Transcript MW PLANNING 2-2007-resume

1
The aim of this Course is to Give
a) Basic notions in Radio Propagation at
microwave frequencies,
b) application to Radio Link Design in the
frequency range from about 450 MHz up to 60
GHz.
• Means :
- Course notes
- Lab simulators
2
•
Prerequisites: - basic notions in:
- Modulation techniques,
- Radio equipment and systems
- Elementary electromagnetic physics.
• Conclusion :
• Course objective : actively involving the
reader in navigating through the text and in
practicing with exercises in the field of
microwave link design.
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 In telecommunications, information can be
analog or digital.
 since the 1970’s , MW Analog systems have
been almost completely replaced by digital
systems.
Now even analog traffic, such as voice calls,
are converted to digital signals ( sampling), to
facilitate long distance transmission and
4 switching.
 Terrestrial MW systems have been used since
the 1950’s( wartime radar technology).
 Today, modern digital microwave radio is world
widely deployed to transport information over
distances of up to 60 kilometers ( sometimes
farther).
Microwave radio is totally transparent to the
information carried : which can be voice, data,
video, or a combination of all three.
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• Transport can be in a variety of formats :
 circuit-switched Time Division Multiplex
(TDM)
 packet-based data protocols such as ATM,
Frame Relay or IP, Ethernet.
In some cases, packetized data can be
overlaid on a TDM frame structure such as:
- PDH,
- SDH or SONET.
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• Microwave radio advantages over cable/fiberbased transmission:
Rapid Deployment
No right-of-way issues – avoid all obstacles
Any requirement to seek permissions :cost &
time delays.
 Flexibility: simple redeployment & capacity
adjustment.
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
Losing customers ≠ Losing assets as in
Cables & fibers
• Easily crosses city terrain (extremely restricted,&
very expensive, to install fiber in city terrains and
street crosses).
• Operator-owned infrastructure - no reliance on
competitors.
• Low start-up capital costs : independent of the
link distance.
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 Minimal operational costs.
Radio infrastructure already exists (rooftops,
masts and towers).
 Microwave radio is not susceptible to
catastrophic failure ( cable cuts,) and can be
repaired in minutes instead of hours or days.
Better resistance to natural disasters (flood,
earthquakes).
 where the fiber was not always available (the
radio is only choice)
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 Fiber is very cost effective where extremely
high bandwidths are required.
However, in the access portion of the network,
where the maximum capacity requirements are
less than STM-4, radio has an obvious
advantage.
 Note STM1 = 155 Mbits ; STMn = n*155Mbits
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The 3 basic components of the radio terminal
Two radio terminals are required to establish a
MW “hop”.
1- digital modem interfaces with digital
terminal equipment, converting customer
traffic to a modulated radio signal;
2- a radio frequency (RF) unit : Frequency
converter + RF amplifier up to around 1 watt.
3- a passive parabolic antenna to transmit and
receive the signal.
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2-Basic configurations for MW terminals
 1-Non-protected, ( 1+0) :
- Any major failure component will result in
a loss of customer traffic.
- cost-effective when traffic is non-critical, or where
alternate traffic routing is available.
 2-protected (1+1) : Main + hot standby (Monitored
Hot Standby (MHSB))
- twice expensive, but No loss of customer traffic.
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•
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



3- Space Diversity
4- Frequency Diversity
5- Polarization diversity
6-Angle diversity
 In addition, Some radios are fitted to use
ODU attached directly to the back of the
antenna, eliminating antenna feed lines and
attendant feed line losses).
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• Used to reinforce the radio dispersive fade margin . The new technology
of Mw radio don’t need this type of diversity
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•
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• Two very important characteristic of digital
MW transmission is:
A- immunity to noise
B- the ability of the radio to operate in the
presence of adverse environmental
conditions.
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A- immunity to noise
• Noise refers to the effects caused by unwanted
electromagnetic signals that interfere and
corrupt the received signal.
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• Microwave systems operate in so-called
“licensed” frequency bands between 2 and
38 GHz (tightly regulation the use of these
frequencies ensure that each operator will not
cause interference to other links operating in
the same area).
• The frequency band characteristics are also
tightly specified on a worldwide basis by ITU.
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• Equipment are controlled, to meet stringent
specifications ( ITU standards, National as FCC
and ETSI).
