Download Basic Antennas

Key Points
1. Principals of EM Radiation
2. Introduction to Propagation & Antennas
3. Antenna Characterization
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1. Principals of Radiated electromagentic (EM) fields
two laws (from Maxwell Equation)
1. A Moving Electric Field Creates a Magnetic (H) field
2. A Moving Magnetic Field Creates an Electric (E) field
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An AC current i(t), flowing in a wire produces an EM field
Assume i(t) applied at A with length l = /2
• EM wave will travel along the wire until it reaches the B
• B is a point of high impedence  wave reflects toward A and is reflected
back again
• resistance gradually dissipates the energy of the wave
• wave is reinforced at A
 results in continuous oscillations of energy along the wire and a high
voltage at the A end of the wire.
A
l = /2
B
c  3 108m/s
l = /2: wave will complete one cycle from A to B and back to A
 = distance a wave travels during 1 cycle
f = c/ = c/2l
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Dipole antenna: 2 wires each with length l = /4
• attach ends to terminals of a high frequency AC generator
• at time t, the generator’s right side = ‘+’ and the left side = ‘−’
−
+
B
i(t)
l = /4
current distribution at time t
A
−
------------------------------------------------------------------------
A
+
++++
+++++++
+++++++++++
+++++++++++++++
+++++++++++++++
B
voltage distribution at time t
• electrons flow away from the ‘−’ terminal and towards the ‘+’ terminal
• most current flows in the center and none flows at the ends
• i(t) at any point will vary directly with v(t)
¼ cycle after electrons have begun to flow  max number of electrons will
be at A and min number at B
vmax(t) is developed
i(t) = 0
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Standing Wave
• center of the antenna is at a low impedance: v(t)  0, imax(t)
• ends of antenna are at high impedence: i(t)  0, vmax(t)
• maximum movement of electrons is in the center of the antenna at all
times
 Resonance condition in the antenna
• waves travel back and forth reinforcin
• maximum EM waves are transmitted into at maximum radiation
EM patterns on Dipole Antenna:
• sinusoidal distribution of charge exists on the antenna that reverses
polarity every ½ cycle
• sinusoidal variation in charge magnitude lags the sinusoidal variation in
current by ¼ cycle.
• Electic field E and magnetic field H 90 out of phase with each other
• fields add and produce a single EM field
• total energy in the radiated wave is constant, except for some absorption
• as the wave advances, the energy density decreases
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POLARIZATION
• EM field is composed of electric & magnetic lines of force that are
orthogonal to each other
• E determines the direction of polarization of the wave
vertical polarization: electric force lines lie in a vertical direction
horizontal polarization : electric force lines lie in a horizontal direction
circular polarization: electric force lines rotate 360 every cycle
An antenna extracts maximumenergy from a passing EM wave when it is
oriented in the same direction as E
• use vertical antenna for the efficient reception of vertically polarized
waves
• use horizontal antenna for the reception of horizontally polarized waves
if E rotates as the wave travels through space  wave has. horizontal and
vertical components
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Ground wave transmissions missions at lower frequencies use vertical
polarization
• horizontal polarization E force lines are parallel to and touch the earth.
earth acts as a fairly good conductor at low frequencies  shorts out
• vertical electric lines of force are bothered very little by the earth.
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2. Introduction to Antennas and Propagation
Types of antennas
• simple antennas: dipole, long wire
• complex antennas: additional components to
shape radiated field
provide high gain for long distances or weak signal reception
size  frequency of operation
• combinations of identical antennas
phased arrays electrically shape and steer antenna
transmit antenna: radiate maximum energy into surroundings
receive antenna: capture maximum energy from surrounding
• radiating transmission line is technically an antenna
• good transmission line = poor antenna
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Major Difference Between Antennas And Transmission Lines
• transmission line uses conductor to carry voltage & current
• radio signal travels through air (insulator)
• antennas are transducers
- convert voltage & current into electric & magnetic field
- bridges transmission line & air
- similar to speaker/microphone with acoustic energy
Transmission Line
• voltage & current variations  produce EM field around conductor
• EM field expands & contracts at same frequency as variations
• EM field contractions return energy to the source (conductor)
• Nearly all the energy in the transmission line remains in the system
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Antenna
• Designed to Prevent most of the Energy from returning to Conductor
• Specific Dimensions & EM wavelengths cause field to radiate
several  before the Cycle Reversal
- Cycle Reversal - Field Collapses  Energy returns to Conductor
- Produces 3-Dimensional EM field
- Electric Field  Magnetic Field
- Wave Energy Propagation  Electric Field & Magnetic Field
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Antenna Performance depends heavily on
• Channel Characteristics: obstacles, distances temperature,…
• Signal Frequency
• Antenna Dimensions
transmit & receive antennas
theoretically are the same (e.g. radiation fields, antenna gain)
practical implementation issue:
transmit antenna handles high power signal (W-MW)
- large conductors & high power connectors,
receive antenna handles low power signal (mW-uW)
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Propagation Modes – five types
(1) Ground or Surface wave: follow earths contour
