by the antennas.

Due to the gain of antennas, the RF signals may be amplified when the antennas transmit or receive the signals.

Pt

Spherical wave plane waveZs

Pr Zl

Vs

Transmit antenna receive antenna

GuidedEM wave

EM fieldsIn free-space

GuidedEM wave

Pt

ZsZant Pt

Vs


3.1 Introduction3.1 Introduction--what is an antennawhat is an antennaWire: monopole, dipole, loop, helix, ...

Aperture: horn, waveguide, ...

Microstrip: dipole, patch, ...

Array: wire, aperture, microstrip, waveguide, ...

Reflector: parabolic, corner, ...

Lens: convex, concave, …

……

antennas

For satellite communications, we need light-weight high-gain antennas satellites and mobile users, and high-gain antennas for earth stations. The wire structures and microstrip antennas are often used for satellites and mobile users. And the reflector, aperture and array antenna with very high gain are commonly used for earth stations.


3.1 Introduction3.1 Introduction--some products for satellite comm.some products for satellite comm.

D=2.4m

Antenna Size 2.4 M (96 in.)Operating Frequency 3.625 - 4.2

GHzMidband Gain (± .2dB) 37.5 dBi3 dB Beamwidth 2.1°Antenna Noise Temperature (linear)

20° elevation 30° elevation

33K 31K

First Sidelobe (typical) -20 dBCross-Pol Isolation (linear) >30 dB (on

axis)VSWR 1.3:1 Max.Feed Interface CPR 229 FInsertion Loss 0.2 dB Max.

C-Band Dual Pol Rx Only Antenna receive VSAT antennas

D=1.2m


Model 9322-800/16-inch Disk Arrayfor Vehicles

Frequency: L-bandGain:9 dBiSteered in: AzimuthFeatures: 20° - 65° elevation coverage CP "disk" array. 16 inch diameter, 1 inch high.


GPS antennas



Low loss "suspended technology" dual polarized radiating elements. (Courtesy Saab Ericsson Space, Sweden).


3.2 System Parameters3.2 System Parameters

Antenna SystemParameters

To describe the characteristics and performance of an antenna, we have defined many parameters.

Here we mainly introduce some antenna parameters, which are related to systems. Using them, we shall discuss antenna effects on system performance.

For example, compared with the near-field parameters such as impedance matching, we are more interested in far field characteristics of an antennas, such as radiation pattern, gain, polarization and so on.

Reciprocity theorem

Impedance matching

Radiation by an antenna

Far-field distance

Radiation intensity

Radiation patterns

Directivity

Radiation efficiency

Gain

Aperture efficiency

Effective area

Antenna polarization


3.2 System Parameters: 3.2 System Parameters: Reciprocity TheoremReciprocity Theorem

I

Antenna B

R R EMF

Antenna B

TIT EMF

Antenna A Antenna A

Basically passive antennas are reciprocal devices.The reciprocity theorem for the antennas states that if a current I is induced in output of the antenna B (operating in receiving mode) by applied electromagnetic fields at input of the antenna A (operating in transmitting mode), then the same electromagnetic fields at the input of the antenna B will induce the same current I at the output of the antenna A.


3.2 System Parameters: 3.2 System Parameters: Reciprocity TheoremReciprocity Theorem

Important consequences

•In a linear antenna system, the power delivered in either direction (ant. 1 to ant. 2 or ant. 2 to ant.1) is the same.•The radiation pattern for an antenna operating in the transmitting mode is the same as the that in the receiving mode.

It suggests that we can determine the property of an antenna operating in either transmitting or receiving mode because the receiving and transmitting characteristics of the antenna are identical at any specific frequency.

This simplifies antenna analysis and measurements greatly.


3.2 System Parameters:3.2 System Parameters:Impedance matchingImpedance matchingIn the analysis and design of an antenna, the impedance matching is a key parameter to measure the capability to delivery energy between the antenna and the feed or load.

Using an equivalent circuit of an antenna in a transmitting mode, the antenna system can be divided into source, transmission line and radiator. If the impedance is matched between the source or antenna and transmission line, there will not be the reflection at the interface of source-transmission line or antenna-transmission line. As a result, maximum energy will be transmitted from the source to the antenna.

Usually we can define some parameters to assess the matching condition, such as VSWR or return loss based on the reflecting coefficient.

Besides the ohmic loss in the source, transmission line and antenna, the impedance mismatching also result in the loss.