• This is in contrast to the “unlicensed” frequency
bands of 2.4 and 5.8 GHz :
No control, Unlicensed systems themselves
incorporate countermeasures to avoid noise and
interference, such as spread-spectrum
transmission
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B- the ability of the radio to operate in the
presence of adverse environmental
conditions.
 a perception that microwave is still unreliable
due to “fading” .
This is largely a remnant of the analog days.
However, digital radio systems today are able
to counteract fading effects in a number of
ways
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 Fading is known to occur as a result of
primarily two phenomena.
1- Firstly, multipath interference affects mainly
lower frequencies below 18 GHz.
This happens when the reflected signal arrive
slightly later than the direct signal path .it
reduces the ability of the receiver to
correctly distinguish the data carried on the
signal.
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• Fortunately, modern radio systems can
compensate for this form of interference
through countermeasures such as:
signal equalization [using DSP-filtering to
cancel the echoes (pre-echoes & postechoes) due to Multipath].
 Forward Error Correction,
diversity receiver configurations.
Multipath fade measurement parameter is often
called the reliability of the link ) )‫( (ذا ثقة‬24
2Secondly, precipitation, mostly in the form of
rain, can severely affect microwave radio
systems in the higher frequencies above 18
GHz.
Microwaves cannot penetrate rain, so :
the heavier the downpour, and the higher the
frequency, the greater the signal attenuation.
Rain fade measurement parameter is often
called the availability of the link
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Although there is noway to counteract rain fade
other than higher transmission power.
The mechanisms of rain fade are very well
understood:
models have been developed by the ITU to
enable links to be planned within extremely
accurate tolerances based upon particular
rainfall profiles.
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• Conclusion :
As a result, modern microwave systems can be
designed for extremely high link total availabilities
in excess of 99.999%, translating to link
downtimes of literally seconds annually, which is
easily comparable to that provided by supposed
“error-free” optical fiber systems.
• Note : total availability concerns the 2 types of the
fade
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Microwave applications
Mobile Cellular Networks
• to provide service for customers and to generate
immediate revenue, cellular carriers need to
connect their cell sites to switching stations, and
have chosen microwave due to:
• its reliability
• speed of deployment
• cost benefits over fiber or leased-line alternatives.
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• Microwave radio will be heavily deployed in
the emerging 2.5 and 3G mobile infrastructures:
More data usage
• greater numbers of cell sites.
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 Last Mile Access
 A significant proportion of business premises lack
broadband connectivity : Wireless provides the
perfect medium for connecting new customers to
overcome the last mile bottleneck.
 Even if an operator chooses to use unlicensed or
multi-point wireless technologies to connect
customers, high capacity microwave provides the
ideal solution for backhaul of customer traffic
from access hubs to the nearest fiber
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• Private Networks:
 Companies now have high speed LAN /
WAN network requirements and need to
connect parts of their business in the same
campus, city or country.
Microwave radio is able to provide rapid, high
capacity connections that are compatible with
Fast and Gigabit Ethernet data networks,
enabling LANs to be extended without reliance
on fiber build-out.
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• Disaster Recovery
 Natural (earthquakes, floods, hurricanes ) and
man-made (terrorist attack and wars) disasters
can wreak havoc on a communications network:
Microwave is often used to restore
communications when transmission equipment
has been damaged by or other natural disasters,
or man-made conflicts such as ( Kuwait, Serbia
and Kosovo)
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The Digital Divide
Microwave radio plays a key role in
bridging the digital divide :
quickly establish a network of access
hubs and high-speed backhaul network to
bring advanced communications services
to areas that would normally have to wait.
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Developing Nations
Microwave has traditionally allowed developing
nations the means of establishing state-of-theart telecommunications quickly over often
undeveloped and impractical terrain (
deserts, jungle or frozen terrain where laying
cable would be all but impossible.
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 Control and Monitoring
 Public transport organizations, railroads, and
other public utilities are major users of microwave.
These companies use microwave to carry control
and monitoring information to and from power
substations, pumping stations, and switching
stations.
