What is an Antenna?

An antenna is defined by the IEEE as a “transmitting

or receiving system that is designed to radiate or receive

electromagnetic waves” . An antenna can be of any

shape or size. A list of some common types of antennas

is wire, aperture, microstrip, reflector, and arrays. Each

antenna configuration has a radiation pattern and design

parameters, in addition to their benefits and

drawbacks. In this section we will describe common

antenna types and their benefits and drawbacks. In

addition, we will discuss fundamental parameters of each

antenna configuration.

Types of Antennas

The “IEEE Standard of Terms for Antennas” has not

been updated since 1983 and the terms/definitions do

not describe many of the new antennas discovered

since, according to David V. Thiel of the Griffith

University .

He proposes antennas should be grouped by

categories. The following are the proposed grouping:

wire antennas (e.g., dipoles and loops), aperture

antennas (e.g., pyramidal horns), reflector antennas

(e.g., parabolic dish antennas), microstrip antennas

(e.g., patches), dielectric antennas (e.g., dielectric

resonant antennas), and active integrated antennas,

lens antennas (sphere), and antenna arrays.

Wire Antenna

A wire antenna is an antenna that is made of a

conductive wire. Wire antennas can come in different

configurations and some of these configurations are

dipoles, helix, and loop . Wire antennas can be seen

everywhere in daily lives. Some examples of wire

antennas are on automobiles as radio antennas, and on

buildings as transmitting or receiving antennas. Figure 3

shows an example of a wire antenna of a car.

Example of Wire Antenna

Wired antennas have an omni-directional radiation

pattern and the monopole antenna, a type of wire

antenna, comes standard with wireless routers.

Dipole Antenna

A dipole defined by the Merriam Webster Diction is

“a pair of equal and opposite electric charges or

magnetic poles of opposite signs separated especially by

a small distance” .

A simple design and radiation pattern of a

dipoleantenna can be shown as:-

Figure 4 Dipole Antenna and Radiation Pattern

A simple design of a dipole antenna is to make the

length of the antenna λ/2, where wavelength λ is equal

to the speed of light over the center frequency the

antenna is mean to operate at. At the feed of a center

fed dipole, the current is at its peak and lowest at the

ends of conductors, or wings. Table 1 shows as the

length of the dipoles in terms of λ, the directivity of the

antenna corresponding to the length.

There is an increase in gain with each increase in

length. In order to achieve a significant gain, the dipole

length must be large in compared to the wavelength.

Trying to achieve high gain at lower frequencies is an

issue because the antenna can be massive, heavy, and

costly. The radiation pattern of a typical dipole can be

seen as:-

Dipole Radiation Pattern

The radiation pattern of a dipole is in all directions.

In addition, the radiation pattern looks similar to that of

a donut. There are many different variations of dipole

antennas and some common types are biconical,

bowtie, and blade dipoles.

Biconical Dipoles

Biconical dipoles are defined as two conical

conductors that have are symmetrical about an axis

and vertex . An example of a biconical dipole can be

seen as-

Biconical Dipole

Biconical dipoles are considered part of the

broadband dipole, being able to operate at a wide range

of frequencies. The dipole feed is located at the center

where both the cones meet. The antenna radiation

pattern is similar to that of a regular dipole and the only

real difference is the allowable bandwidth of this

antenna is considerably higher than the dipole and can

commonly achieve bandwidths of four to one.

Sometimes these cones are made out of a solid

metal conductor which can be heavy and costly.

Bowtie Antennas

Bowtie Antenna is another broadband antenna. It

also has a similar omnidirectional radiation pattern

compared to the traditional dipole. A design of a bowtie

antenna is seen as

Bowtie Dipole

Instead of being constructed with a conductor

sheet, bowtie antenna can constructed using a wire to

form the same shape. This is beneficial because it is

lower in because less is being used metal and decreases

wind resistances. The bow tie antenna is center fed like

a dipole. Figure shows an illustration of a wire bowtie

antenna.

Wire Bowtie Antenna

As you can see, the metal used to construct this

dipole is significantly less than a tradition bowtie

antenna constructed from sheet metal. This method will

lower production cost and decrease the weight of

antenna.

