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15 Νοε 2013 (πριν από 4 χρόνια και 6 μήνες)

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1

Antennas

Sources:

(1) Course materials developed by CDR Hewitt Hymas, USN

(2) Frenzel, Principles of Electronic Communication Systems, 3
rd

ed., McGraw Hill, 2008

Antenna Fundamentals

a. Describe the purpose of an antenna.

b. Define Reciprocity.

c.
State and describe the various polarizations possible in antennas.

d. Calculate the far
-
field distance for a given antenna.

e. Describe the construction of an antenna from a transmission line.

Antennas

a device that provides a transition between guided el
ectromagnetic waves in wires and electromagnetic waves in
free space.

Reciprocity

Antennas can usually handle this transition in both directions (transmitting and receiving EM waves).
This property is called
reciprocity
.

The antenna’s size and shape
largely determines the frequencies it can handle and how it radiates
electromagnetic waves.

Antenna polarization

The polarization of an antenna refers to the orientation of the
electric field

it produces.

o

Polarization is important because the receiving a
ntenna should have the same polarization as the
transmitting antenna to maximize received power.

Types of Polarization

o

Horizontal Polarization

2

o

Vertical Polarization

o

Circular Polarization

Electric and magnetic field rotate at the frequency of the
transmitter

Used when the orientation of the receiving antenna is unknown

Will work for both vertical and horizontal antennas

Right Hand Circular Polarization (RHCP)

Left Hand Circular Polarization (LHCP)

Both antennas must be the same orientation (RHCP or

LHCP)

Wavelength and antennas

recall

that wavelength (

) and frequency (
f
) of an electromagnetic wave in free space are related by the
speed of light (
c
)
. (
c
=3.0x10
8

m
/
s
)

The dimensions of an antenna are usually expressed in terms of
wavelength

(

).

o

Low frequencies imply long wavelengths, hence low frequency antennas are very large.

o

High frequencies imply short wavelengths, hence high frequency antennas are usually small.

What to look for in Antennas

Freq/Wavelength

Beam Pattern

Bandwidth

Gain

Basic

Antenna

An antenna can be a length of wire, a metal rod, or a piece of metal tubing.

Antennas radiate most effectively when their length is directly related to the wavelength of the
transmitted signal.

Most antennas have a length that is some fraction of
a wavelength.

One
-
half and one
-
quarter wavelengths are most common.

isotropic point source
-

Power fro
m an isotropic point source is
equally distributed in all directions
.
It
is completely
unfocused
.

Antenna radiates equally in all directions
.

3

Antenna
gain (
G
)

Because an antenna is a passive device, the power radiated can not be greater than the input power.

The ability of an antenna to focus electro
-
magnetic energy is defined by its
gain
.

Antenna gain is expressed as a ratio of the effective radiated o
utput power (P
out
) to the input power (P
in
)

The
gain

of an antenna is a measure of power transmitted
relative

to that transmitted by an isotropic
source.

Antenna gain relative to an isotropic source is expressed in decibels as
dBi
.

r

The

(ERP or EIRP) is the gain of an antenna (with respect to an isotropic
radiator) multiplied by its input power.

For example, a highly directional antenna with a gain of 7 has an input power of 1
-
kW. Its ERP is
therefore 7
kW.

Dipole Antenna

One of the most widely used antenna types is the half
-
wave dipole.

The half
-
wave dipole, also called a doublet, is formally known as the Hertz antenna.

A dipole antenna is two pieces of wire, rod, or tubing that are one
-
quarter
wavelength long at the
operating resonant frequency.

Electric and Magnetic Fields around a Transmission Line

Converting a Transmission Line into an Antenna

4

The Dipole Antenna

Three
-
dimen
sional radiation pattern for a dipole

5

Antenna Losses

o

o

Resistive Loss

o

Results in heat due to resistance of conductor

Dipole Definitions

The dipole has an impedance of 73
Ω

at its center, which is the radiation resistance.

o

An antenna
ideally appears as a resistor to the transmitter. This “radiation resistance” does not
dissipate power in the form of heat; the power is dissipated as radiated electromagnetic energy.

An antenna is a frequency
-
sensitive device, and a particular antenna can

be operated over a range of
frequencies (BW).