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• A- Understanding db & db units :
A- db : The ratio of 2 signals may be
expressed in db by :
in case of voltages : V1/V2
( V2/V1)db = 20 log10 ( V2/V1)
in case of Powers : P1/P2
( P2/P1)db = 10 log10 ( P2/P1)
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• Example :
a signal if 10 w is applied to long
transmission line . The power measured at
the load end is 7 W. What is the loss in db
• Solution :
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Table of some common ratios
Ratio ®
Factor
1
2
10
100
Power ratio (db)
10 log10 R
0.00
3.01
10.00
20.00
Voltage ratio (db)
20 log10 R
0.00
6
20
40
1:1
2:1
10:1
100:1
1000:1
1000
30.00
60
1/10
1/100
1/1000
0.1
0.01
0.001
-10
-20
-30
-20
-40
-60
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30 dB is an increase of 1000X in power
20 dB is an increase of 100X in power
10 dB is an increase of 10X in power
6 dB is an increase of 4X
in power
3 dB is an increase of 2X
in power
2 dB is an increase of 1.6X
in power
1 dB is an increase of 1.25X in power
0 dB is no increase or decrease in power
-1 dB is a decrease of 20% in power
-2 dB is a decrease of 37% in power
(roughly a decrease of 1/3)
-3 dB is a decrease of 50% in power
-6 dB is a decrease of 75% in power
-10 dB
is a decrease of 90% in power
-20 dB is a decrease of 99% in power
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-30 dB is a decrease of 99.9% in power
• B) : - db-power units
- dbw : is the unit of power expressed relatively to 1W
P(dbw) = 10 log10 P(w)
- dbm : is the unit of power expressed relatively to
1mW
P(dbm) = 10 log10 P(mw)
Attention :
0dbw = 30dbm= 1W
0 dbm = -30dbw = 1mw
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P (dbm) = P(dbW) + 30
P(dbW) = P (dbm) -30
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• Example Problem
• If the two antennas in the drawing are "welded"
together, how much power in dbm will be measured at
point A? (Line loss L1 = L2 = 0.5) –suppose no ideal
antenna coupling
• Multiple choice:
a. 16 dBm
b. 28 dBm
c. 4 dBm
d. 10 dBm
e. < 4 dBm
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• Answer:
• The antennas do not act as they normally
would since the antennas are operating in the
near field. They act as inefficient coupling
devices resulting in some loss of signal. In
addition, since there are no active components,
you cannot end up with more power than you
started with.
The correct answer is "e. < 4 dBm."
• 10 dBm - 3 dB - small loss -3 dB = 4 dBm small loss
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Example :
Convert 10dbm in dbw ; -2dbw in dbm
Solution : given P(dbW) = P (dbm) -30 = 10-30 =
-20 dbW
P (dbm) = P(dbW) + 30 = -2 + 30 = 28 dbm
Example : consider the 2 following configurations
10dbm
?
Gain 3db
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Gain 10db
C) - db-voltage units
- dbmv : is used in RF receiver in which the
system
impedance is 50 Ω.
It is the unit of voltage expressed relatively to
1mv
v(dbmv) = 20 log10 v(mv)
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- dbµv : is used in RF receiver in which the
system impedance is 50 Ω. It is the unit of
voltage expressed relatively to 1µv :
v(dbµv) = 20 log10 v(µv)
Example : The received RF effective voltage
at the input of radio receiver is 0.5mv
. Find the input voltage in dbµv & the input
power in dbm
Solution
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-Field db units : Electromagnetic field
a- Electric field E in dbµv /m
E (dbµv/m) = 20 log10 E (µv/ m)
b- Magnetic field H = E/377 where H in A/m
and E in v/ m
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•
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Power and Field Db-units
•
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c) received power in dBm at the RX-antenna
where Gr is the RX-antenna gain.
Pdbm = Edbµv/m + Gr - 20 log (FMHZ) – 77.2
In case of isotropic Rx-antenna
Pdbm = Edbµv/m - 20 log (FMHZ) – 77.2
Received voltage into 50 input receiver:
P (dBm) = U (dBµV) - 107
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• Example:
The received RF effective voltage at the input of
radio receiver is 0.5mv
a) Find the input voltage in dbµv & the input
power in dbm.
b) knowing that the receiver’s antenna has 20db
Gain and the transmitted frequency is 10GHZ,
Find the Electric field at the antenna location In
dbµv/m and V/m .
Deduce the Magnetic field value.
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Exercises on Db 
• A cable has 6 dB signal loss . Find the signal at the
output of this cable ,knowing that the input signal is
1mW.
• an amplifier has 15 dB of gain. Find the signal at the
output of this amplifier ,knowing that the input signal
is 1mW.
• Complete the following sentences:
– a)Every time you double (or halve) the power level, you add
(or subtract) ……. dB to the power level. This corresponds to
a ……. percent gain or reduction.
