ANTENNA

-A structure that is generally a metallic object, often a wire or group of wires, used to convert high
frequency signals into electromagnetic waves and vice versa

-A device whose function is to radiate electromagnetic energy and/or intercept electromagnetic

Basic Consideration: Maximum Power Transfer

FIELDS OF AN ANTENNA

INDUCTION FIELD

Considered to extend out from the antenna to a distance of D2/8λ

Near Field or Fresnel Region

TRANSITION ZONE

Zone between the two(2) regions

RADIATION FIELD

Considered to extend out from a distance of 2D2/λ

Far Field or Fraunhoffer Region

BASIC FORMULAS

T = 1/f

Where: T – time

F - frequency

λ = c/f

Where: λ – wavelength

F – frequency

C – velocity in free space

CHARACTERISTICS OF ANTENNAS

RADIATION PATTERN

A line drawn to join points in space which have equal field intensity due to the source

Major Lobe – the direction of maximum radiation

Minor Lobe – the direction of minimum radiation

Null – the direction with radiation intensity equal to zero.

b. Power losses

a. Antenna height

c. Terminations at its end

d. Thickness of the antenna wire

PRINCIPLE OF RECIPROCITY

The characteristics of antennas, such as impedance and radiation pattern are identical, regardless of
use for reception or transmission.

ISOTROPIC ANTENNA

An antenna that radiates uniformly in all directions in space

DIRECTIVE GAIN

The ratio of the power density in a particular direction of one antenna to the power density that
would be radiated by an isotropic antenna.

HERTZIAN DIPOLE: 1.5 : 1 (1.76 dB)

HALF WAVE DIPOLE: 1.64 : 1 (2.15 dB)

NOTES:

The longer the antenna, the higher the directive gain

Non resonant antennas have higher directive gain than resonant antennas

The directive gain of all practical antennas is greater than unity

DIRECTIVITY, D

Maximum directive gain

The gain in the direction of one of the major lobes in the antenna’s radiation pattern.

POWER Overall gain considering losses and efficiency

Ap = %D

Where: % - antenna efficiency

D - directivity

ANTENNA RESISTANCE

The ratio of the applied voltage to the flowing current

1RADIATION RESISTANCE, Rr
AC resistance
 The ratio of the power radiated by the antenna to the square of the current at the feedpoint.

2. LOSS RESISTANCE, Rd

Antenna and ground resistance

Discharge or corona effects

Losses in imperfect dielectric very near the antenna

Eddy current loss

ANTENNA EFFICIENCY

% = Rr / Rr + Rd

The ratio of radiation resistance to the total system resistance

An antenna has a radiation resistance of 72 ohms, a loss resistance of 8 ohms, and a power gain of
16. What efficiency and directivity does it have.

a. 90% and 17.78

b. 10% and 17.78

c. 90% and 14.4

d. 10% and 14.4

To produce a power density of 1 mw/m2 in a given direction, at a distance of 2 km, an antenna

a. 11.25 dB
b. 13.21 dB
c. 10 dB
d. 6 dB

EFFECTIVE RADIATED POWER (ERP)

The product of the power fed to an antenna and its power gain

ERP = Total Radiated Power x Power Gain

EFFECTIVE ISOTROPIC RADIATED POWER (EIRP)

The power radiated by an antenna in its favored direction, taking the gain of the antenna into account
as referenced to an isotropic radiator.

BANDWIDTH

The operating frequency range of an antenna

Ratio of the power at the optimum direction of the antenna to that of the power 180 degrees from
the optimum direction

BEAMWIDTH

The degree of concentration of the antenna’s radiation

Angular separation between two half power points in a major lobe of an antenna radiation pattern

POLARIZATION

Space orientation of the waves that the antenna radiates

The electric field vector is always parallel to the antenna elements.

Ex. What is the wavelength of a 500 MHz signal?

a. 60 cm

b. 6 m

c. 0.06 m

d. 60 m

Ex. What is the electrical length of an antenna operating at a frequency of 500 kHz?

a. 500 m

b. 570 m

c. 600 m

d. 630 m

GROUNDING SYSTEMS

EFFECTS OF GROUND ON ANTENNAS

Whereas an ungrounded antenna with its image forms an antenna array, the bottom of the grounded
antenna is joined to the top of the image; the system acts as an antenna of double size.

GROUND SCREEN

A network of buried wires directly under the antenna, consisting of a large number of radials
extending from the base of the tower, like spokes on a wheel, and placed 15 and 30 cm below the
ground

COUNTERPOISE

A substitute for ground screen in areas of low conductivity, i.e. rock, mountains and antennas on top
of buildings
ANTENNA HEIGHT

ANTENNA HEIGHT

The actual antenna height should at least be λ/4, but where this is not possible, the effective height
should correspond to λ/4.

