ANTENNA THEORY

ENGR. DIVINO FIEL A. DE BIEN

ECE # 38900
Definition
 A conductor that can radiate or
receive electromagnetic signal.
◦ Transmission - radiates
electromagnetic energy into space
◦ Reception - collects
electromagnetic energy from space
 Principle of Reciprocity
◦ Characteristics of antenna are
independent whether it is used as a
transmitting or a receiving antenna.
The Isotropic Radiator
 An ideal isotropic radiator would radii all
the electrical power supplied it and would
do so equally in all directions.
 It would also be a point source, that is, it
would have zero size.

r – any fixed distance from the source to

r
where the intensity is measured
Half-wave dipole antenna
 The word dipole simply means it has two parts, as
shown.
 A half-wave dipole is sometimes called a Hertz
antenna
 95% of one-half the wavelength measured in free
space.

one half wavelength


4

balanced feederline
Design Problem # 1
 Calculate the length of a half-wave dipole
for an operating frequency of 20MHz.
one half wavelength

balanced feederline
Radiation Resistance
 The portion of an antenna's input
impedance that is due to power radiated
into space

Prad Prad Rrad

  
PT Prad  Pd Rrad  Re
Where:
η = antenna efficiency
Prad = power radiated by an antenna (W)
Pd = power dissipated in antenna (W)
Rrad = radiation resistance (ohms)
Re = effective antenna resistance (ground resistance, ohms)
Design Problem # 2
 A dipole antenna has a radiation
resistance of 67 Ω and a loss resistance of
5 Ω, measured at the feed point. Calculate
the efficiency.
Antenna Characteristics
 Radiation Pattern
 Gain and Directivity
 Beamwidth
 Front-to-Back Ratio
 Major and Minor Lobes
 EIRP and ERP
 Impedance
 Polarization
 Ground Effects
 Others
Radiation Pattern
 a polar diagram or
graph representing
field strengths or
power densities at
various angular
positions relative
to an antenna.
Radiation or Induction Field
 NEAR FIELD or INDUCTION
FIELD or FRESNEL FIELD
◦ field pattern that is close to the 2D 2
antenna R
 FAR FIELD or RADIATION 
FIELD or FRAUNHOFER Where:
FIELD R = the distance
◦ field pattern that is at great from the antenna
distance from the antenna where D = dimension of
power that reach this region the antenna
continues to radiate outward and
is never returned to the antenna.  = wavelength of
the transmitted
signal
Design Problem # 3
 Determine the distance from a /2 dipole
to the boundary of the far-field region if
the /2 dipole is used in a 150 MHz
communication system.
Gain and Directivity
 DIRECTIVE GAIN – it is PD
the ratio of te power D
density in a particular
PD (ref )
direction of one antenna to
the power density that AP  D
would be radiated by an
omnidirectional antenna AP dB  10 log D
 POWER GAIN – it is a
comparison of the output Where:
power of an antenna in a η = antenna efficiency
PD = power density radiated
certain direction to that of PD(ref) = power density of
an isotropic antenna reference antenna
Antenna Gain
 Relationship between antenna gain and effective
area
4Ae 4f Ae 2
G 
2 c2
 G = antenna gain
 Ae = effective area
 f = carrier frequency
 c = speed of light (» 3 ´ 108 m/s)
  = carrier wavelength
Beamwidth
beamwidth
 the angle created
by comparing the
half power points
(3dB) on the main
• A Radiated
energy is
focused in
Max
radiation lobe to power a specific
its maximum direction
power point.
 It is the angular
separation
between the two
half power points
on the power antenna Power 3dB
density radiation 2 dipole
down from
pattern. maximum
point A
Bandwidth
 refers to the range of frequencies the
antenna will radiate effectively.
Front-to-Back Ratio

