1. 1
Key Points
1. Principals of EM Radiation
2. Introduction to Propagation & Antennas
3. Antenna Characterization
2. 2
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
3. 3
c ≈ 3 ×108
m/s
l = λ/2: wave will complete one cycle from A to B and back to A
λ = distance a wave travels during 1 cycle
f = c/λ = c/2l
l = λ/2
A B
Assume i(t) applied at A with length l = λ/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.
An AC current i(t), flowing in a wire produces an EM field
4. 4
Dipole antenna: 2 wires each with length l = λ/4
• attach ends to terminals of a high frequency AC generator
• at time t, the generator’s right side = ‘+’ and the left side = ‘−’
• 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)
current distribution at time t
− +
i(t) l = λ/4
A B
− +
++++
+++++++
+++++++++++
+++++++++++++++
+++++++++++++++
-----
-----------
----------------
--------------------
------------------------
voltage distribution at time t
A B
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
5. 5
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
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
6. 6
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
7. 7
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.
8. 8
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
2. Introduction to Antennas and Propagation
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
9. 9
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
10. 10
Antenna
• Designed to Prevent most of the Energy from returning to Conductor
• Specific Dimensions & EM wavelengths cause field to radiate
several λ 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
11. 11
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)
Antenna Performance depends heavily on
• Channel Characteristics: obstacles, distances temperature,…
• Signal Frequency
• Antenna Dimensions
12. 12
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
(2) Space Wave
• Line of Sight (LOS) wave
• Ground Diffraction allows for greater distance
• Approximate Maximum Distance, D in miles is
(antenna height in ft)
• No Strict Signal Frequency Limitations
rxtx hh 22 +D =
hrx
htx
14. 14
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 altitude
(miles)
Frequency
Range
Availability
D 20-25 several MHz day only
E 55-90 20MHz day, partially
at night
F1 90-140 30MHz 24 hours
F2 200-250 30MHz 24 hours
F1 & F2 separate during daylight, merge at night
15. 15
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
16. 16
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
angle of radiation
ionosphere
Critical Angle
17. 17
(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
18. 18
total loss = Gt + Gr – path loss (dB)
Free Space Path Loss equation used to determine signal levels
over distance
G = antenna gain: projection of energy in specific direction
• can magnify transmit power
• increase effective signal level at receiver
2
4
=
c
fd
P
P
r
t π
c
fdπ4
log20 10 (dB)
19. 19
(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
20. 20
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
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
antenna design & relative signal intensity determines relative field
pattern
21. 21
forward gain = 10dB
backward gain = 7dB
+10dB
+7dB
+ 4dB
0o
270o
180o
90o
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
22. 22
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
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
(i) antenna field pattern = general shape of signal intensity in far-field
23. 23
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
24. 24
0o
270o
180o
90o
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
100 uV/m
practically
• distance between meter & antenna kept constant
• antenna is rotated
• plot of field strength versus angle is made
25. 25
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
26. 26
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
(ii) Antenna Gain
27. 27
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
28. 28
Rotational Antennas can vary direction of antenna gain
Directional Antennas focus antenna gain in primary direction
• 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
29. 29
(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
0o
270o
180o
90o
beam
width
null
30. 30
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%
31. 31
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 Design Basics
Antenna Dimensions
• operating frequencies determine physical size of antenna elements
• design often uses λ as a variable (e.g. 1.5 λ length, 0.25 λ spacing)
32. 32
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
33. 33
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
34. 34
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
35. 35
Antenna Fundamentals
Dipole Antennas (Hertz): simple, old, widely used
- root of many advance antennas
• consists of 2 spread conductors of 2 wire transmission lines
• each conductor is ¼ λ in length
• total span = ½ λ + small center gap
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 = 73Ω
• easily matched to many transmission lines
gap
¼ λ¼ λ
½ λ
Transmission
Line
+v
-v
i
High Impedance 2k-3kΩ
Low Impedance 73Ω
36. 36
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
37. 37
Azimuth Pattern
Elevation Pattern
Polar Radiation Pattern
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
38. 38
½λ 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 λ 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
39. 39
Multi-Band Dipole Antennas
Transmission
Line
λ1/4C
L
C
L
λ1/4
λ2/4λ2/4
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
40. 40
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
41. 41
Elementary Antennas
low cost – flexible solutions
Long Wire Antenna
• effective wideband antenna
• length l = several wavelengths
- used for signals with 0.1l < λ < 0.5l
- frequency span = 5:1
• drawback for band limited systems - unavoidable interference
• near end driven by ungrounded transmitter output
• far end terminated by resistor
- typically several hundred Ω
- impedance matched to antenna Z0
• transmitter electrical circuit ground connected to earth
Antenna
Transmission
Line
earth ground
R=Z0
42. 42
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
angular radiation pattern
horizon
feed
polar ration pattern
43. 43
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
44. 44
Folded Dipole Antenna
- basic ½λ dipole folded to form complete circuit
- core to many advanced antennas
- mechanically more rugged than dipole
- 10% more bandwidth than dipole
- input impedance ≈ 292 Ω
- close match to std 300Ω twin lead wire transmission line
- use of different diameter upper & lower arms allows
variable impedance
λ/2
45. 45
Loop & Patch Antenna – wire bent into loops
Patch Antenna: rectangular conducting area with || ground plane
Area A
N-turns
V = maximum voltage induced in receiver by EM field
B = magnetic field strength flux of EM field
A = area of loop
N = number of turns
f = signal frequency
k = physical proportionality factor
V = k(2πf)BAN
Antenna
Plane
46. 46
• 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
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
47. 47
552.14 dB
Dipole
3600 dBIsotropic
Beamwidth
-3 dB
Gain (over
isotropic)
ShapeName Radiation Pattern
20
30
50
200
25
14.7 dB
10.1 dB
-0.86 dB
3.14 dB
7.14 dB
Parabolic
Dipole
Helical
Turnstile
Full Wave
Loop
Yagi
Biconical
Horn
1515 dBHorn
360x20014 dB