TOP LOADING

A good method of increasing radiation resistance by having a horizontal portion at the top of the
antenna

Effect: to increase the current at the base of the antenna and to make the current distribution more
uniform

EFFECTIVE LENGTH

Antennas behave as though (electrically) they were longer than their physical length

END EFFECT

The result of physical antennas having finite thickness, instead of being infinitely thin.

ANTENNA COUPLING AND IMPEDANCE MATCHING

ANTENNA COUPLING

A network composed of reactances and transformers, which may be lumped or distributed, to provide
impedance matching

REASONS FOR COUPLING

To provide the transmitter with the correct value of load resistance

To tune out the reactive component of the antenna impedance

To prevent illegal radiation of spurious frequencies

ANTENNA COUPLERS

Direct Coupler

The antennas are coupled directly to their transmitters

pi Coupler

Affords a wider reactance range, giving adequate harmonic suppression

Symmetrical pi Coupler

Used for balanced lines

IMPEDANCE MATCHING
Stub Matching

Accomplished by connecting the coax or twin lead to the stub and sliding the connections up or down
the stub until the proper SWR is indicated by a meter connected in the system.

Delta Matching

Accomplished by spreading the ends of the feedline and adjusting the spacing until optimum
performance is reached.

Gamma Matching

A sliding clamp is included in the assembly to permit fine tuning for minimum SWR at the time of
installation

Quarter Wave Matching

A section of transmission line one quarter wavelength long placed between the load and the line

BALUN

Used to connect an unbalanced (coaxial line) to a balanced antenna

CURRENT FED (LOW Z FEED)

An antenna is said to be current fed if it is fed at the point of current maximum

Includes all feed point impedances below 600 ohms

Ex. Center fed half wave dipole or Marconi antenna

VOLTAGE FED (HIGH Z FEED)

An antenna is said to be voltage fed if it is fed at the point of voltage maximum

Includes all feed point impedances in excess of 600 ohms

Ex. Center fed full wave dipole

ISOTROPIC ANTENNA

a standard reference antenna , radiating equally in all directions, so that the radiation pattern is
spherical.

ELEMENTARY DOUBLET

A theoretical antenna shorter than a wavelength used as a standard to which all other antenna
characteristics can be compared

E= 60pi le I sin θ / λr

where:

E – field strength

r – distance

Le – antenna length

I – antenna current

θ – angle of axis and point of maximum radiation

Ex. An elementary doublet is 10 cm long. If the 10 MHz current flowing through it is 2 A, what is the
field strength 20 km away from the doublet in a direction of maximum radiation?

a. 6.28 uV/m

b. 62.83 uV/m

c. 15.92 uV/m

d. 1.59 uV/m
DIPOLE

An antenna made up of two wires bent at 90 degrees to each other so as to be in the same line and
signal is fed at the center

HALF WAVE DIPOLE

Length is λ/2 and radiation pattern is a toroid (bidirectional)

NON-RESONANT ANTENNA

One in which there are no standing waves

Radiation pattern is directional

Standing waves are suppressed by the use of a correct termination to ensure that no power is
reflected, so that only a forward traveling wave will exist.

LONG WIRE ANTENNA

Lengths in the order of several wavelengths

When an antenna is 2 or more wavelengths long, it provides gain and a multilobe radiation pattern.

When terminated at one end, it becomes unidirectional.

RHOMBIC ANTENNA

Non-resonant antenna used for long distance sky wave transmission or reception of horizontally
polarized waves over distances from 200 to over 3000 miles at frequencies from 4 to 22 MHz

Consists of non-resonant antenna elements arranged differently, i.e. planar rhombus

Length of equal radiators = 2 to 8 ‘s

Angle of tilt: 40 to 75˚

Rt = 800 ohms

Rin = 650 to 700 ohms

Standing waves exist, caused by the presence of both a reflected traveling wave and the forward
wave.

Antenna whose length is a multiple of λ/4’s

HERTZ ANTENNA

An antenna system in which the ground is not an essential part

Half wave antenna used for frequencies above 2 MHz

Half Wave Dipole

MARCONI ANTENNA

Grounded Quarter Wavelength antenna

Quarter Wavelength antenna used for frequencies below 2 MHz; omnidirectional

Vertical Monopole

ANTENNA ARRAYS

ANTENNA ARRAY

A radiating system consisting of individual radiators or elements placed close together so as to be

DRIVEN ELEMENT

Element of an array connected to the output of the transmitter

Radiation not directly connected to the output of the transmitter

Receives energy through the induction field of a driven element

REFLECTOR

A parasitic element longer than the driven element and close to it reduces signal strength in its own
direction and increases it in the opposite direction.