At the resonant frequency, the antenna appears to be a pure resistance of 73 Ω

Dipole
Antenna gain (
G
)

A dipole antenna gain is 1.64

A half
-
wave dipole antenna has a power gain of 1.64 (or 2.15 dB) over an
isotropic source.

Antenna gain relative to a dipole antenna can be expressed in decibels as
dBd
.

Thus, an antenna with a gain of 3 dBd would have a gain of 5.15 dBi (3 dB + 2.15 dB)

Dipole Antenna Lengths

A dipole resonates best when it is approx. 95% of
the actual “half
-
wavelength length”

Shortcut:

L
feet

= 468/f
MHz

(This is in Feet)

1 ft = .3048 m

Dipole hung vertically is closest to an isotropic radiator

6

Bottom of dipole antenna should be at least ½ a wavelength off the ground

o

May make total structure height unreasonable

Conical Antenna

A common way to increase bandwidth is to use a version of the dipole antenna known as the conical
antenna.

The center radiation resistance of a conical antenna is much higher than the 73
Ω

usua
lly found when
straight
-
wire or tubing conductors are used.

The primary advantage of conical antennas is their tremendous bandwidth.

They can maintain a constant impedance and gain over a 4:1 frequency range.

Antenna Beam Width

Marconi or
Ground
-
Plane Vertical Antenna

The quarter
-
wavelength vertical antenna, also called a
Marconi antenna is widely used.

It is similar in operation to a vertically mounted
dipole antenna.

7

The Marconi antenna is half the length of a dipole antenna.

The earth
acts as a type of electrical “mirror,” effectively

providing the other quarter wavelength making
it equivalent

to a vertical dipole.

Half the length of a dipole

Can be located at earth level without degrading performance

Has omni
-
di
rectional radiation pattern similar to dipole

Gain is slightly lower than a dipole (about 1 dB less), but for our purposes we will consider them the
same

Antenna is extremely dependent on conductivity of the earth

Using a counterpoise

will improve conductivity

Counterpoise

Sometimes connecting a monopole antenna to the ground is not feasible. Create a ground.

o

Antennas mounted on buildings or towers

o

Soil is highly resistive (dry)

8

A
counterpoise

is a flat structure of wire
or screen that forms an artificial
reflecting surface for the monopole
antenna if the actual earth cannot be used

Counterpoise requirements

o

Must be at least equal to or larger
than the antenna.

o

Should extend in equal distances
from the antenna.

o

Must be insulated from the
ground.

The performance of a
quarter
-
wave
antenna (either well
-
grounded or using a
counterpoise) is essentially the same as a
half
-
wave dipole antenna.

antenna impedance

a. Compute the length of one
-
quarter wavelength and one
-
half wavelength antennas, given frequency of operation.

b. Compare and contrast a monopole antenna vs. dipole antenna (physical construction and radiation pattern).

c. Define parasitic
element, reflector, and director and explain their role in an array antenna.

d. Explain the purpose of a counterpoise.

e. List the basic antenna types and give the characteristics of each.

f. Identify the 3dB down points and their significance given a 2
-
D

g. Calculate “Effective Radiated Power” (ERP) given a specific antenna type.

h. Define the relationship between beam width and antenna gain for a dipole antenna.

i. Define antenna array and phased array.

j. Perform calculations o
f Front
-
to
-
Back ratio.

k. Describe the importance of impedance matching.

l. Describe antenna gain in terms of dBi and dBd.

We have
described the half
-
wave dipole antenna (which actually resonates at
95% of the actual half
-
wavelength
leng
th

as indicated in the s
hortcut

formula
L
feet

= 468/
f

MHz

(
where the antenna length,
L
, is in f
eet)
.

We have also described the
quarter
-
wavelength vertical
monopole
antenna

(the so
-
called
Marconi antenna
)
which

provided for a smaller antenna, but at the price of a lower gain compared to a half
-
wave dipole.

But sometimes we need antennas even small than a
quarter
-
wavelength
. Consider a cordless telephone
operating at 50 MHz. What is the length of a quarter wav
elength antenna?

In general, meeting this quarter wavelength requirement is toughest in mobile applications.