– b) ……dB gain/loss corresponds to a tenfold
increase/decrease in signal level.
• A 20 dB gain/loss corresponds to a ……….-fold
increase/decrease in signal level.
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• Exercises on Dbm (dB milliWatt)
A signal strength or power level 0 dBm is defined
as …. mW (milliWatt) of power into a terminating
load such as an antenna or power meter.
• Small signals are negative numbers. For example,
typical 802.11b WLAN cards have +15 dBm
(….mW) of output power. They also specify a -83
dBm (………pW.) RX sensitivity (minimum RX
signal level required for 11Mbps reception).
Additionally, a) 125 mW is ….. dBm, and
b) ….mW is 24 dBm.
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CH2- Antenna and space
propagation
Recommended Software Andrew
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Antenna Basic questions
which cause some antennas to accept one wave
and reject others?:
 The physical size of an antenna : defines the
efficiently radiated or received frequency
 The shape of the antenna determine the
directivity of an antenna
The property of polarization describes the
angular pointing of the EM field vector
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Antenna Electromagnetic field radiation :
General discussion
An antenna serves two basic functions:
1- it matches the characteristic impedance of
the transmission line to the intrinsic impedance
of free space (To avoid any reflections back to
the source or load)
2 - Second, the antenna is designed to direct the
electromagnetic radiation in the desired
direction.
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• Isotropic Point Radiator
 It is a fictitious ideal isotropic point radiator. it would
radiate power equally well in all directions in a volume
sense. it would have an omni directional pattern in all
planes. All real antennas have some directivity.
Isotropic antenna practically doesn’t exist
 Omni-directional fictive
Hertz Isotropic point
antenna G=1 fold or
G = 0 Dbi
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• Radiation Pattern
The radiation pattern is a plot of the relative
strength (more often power density in db )of
the antenna radiation as a function of the
orientation in a given plane.
• Example :radiation pattern of
ANTMAN :ANDREW CORPORATION
MODNUM:FP10-34
•
LOWFRQ:3400; HGHFRQ:3900
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ANTMAN:
61
ANDREW
CORPORATIO
N
MODNUM:
FP10-34
PATNUM:
6605
POLARI:
H/H & V/V
NUPOIN:
37
-180
-59
-125
-59
-125
-56
-110
-56
-85
-38
DTDATA:
19790309
LOWFRQ:
3400
-35
-38
HGHFRQ:
3900
-30
-33
GUNITS:
DBI/DBR
-25
-33
-15
-29
LWGAIN:
37
-15
-25
MDGAIN:
38.3
-8
-19
HGGAIN:
38.8
-4
-17
-2.8
-17
AZWIDT:
1.9
-2.5
-11.3
ELWIDT:
1.9
-2
-5.4
ATVSWR:
1.06
-1.5
-2.5
-1
-1.1
FRTOBA:
60
-0.5
-0.3
ELTILT:
0
0
0
Continuous H/H
&V/V
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POLARI:
H/V & V/H
NUPOIN:
13
0.5
-0.3
1
-1.1
-180
-60
1.5
-2.5
-105
-60
2
-5.4
-15
-42
2.5
-11.3
-10
-41
2.8
-17
-4
-37
4
-17
-2
-28
8
-19
15
-25
0
-28
15
-29
2
-28
25
-33
4
-37
30
-33
10
-41
35
-38
15
-42
85
-38
105
-60
110
-56
180
-60
125
-56
125
-59
180
-59
0
10
20
30
40
50
60
70
Expanded Scale
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63
0
5
10
15
20
40
60
80
100
120
140
160
180
• Antenna Gain
• Ratio of the power density at a particular
location from an antenna with directivity to the
power density from an ideal isotropic antenna
radiating the same power:
The power is taken away from some directions
and added to the power in other directions, and
the result is :
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• The antenna reference
- most often used is :
the hypothetical
(Gain units dbi).
- Sometimes a "real-life" antenna such as
the
(Gain units dbd).
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Figure 1: Half-wave dipole vs. isotropic antenna
• antenna reference
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• Reciprocity
• Basically, it states that the properties of the antenna
used for transmission will be same as when used for
reception.
• In realistic terms, the transmitting antenna must be
constructed to handle a much larger power level than
at the receiver
• The best interpretation is to assume similar field
patterns and impedance properties of a given
antenna used in the TX or RX.