S

standing

wave

Za=Ra+jXaZs=Rs+jXs

source antenna

transmission lineZo

Zs= Zo*Zo= Za*

Impedancematch

VSWR orreturn loss


3.2 System Parameters: 3.2 System Parameters: Radiation by an antennaRadiation by an antennaAt large distance, the radiated electric field by an antenna can be expressed as

( ) ( )[ ] V/m,ˆ,ˆ),,(r

eFFrEjkr−

+= φθφφθθφθ φθ

r

the electric field vector;unit vectors in the spherical coordinate system;the radial distance from the originthe free-space propagation constant with wavelength λpattern functions, independent of distance r( ) ( )φθφθ

φθ

φθ

φθ ,,,

ˆ,ˆ),,(

FFkr

rrwhere E

The electric field propagates in the radial direction, with a phase delay of (jkr) and amplitude attenuation of 1/r. The electric field may be polarized in either θ or φ directions, but not in the radial direction.


3.2 System Parameters: 3.2 System Parameters: Radiation by an antennaRadiation by an antenna

o

EH

ηφ

θ −=o

EHηθ

φ =The associated magnetic field is and

Ω= 377oη : the wave impedance of free space

The magnetic field vector is also polarized in either θ or φ directions only.

The Poynting vector for EM waves can be given by the cross product of E and H fields:

2* W/mHESrrr

×= : the radiated power density

( ) ( ) 2* W/mRe21Re

21 HESSavg

rrrr×== : time-average Ponyting vector


3.2 System Parameters: 3.2 System Parameters: FarFar--field zonefield zone

At large distances the near fields of an antenna can be negligible. So, the fields can be expressed simply as TEM waves. Moreover, the fields can be considered as a plane wave with ideal planar phase. This distance is called far-field distance and can be determined by maximum dimension of an antenna, D.

m2 2

λDr fieldfar =−

This relationship is derived from the condition that the actual spherical wave front radiated by the antenna departs less than π/8=22.5o from a true plane wave front over the maximum extent. Or the angular field distribution is essentially independent of the distance r.

However, for small antennas, far-field distance rfar-field is at least 2λ.


3.2 System Parameters: 3.2 System Parameters: FarFar--field zonefield zone--ExampleExample

A dish for DBS reception is 1.2m in diameter and operates at 12.4GHz. 1) Find its far-field distance. 2) Evaluate the critical diameter of the dish which makes the DBS satellite

not locate within its far-field zone.Solution:1) The wave length is λ=c/f=2.42 cm.

So, the distance rfar-field=2 (D)2/ λ=119 m<<36,000,000 m (The distance from a DBS satellite to the dish is 36,000km.)

2) The far-field distance is 36,000km.So, the diameter D =[rfar-field λ/2]1/2= 660 m

That means that the DBS satellite will not locate in the far-field region of the dish of a diameter of <660 meter.

1.2m


3.2 System Parameters: 3.2 System Parameters: Radiation IntensityRadiation Intensity

The radiation intensity can be defined as

( ) ( ) ( ) WRe2

1Re2

Re2

),(2222

222

φθφθ ηηφθ FFEErSrSrU

ooavg +=+===

rr

The radiation intensity is the radiation power per unit solid angle since the radial dependence has been removed. It gives the variation in radiated power versus position around antenna. So, we can find the total power radiated by the antenna by integrating the Ponyting vector over surface of a sphere of radius r. The sphere encloses the antenna.

Wsin),(sinˆ2

0 0

2

0 0

2rad φθθφθφθθ

π

φ

π

θ

π

φ

π

θddUddrrSP avg ∫ ∫∫ ∫ = == =

=•=r


3.2 System Parameters: 3.2 System Parameters: Radiation patternRadiation pattern

An important fact is that an antenna is a directional device and characterised in terms of a radiation pattern.

Radiation pattern: a plot of the relative far-field strength or power transmitted or received by the antenna versus position around antenna at a fixed distance from antenna. Thus the pattern can be plotted from the pattern functions Fθ and Fφ versus θ (in an elevation plane) and φ (in an azimuthal plane) of a spherical co-ordinate system at one frequency one polarization and one plane cut. The polarization of the antenna determines the plotting Fθ and Fφ . The radiation patterns can be measured at a constant radius r. And the plots also can be 2 or 3-dimensional.

Radiation Pattern for fields:

Radiation Pattern for power:

f (θ , φ)= Fθ or φ (θ , φ)/Fmax

p (θ , φ)= F2θ or φ (θ , φ)/Pmax



mainlobe

elevationplane

azimuthplane

minorlobes

x

z

y

3-dimensional radiation patternsUsually, we normalize the power at a certain point by the maximum power at the fixed radius.

This trace is the received power or field at a constant radius, called the power or field pattern.

There are main lobe or beam and some side lobes or minor lobes.