• affected by natural and man-made terrain
• salt water forms low loss path
• several hundred mile range
• 2-3 MHz signal
htx
(2) Space Wave
• Line of Sight (LOS) wave
• Ground Diffraction allows for greater distance
• Approximate Maximum Distance, D in miles is
hrx
D = 2htx  2hrx
(antenna height in ft)
• No Strict Signal Frequency Limitations
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(3) Sky Waves
• reflected off ionosphere (20-250 miles high)
• large ranges possible with single hop or multi-hop
• transmit angle affects distance, coverage, refracted energy
refracted
wave
ionosphere
reflected
transmitted
wave
wave
skip distance
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Ionosphere
• is a layer of partially ionized gasses below troposphere
- ionization caused by ultra-violet radiation from the sun
- affected by: available sunlight, season, weather, terrain
- free ions & electrons reflect radiated energy
• consists of several ionized layers with varying ion density
- each layer has a central region of dense ionization
Layer
D
E
F1
F2
altitude
(miles)
20-25
55-90
Frequency Availability
Range
several MHz day only
20MHz
day, partially
at night
90-140
30MHz
24 hours
200-250
30MHz
24 hours
F1 & F2 separate during daylight, merge at night
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Usable Frequency and Angles
Critical Frequency: frequency that won’t reflect vertical transmission
- critical frequency is relative to each layer of ionosphere
- as frequency increases  eventually signal will not reflect
Maximum Usable Frequency (MUF): highest frequency useful for
reflected transmissions
- absorption by ionosphere decreases at higher frequencies
- absorption of signal energy = signal loss
- best results when MUF is used
Frequency Trade-Off
• high frequency signals eventually will not reflect back to ground
• lower frequency signals are attenuated more in the ionosphere
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Critical Angle
angle of radiation: transmitted energy relative to surface tangent
- smaller angle requires less ionospheric refraction to return to earth
- too large an angle results in no reflection
- 3o-60o are common angles
critical angle: maximum angle of radiation that will reflect energy
to earth
Determination of minimum skip distance:
- critical angle - small critical angle  long skip distance
- height of ionosphere - higher layers give longer skip distances
for a fixed angle
multipath: signal takes different paths to the destination
ionosphere
angle of radiation
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(4) Satellite Waves
Designed to pass through ionosphere into space
• uplink (ground to space)
• down link (space to ground)
• LOS link
Frequencies >> critical frequency
• penetrates ionosphere without reflection
• high frequencies provide bandwidth
Geosynchronous orbit  23k miles (synchronized with earth’s orbit)
• long distances  result in high path loss
• EM energy disperses over distances
• intensely focused beam improves efficiency
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Free Space Path Loss equation used to determine signal levels
over distance
2
Pt  4fd 


Pr  c 
 4fd 
20 log 10 

 c 
(dB)
G = antenna gain: projection of energy in specific direction
• can magnify transmit power
• increase effective signal level at receiver
total loss = Gt + Gr – path loss (dB)
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(5) radar: requires
• high gain antenna
• sensitive low noise receiver
• requires reflected signal from object – distances are doubled
• only small fraction of transmitted signal reflects back
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3. Antenna Characterization
antennas generate EM field pattern
• not always possible to model mathematically
• difficult to account for obstacles
• antennas are studied in EM isolated rooms to extract key
performance characteristics
antenna design & relative signal intensity determines relative field
pattern
absolute value of signal intensity varies for given antenna design
- at the transmitter this is related to power applied at transmitter
- at the receiver this is related to power in surrounding space
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Polar Plot of relative signal strength of radiated field
• shows how field strength is shaped
• generally 0o aligned with major physical axis of antenna
• most plots are relative scale (dB)
- maximum signal strength location is 0 dB reference
- closer to center represents weaker signals
90o
forward gain = 10dB
backward gain = 7dB
0o
180o
270o
+10dB
+7dB
+ 4dB
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radiated field shaping  lens & visible light
• application determines required direction & focus of signal
• antenna characteristics
(i) radiation field pattern
(ii) gain
(iii) lobes, beamwidth, nulls
(iv) directivity
(i) antenna field pattern = general shape of signal intensity in far-field
far-field measurements measured many wavelengths away from
antenna
near-field measurement involves complex interactions of decaying
electrical and magnetic fields - many details of antenna construction
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Measuring Antenna Field Pattern
field strength meter used to measure field pattern
• indicates amplitude of received signal
• calibrated to receiving antenna
• relationship between meter and receive antenna known
measured strength in uV/meter
received power is in uW/meter
• directly indicates EM field strength
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Determination of overall Antenna Field Pattern
form Radiation Polar Plot Pattern
• use nominal field strength value (e.g. 100uV/m)
• measure points for 360o around antenna
• record distance & angle from antenna
• connect points of equal field strength
practically
• distance between meter & antenna kept constant
90o
• antenna is rotated
• plot of field strength versus angle is made
0o
180o
270o
100 uV/m
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Why Shape the Antenna Field Pattern ?
