ANTENNAS

Engr. Pablo Barrac Asi

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.

ANTENNA

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

Basic Consideration: Maximum Power

Transfer

FIELDS OF AN ANTENNA

INDUCTION FIELD
Considered to extend
out from the antenna
to a distance of D
2/8
Near Field or Fresnel
Region

TRANSITION
ZONE
Zone between
the two(2)
regions

RADIATION
FIELD
Considered to
extend out from a
distance of 2D2/
Far Field or
Fraunhoffer Region

BASIC
FORMULAS
T = 1/f

Where: T time
F - frequency

= c/f
Where:
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.

FACTORS
WHICH
DETERMINE THE
RADIATION
PATTERN OF AN
ANTENNA

a. Antenna
height

b. Power
losses
c.
Terminations
at its end
d. Thickness of the
antenna wire

PRINCIPLE
OF
RECIPROCIT
Y
ISOTROPIC
ANTENNA

The characteristics of antennas,

such as impedance and
radiation pattern are identical,
regardless of use for reception or
transmission.
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:
HALF WAVE DIPOLE:
1.5 : 1 (1.76 dB)
1.64 : 1 (2.15 dB)

The longer the antenna, the

higher the directive gain

NOTES:

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
the major lobes in the antennas radiation patte
of

POWER
GAIN

Overall gain considering

losses and efficiency
Ap = %D
Where: % - antenna
efficiency
D - directivity

ANTENNA
RESISTANCE

1. RADIATION
RESISTANCE, Rr

The ratio of the

applied voltage
to the flowing
current

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
Losses in
ground
imperfect
resistance
dielectric very
Discharge or
near the
corona effects
Eddy current
antenna
loss

ANTENNA
EFFICIENCY
% = Rr / Rr + Rd
The ratio of the power radiated by the
antenna to the power delivered at the
feedpoint
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 radiates
a total of 180 w. An isotropic antenna would have to
radiate 2400 w to produce the same power density at
that distance. What, in dB, is the directive gain of the
practical antenna?
a. 11.25 dB
b. 13.21 dB
c. 10 dB
d. 6 dB

EFFECTIVE RADIATED
POWER (ERP)

EFFECTIVE ISOTROPIC
RADIATED POWER (EIRP)

The product of the power

fed to an antenna and its
power gain.
ERP = Total Radiated
Power x Power Gain

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

FRONT TO BACK
RATIO
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
antennas 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.

PHYSICAL
LENGTH

Actual length of the

antenna
L =/2; = c/f
/2 is the shortest length
of a conductor which will
resonate at a given
frequency

ELECTRICAL LENGTH
Dependent upon the
velocity coefficient or
velocity factor
L=k
where k is the dielectric
constant

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.

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.

COUNTERPOIS
E
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 /4, but
where this is not possible, the
effective height should
correspond to /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

EFFECTIV
E 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 tune out the reactive component of the

antenna impedance
To provide the transmitter with the correct value
of load resistance
To prevent illegal radiation of spurious
frequencies

ANTENNA COUPLERS
Direct Coupler

The antennas are coupled directly to their

transmitters

Coupler

Affords a wider reactance range, giving

adequate harmonic suppression

Symmetrical
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.

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

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

IMPEDANCE MATCHING
Quarter
Wave
Matchin
g
BALUN

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

REVIEW QUESTIONS

1. Device that converts high frequency current

into electromagnetic waves.
a. antenna
b. loudspeaker
c. microphone
d. lightning arrester

2. A polar diagram or graph representing field

strengths or power densities at various angular
positions relative to an antenna.
a. Venn Diagram
b. Figure 8 pattern
c. Lissajous figure
d. Radiation Pattern

3. Refers to the orientation of the electric field

radiated from an antenna.
a. radiation
b. polarization
c. beamwidth
d. bandwidth

4. Pertains to a wire structure placed below the

antenna and erected above the ground which
is a form of capacitive grounding system.
a. image
b. counterpoise
c. antenna orientation
d. polarization

5. What is the technique used to electrically

increase the antenna length?
a. loading
b. using image antenna
c. using antenna arrays
d. increasing antenna height

6. Antenna supported by insulators seems

electrically longer than its physical length due
to
a. image
b. reflection
c. end effect
d. broadside effect

7. The ratio of the power radiated by the

antenna to the total input power.
a. power gain
b. directive gain
c. antenna efficiency
d. radiation efficiency

8. The ratio of the front lobe power to the back

lobe power
a. front to side ratio
b. front to back ratio
c. back to front ratio
d. minor to major ratio

9. The standard reference antenna for directive

gain
a. infinitesimal dipole
b. isotropic antenna
c. elementary doublet
d. half wave dipole