Helix Antenna

A helix antenna is defined as an antenna whose

configuration relates to a helix . The helix antenna is

relatively light weight because it is constructed using a

metal conductor wire, a center support the helix

structure, and is usually attached to a ground plane at

the base. An example of a helix antenna is seen in

Basic Helix Antenna Configuration

The lossless gain of a Helix Antenna is given by N= Number of turns

C= Circumference

of Helix S = spacing

between turnsEquation 7.

N= Number of turns C=Circumference of Helix S = spacing between turns

The gain is dependent of the number of turns, the

circumference of the helix, the spacing between turns,

and the wavelength. Designers can increase the gain of

the antenna by adding additional turns which will

increase the length of the antenna. Another key

characteristic is the input impedance of the antenna.

This can be obtained using the equation:-

C =Circumference of helix

The resistance of the antenna is dependent of the

circumference of the helix and the wavelength. The

following Figure shows a basic configuration of a helix

antenna.

Figure:-Basic Helix Antenna Configuration

The coaxial cable is connected to the feed is label as

C, R is the reflector base, B is the center support, E is the

support for the helix, and S is the wire of the helix

antenna that is radiating or receiving electromagnetic

waves. Other design parameters that needs to be

consider when designing a helix antenna are the pitch

angle (arctan (S/ pi*D)), the total length of the antenna

(NS), and total length of wire (N*Length of one turn)

There are two operational modes for a helix

antenna: axial mode, and normal mode. In normal

mode the spacing between helixes and the diameter of

the helixes are small in comparison with the

wavelength. The radiation pattern is along the helical

direction and it is similar to that of a dipole. In axial

mode, the antenna functions like a directional antenna

and the spacing between elements is λ/4. The antenna

radiates at the top of the helix along the axis of the

antenna. The radiation pattern of both operation modes

can be seen in

Radiation Pattern of Helix Antenna

Loop AntennaThe “IEEE Standard Definitions of Terms for

Antennas” defines a loop antenna as “an antenna

whose configuration is that of a loop” [5]. This loop can

be in the shape of a square, rectangle, circle trip, and

many other geometric shapes. There are two different

categories to loop antennas: electrically small or

electrically large . Electrically small antennas are defined

as antennas that loop length is less than one-tenth of

wavelength. Wavelength, λ, is the ratio of the speed of

light over the frequency at which the antenna is

designed to operate at. Small loop antennas are

sometimes called the magnetic loop because it acts like

an inductor.

Electrically large loop antennas are defined as

antennas that have a loop length of approximately λ.

Figure shows examples of loop antennas.

Loop Antenna Example

The radiation pattern of a loop antenna is

omnidirectional which is similar to the dipole. Radiation

pattern of small circular and rectangular loop antenna:-

Aperture antennas

An aperture antenna is an antenna that contains an

opening in which electromagnetic waves are transmitted

or received through . Aperture antennas can be many

different shapes. Popular configurations of an aperture

antenna are waveguides and horns . Aperture antennas

are used widely in aircrafts because the can be covered

with a dielectric. This dielectric protects the antenna

from the environments that an aircraft is exposed to. A

waveguide is an antenna that guides an electromagnetic

wave. It consists of a conductive wall that is hollow in

the inside for the wave to travel. A horn antenna is “an

antenna consisting of a waveguide section in which the

cross-sectional area increases towards an open end

which is the aperture”. A typical horn antenna is

Example of a Horn Antenna [

There are three types of horn antennas: 1) E-plane sectoral horn, 2) H-plane sectoral horn, and

3) pyramidal horn

H-Plane Sectoral horn has a wider width to of the

aperture while E-Plane Sectoral horn has a wider

height. The pyramidal horn has approximately equal

width and height.

E-Plane Horn (Left), H-Plane Horn (Middle), Pyramidal Horn (Right)

Reflector AntennaReflector antennas redirect electromagnetics and

refocus it in a certain direction. This type of antenna is

commonly used for space crafts for long distance

communication . Several common types of reflector

antennas are the plane reflector, the corner reflector,

and the parabolic reflector. A plane reflector is flat

reflector made of a conductor. The electromagnetic

waves redirects concept can be compared to sunlight

hitting a mirror.

A corner reflector usually consists of two plane

reflectors joined together at an angle. Typically these

two plane reflector joins together to form a 90 degree

angle. Figure 16 shows the concept of a 90 degree

corner reflector.

Concept of Corner Reflector [24]

Parabolic reflectors are shaped like a parabola.