 The ratio expressed in dB of the output

in the most optimum direction to the
output 180 degrees away from the
optimum direction
Major and Minor Lobes
 Front lobe –
major lobe; lobe
that receives the
most energy
 Side lobes –
lobes adjacent to
the front lobe
 Back lobes –
lobes in direction
exactly opposite
the front lobe
ERP and EIRP
 ERP (Effective Radiated Power): is the
radiated power (transmit power times
antenna gain) with respect to a dipole
antenna within a given geographic area.
 EiRP (Effective Isotropic Radiated Power):
is the radiated power from an isotropic
antenna.
EiRP = ERP + 2.15 (dB)
Polarization
 this refers to the direction in space of the
E field radiated by the transmitting system
Ground Effects
 The ground is a good
conductor for low and
medium frequencies and
acts as a large mirror for
the radiated energy.
 By use of the monopole
antenna, which is
quarter- wave in physical
length.
 Monopole Antennas uses
the ground as the other
quarter-wavelength,
making the antenna
electrically half-
wavelength.
Earth Mat (Ground Screen)
 A network of up to 120 buried wires 15
to 30 m below the ground under the
antenna.
 The ground screen reduces ground
absorption losses that occur when
antenna is erected over ground with poor
conductivity.
 The height of the antenna can be set
accurately, and the radiation resistance
can be determined more accurately.
Counterpoise
 Consists of a structure made of wire
erected at a distance above the ground
and insulated from the ground. This is
installed above the ground
Review Questions
1. For an antenna with input power of 100
W, rms current of 2 A and effective
resistance of 2 ohms, determine the
a) antenna radiation resistance;
b) antenna efficiency;
c) power radiated from the antenna.
2. Determine the power gain in dB for an
antenna with a directive gain of 50 dB
and efficiency of 75%
Review Questions
3. Determine the EIRP in dBm for an antenna
with directivity of 33 dB, efficiency of 82%
and input power of 100W.
4. Find the power density at a point 20 km
from an antenna with input power of 1kW
and power gain of 23 dB.
5. A half‐wave dipole is driven with a 10‐W
signal at 200 MHz. A receiving dipole100
km away is aligned such that the gain is cut
in half. Determine the receive power and
voltage into the 73‐ohm receiver.
Review Questions
6. Determine the distance from a parabolic
reflector with diameter (D) = 4.5 m to
the boundary of the far-field region if
the parabolic reflector is used for Ku-
Band transmission of a 12 GHz signal.
ANTENNA THEORY

BASIC ANTENNA
Elementary Doublet
 an electrically short dipole

60Il sin  30I l sin  2 2 2

E PD 
R R
2 2

Where: E = electric field intensity (V/m)

I = dipole current (A, rms)
l = end‐to‐end length of the dipole
R = distance from the dipole
λ = wavelength
 = angle between the axis of the antenna and
the direction of radiation
Design Problem # 3
 An elementary doublet is 8 cm long. If the
20 MHz current flowing through it is 3A,
what is the field strength 25 km away
from the doublet, in a direction of
maximum radiation?
Marconi Antenna
 Monopole (single)
antenna one-quarter
wavelength long
mounted vertically
with the lower end
either directly
connected to ground
or through a coupling
network.
 Current maxima
occurs at the
grounded ends
GENERAL TYPES OF
ANTENNA LOADING
 TOP LOADING – the loading component
is attached at the top of the antenna
structure
 CENTER LOADING – the loading
component is placed along the antenna
structure, approximately half way between
the feedpoint and the end.
 BASE LOADING – the loading
component is located at the bottom of
the base antenna structure
Antenna Loading
ANTENNA THEORY

ANTENNA ARRAYS
 Antenna Arrays
 Formed when two or more
antenna elements are
combined to form a single
antenna
 Increase the directivity of the
antenna and concentrates
radiated power within a small
geographic area
 Antenna elements can be
driven or parasitic.
◦ Driven elements are directly
connected to the transmission
line and receive power from the
source.
◦ Parasitic elements receive
energy through mutual
induction with a driven or
another parasitic element.
Collinear Array
 It is comprised of quarter-wave coaxial sections with inner
and outer conductors transposed at each junction.
 The number of elements is increased, the gain increases and
the beamwidth decreases.

(a) single half-

wave dipole,

(b) two-
element array,

(c) Three
element
array
Broadside Array
 Made by placing
several resonant
dipoles of equal size
in parallel with each
other and in a
straight line. All
elements are fed in
phase from the
source
 Radiates at right
angles to the plane
of the array and
very little to the
direction of the
plane
End‐fire Array
 Same element configuration as the
broadside array except that the
transmission line is not crisscrossed
between elements
Rhombic Antenna
 A nonresonant antenna suited for HF
transmission
 Made up of four nonresonant elements
terminated in a resistor
ANTENNA THEORY

SPECIAL‐PURPOSE
ANTENNAS
Folded Dipole
 A single antenna made up of two elements
 Input impedance is equal to half‐wave
impedance (72 Ω) times the square of the
number of folded wires. (22 * 72 = 288 Ω)
Yagi‐Uda Antenna
 A linear array consisting of a dipole and two or
more parasitic elements: one reflector and one or
more directors
 Commonly used for VHF TV transmission
Turnstile Antenna
 Formed by placing two dipoles at right
angles to each other (90 degrees out of
phase)
 Radiation pattern produces nearly an
omnidirectional pattern
Log‐Periodic Antenna
 Consists of several dipoles of different length and spacing that are
fed from a single source at the small end. The transmission line is
crisscrossed between the feedpoints of adjacent pairs of dipoles
 Advantage: independent of radiation resistance and radiation
pattern to frequency
 Not a type of antenna but a class of antenna
 Physical structure is repetitive, making electrical characteristics
repetitive as well
Log‐Periodic Design

Where:
1 Rn Ln R = dipole spacing
  L = dipole length
 Rn 1 Ln 1 τ = design ratio (less than 1)

R2 R3 R4 1 L2 L3 L4 For a typical design:

      τ = 0.7; α = 30°
R1 R2 R3  L1 L2 L3
 Helical Antenna
 A broadband VHF or UHF antenna suited for applications for which
radiating circularly‐polarized electromagnetic waves are required
 Mounted on a ground plane made up of either solid metal or metal screen
 Two modes of propagation are available: normal and axial.