10. The gain of a hertzian dipole with respect

to an isotropic antenna
a. 1.76 dB
b. 2.15 dB
c. 1.5 dB
d. 1.64 dB

11. A half wave dipole antenna is capable of radiating

2000 watts and has a 2.15 dB gain over an isotropic
antenna. How much power must be delivered to the
isotropic antenna to match the field strength of the
directional antenna?
a. 1640 watts
b. 3280 watts
c. 4300 watts
d. 3520 watts

12. An ungrounded antenna near the

ground
a. acts as a single antenna of twice the
height
b. is unlikely to need a ground screen
c. acts as an antenna array
d. must be horizontally polarized

13. Top loading is sometimes used with an

antenna in order to increase its
a. effective height
b. bandwidth
c. beamwidth
d. input capacitance

14. Very low signal strength in an

antenna
a. minor lobes
b. nulls
c. antenna patterns
d. major lobes

15. A horizontal antenna is ______

polarized.
a. vertically
b. horizontally
c. centrally
d. circularly

16. An antenna with unity

gain.
a. rhombic
b. half wave dipole
c. isotropic
d. whip

17. What is the front to back ratio of an

antenna which radiates 500 watts in a
northernly direction and 50 watts in a
southernly direction?
a. 25000 dB
b. 10 dB
c. 100 dB
d. 20 dB

18. Good grounding is important

for
a. Horizontal antennas
b. Broadside array
c. Vertical antennas
d. Yagi Uda Antennas

19. If the radiated power increases 10.89 times,

the antenna current increases by
a. 3.3 times
b. 6.6 times
c. 1.82 times
d. 10.89 times

20. Shortening effect of an antenna that makes

it appear as if it were 5% longer
a. end effect
b. flywheel effect
c. skin effect
d. capture effect

21. If an antenna is too short for the wavelength

being used, the effective length can be
increased by adding
a. capacitance in series
b. inductance in series
c. resistance in parallel
d. resistance in series

22. Actual height of an antenna should be at

least
a. 1
b. /2
c. /4
d.

23. The directivity pattern of an isotropic

radiator
a. figure 8
b. a sphere
c. unidirectional cardioid
d. parabola

24. A Hertz antenna is operating on a frequency of

2182 kHz and consists of a horizontal wire that is
hanged between two towers. What is the frequency of
its third harmonic?
a. 727 kHz
b. 6546 kHz
c. 436 kHz
d. 6.546 kHz

25. What is the gain of an antenna over a half

wavelength dipole when it has 6 dB gain over an
isotropic radiator?
a. 6 dB
b. 8.1 dB
c. 3.9 dB
d. 10 d

BASIC TYPES OF
ANTENNAS

ISOTROPIC ANTENNA

ELEMENTARY DOUBLET

a standard reference antenna ,

radiating equally in all
directions, so that the radiation
pattern is spherical.

A theoretical antenna shorter

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

 = 60 le I sin / r
Where

angle of axis and point of maximum

radiation
Le antenna
electric field
length
intensity
I antenna current
r antenna
length

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 /2 and
radiation pattern is a
toroid (bidirectional)

VOLTAGE AND CURRENT CHARACTERISTICS

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
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

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.

RESONANT ANTENNA
Standing waves exist, caused by
the presence of both a reflected
traveling wave and the forward
wave.
Antenna whose length is a
multiple of /4s

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 within each
others induction
field

DRIVEN
ELEMENT

Element of an array
connected to the
output of the
transmitter

PARASITIC
ELEMENT

Radiation not directly

connected to the output
of the transmitter
Receives energy through
the induction field of a
driven element

REFLECTOR

DIRECTOR

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.

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: /2 or /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

LOG PERIODIC ANTENNA

Main feature is frequency
independence for both
radiation resistance and
pattern

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.

a type of log periodic

antenna

UHF AND MICROWAVE

ANTENNAS

PARABOLIC ANTENNA
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.

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.

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

Where:
D= mouth diameter, meters

= 70 /
D
0 = 2

0 = beamwidth between nulls,

degrees
= beamwidth
between half power
points, degrees
= wavelength,
meters

GAIN OF A PARABOLIC
ANTENNA
Ap = 6 ( D/)2
Where:
D= mouth diameter,
Ap =meters
Power Gain
= 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

Ex. Calculate the gain of the paraboloid

reflector in the previous problem.
a. 9600
b. 2400
c. 1600
d. 2800

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 SUPPORT
STRUCTURES

ANTENNA SUPPORTING STRUCTURES

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

MAST

TOWER

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

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

REVIEW QUESTIONS

A non-resonant antenna that is capable of

operating satisfactorily over a relatively wide
bandwidth, making it ideally suited for HF
transmission.
a. end-fire array
b. rhombic
c. broadside array
d. log periodic
1.