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•
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• Antenna Reciprocity
c- Power density of electromagnetic
energy in W/m2 :
An ideal isotropic point radiator transmitting
power PT. The power density pd upon the
surface of the sphere of radius r will be equal at
all points and will be
In free space propagation ( far field ) E in v/m,
Pd = E2/ 377
• Sometimes we define the radiation intensity as
• the unit of U is watts / steradian. Know that
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• Power density of
electromagnetic
energy in W/m2 
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• the radiation intensity in
watts / steradian
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• Solid Angles
• solid angle spanned by a cone is measured by
the area of intersection of the cone with a
sphere: differential solid angle can be assigned
a direction.
• Unit: steradian
• (full sphere = 4)
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• Example :
• 1-The power density 10 km from a transmitting antenna is 0.06
ìW/m2. Determine the radiation intensity.
• 2-The radiation intensity from a transmitting antenna is 50 W/sr.
Determine the power density of a receiving antenna located 25
km from the transmitting station.
Solution
• 1-
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• 2- E.M Radiation From an Antenna
• Time-varying voltages and currents in an antenna
produce time-varying electric and magnetic fields
that travel radially away from the antenna at a
velocity determined by the medium in which the
electromagnetic fields are propagating.
• There are two distinct regions of electric and
magnetic fields surrounding an antenna: near
field and far field.
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• They are defined by the distance from the
antenna as a function of the wavelength of
the electromagnetic radiation and size of the
antenna D.
•
• The fields in the far field are transverse fields; i. e.,
the electric and magnetic field intensities are
transverse to the direction of propagation: This
condition is referred to as plane wave propagation.
– Both transverse and radial electric and magnetic
field intensities exist in the near field region.
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• The radiation patterns that describe the
radiation intensity of the antenna as a function
of angle are usually patterns for the far field.
• Example 1: Determine the distance from a 100MHz half-wavelength dipole to the boundary
between the near field and the far field.
• Solution:  = 3 m. Therefore, the length of the
dipole is 1.5m. The distance to the far field can
then be determined
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Example 2
• A parabolic reflector antenna with a diameter D operates
at 2.3 GHz . Determine the far field distance for this
antenna.
• Case1 : D= 1m = 0.13 m. Rff = 2D2/ = 15m
• Case2 : D= 20m = 0.13 m. Rff = 2D2/ = 6Km
D=20m then Rff = 6Km :The preceding analysis shows
that the far-field distance for a high-gain antenna can
be very large. The measurement of the far field radiation
pattern for a large antenna operating at a high frequency
can be a very difficult task.
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• 15-4 Radiation Patterns
• In general, the radiation pattern of an antenna is
a three-dimensional plot of the relative strength
or radiation intensity of an antenna as a function
of the coordinate systems.
• Since it is difficult to present 3-D information,
typically the radiation patterns are shown as a
pair of two-dimensional plots:
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• Open-ended waveguide sections
3-D Amplitude
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Open-ended waveguide sections
•
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E-plane
H-plane
Figure - Comparison of rectangular- and polarcoordinate graphs for an isotropic source.
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• Figure - Anisotropic radiator : Rect. & Polar
Coordinates
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• - Polar-coordinate graph for anisotropic
(directive) radiator.
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• The first plot shows the radiation intensity as a
function of the angle in the plane of the electric
field intensity vector - E plane pattern and the
second plot shows the radiation intensity as a
function of the angle in the plane of the
magnetic field - H plane pattern.
• The E plane and H plane are orthogonal to
each other and are referred to as the principal
plane patterns.
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• Typical E and H plane plots:
• Consider a simple half-wave dipole antenna
aligned with the y-axis with the center at the
origin. A typical
• E plane radiation pattern for a half-wave dipole
antenna is shown in Figure (a). This plane is
the –y z plane in this case. A typical H plane
radiation pattern for a half-wave dipole is shown
in Figure (b).
• This plane is the –x z plane for the orientation
given. Note that the pattern is omnidirectional in
the H plane.
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•
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• Normalized Gain Functions
• Radiation patterns are often normalized to the
maximum gain
by dividing the gain
as a function of the two angles by the maximum
gain to obtain the normalized gain. The
normalized gain will be represented as
• This means that the normalized maximum gain is
1, and the gain at other angles is less than 1.
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• Since normalized antenna patterns cover
a significant dynamic range, typically
from 1 down to 10-4 or less, antenna
radiation patterns are normally plotted in
decibels on a linear scale, usually on a
polar plot.