• transmit antennas: produce higher effective power in direction of
intended receiver
• receive antennas: concentrate energy collecting ability in
direction of transmitter
- reduced noise levels - receiver only picks up intended signal
• avoid unwanted receivers (multiple access interference = MAI):
- security
- multi-access systems
• locate target direction & distance – e.g. radar
not always necessary to shape field pattern, standard broadcast is
often omnidirectional - 360o
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(ii) Antenna Gain
Gain is Measured Specific to a Reference Antenna
• isotropic antenna often used - gain over isotropic
- isotropic antenna – radiates power ideally in all directions
- gain measured in dBi
- test antenna’s field strength relative to reference isotropic antenna
- at same power, distance, and angle
- isotropic antenna cannot be practically realized
• ½ wave dipole often used as reference antenna
- easy to build
- simple field pattern
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Antenna Gain  Amplifier Gain
• antenna power output = power input – transmission line loss
• antenna shapes radiated field pattern
• power measured at a point is greater/less than that using
reference antenna
• total power output doesn’t increase
• power output in given direction increases/decreases relative to
reference antenna
e.g.
a lamp is similar to an isotropic antenna
a lens is similar to a directional antenna
- provides a gain/loss of visible light in a specific direction
- doesn’t change actual power radiated by lamp
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• transmit antenna with 6dB gain in specific direction over isotropic
antenna  4 transmit power in that direction
• receive antenna with 3dB gain is some direction receives 2 as
much power than reference antenna
Antenna Gain
often a cost effective means to
(i) increase effective transmit power
(ii) effectively improve receiver sensitivity
may be only technically viable means
• more power may not be available (batteries)
• front end noise determines maximum receiver sensitivity
Rotational Antennas can vary direction of antenna gain
Directional Antennas focus antenna gain in primary direction
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(iii) Beamwidth, Lobes & Nulls
Lobe: area of high signal strength
- main lobe
- secondary lobes
Nulls: area of very low signal strength
Beamwidth: total angle where relative signal power is 3dB
below peak value of main lobe
- can range from 1o to 360o
Beamwidth & Lobes indicate sharpness of pattern focus
90o
beam
width
0o
180o
null
270o
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Center Frequency = optimum operating frequency
Antenna Bandwidth  -3dB points of antenna performance
Bandwidth Ratio: Bandwidth/Center Frequency
e.g. fc = 100MHz with 10MHz bandwidth
- radiated power at 95MHz & 105MHz = ½ radiated power at fc
- bandwidth ratio = 10/100 = 10%
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Antenna Design Basics
Main Trade-offs for Antenna Design
• directivity & beam width
• acceptable lobes
• maximum gain
• bandwidth
• radiation angle
Bandwidth Issues
High Bandwidth Antennas tend to have less gain than
narrowband antennas
Narrowband Receive Antenna reduces interference from adjacent
signals & reduce received noise power
Antenna Dimensions
• operating frequencies determine physical size of antenna elements
• design often uses  as a variable (e.g. 1.5  length, 0.25  spacing)
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Testing & Adjusting Transmitter  use antenna’s electrical load
• Testing required for
- proper modulation
- amplifier operation
- frequency accuracy
• using actual antenna may cause significant interference
• dummy antenna used for transmitter design (not antenna design)
- same impedance & electrical characteristics
- dissipates energy vs radiate energy
- isolates antenna from problem of testing transmitter
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Testing Receiver
• test & adjust receiver and transmission line without antenna
• use single known signal from RF generator
• follow on test with several signals present
• verify receiver operation first  then connect antenna to
verify antenna operation
Polarization
• EM field has specific orientation of E-field & M field
• Polarization Direction determined by antenna & physical orientation
• Classification of E-field polarization
- horizontal polarization : E-field parallel to horizon
- vertical polarization: E-field vertical to horizon
- circular polarization: constantly rotating
33
Transmit & Receive Antenna must have same Polarization for
maximum signal energy induction
• if polarizations aren’t same  E-field of radiated signal will try to
induce E-field into wire  to correct orientation
- theoretically no induced voltage
- practically – small amount of induced voltage
Circular Polarization
• compatible with any polarization field from horizontal to vertical
• maximum gain is 3dB less than correctly oriented horizontal or
vertically polarized antenna
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Antenna Fundamentals
Dipole Antennas (Hertz): simple, old, widely used
- root of many advance antennas
• consists of 2 spread conductors of 2 wire transmission lines
½
• each conductor is ¼  in length