9

So…why do we want quarter wavelength antennas anyway (as opposed to one
-
tenth or one
-
one
-
hundredth
wavelength
antennas

the antenna length being a quarter wavelength (or greater)?

/4 antennas are desirable because their impedance is purely resistive.

For example,

the impedance of a

/4 antenna is 36.6 + j0

.

the impedance of a

/8 antenna is 8

j500

.

If a
vertical antenna is less than a quarter wavelength, it no longer resonates at the desired operating

frequency

(it looks more like a capacitor).

The capacitive load does not accept

energy from the transmitter well.

How can we
remedy this,
i.e., how can w
e
make an antenna “
appear”

to be

/4 .

To compensate

for this unwanted capacitance

The coil is often
variable in order to tune the antenna for different frequencies.

Somewhat related to the ides above is the notion of
impedance matching
. To ensure maximum power transfer
from the tranmitter to the antenna, the impedance of the tranmission line must match the impedance of the
antenna iteself.
We
can modify the antenna impedance by adjusting ita length, or by adding circuit elements to
match the impedance.

Directional

Antennas

For many applications, we desire to focus the energy over a more limited range
.
Directional antennas

have this
capability
.

Because energy is only sent in the desired direction, the possibility of interference with other stations is
reduced

The reduced beamwidth results in increased gain

Controlling the direction of

the beam improves information security

Frequencies can be reused (wireless modems)

Directional antennas don’t work well in mobile situations

More complex

10

Consider the highly directional antenna whose beam
pattern is shown below.

From Frenzel,
Principles of Electronic Communication Systems
, McGraw Hill, 2008

What is the
beam
width of this directional antenna?

Will a station who is located at
120

(as indicated above) interfere with me? Will I interfere with him?

Will a station who is located at
255

(as indicated above) be able to eavesdrop on my communications?

Suppose the receiver I am communicating with (at
0
) requires that the signal received by him be 1 pW.
Will I have to transmit more power or less power than if I were using an omnidirectional antenna?
W
hy?

Antenna Arrays
. So, how do we create a highly directive antenna (i.e., an antenna with high gain)?

antenna array
.
An
antenna array

is group of antennas or antenna elements arranged to
provide the desired directional characteristics.

Put another
way, an antenna array can be used
to “shape” a beam
.

11

Parasitic Array

A parasitic array consists of a basic antenna (called the driven element

such as a half
-
wave
dipole) which receives its signal from a transmission line, plus one or more elements

that are not connected to
the transmission line (called parasitic elements). So, the parasitic elements are not electrically connected.

The parasitic elements, despite their name, are actually very helpful! The parasitic elements are placed in
parall
el with and near the driven elements, and r
adiation from the driven element excites the parasitic elements
.

There are t
wo
t
ypes

of parasitic elements
:

Reflectors

Are made electrically long, 5% longer than the driven element

Reverses the direction of
energy emitted from rear of antenna

Directors

Made electrically short, 5% shorter than the driven element

Reinforces and focuses energy from the front of the antenna

Consider the half
-
wave dipole with a single half
-
wave parasitic element below.

S
hown is
(very simplistic) with and without the reflector.

The driven element
(the half
-
wave dipole)

This induces voltages and currents in the
parasitic element causing it to radiate also.

This radiation is reflected back and arrives

back at the dipole in
phase

with the dipoles radiation, reinforcing the radiation in the direction away from the reflector.

An antenna made up of a driven element and one or more parasitic elements is termed a

Yagi antenna
.
Most
Yagi antennas have 1 reflector and 1
-
20 directors

The simplest Yagi, consisting of a driven element and one reflector, shown on the bottom of the prior page, has
a gain of about 3 dB over a dipole.

12

More is Better

More parasitic
elements means more gain

The greater the number of directors, the higher the gain and the narrower the beam angle

Here is a Yagi with one director and one reflector. This is a three
-
elem
ent Yagi.

Most Yagi antennas have 1 reflector and 1
-
20 directors. There was a time when every home in America was
equipped with a Yagi antenna. Do you know why?