 G (db) = 10 log G (folds)
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• Antenna Beamwidths and Sidelobes
An ideal antenna would have a radiation pattern
whose normalized gain is 1 over the desired
angular beamwidth and 0 at all other angles.
• Beamwidth
The antenna beamwidth is defined as the
included angle between the -3 dB (Half power
gain) points on the normalized gain pattern.
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•
90
Lobes
• The main lobe is the antenna beam defined
between the first null on either side of the
maximum gain angle.
• Typically for high-gain antennas,
the null-to-null beamwidth is 2.5 times
the 3-dB beamwidth.
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• An antenna will usually radiate some power in
undesired directions. The radiation pattern of
the Figure has several sidelobes.
• The levels of the sidelobes determine how
much power is radiated in these undesired
directions.
• If the antenna is a receiving antenna, the
sidelobes will determine the levels of
undesired signals that could be received.
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• Backlobe
• Another undesired part of the radiation pattern
when single direction transmission is desired is
the backlobe.
• A quality factor called the front-to-back ratio is
important in these cases. As shown in the
Figure , the absolute value of the front-to-back
ratio of a dipole is 1, which in decibel form
would be 0dB.
• A dipole cannot tell if the signal is coming from
the front or back of the antenna.
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• 15-6 Directivity and Antenna Gain
• There are two commonly employed terms used to
describe the radiation characteristics of an antenna:
directivity and antenna gain.
Directivity is a characteristic of the radiation pattern
of an ideal lossless antenna while the antenna gain
includes the ohmic losses of the antenna physical
structure.
• Directivity : The directivity D of an antenna is defined
from the radiation pattern as
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• Antenna Gain
Antenna gain is defined as the ratio of the maximum radiation
intensity Umax to the maximum radiation intensity Uref from a
reference antenna with same power input to the antenna.
The difference between directivity and gain is that directivity is
referenced to the power radiated by the antenna, while gain is
referenced to the power delivered by the transmission line to the
antenna. Therefore, gain is always less than or equal to directivity,
the difference being the power dissipated in the antenna ohmic
losses.
• Normally, antenna gain is expressed as a power ratio and is usually
specified in decibels as
96
• The value of gain depends on the gain of the
reference antenna. It is important to know what
reference has been used for the antenna gain.
Two of the common references are as follows:
• 1. a lossless isotropic antenna, in which the
radiation intensity is uniform over the sphere
surrounding the antenna, i. e., all 4 steradians
• 2. a reference dipole.
• The lossless isotropic antenna is a theoretical
concept and has never been realized in practice,
while the dipole is readily available.
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• Antenna measurements of gain are usually referenced
to a standard dipole for low-gain antennas or to a
standard-gain horn for higher-gain antennas.
• Accurate theoretical calculations of the gain
referenced to a lossless isotropic antenna are possible
for both the standard dipole and the standard-gain
horn.
• The absolute gain of a standard half-wave dipole with
respect to an isotropic radiator is 1.643 or 2.16 dB
Dbi = Dbd +2,16
98
99
Exercise in dbd &dbi
• dBd (dB dipole)
The gain an antenna has over a dipole antenna at the
same frequency. A dipole antenna is the ….. ., least
gain practical antenna that can be made.
• The term dBd generally is used to describe antenna
gain for antennas that operate under 1GHz
(1000Mhz), ( manufacturers calibrate their equipment
using a simple dipole antenna as the standard. Then
they replace it with the antenna they are testing. The
difference in gain (in dB) is reference to the signal
from the dipole).
100
• dBi (dB isotropic)
Unfortunately, an isotropic antenna cannot be made in
the real world, but it is useful for calculating theoretical
fade and System Operating Margins. The gain of
Microwave antennas (above 1 GHz) is generally given
in dBi. A dipole antenna has 2.14 dB gain over a 0 dBi
isotropic antenna.
• So if an antenna gain is given in dBd, not dBi, ……
2.15 to it to get the dBi rating.
• For example, if an antenna has 5 dBd gain, it would
have ………. dBi gain.
101
• Antenna Efficiency
• It depends on the ohmic losses of the antenna. It
is the ratio of the total power radiated from the
antenna / the power delivered to the antenna
from the transmission line.
• It is also equal
• where D and G are the absolute values of
directivity and gain.
102
1- An antenna is transmitting 200 W of power. The maximum power
density at a distance of 10 km is 3.184 mW/m2. Determine the
directivity of the antenna.