Electromagnetic waves can be focused into a beam and

aimed at locations with accuracies. Because of this

characteristic, parabolic are commonly used by dish TV

companies, and satellite communication. Figure shows

an example of a parabolic antenna.

The losses gain or directivity of a reflector antenna

can be found by knowing the wavelength and the cross-

sectional aperture.

The following Equation is the equation for the

directive for a reflector antenna.

Directivity of a reflector antenna

The true gain of a reflector antenna takes into

account radiation, aperture taper, spillover, and

achievement losses. Taking those factors into

consideration, the gain of a reflector antenna can be

found using eqn:-

Microstrip Antenna The microstrip antenna, sometimes called a patch

antenna, is defined as an antenna which consists of a

thin metallic conductor bonded to a thin grounded

dielectric substrate . Microstrip antennas are low

profile, small in volume, and have low production cost .

The feed can be connected directly to the conductor on

the same substrate. The antenna design can be printed

onto ceramic substrate which eliminates the need for

an adhesive to bond the conductor to the substrate.

shows a single rectangular patch antenna configuration.

Basic Patch Antenna Design

The patch antenna can operate from the ranges from 1GHz to 6GHz. At lower frequency the antenna can be

large in size and may not be practical.

To design an antenna to resonance at a desired frequency, Equation 11 can be used

.

h = Height of dielectric substrate

W = the width of the patch

€R = Relative Dielectric of substrate

There are many different variations of shape for patch antenna’s radiating

element. Some common shapes are square, circle, ellipses, triangle, circular ring,

and dipole. The more commonly used shapes are square, rectangle, dipole, and

circle are used because they are easier to analyze than other shapes.

Antenna function

Space wave

Guided wave

• Transformation of a guided EM wave (in waveguide/ transmission line ) into an EM wave freely propagating in space (or vice versa)

– Transformation from time-function into RF wave (= vectorial field dependent on time and 3 space-dimensions)

– The specific form and direction of the wave is defined by the antenna structure and the environment

• Transmission line

– Power transport medium – the transition ideally without power reflections (matching devices!)

• Radiator

– Must radiate efficiently – must be of a size comparable with the half-wavelength

• Resonator

– Unavoidable - for broadband applications resonances must be attenuated

Monopole (dipole over plane)

Sharptransition region

Uniform wave traveling along the line

Thin radiator

High-Q Narrowband

Smooth transition region

Low-Q Broadband

Thick radiator

If there is an inhomogeneity (obstacle, or sharp transition), reflections, higher field- modes and standing wave appear.

With standing wave, the energy is stored in, and oscillates from electric energy to magnetic one and back. This can be modeled as a resonating LC circuit with

Q = (energy stored per cycle) / (energy lost per cycle)

Dipole, Slot & INF antennas

• Slot antenna: a slot is cut from a large (relative to the slot length) metal plate.

• The center conductor of the feeding coaxial cable is connected to one side of the slot, and the outside conductor of the cable - to the other side of the slot.

– The slot length is some (/2) for the slot antenna and (/4) long for the INF antenna.

• The INF and the slot antennas behave similarly.• The slot antenna can be considered as a loaded version of

the INF antenna. The load is a quarter-wavelength stub, i.e. a narrowband device.

• When the feed point is moved to the short-circuited end of the slot (or INF) antenna, the impedance decreases. When it is moved to the slot center (or open end of the INF antenna), the impedance increases

Antennas for laptop applications

• Patch and slot antennas derived from printed-circuit and micro-strip technologies

• Ceramic chip antennas are typically helical or inverted-F (INF) antennas, or variations of these two types with high dielectric loading to reduce the antenna size

• Patch and slot antennas are

– Cheap and easy to fabricate and to mount

– Suited for integration

– Light and mechanically robust

– Have low cross-polarization

– Low-profile - widely used in antenna arrays

spacec–rafts, satellites, missiles, cars and other mobile applications

Aperture-antenna

• Aperture antennas

Powerabsorbed: P

[watt]

Effective aperture:

A[m2]

EM wave

Power density:

PFD [w/m2]

A = A*PFD

derived from waveguide technology (circular, rectangular)

• Can transfer high power (magnetrons, klystrons)

• Above few GHz• The aperture concept is

applicable also to wired antennas. For instance, the max effective aperture of linear /2 wavelength dipole antenna is 2/8

Leaky-wave antennas

• Derived from millimeter- wave guides (dielectric guides, microstrip lines, coplanar and slot lines).