DIRECTOR

A parasitic element shorter than the driven one from which it receives energy; tends to increase
radiation in its own direction

BROADSIDE ARRAY

Simplest array which consists of a number of dipoles of equal size, equally spaced along a straight line
with all dipoles fed in the same phase from the same source.

Typical antenna length: 2 to 10 ‘s

Typical spacing: λ/2 or λ/4

Number of elements: dozens

ENDFIRE ARRAY

Physical arrangement is the same as that of the broadside array

The magnitude of the current in each element is still the same as in every other element, there is now
a phase difference between these currents.

TURNSTILE ARRAY
Consists of two horizontal, half wave antennas mounted at right angles to each other

YAGI UDA ANTENNA

An array consisting of a driven element and one or more parasitic elements arranged collinearly and
close together.

FOLDED DIPOLE

Single antenna which consists of 2 elements, one is fed directly and the other coupled conductively at
the ends.

LOG PERIODIC ANTENNA

Main feature is frequency independence for both radiation resistance and pattern

Bandwidths of 10:1 are achievable with ease

Radiation patterns: uni and bidirectional

PYRAMIDAL ANTENNA

It looks and works in much the same way as a standard log periodic antenna, with one big
difference: the two halves of the transmission line are separated and positioned as a V, so each half
of the transmission line is in effect a single wire transmission line.

UHF AND MICROWAVE ANTENNAS

PARABOLIC ANTENNA

All waves coming from the source and reflected by the parabola will travel in the same distance by the
time they reach the directrix, no matter from what point on the parabola they are reflected.

Works on the principle of a parabola

Parabola – a plane curve defined as the locus of a point which moves so that its distance from another
point (called the focus) plus its distance from a straight line (directrix) is constant.
CASSEGRAIN ANTENNA

an antenna in which the radiator is mounted at or near the surface of a concave main reflector and is
aimed at a convex secondary reflector slightly inside the focus of the main reflector

BEAMWIDTH

Ф = 70λ / D

Ф0 = 2 Ф

Where:

D= mouth diameter, meters

Ф0 = beamwidth between nulls, degrees

Ф = beamwidth between half power points, degrees

λ = wavelength, meters

GAIN OF A PARABOLIC ANTENNA

Ap = 6 ( D/λ)2

Where: D= mouth diameter, meters

Ap = Power Gain

λ = wavelength, meters

Ex. Calculate the beamwidth between nulls of a 1m paraboloid reflector used at 6GHz.

a. 3.5˚

b. 7˚
c. 1.75˚

d. 14˚

HORN ANTENNA

Ideal as primary feed antenna for parabolic reflectors and lenses

LENS ANTENNA

Used as a collimator of frequencies in excess of 3 GHz

HELICAL ANTENNA

Broadband VHF and UHF antenna which is used when it is desired to provide circular polarization
characteristics

Consists of a loosely wound helix, backed up by a ground plane, which is simply a screen made of
chicken wire

DISCONE ANTENNA

A combination of a disk and a cone in close proximity

Characterized by an enormous bandwidth for both input impedance and radiation pattern

A constant angle, low gain antenna; omnidirectional

LOOP ANTENNA

Used for direction finding, because they do not radiate in a direction at right angles to the plane of the
loop.

For portable domestic receivers

Circular or square shaped

PHASED ARRAY
Group of antennas, connected to one transmitter or receiver, whose radiation beam can be adjusted
electronically without physically moving parts; used in radars.

WHIP ANTENNA

the most common example of a monopole antenna, an antenna with a single driven element and a
ground plane.

The whip antenna is a stiff but flexible wire mounted, usually vertically, with one end adjacent to a
ground plane.

SLOT ANTENNA

-consists of a metal surface, usually a flat plate, with a hole or slot cut out

-When the plate is driven as an antenna by a driving frequency, the slot radiates electromagnetic waves
in similar way to a dipole antenna.

NOTCH ANTENNA

-An open ended slot antenna

-Since the currents are zero at the middle of the slot, we may cut the ground plane here to make a notch
antenna.

ANTENNA SUPPORTING STRUCTURES

POLE-Used to signify one piece of structure similar to the common telephone pole

MAST-Used to designate a structure made of metal or wood which may be either in a form of a one
piece or sectionalized structure

TOWER-Applied to a very large, high structure, which in most cases, is constructed of metal