103
• 2- An antenna with a directivity of 16 dB is transmitting a power
of 1 kW. Determine the maximum power density at a distance
of 50 km from the antenna.
104
• 3-An antenna has an efficiency of 95% and the
directivity is 33 dB. Determine the antenna gain
in dB.
105
• 15-7 Effective Area of an RX Antenna
(capture area )
. It is the area by which the power density in watts per
unit of area is multiplied to obtain power in watts
delivered to the load.
It is close to the physical area of the antenna.
The effective antenna area Ae of the parabolic
reflector is given by
106
107
• 15-8 Polarization
• By definition, the polarization of an
electromagnetic wave propagating in free
space is the orientation of the electric field
intensity vector relative to the surface of the
earth.
• There are two basic types of polarization: linear
and elliptical.
•
108
• In linear polarization, the electric vector does
not change orientation as it travels away from
an observer : 2 Types : H& V
• In elliptical polarization, the electric vector
rotates as it travels away from an observer,
and the tip of the electric vector traces an
ellipse : sense here mean Clockwise and
anticlockwise.
109
• An antenna transmits vertical, horizontal, right-hand
(RH) circular, or left-hand (LH) circular polarization
depending on the antenna design and orientation.
• There is a significant cross-polarization loss of
approximately 30 dB.
• This loss also occurs in case of cross sens rotation in
circularly polarized antennas.
• This characteristic is used to provide polarization
diversity in communication systems.
110
Polarization Requirements for Various Frequencies
Sky waves
Direct waves
(including satellite)
wave type
Ground wave
frequency Band
Low & Medium Short waves
VHF , UHF,SHF
Polarization
possible to be
used
Vertical
Vertical or Horizontal
Vertical orHorizontal
Polarization
practically to be
used
Vertical
Horizontal
Vertical orHorizontal
Why
111
a) Less auto and
electro ignition,
The earth is
b) Less building
fairly good
absorption
conductor
c) More simple
,short out Eh
support antenna
structure
No ionosphere entry
or reflexion
• 15-9 Antenna Impedance and Radiation
Resistance
• when it is excited by an appropriate AC source
The antenna acts like an a complex
impedance to the source providing power to it
Z = R + jX
• This impedance can be measured by an
appropriate RF bridge .
112
• Antenna Impedance
• Ideally, it should be purely resistive R at the
frequency of operation and equal to the
characteristic impedance of the line connected
to it.
• Self Impedance of the isolated antenna
If an antenna is isolated from ground and any
other surrounding objects, this impedance is
the self-impedance of the antenna & at the
resonance it is purely resistive ( Z= R +j0)
113
• Mutual impedance of the antenna.
When other antennas, objects, or ground is
near the antenna, the currents flowing in these
objects have an influence on the antenna
impedance.
The antenna impedance is then determined both by
the self-impedance of antenna and by a mutual
impedance between the antenna and the nearby
objects.
114
Radiation Resistance
• The radiation resistance is the real part of the
complex antenna impedance of a lossless
antenna. It is equal to
• Where : - Prad is the amount of this energy
leaving a sphere surrounding the antenna per
unit of time is the power radiated by the
antenna.
• - Irms is the rms value of the antenna current
magnitude at the input terminals of the antenna.
115
• Example : An antenna has an rms current of 3
A flowing into the antenna, and it is transmitting
1 kW of power. Determine the radiation
resistance of the antenna.
• Solution
• The radiation resistance is determined from
(15-29).
116
- Simple Dipoles
- Folded Dipoles
- Antennas Above a Ground Plane
- Monopole Antenna
- Waveguides and horn antennas
- Parabolic antennas
117
•
• Simple Dipoles
- Folded Dipoles
- Antennas Above a
Ground Plane
•
- Monopole Antenna
118
• Dipoles
• One of the simplest and most commonly used
antennas is the half-wave dipole, formed from a
two-wire parallel transmission line as shown in The
following figure.
• Starting with an open-ended line, which has a
voltage maximum at the open end and a voltage
minimum back from the open end, the two
conductors are bent 90o from the transmission line
as illustrated in the Figure.
119
• The theoretical length of the antenna is the
• Diameter d of the wires is assumed to be much
smaller than the length, and the spacing D at
the feed point must be small compared with the
length.