With the help of the radiation pattern, we can know maximum radiation direction easily. This is helpful for us to design the RF link. As said, the purpose of communications is delivery the information to the desired destination (in the certain direction) by the RF energy.



mainlobe

sidelobe

back lobe

minorlobes

x

First null beamwidthFNBW

Half-power beamwidthHPBW

minorlobes

z

y

3-dimensional pattern

FNBW

HPBWSide lobes

mainlobe

Side lobe back lobe

π π/2 0 π/2 π θ

Radiationintensity

back lobe

2-dimensional patternThis is a two dimensional radiation pattern in a specific cut. It displays radiated field distribution in a specific plane. Using the 2-D pattern, we can easily define the main lobe, side lobes, back lobe, first null beamwidth and also half-power beamwidth.



Absolutegain, dB

elevationangle

azimuthangle

RelativegaindB

0

For satellite communications, we can plot the radiation patterns in this way.

This figure shows the field or power distribution of the coverage region illuminated by a satellite antenna.

There is maximum power near the origin.

radiation pattern for satellite antenna


3.2 System Parameters: 3.2 System Parameters: Radiation patternRadiation patternDistribution of E-fieldfeed

reflector (aperture)

Da practical

ideal

reflector antennaHere we consider the antennas most commonly used in satellite communication systems, namely, reflector antenna.This is a typical reflector antenna sometimes used in earth station. The reflectorantennas can focus transmissions within desired areas with very high gains. The radiation patterns of aperture antennas depend on the field distribution patterns across the antenna aperture.



aDNλ

=Θ

N: a constant dependent on aperture distributionN=58 uniform distributionN=70 tapered distribution

Da: antenna diameterλ: operating wavelength

So, the half-power beam-width depends on the aperture field distribution, antenna diameter, and operating frequency.

To minimized spill-over of energy, the field distribution is usually tapered across the aperture with the maximum at the center of the aperture. Usually, we describe the ability to focus the energy on a certain area in terms of half-power beam-width.

We can evaluate it from this approximate relationship.

HPBW:half-power beam-width

(radian)

Da


3.2 System Parameters: 3.2 System Parameters: DirectivityDirectivity

Directivity

Directivity is one measure of focusing property of an antenna. It is defined as the ratio of maximum radiation intensity in the main beam to average radiation intensity over all space .

avgavgavg PP

PU

UUD maxmaxmax 4

===π

Umax & Uavg: maximum & average radiation intensityPmax & Pavg : maximum & average radiated power

π4r

avgPP =

The average power radiated from an antenna is the power over unit solid angle.

Pr: the total radiated power from an antenna.


3.2 System Parameters: 3.2 System Parameters: DirectivityDirectivity

( ) AddF

FD

Ω==

∫ ∫π

φθθφθπ π π

φθ

φθ 4

sin,4 2

0. 0 or

maxorΩA: beam solid angle

ΘΘ

ΘΘ≈

Ω=

deg2deg1

221

)/180(4

44

ππ

ππ radrad

A

D

We also can calculate the directivity using pattern functions. This is an exact expression to calculate the maximum directivity. But, in practice, it seems not easy to use it.

So, for the directional patterns, we can use the simple expression, such as two half-power beam-widths in radian or degree to approximate the beam solid angle.

Θ1rad: HPBW in one plane in radianΘ2rad: HPBW in a plane at a right angle to the

other plane in radianΘ1degree: HPBW in one plane in degreeΘ2degree: HPBW in a plane at a right angle to

the other plane in degree


3.2 System Parameters: 3.2 System Parameters: EfficiencyEfficiency

However, the definition of the directivity does not consider the efficiency of an antenna because Pr is only related to the actual power radiated into free space.

In fact, some power in an antenna is definitely lost as a result of spill-over, blockage of RF energy by sub-reflector and supporting structures, manufacturing defects, ohmic and mismatching losses. Such losses reduce the power delivered from the the input of an antenna to free space. The radiated power is lower than the input power of the antenna. We can measure the difference by radiation efficiency.

in

loss

in

lossin

in

rad 1PP

PPP

PPerad −=

−==

Prad : the power radiated by the antennas Pin: the power applied to the input of the antennaPloss: the power lost in the antenna

?Question:Do impedance mismatch at the input or output of antennas, polarization mismatch with the receive antenna contribute to the loss of transmit power? How to eliminate them?


3.2 System Parameters: 3.2 System Parameters: GainGain

Another useful measure describing the performance of an antenna id the gain.

Although the gain of the antenna is closely related to the directivity, it is a measure to take into account the efficiency as well as its directional capabilities. Remember that the directivity is a measure only describing the directional properties of the antenna and so controlled only by the pattern. Therefore, the gain function is related to directivity and efficiency as described in its definition.

