1. Transmission Media Key Learning Points
Characteristics of Transmission Lines
Transmission Line Models
Characteristic Impedance and Impedance Matching
Antenna Characterization
Antenna Field Pattern
Antenna Gain
Antenna Design

2. Transmission Lines
wire carries signal (current and voltage)
cable includes wire, connectors, insulation, etc.
goal: efficiently transfer signal energy from source to load over a
cable
high frequency issues
line and load impedance
termination
signal reflections

3. Low Frequency Signals
primarily a resistive circuit that depends on wire thickness
thicker wires carry more current without overheating
thinner wires have higher resistance
Vdrop = IRwire
Rwire = wire resistance, depends on thickness and length
e.g. WG 22 = 16.5/1000ft
Higher Frequencies require more accurate model
transmission line modeled as RLC circuit
depends on material, diameter, spacing, insulation
L & C are negligible at low frequencies, significant at high
frequencies

4. Z0 = characteristic impedance of line
properties of Z0
characterizes wire at varying signal frequencies
determines efficiency of signal energy propagation
optimal value relative to source and load impedance
magnitude due to voltage/current ratio looking into cable
determined by L & C
affect is similar to LPF
affects amplitude, phase and system ground

5. Z = impedance  ratio of voltage to current
v = Ri
v =
i = C
V = RI
V = jwLI
V = I .
jwC
time frequency Z R
jwL
1 = - j
jwC wC
Z = R + jX
R = resistance
X = reactance
X > 0  inductive impedance (current lags voltage)
X < 0  capacitive impedance (current leads voltage)

6. power transfer from source to line
maximum when impedances are complex conjugate (R jX)
impedance mismatch results in wasted energy
G = conductance
inverse of dielectric resistance between shield & conductor
results from small current leakage between them
R = DC resistance of wire, proportional to length & thickness
C = wire’s inherent capacitance
L = wire’s inherent inductance

7. Lumped Sum Parameter Model - Coax Cable
uses discrete R,L,C,G components
physically, parameters exist continuously over cable, specified in
electrical units per meter
C = shunt capacitance
G = shunt conductance
R = series resistance
L = series inductance R C G L

8. Z0 = impedance of infinite line length
Z0 = impedance of finite line terminated by resistance Z0
Z0 =
often in practice DC resistance and current leakage are very low
Z0 =

9. Impedance of Coaxial Cable in terms of physical dimensions
Z0 =
D = outer coax diameter
d = inner conductor diameter
 = dielectric constant of inner material ( )
Other Types of Transmission Lines with Similar Models

10. Line and Load Impedance Matching
signal source not always a physical generator
signal load = receiver of the signal Tx Rx
transmission
line
source/load
electrical length =

11. Impedance Mismatch ZL  Z0
reflections from load end cause periodic repetition of voltage
voltage & current cycles
Value of Z0 cycles from inductive to capacitive - depends on
where it is measured
Impedance Matched: ZL = Z0
transmission line terminated with Z0
transmission line impedance is constant
flat line (non-resonant) no reflected energy – all absorbed by load
ZL = Load Impedance
Z0 = Characteristic Impedance of Transmission Line i

12. impedance of components determined by dimensions & characteristics
antenna
cable
transistor
amplifiers
often not practical to change impedance
goal: make signal source see desired load value - even if
physical value is different

13. Stub Matching: short piece of cable with end open or shorted
shorted end preferred – radiates less energy
acts like a reactance, jX, placed in parallel with transmission line
varying position & length of stub  stub takes on full range of jX
impedance of stub varies with position due to phase difference
between current & voltage

14. Antennas & Propagation
Antennas are designed to radiate & receive signals
Design & selection impacted by
- application
- location
- channel characteristics & signal propagation
Antenna’s performance characterization
- shape of the transmitted signal field
- ability to reject signals to the side of main line of strength
- bandwidth capabilities

15. 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 array electrically shape and steer antenna

16. Propagation & Antennas
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
antennas are transducers
- convert voltage & current into electric & magnetic field
- bridges transmission line & air
- similar to speaker/microphone with acoustic energy
EM field = electromagnetic field

17. 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)
All of the energy in the transmission line remains in the system
Antenna
Designed to Prevent most of 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

18. 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)

19. Ground or Surface wave: follow earths contour
Propagation Modes
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
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
hrx
htx
D =

20. reflected off ionosphere (20-250 miles high)
Sky Waves
reflected off ionosphere ( miles high)
large ranges possible with single hop or multi-hop
transmit angle affects distance, coverage, refracted energy
ionosphere
transmitted
wave
reflected
refracted
skip distance
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

21. 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

22. 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
(dB)
total loss = Gt + Gr – path loss (dB)

23. Antenna Characterization
EM field pattern developed by antenna
not always possible to model mathematically
difficult to account of obstacles
antennas are studied in EM isolated rooms to extract key
performance characteristics
relative signal intensity used relative field pattern determined by
antenna design
absolute value of signal intensity varies for given antenna design
- at transmit antenna is related to power applied at transmitter
- at receive antenna is related to power in surrounding space

24. 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
forward gain = 10dB
backward gain = 7dB
+10dB
+7dB
+ 4dB 0o
270o
180o
90o
beam
width
null

25. radiated field shaping  lens & visible light
application determines required direction & focus of signal
antenna characteristics
- radiation field pattern
- gain
- lobes
- beamwidth
- directivity
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
- complicated details of antenna design involve near-field
measurements

26. 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

27. 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
antenna is rotated
plot of field strength versus angle is made 0o
270o
180o
90o
100 uV/m

28. 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
- receiver only picks up intended signal
avoid unwanted receivers:
- security
- multi-access systems
locate target direction & distance – e.g. radar
not always necessary to shape field pattern
standard broadcast often omnidirectional - 360o

29. 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 (reference to isotropic antenna)
- 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

30. 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

31. lamp  isotropic antenna
lens  directional antenna
 lens provides a gain/loss of visible light in a specific direction
 lens doesn’t change actual power radiated by lamp
e.g.
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 twice
as much power than reference antenna

32. 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

33. Antenna Design results in 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
beam
width
lobe
null

34. Antenna Design – Spectral Parameters
Center Frequency - optimum operating frequency
Antenna Bandwidth  -3dB points of antenna performance
Bandwidth Ratio: Bandwidth/Center Frequency
e.g. let fc = 100MHz with 10MHz bandwidth
- radiated power at 95MHz & 105MHz = ½ radiated power at fc
- bandwidth ratio = (105-95)/100 = 10%

35. Bandwidth Issues
High Bandwidth Antennas tend to have less gain than
narrowband antennas
Narrowband Receive Antenna
- reduces interference from adjacent signals
- reduces received noise power
main trade-offs for Antenna Design
directivity & beam width
acceptable lobes
maximum gain
bandwidth
radiation angle

36. 55
2.14 dB
                              
Dipole
                         
360
0 dB
Isotropic
Beamwidth -3 dB
Gain (over isotropic)
Shape
Name
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 15
15 dB
Horn
360x200
14 dB