120
121
Practically Mounted dipole
•
122
• Input Impedance of Dipole
• A voltage minimum and a current maximum
occur at the feed point, which means that the
impedance is a minimum at that point.
• The actual value of the impedance of the halfwave dipole is 73+ 42.5 j .
123
• The reactive component can be eliminated by
tuning the antenna, which is accomplished by
shortening the length by about 5% from 0.5
to 0.475, corresponding to approximately 95%
of the theoretical length.
• When properly tuned, the half-wave dipole has
an impedance of 73  resistive, which for a
lossless dipole is the radiation resistance of the
antenna.
124
• The dipole is a balanced antenna and must,
therefore, be fed by a balanced transmission
line.
• Since the most common transmission line
providing the best impedance match is coaxial
cable, a balun must be used to properly connect
a coaxial cable to a dipole.
125
• Radiation Patterns
• The E plane and H plane radiation patterns of the half-wave
dipole were shown in the following Figure as examples of
patterns.
• The E plane pattern is like a doughnut with two maxima
broadside to the dipole and a null at both ends of the
dipole.
The 3-dB beamwidth is 78o. The isotropic power gain or
directivity for a lossless /2 dipole is 1.643 folds or 2.16 dB.
•
126
127
• As shown in the Figure , the polarization of the
dipole is parallel to the dipole.
• The H plane pattern is illustrated in the Figure
and is a uniform circular pattern with a
constant gain for an angle of 360o about the
dipole.
128
• Effective Area
The effective area of a dipole can be
determined from the isotropic gain and is
for f= 98MHz(=3m) ; Ae =1.218m2
129
• Folded Dipole
A folded dipole is constructed from a /2 length of
300- twin-lead transmission line (see figure).
The combination of the 73- self-impedance of the
dipole and the mutual impedance from the parallel
conductor connected at both ends increases the
antenna impedance of the folded dipole to 280 .
• Therefore, the folded dipole is a balanced 280-
antenna, which closely matches the 300- twin-lead
balanced transmission line.
130
•
131
Practically mounted Folded antenna
•
132
• The folded dipole is the perfect antenna for stations
that require a truly professional antenna for their
broadcasting. One powerful advantage of a folded
dipole antenna is that is has a wide bandwidth, in fact
a one octave bandwidth. This is the reason it was
often used as a TV antenna for multi channel use.
Folded dipole antennas were mainly used in
conjunction with Yagi antennas.
• No tuning or adjustment is needed for any
frequency on the band which makes this antenna
the only one to use if you plan to move your
transmitters frequency often.
133
Specifications for the folded professional antenna
Max Power Input
300 Watts
Impedance
50 Ohms
Gain
0dBd
VSWR
Better than 1:1.5 (88 - 108 MHz)
Frequency Range
88 - 108 MHz (No Tune)
Connector
N-Type Female
Dimension
1600mm(height) x 150mm x 35mm
Weight
2kg
134
135
• 15-11 Antennas Above a Ground Plane
A ground plane is a uniform good ground plane
surface beneath an antenna- constructed from
good conductors, or at some frequencies, the
earth acts as a good ground plane.
• Electromagnetic fields cannot exist in a
perfect conductor, and any wave incident upon
a perfect conductor will be reflected.
136
• Figure illustrates this situation. To satisfy the
boundary condition that no tangential
component of electric field can exist there, the
reflected wave will be shifted in phase by
180o.
• A concept known as image theory is used to
determine the characteristics of an antenna
above a ground plane.
137
• The reflected wave is like a direct wave from an
identical antenna located within the ground
plane the same distance from the boundary as the
real antenna is above the ground plane.
• This situation is illustrated in the following Figure.
• The image antenna is similar to an image
formed in a mirror at optical frequencies.
138
•
139
•
140
• Monopole Antenna
• An important and commonly used antenna is
the /4 monopole antenna on a ground plane
as shown in the Figure.
• This is an unbalanced antenna since the feed
point is between the monopole and ground and
it has vertical polarization.
• The radiation resistance is 36.5 , and the
antenna impedance has a reactive component
of 21 j . When the monopole is located close
to the ground plane, the image antenna forms a
dipole as shown in the previous Figure .
141
• The E plane radiation pattern is that of a /2
dipole with only one-half of the pattern above
the ground plane.
• The ground plane can be achieved either by a
grounded metal disc or by radial wires as
shown in the following Figure.
•
• The roof of a vehicle such as a car or truck
can form a ground plane for a /4 monopole.
142
•
143