• For frequencies > 30 GHz, including infrared

• Subject of intensive study.– Note: Periodical

discontinuities near the end of the guide lead to substantial radiation leakage (radiation from the dielectric surface).

Image Theory

• Antenna above perfectly conducting plane surface

• Tangential electrical field component = 0– vertical components:

the same direction– horizontal

components: opposite directions

• The field (above the ground) is the same as if the ground is replaced by an mirror image of the antenna

+

-

Elliptical polarization: change of the rotation sense!

•

The Arecibo Observatory Antenna System

The world’s largest single radio telescope

304.8-mspherical reflector

National Astronomy and Ionosphere Center (USA), Arecibo, Puerto Rico

The Arecibo Radio Telescope

[Sky & Telescope

Lens antennas

Lenses play a similar role to that of reflectors in reflector antennas: they collimate divergent energyOften preferred to reflectors at frequencies > 100 GHz.

Radiation pattern

• The radiation pattern of antenna is a representation (pictorial or mathematical) of the distribution of the power out-flowing (radiated) from the antenna (in the case of transmitting antenna), or inflowing (received) to the antenna (in the case of receiving antenna) as a function of direction angles from the antenna

• Antenna radiation pattern (antenna pattern):– is defined for large distances from the antenna,

where the spatial (angular) distribution of the radiated power does not depend on the distance from the radiation source

– is independent on the power flow direction: it is the same when the antenna is used to transmit and when it is used to receive radio waves

– is usually different for different frequencies and different polarizations of radio wave radiated/ received

Power- orfield-strength meter

Turntable

ant

Generator

Power pattern vs. Field pattern

Anten under test

AUT

Large dist Auxiliary

antenna

The power pattern is the measured (calculated) and plotted received power: |P(θ, ϕ)| at a constant (large) distance from the antenna

• The amplitude field pattern is the measured (calculated) and plottedelectric (magnetic) field

The power pattern and the field patterns are inter-related for plane wave:

P(θ, ϕ) = (1/)*|E(θ, ϕ)|2 = *|H(θ, ϕ)|2

P = powerE = electrical field

component; H = magnetic

field component vector

= 377 ohm (free-space, plane wave impedance)

intensity, |E(θ, ϕ)| or|H(θ, ϕ)| at a constant (large) distance from theantenna

Reference antenna (/2 dipole)

Reference antenna ( /2 dipole)

‘Biquad’

Biquad antenna

‘Cantenna’

Cantenna

3-D pattern

3-D pattern

• Antenna radiation pattern is3-dimensional

• The 3-D plot of antenna pattern assumes both angles θ and ϕ varying, which is difficult to produce and to interpret

2-D pattern

Two 2-D patterns

• Usually the antenna pattern is presented as a 2-D plot, with only one of the direction angles, θ orϕ varies

• It is an intersection of the 3-D one with a given plane– usually it is a θ = const

plane or a ϕ= const plane that contains the pattern’s maximum

Example: a short dipole on z-axis

Principal patterns• Principal patterns are the 2-D patterns of linearly polarized

antennas, measured in 2 planes

1. the E-plane: a plane parallel to the E vector and containing the direction of maximum radiation, and

2. the H-plane: a plane parallel to the H vector, orthogonal to the E-plane, and containing the direction of maximum radiation

Example

Isotropic antenna

• Isotropic antenna or isotropic radiator is a hypothetical (not physically realizable) concept, used as a useful reference to describe real antennas.

• Isotropic antenna radiates equally in all directions.– Its radiation pattern is represented by

a sphere whose center coincides with the location of the isotropic radiator.

Directional antenna

• Directional antenna is an antenna, which radiates (or receives) much more power in (or from) some directions than in (or from) others.

– Note: Usually, this term is applied to antennas whose directivity is much higher than that of a half-wavelength dipole.

Omnidirectional antenna

• An antenna, which has a non- directional pattern in a plane

– It is usually directional in other planes

Pattern lobes

Pattern lobe is a portion of the radiation pattern with a local maximum

Lobes are classified as: major, minor, side lobes, back lobes.

Pattern lobes and beam widths

Example

Beamwidth

• Half-power beamwidth (HPBW) is the angle between two vectors from the pattern’s origin to the points of the major lobe where the radiation intensity is half its maximum

• Often used to describe the antenna resolution properties

» Important in radar technology, radioastronomy, etc.