DeP

PG radin

rad

power,input power,radiation maximum

==Gain:

rad Pe =

in

radP


3.2 System Parameters: 3.2 System Parameters: GainGainGain for an aperture antenna

2e4

λπ AG = Ae: the effective aperture (area) of the antenna

ape

emape

2ape

aperad2em

rad 4

AAe

DeeAeG

=

==

λπ

λπ

eape : efficiency of apertureAape=π(Dape/2)2 the physical area of aperture of the antennaDaee: diameter of aperture

For circular aperture

As example, we calculate the gain for an aperture antenna using its aperture dimension, Ae. The Ae is determined by the field distribution on the aperture and radiation efficiency of the antenna. For a circular aperture, we can calculate its aperture area Aape with its diameter. eape is the efficiency of the aperture. The efficiency of a typical parabolic antenna is 50-70%.

In addition, it is clear that the gain of an antenna increases with an increase in its aperture size. So, the larger the diameter of an antenna is, the higher the gain of an antenna is.


3.2 System Parameters: 3.2 System Parameters: Effective areaEffective area

The important parameters, such as antenna directivity, efficiency, and gain have been discussed above in terms of transmit antennas. In fact, they all can apply to receive antennas as mentioned in Reciprocity Theorem. However, for a receive antenna, we need a parameter to measure its ability to receive power for a given incident plane wave.

From the calculation of the gain for an aperture antenna, we know that the received power is proportional to the effective aperture (area), Ae and incident power density.

eavg ASPr = Savg: incident power densityreceived power:

πλ4

2

maxrademax DeA =Maximum effective aperture (area)

The maximum effective area of any antenna is proportional to the maximum directivity of the antenna and the operating wavelength. (C. A. Balanis: Antenna Theory-analysis and design, 2nd edition,Wiley, 1997)


3.2 System Parameters: 3.2 System Parameters: PolarizationPolarization

Polarization of an EM wave describes the orientation of radiated electric field vector in space and is usually a function of time. The polarization is determined by the antenna or the feed of an antenna. For example, when a plane wave is propagating along the z-axis, the electric field can be expressed using its x- and y-components.

( ) jkzyx eEyExE −+= ˆˆ

r

Ex and Ey are the amplitude of the x- and y-components of the EM wave and in-phase.If Ex =0, and Ey=1, or Ex =1, and Ey=0, the EM wave is linearly polarized in the y- or x-direction. If Ex =1, and Ey=1 (in phase), the EM wave is linearly polarized in the 45o direction.



For the case of the x- and y-components with arbitrary amplitude and phase, the EM wave operates in an elliptical polarization. This can be considered the general case as illustrated in this figure. The waves is propagating in –z direction. Usually, we use the axial ratio to express the polarization. The linear and circular polarization are just the special cases of the elliptical case.For elliptical or circular cases, there are right-hand and left-hand polarized polarization. The rotations are clockwise and counter-clockwise for right and left-hand case respectively.

Ex

Ey

axial ratio:

x

y

EE

AR =AR=1 & phase difference is 90o

: circular AR=0 (Ey=0): linear (horizontal)

AR=∞ (Ex=0): linear (vertical)

Otherwise:

elliptical

Propagationdirection

right-hand: clockwise

left-hand:counter-clockwise

ω

E

right-hand: clockwise

left-hand: counterclockwisey

x

Elliptically polarized wave


The polarization characteristics of an antenna (in transmitting mode) are defined by the polarization of the wave it transmitted. (For example, a transmitting dipole horizontally positioned would produce horizontally polarized waves.On the other hand, it properly receives the maximum energy with the horizontal polarization.) For the maximum received power, the receive antenna must be in the same polarization and point to the transmit antenna. We can use polarization match factor to measure the former and misalignment the latter.

R

Eα

In the different planesIn the same plane

T R

Eα

linear case

? What will happen as α=90o?TV antenna outdoor


T

Erec=E cosα Erec=E cosα



In satellite communications, the antennas of circular polarization are commonly used. At a certain time, the E-field can be divided into two orthogonal components, which can be received by a circularly polarized antenna or a two-orthogonal-dipole antenna.

Propagationdirection

ω

E

Ev E

Ex

Eh

Ey

Elliptically polarized wave



We can assess the isolation of the polarization I in dB.

polX

polcodB log20

−

−=EE

IIsolation of polarization:

Also, we can use the cross-polarization discrimination XPD assess the loss caused by the depolarization in signal propagation. We will discuss this loss in the next Chapter.

polXpolco

polXpolco

polX

polco

polX

polco

dB

log20

1

1log20

−−

−−

−

−

−

−

−

+=

−

+=

EEEE

EEEE

XPDCross-polarization discrimination


ConclusionConclusion

•The antenna is one of basic elements of a satellite communication link.•Fundamental characteristics of the antenna affect the design of satellite communication link.

impedance matching; radiation efficiency; directivity; gain; aperture efficiency

•The characteristics of propagation are also important for the link design. polarization

Important!!!Effects on link design

Documents

Satellite Communication PPT by Chen, Zhi Ning