Download Advance Communication Systems Lecture 9

Microwave Devices
Introduction
• Microwaves have frequencies > 1 GHz approx.
• Stray reactances are more important as frequency
increases
• Transmission line techniques must be applied to
short conductors like circuit board traces
• Device capacitance and transit time are important
• Cable losses increase: waveguides often used
instead
Waveguides
• Pipe through which waves propagate
• Can have various cross sections
– Rectangular
– Circular
– Elliptical
• Can be rigid or flexible
• Waveguides have very low loss
Modes
• Waves can propagate in various ways
• Time taken to move down the guide varies
with the mode
• Each mode has a cutoff frequency below
which it won’t propagate
• Mode with lowest cutoff frequency is
dominant mode
Mode Designations
• TE: transverse electric
– Electric field is at right angles to direction of
travel
• TM: transverse magnetic
– Magnetic field is at right angles to direction of
travel
• TEM: transverse electromagnetic
– Waves in free space are TEM
Rectangular Waveguides
• Dominant mode is TE10
– 1 half cycle along long dimension (a)
– No half cycles along short dimension (b)
– Cutoff for a = c/2
• Modes with next higher cutoff frequency
are TE01 and TE20
– Both have cutoff frequency twice that for TE10
Usable Frequency Range
• Single mode propagation is highly desirable
to reduce dispersion
• This occurs between cutoff frequency for
TE10 mode and twice that frequency
• It’s not good to use guide at the extremes of
this range
Group Velocity
• Waves propagate at speed of light c in guide
• Waves don’t travel straight down guide
• Speed at which signal moves down guide is
the group velocity and is always less than c
 fc 
vg  c 1   
 f 
2
Guide Wavelength
• Longer than free-space wavelength at same
frequency
g 

 fc 
1   
 f 
2
Impedance Matching
• Same techniques as for coax can be used
• Tuning screw can add capacitance or
inductance
Microwave Tubes
• Used for high power/high frequency
combination
• Tubes generate and amplify high levels of
microwave power more cheaply than solid
state devices
• Conventional tubes can be modified for low
capacitance but specialized microwave
tubes are also used
Magnetron
•
•
•
•
High-power oscillator
Common in radar and microwave ovens
Cathode in center, anode around outside
Strong dc magnetic field around tube causes
electrons from cathode to spiral as they
move toward anode
• Current of electrons generates microwaves
in cavities around outside
Slow-Wave Structure
• Magnetron has cavities all around the outside
• Wave circulates from one cavity to the next
around the outside
• Each cavity represents one-half period
• Wave moves around tube at a velocity much less
than that of light
• Wave velocity approximately equals electron
velocity
Duty Cycle
• Important for pulsed tubes like radar
transmitters
• Peak power can be much greater than
average power
Ton
D 
TT
Pavg  PP D
Crossed-Field and Linear-Beam
Tubes
• Magnetron is one of a number of crossedfield tubes
– Magnetic and electric fields are at right angles
• Klystrons and Traveling-Wave tubes are
examples of linear-beam tubes
– These have a focused electron beam (as in a
CRT)
Klystron
• Used in high-power amplifiers
• Electron beam moves down tube past
several cavities.
• Input cavity is the buncher, output cavity is
the catcher.
• Buncher modulates the velocity of the
electron beam
Velocity Modulation
• Electric field from microwaves at buncher
alternately speeds and slows electron beam
• This causes electrons to bunch up
• Electron bunches at catcher induce
microwaves with more energy
• The cavities form a slow-wave structure
Traveling-Wave Tube (TWT)
• Uses a helix as a slow-wave structure
• Microwaves input at cathode end of helix,
output at anode end
• Energy is transferred from electron beam to
microwaves
Microwave Antennas
• Conventional antennas can be adapted to
microwave use
• The small wavelength of microwaves
allows for additional antenna types
• The parabolic dish already studied is a
reflector not an antenna but we saw that it is
most practical for microwaves
Horn Antennas
• Not practical at low frequencies because of
size
• Can be E-plane, H-plane, pyramidal or
conical
• Moderate gain, about 20 dBi
• Common as feed antennas for dishes
Slot Antenna
• Slot in the wall of a waveguide acts as an
antenna
• Slot should have length g/2
• Slots and other basic antennas can be
combined into phased arrays with many
elements that can be electrically steered
Fresnel Lens
• Lenses can be used for radio waves just as
for light
• Effective lenses become small enough to be
practical in the microwave region
• Fresnel lens reduces size by using a stepped
configuration
Radar
• Radar stands for Radio Dedtection And
Ranging
• Two main types
– Pulse radar locates targets by measuring time
for a pulse to reflect from target and return
– Doppler radar measures target speed by
frequency shift of returned signal
– It is possible to combine these 2 types
Radar Cross Section
• Indicates strength of returned signal from a
target
• Equals the area of a flat conducting plate
facing the source that reflects the same
amount of energy to the source
Radar Equation
• Expression for received power from a target
 PT G 
PR 
3 4
4  r
2
2
Pulse Radar
• Direction to target found with directional
antenna
• Distance to target found from time taken for
signal to return from target
R = ct/2
Maximum Range
• Limited by pulse period
• If reflection does not return before next
pulse is transmitted the distance to the target
is ambiguous
Rmax = cT/2
Minimum Range
•
•
If pulse returns before end of transmitted
pulse, it will not be detected
Rmin = cTP/2
A similar distance between targets is
necessary to separate them
Doppler Radar
• Motion along line from radar to target
changes frequency of reflection
• Motion toward radar raises frequency
• Motion away from radar lowers frequency
Doppler Effect
2v r f i
fD
c
Limitations of Doppler Radar
• Only motion towards or away from radar is
measured accurately
• If motion is diagonal, only the component
along a line between radar and target is
measured
Stealth
• Used mainly by military planes, etc to avoid
detection
• Avoid reflections by making the aircraft
skin absorb radiation
• Scatter reflections using sharp angles