• First-null beamwidth (FNBW) is the angle between two vectors, originating at the pattern’s origin and tangent to the main beam at its base.

» Often FNBW ≈ 2*HPBW

Volts

Equivalent half-power beamwidth representations of an antenna’s radiation pattern.

Anisotropic sources: gain

Hypothetic isotropic antenna

Hypothetic directional antenna

• Every real antenna radiates more energy in some directions than in others (i.e. has directional properties)

• Idealized example of directional antenna: the radiated energy is concentrated in the yellow region (cone).

• Directive antenna gain: the power flux density is increased by (roughly) the inverse ratio of the yellow area and the total surface of the isotropic sphere

– Gain in the field intensity may also be considered - it is equal to the square root of the power gain.

ω

Plane angle: radian

• Angle in radians,ω = lω / r; lω = ω*r

l – lω is the length of the arc segment

supported by theω angle ω in a circle of radius r.

r– There are 2rad in a full

circle

– 1 rad = (360 / 2) deg

•

Solid angle: steradian

• Solid angle in steradians (sr),

= (S)/r2; S= r2

S is the spherical surface area supported by the solid angle in a sphere of radius r

• The steradian is the area cut out by the solid angle, divided by the sphere’s radius squared - ‘squared radian’.

• If the area is S, and the radius is d, then the angle is S/d2 steradians. The total solid angle (a full sphere) is thus 4steradians.

As one radian is 180/π = 57.3 degrees, the total solid angle is 4x (57.3)2 41253 square degrees, one steradian is 3282.806 square degrees, and one square degree is about 305 x 10-6 steradians

Antenna gain measurement

Reference antenna

Measuring equipment

Actual antenna

Measuring equipment

Po = Power delivered to

the reference antenna

S0 = Power received (the same in both steps)

P = Power delivered to the actual antenna

S = Power received

(the same in both steps)

Step 1: reference Step 2: substitution

Antenna Gain = (P/Po) S=S0

Antenna Gains Gi, G

d

• Unless otherwise specified, the gain refers to the direction of maximum radiation.

• Gain is a dimension-less factor related to power and usually expressed in decibels

• Gi “Isotropic Power Gain” – theoretical concept, the

reference antenna is isotropic

• Gd - the reference antenna is a half-wave dipole

Typical Gain and Beamwidth

Type of antenna Gi [dB] BeamW.

Isotropic 0 3600x3600

Half-wave Dipole 2 3600x1200

Helix (10 turn) 14 350x350

Small dish 16 300x300

Large dish 45 10x10

Gain, Directivity, Radiation Efficiency

• The radiation intensity, directivityG( ,) D( ,)

and gain are measures of the ability of an antenna to concentrate power in a particular direction.

• Directivity relates to the power radiated by antenna (P0 )

• Gain relates to the power delivered to antenna (PT)

PT

P0

• : radiation efficiency (0.5 - 0.75)

Antenna gain and effective area

• Effective area: Measure of the effective absorption area presented by an antenna to an incident plane wave.

• Depends on the antenna gain and wavelength2

Ae

4G(, ) [m

2 ]

Aperture efficiency: a = Ae / AA: physical area of antenna’s aperture, square meters

e.i.r.p.

• Equivalent Isotropically Radiated Power (in a given direction):

e.i.r. p. PGi

• The product of the power supplied to the antenna and the antenna gain (relative to an isotropic antenna) in a given direction

Linear Polarization

In a linearly polarized

plane wave the direction of the E (or

H) vector is constant .

Polarization ellipse

Ex• The superposition of two

coherent plane-wave

M components results in an

Ey elliptically polarized wave

•

N The polarization ellipse is• defined by its axial ratio

N/M (ellipticity), tilt angle

and sense of rotation

Elliptical Polarization

LHC

Ex = cos (wt) Ey = cos (wt)

Ex = cos (wt)Ey = cos (wt+pi/4)

Ex = cos (wt) Ey = -sin (wt)

Ex = cos (wt)Ey = cos (wt+3pi/4)

Ex = cos (wt)Ey = -cos (wt+pi/4)

RHCEx = cos (wt) Ey = sin (wt)

:

• At any moment in a chosen reference point in space, there is actually a single electric vector E (and associated magnetic vector H).

• This is the result of superposition (addition) of the instantaneous fields E (and H) produced by all radiation sources active at the moment.

