1. Microwave Devices

2. 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

3. Waveguides Pipe through which waves propagate
Can have various cross sections
Rectangular
Circular
Elliptical
Can be rigid or flexible
Waveguides have very low loss

5. 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

7. Mode Designations TE: transverse electric TM: transverse magnetic
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

8. 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

10. 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

11. 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

13. Guide Wavelength
Longer than free-space wavelength at same frequency

14. Impedance Matching Same techniques as for coax can be used
Tuning screw can add capacitance or inductance

15. 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

16. 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

18. 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

19. Duty Cycle Important for pulsed tubes like radar transmitters
Peak power can be much greater than average power

20. Crossed-Field and Linear-Beam Tubes
Magnetron is one of a number of crossed-field 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)

21. 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

23. 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

25. 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

27. 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

28. 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

30. 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

31. 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

32. 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

33. 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

34. Radar Equation
Expression for received power from a target

35. 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

37. 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

39. 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

40. Doppler Radar
Motion along line from radar to target changes frequency of reflection
Motion toward radar raises frequency
Motion away from radar lowers frequency

42. Doppler Effect

44. 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

46. Stealth Used mainly by military planes, etc to avoid detection
Avoid reflections by making the aircraft skin absorb radiation
Scatter reflections using sharp angles