2. Antenna that is independent of their radiation

resistance and radiation pattern to frequency. It
has bandwidth ratios of 10:1 or greater.
a. loop antenna
b. helical
c. Yagi Uda antenna
d. Log periodic antenna

3. A half wave antenna

a. Hertz
b. Marconi
c. Parabolic
d. Vertical Monopole

4. Antenna which is used very frequently but almost

entirely as a reception antenna and is usually found
at the back of table radios.
a. loop antenna
b. folded antenna
c. rhombic
d. log periodic

5. One of the special purpose antennas which has

broadband VHF and UHF that is ideally suited for
applications for which radiating circular rather than
horizontal or vertical polarized electromagnetic
waves are required.
a. loop antenna
b. phased array
c. folded dipole
d. helical

6. What are the two types of antenna elements?

a. driven and reflector
b. director and reflector
c. parasitic and director
d. driven and parasitic

7. Which one of the following terms does not

apply to the Yagi Uda array?
a. good bandwidth
b. parasitic elements
c. folded dipole
d. high gain

8. Indicate the antenna that is not

wideband.
a. discone
b. folded dipole
c. helical
d. Marconi

9. One of the following is not an omnidirectional

antenna
a. halfwave dipole
b. log periodic antenna
c. discone
d. Marconi

10. One of the following consists of nonresonant

antennas
a. rhombic
b. folded dipole
c. end fire array
d. broadside array

11. Which of the following is best excited

from a waveguide?
a. biconical
b. horn
c. helical
d. discone

12. An antenna that is circularly

polarized
a. parabolic reflector
b. Yagi Uda
c. Helical
d. Circular loop

13. What is the polarization of a discone

antenna?
a. vertical
b. horizontal
c. circular
d. spiral

14. When speaking of antennas, ____ is a section

which would be a complete antenna by itself.
a. image
b. top loading
c. bay
d. quarterwave

15. ______ is an antenna with a number of half

wave antennas in it.
a. antenna array
b. tower
c. omnidirectional
d. rhombic

16. Which antenna radiates an omnidirectional

pattern in the horizontal plane with vertical
polarization?
a. Marconi
b. Discone
c. Horn
d. Helical

17. An antenna with very high gain and very

narrow beamwidth.
a. helical
b. discone
c. horn
d. parabolic dish

18. An open ended slot

antenna
a. helical
b. rhombic
c. notch
d. cassegrain

19. Which antenna is properly

terminated?
a. Marconi
b. Rhombic
c. Dipole
d. Yagi Uda

20. What is the radiation characteristic of a

dipole antenna?
a. omnidirectional
b. bidirectional
c. unidirectional
d. hemispherical

21. An antenna which is one tenth

wavelength long.
a. hertz antenna
b. loop antenna
c. Marconi antenna
d. Elementary doublet

22. What is the minimum number of turns a

helical antenna must have?
a. 3
b. 4
c. 5
d. 6

23. An antenna made up of a number of full

wavelengths
a. elementary doublet
b. log periodic
c. long wire
d. whip

24. Which of the following improves

antenna directivity?
a. driven element
b. reflector
c. director
d. parasitic element

25. The frequency of operation of a dipole

antenna cut to a length of 3.4 m
a. 88 MHz
b. 44 MHz
c. 22 MHz
d. 11 MHz

26. Where are the voltage nodes in a half wave

antenna?
a. at the ends
b. three quarters of the way from the feedpoint
towards the end
c. one half of the way from the feedpoint towards
the end
d. at the feedpoint

27. A simple half wavelength antenna

radiates the strongest signal
a. at 45 degrees to its axis
b. parallel to its axis
c. at right angles to its axis
d. at 60 degrees to its axis

28. An antenna array which is highly directional

at right angles to the plane of the array
a. Broadside
b. End fire
c. Turnstile
d. Log periodic

29. What is the usual electrical length of a

driven element in an HF beam antenna?
a. /4
b. /2
c. 3/4
d. 1

30. The input terminal impedance at the

center of a folded dipole antenna
a. 72
b. 300
c. 50
d. 73

31. The length of a Marconi antenna to operate

with 985 kHz is
a. 200 ft
b. 500 ft
c. 250 ft
d. 999 ft

32. The purpose of stacking elements on an

antenna
a. sharper directional pattern
b. increased gain
c. improved bandpass
d. all of these

33. Which of the following antennas is used for testing

and adjusting a transmitter for proper modulation,
amplifier operation and frequency accuracy?
a. elementary doublet
b. real
c. isotropic
d. dummy