• total span = ½  + small center gap
¼
¼
Distinct voltage & current patterns
driven by transmission line at midpoint
• i = 0 at end, maximum at midpoint
• v = 0 at midpoint, vmax at ends
• purely resistive impedance = 73
+v
• easily matched to many transmission lines
Transmission
Line
gap
i
-v
High Impedance 2k-3k
Low Impedance 73
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E-field (E) & M-field (B) used to determine radiation pattern
• E goes through antenna ends & spreads out in increasing loops
• B is a series of concentric circles centered at midpoint gap
E
B
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3-dimensional field pattern is donut shaped
antenna is shaft through donut center
radiation pattern determined by taking slice of donut
- if antenna is horizontal  slice reveals figure 8
- maximum radiation is broadside to antenna’s arms
Azimuth Pattern
Polar Radiation Pattern
Elevation Pattern
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½ dipole performance – isotropic reference antenna
• in free space  beamwidth = 78o
• maximum gain = 2.1dB
• dipole often used as reference antenna
- feed same signal power through ½  dipole & test antenna
- compare field strength in all directions
Actual Construction
(i) propagation velocity in wire < propagation velocity in air
(ii) fields have ‘fringe effects’ at end of antenna arms
- affected by capacitance of antenna elements
1st estimate: make real length 5% less than ideal - otherwise
introduce reactive parameter
Useful Bandwidth: 5%-15% of fc
• major factor for determining bandwidth is diameter of conductor
• smaller diameter  narrow bandwidth
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Multi-Band Dipole Antennas
use 1 antenna  support several widely separated frequency bands
e.g. HAM Radio - 3.75MHz-29MHz
Traps: L,C elements inserted into dipole arms
• arms appear to have different lengths at different frequencies
• traps must be suitable for outdoor use
• 2ndry affects of trap impact effective dipole arm length-adjustable
• not useful over 30MHz
2/4
L
2/4
L
C 1/4
1/4 C
Transmission
Line
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Transmit Receive Switches
• allows use of single antenna for transmit & receive
• alternately connects antenna to transmitter & receiver
• high transmit power must be isolated from high gain receiver
• isolation measured in dB
e.g. 100dB isolation 10W transmit signal  10nW receive signal
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Elementary Antennas
low cost – flexible solutions
Long Wire Antenna
• effective wideband antenna
• length l = several wavelengths
- used for signals with 0.1l <  < 0.5l
- frequency span = 5:1
Transmission
Line
R=Z0
earth ground
• drawback for band limited systems - unavoidable interference
• near end driven by ungrounded transmitter output
• far end terminated by resistor
- typically several hundred 
- impedance matched to antenna Z0
• transmitter electrical circuit ground connected to earth
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practically - long wire is a lossy transmission line
- terminating resistor prevent standing waves
Polar radiation pattern
• 2 main lobes
- on either side of antenna
- pointed towards antenna termination
• smaller lobes on each side of antenna – pointing forward & back
• radiation angle 45o (depending on height)  useful for sky waves
feed
horizon
polar ration pattern
angular radiation pattern
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poor efficiency:
transmit power
- 50% of transmit power radiated
- 50% dissapated in termination resistor
receive power
- 50% captured EM energy converted to signal for reciever
- 50% absorbed by terminating resistor
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Folded Dipole Antenna
- basic ½ dipole folded to form complete circuit
- core to many advanced antennas
/2
- mechanically more rugged than dipole
- 10% more bandwidth than dipole
- input impedance  292 
- close match to std 300 twin lead wire transmission line
- use of different diameter upper & lower arms  allows
variable impedance
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Loop & Patch Antenna – wire bent into loops
Patch Antenna: rectangular conducting area with || ground plane
V = k(2f)BAN
V = maximum voltage induced in receiver by EM field
B = magnetic field strength flux of EM field
N-turns
A = area of loop
N = number of turns
f = signal frequency
k = physical proportionality factor
Area A
Antenna
Plane
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Radiation Pattern
• maximum  to center axis through loop
• very low broadside to the loop
• useful for direction finding
- rotate loop until signal null (minimum) observed
- transmitter is on either side of loop
- intersection with 2nd reading pinpoints transmitter
• Loop & Patch Antennas are easy to embed in a product (e.g. pager)
• Broadband antenna - 500k-1600k Hz bandwidth
• Not as efficient as larger antennas
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Name
Isotropic
Shape
Gain (over Beamwidth
isotropic)
-3 dB
0 dB
360
2.14 dB
55
Turnstile
-0.86 dB
50
Full Wave
Loop
3.14 dB
200
Yagi
7.14 dB
25
Helical
10.1 dB
30
Parabolic
Dipole
14.7 dB
20
Horn
15 dB
15
Biconical
Horn
14 dB
360x200
Dipole
Radiation Pattern
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