Although the Yagi antenna does a good job at directing (and receiving) energy from

the forward direction (in
the main lobe), it has a somewhat pesky back lobe. Thus, for a Yagi antenna, we are often interested in the ratio
of the power radiated in the forward direction to the power radiated in the backward direction, or the front
-
to
-
ba
ck (F/B) ratio.
In fact, m
ost Yagi antennas are designed to maximize F/B ratio rather than gain. This
minimizes the radiation and reception from the rear of the antenna.

13

The front
-
to
-
back ratio (F/B ratio) is the ratio of the power radiated in the forwa
rd direction to the power

/10log
f
b
P
F B
P

dB

where
P
f

is the f
orward power

and
P
b

is the b
ackward power
.

If the radiation patterns are plotted in decibels,

the F/B ratio is simply the difference between

the forward value
and the backward value,

in dB

Impact of a Reflector and Director

Comparing several radiation patterns will demonstrate the effect of
adding a reflector and directors to a
Y
agi antenna
.

------------------------------------------------------------------------------------------

14

------------------------------------------------------------------------------------------

------------------------------------------------------------
------------------------------

15

Driven

Array

A parasitic array, such as the Yagi,
produces good forward gain

b
ut has a limited operating
frequency range
.
A
driven array
, as opposed to a parasitic array,

is a multi
-
element antenna in which
all

of the
elements are excited through a transmission line.

Some examples:

A
collinear array

consists of 2 or more dipoles connected end
-
to
-
end.
The r
p
attern is similar

to
a
dipole, but more concentrated

in the horizontal plane
.

A
array

is a stacked collinear antenna
.
The broadside array results in increased directivity in both the
horizontal and vertical plane.

In the
log
-
periodic antenna
, the l
engths of driven elements are related logarithmically
.
The longest element has
a
length of ½ the wavelength of the lowest frequency

and t
he shortest element is ½ the wavelength of the
highest frequency
. The a
dvantage is very wide bandwidth
.

16

The Phased Array

Antennas can be driven in sets to produce directional radiation patterns
.
B
y controlling
the amplitude and phase of the RF current driving each antenna, an infinite number of radiation patterns can be
created
. The potential benefits can be seen from this simple t
wo
-
element phased Marconi array
.

The ability to shape and elec
tronically steer a beam has resulted in advanced technology
.
Eliminating rotating
antennas saves weight and significant maintenance costs
.

17

Examples

Example Problem 1:

(a)
What is the front to back ratio for the radiation pattern shown below?

(b
)
What is the power ratio?

Example Problem 2: What is the length of the driven element in a Yagi at 290 MHz?

18

Example Problem 3:
A radio station has an ERP of 25kW and an input power of 1.73 kW. What is the gain of
the antenna?

Example Problem 4:
The Cisco Multiband Wall
-
Mount antenna (AIR
-
ANTM5560P
-
R) is used in wireless
networking applications and has the following radiation pattern. Answer the following questions regarding this
antenna.

(a)
What is the front
-
to
-
back rati
o

in dB?

(b)
What is the gain of this
a
ntenna in dBd?

(c) It can be shown that the gain of an antenna in dB is related to the bean width,
B
, in degrees by the formula

203
20log
G dB
B

. What is the gain of the antenna
whose pattern is shown above?

19

Examples

Example Problem 1:
How long would a dipole antenna be for AM 1100?

(
Calculate using wavelength and
shortcut
)

Example Problem 2: An antenna is transmitting a signal at 24MHz. An engineer is hired
to produce an antenna
radiation pattern. What is the minimum distance the engineer can take measurements for a valid antenna

Example Problem 3: The ballistic submarine, USS Alaska, has gone alert. They must stream an antenna to ge
t
their alert signal. If the alert signal is transmitted at 30 KHz, how far should they stream their antenna? (The
antenna being streamed is a straight wire)

Example Problem 4: What is the length of a wire dipole antenna at 16 MHz?

Example
Problem 5: A dipole antenna has a length of 27 feet. What frequency does it operate at?

Example Problem 6: The power applied to an antenna with a gain of 4 dB is 5 W. What is the ERP?

Example Problem 7: An antenna has a dBd gain of 6. What is
its gain with respect to an isotropic antenna?

Example Problem 8:
A transmitter feeds a half
-
wave dipole antenna with 100 watts of power.

Calculate the