• The separation of fields by their wavelength, polarization, or direction is the result of ‘filtration’.

Polarization Efficiency

• The power received by an antennafrom a particular direction is maximal if the polarization of the incident wave and the polarization of the antenna in the wave arrival direction have:

– the same axial ratio

– the same sense of polarization

– the same spatial orientation

.

2

Polarization filters/ reflectors

Wall of thin parallel wires (conductors)

|E1|>0 |E | = 0

|E1|

>0|E2| ~ |E2|

Vector E wires

Vector E wires

Reflecting

Wire distance ~ 0.1

Transparent

• At the surface of ideal conductor the tangential electrical field component = 0

Gen

erat

or

Transmitting antenna equivalent circuit

Transmitter Transm. line Radio wave

jXG

RG

VG

jXA

Rr

Rl

The transmitter with the transmission line is represented by an (Thevenin) equivalent

generatorThe antenna is represented by its input impedance

(which is frequency-dependent and is

influenced by objects nearby) as seem from the generator

jXA represents energy stored in electric (Ee) and magnetic (Em) near-field components; if |Ee| = |Em| then XA = 0 (antenna resonance)

Rr represents energy radiated into space (far-field components)Rl represents energy lost, i.e. transformed

2

Power vs. field strength

Pr E E

Z0

Pr Z0

E E2 E

2

EH

Z0Z0 377 ohms

for plane wave in free

space

Ant

enna

Receiving antenna equivalent circuit

Radio wave

Antenna

Transm.line

Receiver

jXA

Rr

Rl

VA

jXL

RL

The antenna with the transmission line is represented by an (Thevenin) equivalent generator

The receiver is represented by its input impedance as seen from the antenna terminals (i.e. transformed by the transmission line)

VA is the (induced by the incident wave) voltage at the antenna terminals determined when the antenna is open circuited

PA

/ P

Am

ax

Power transfer

1

0.5

0 0.1 110

RA / RG; (XA+XG = 0)

• The maximum power is delivered to (or from) the antenna when the antenna impedance and the impedance of the equivalent generator (or load) are matched

• When the impedances are matched

– Half of the source power is delivered to the load and half is dissipated within the (equivalent) generator as heat

– In the case of receiving antenna, a part (Pl) of the power captured is lost as heat in the antenna elements, the other part being reradiated (scattered) back into space

• Even when the antenna losses tend to zero, still only half of the power captured is delivered to the load (in the case of conjugate matching), the other half being scattered back into space

Satellite antennas (TV)

• Not an array!

Owens Valley Radio

Observatory Array

The Earth’s atmosphere is transparent in the narrow visible-light window(4000-7000angstroms) and the radio band between 1 mmand 10 m.

New Mexico Very Large Array

27 antennas along 3 railroad tracks provide baselines up to 35 km. Radio images are formed by correlating the signals garnered by each antenna.

R Struzak

2 GHz adaptive antenna array

• A set of 48 2 GHz antennas

• Switched beam antennas

– Based on switching function between separate directive antennas or predefined beams of an array

• Space Division Multiple Access (SDMA) = allocating an angle direction sector to each user– In a TDMA system, two users will be

allocated to the same time slot and the same carrier frequency

– They will be differentiated by different direction angles

Adaptive (“Intelligent”) Antennas

• Array of N antennas in a linear, circular, or planar configuration

• Used for selection signals from desired sources and suppress incident signals from undesired sources

• The antenna pattern track the sources

• It is then adjusted to null out the interferers and to maximize the signal to interference ratio (SIR)

• Able to receive and combine constructively multipath signals

Relative distance, Br

Z /

377

Field impedance

100

10

1

0.1

0.01

Short dipole

Small loop

Field impedanc e

Z = E/Hdepends

on the antennatype and

0.01 0.1 1 10100

Distance / (lambda/ 2Pi)

on distance

Far-Field, Near-Field

• Near-field region:

– Angular distribution of energy depends on distance from the antenna;

– Reactive field components dominate (L, C)

• Far-field region:

– Angular distribution of energy is independent on distance;

– Radiating field component dominates (R)

– The resultant EM field can locally be treated as uniform (TEM)

• The EM field at large distances from an antenna can be treated as originated at a point source - fictitious volume-less emitter.

• The EM field in a homogenous unlimited medium at large distances from an antenna can be approximated by an uniform plane TEM wave