Power Gain of a Helical Antenna 3‐dB Beamwidth

  D  2 NS  52
Ap dB   10 log 15 

     

D   NS  
Where:
Ap (dB) = antenna power gain (dB) S = pitch
D = helix diameter (m) λ = wavelength
N = number of turns
Design Problem # 4
 Determine the power gain and
beamwidth for an end‐fire helical antenna
with the following parameters:
◦ helix diameter = 0.1 m
◦ number of turns = 10
◦ pitch = 0.05 m
◦ frequency of operation = 500 MHz
ANTENNA THEORY

UHF AND MICROWAVE

ANTENNAS
Parabolic Antenna
 A high-gain
reflector antenna
used for radio,
television and data
communications,
and also for
radiolocation
(radar), on the
UHF and SHF
parts of the
electromagnetic
spectrum. The biggest facility for satellite
communication in Raisting, Bavaria,
Germany.
Parabolic Antenna
 Consists of a parabolic
reflector and the feed
mechanism
 Feed mechanism radiates
the energy toward the
reflector (center feed, horn
feed, Cassegrain feed)
 Parabolic reflectors are
sometimes called parabolic
dish antennas
 All waves radiated toward
the parabola from the
focus will be in phase when
they reach the directrix,
regardless from which
point on the parabola they
are reflected
Types of Parabolic
 Off-set antenna
◦ the feed element is still located at
the focal point, which because of the
angles utilized Gregorian

◦ usually located below the reflector

so that the feed element and
support do not interfere with the
main beam
 Gregorian antenna
◦ can be identified by the fact that it
uses a concave sub-reflector
 Cassegrain antenna
◦ uses a convex sub-reflector
Design Parameters
 Parabolic Antenna Beamwidth
Where:
70
  = Beamwidth at 3dB
D = diameter (m)
D λ = wavelength

 Beamwidth between nulls:

Where:
0  2  = Beamwidth at 3dB
0 = Beamwidth between nulls

 Parabolic Antenna Power Gain

Where:
 D 
2

G  
G = antenna power gain
 D = diameter (m)
   λ = wavelength
Design Problem # 5
 Determine the beamwidth and transmit
and receive power gains of a parabolic
antenna with the following parameters:
dish diameter of 2.5 m, frequency of
operation of 4 GHz, and a 55% efficiency.
Horn Antenna
 To overcome the difficulties in radiating energy using a
waveguide, the mouth of the waveguide maybe opened out,
as was done to the transmission line, but this time an
electromagnetic horn results instead of the dipole.
◦ Sectoral horn – flares out in one direction only.
◦ Pyramidal Horn – flares out in both direction and has the shape of a
truncated pyramid
◦ Conical Horn – flares out in both directions and is a logical
termination for a circular waveguide.
Review Questions
 Calculate the ERP from a Yagi-Uda antenna
driven with 500 W. (ANS. 2500W)
 An antenna has a maximum forward gain of
14 dB at its 108 MHz center frequency. Its
reverse gain is -8dB. Its beamwidth is 36
and the bandwidth extends from 55 to 185
MHz. Calculate:
◦ Gain at 18 from maximum forward gain (ANS.
11dB)
◦ Bandwidth (ANS. 130 MHz)
◦ F/B ratio (ANS. 22 dB)
◦ Maximum gain at 185 MHz (ANS. 11 dB)
Review Questions
 A /2 dipole is driven with a 5W signal at
225 MHz.A receiving dipole 100km away
is aligned so that its gain is cut in half.
Calculate the received power and voltage
into a 73 receiver. (ANS. 7.57 pW,
23.5V)
 If a field intensity of 25 mV/m develops
2.7 V in a certain antenna, what is its
effective height? (ANS. 108m)
THE END
The Yagi Antenna is a directional
antenna invented by Dr. Hidetsugu Yagi
of Tohoku Imperial University and his
assistant, Dr. Shintaro Uta.
Dr. Yagi's invention was ahead of its
time (patented in 1926) and therefore
not understood in Japan.
Its value was, however, accepted in
Europe and North America, where it
entered commercial production.
It is said that people in Japan realized
the true value of the Yagi Antenna in
World War 2 when it was discovered
that the invention was used as a radar
antenna by the Allies.