CHAPTER 10
Microwave Techniques
1. Microwaves are radio signals in the frequency range from I to 300 GHz.
2. The RF spectrum below UHF is mostly already fully occupied leaving little or no room for the growth of new radio services.
3. At microwave frequencies, tremendous bandwidth is available for new radio services as well as for wide-bandwidth signals such as TV, multiplexed signals, or computer data.
4. The microwave frequencies are used primarily for telephone communications, radar, and satellite communications.
5. Other microwave applications include cable TV, space communications, radio astronomy, and heating.
6. The primary benefit of microwaves is wide bandwidth availability.
7. The main disadvantages of microwaves are that they are limited to line-of-sight transmission distances, conventional components are not usable, and circuits are more difficult to analyze and design.
8. The allocation of the RF spectrum is handled by the Federal Communications Commission (FCC) in the United States.
9. Balanced transmission line is not used for microwaves because of radiation losses. Coaxial cable is not used because of its high attenuation.
10. The preferred transmission line for microwaves is waveguides.
11. Because of the short physical length of transmission lines at microwave frequencies, quarter- and half-wave lines are commonly used for tuned circuits and filters.
12. Two printed circuit board implementations of transmission lines, called stripline and microstrip, are widely used to create resonant circuits and filters.
13. A waveguide is a hollow metal pipe with a circular or rectangular cross section used for carrying microwave signals from one place to another.
14. A waveguide acts like a high-pass filter, passing all frequencies above its cut-of frequency and rejecting those below it.
15. The cutoff frequency fco of a waveguide depends upon its physical size. For a rectangular waveguide, it is 300/2a. where a is the wide dimension of the waveguide in meters.
16. The microwave signal carried by a waveguide is made up of electric (E) and magnetic (H) fields that bounce off the walls of the waveguide as they propagate along its length.
17. The modes of a waveguide describe the various patterns of electric and magnetic fields that are possible.
18. A transverse electric (TE) mode is one where the electric field is transverse or perpendicular to the direction of propagation.
19. A transverse magnetic (TM) mode is one where the magnetic field is perpendicular to the direction of propagation.
20. Waveguides are available in standard lengths and sizes, and special pieces are used for right-angle bends and 90° twists.
21. Half-wavelength sections of waveguides with shorted or closed ends are known as resonant cavities since they "ring" or oscillate at the frequency determined by their dimensions.
22. Cavity resonators are metallic chambers of various shapes and sizes that are used as parallel-tuned circuits and filters. They have a Q of up to 30,000.
23. Point-contact and Schottky or hot-carrier diodes are widely uses as mixers in microwave equipment as they have low capacitance and inductance.
24. Varactor diodes are widely used as microwave frequency multipliers. Multiplication factors of 2 and 3 are common with power levels up to 20 Wand efficiencies up to 80 percent.
25. Step-recovery or snap-off diodes are also widely used as frequency multipliers with multiplication factors up to 10, power ratings up to 50 W. and efficiencies approaching 80 percent.
26. A Gunn diode is a microwave semiconductor device used to generate microwave energy. When combined with a microstrip, stripline or resonant cavity, simple low power oscillators with frequencies up to 50 GHz are easily implemented.
27. Both IMPATT and TRAPATT diodes are GaAs devices operated with high reverse bias to produce avalanche breakdown. Both are used in microwave oscillators.
28. A klystron is a vacuum tube used for microwave amplification and oscillation.
29. Klystrons use a cavity resonat or to velocity modulate an electron beam which imparts energy to another cavity, producing power amplification. Klystrons are available which produce from a few to many thousands of watts.
30. A single-cavity reflex klystron is used as a microwave oscillator.
31. Klystrons are being gradually replaced by Gunn diodes and traveling-wave tubes.
32. A magnetron is a diode vacuum tube used as a microwave oscillator in radar and microwave ovens to produce powers up to the megawatt range.
33. In a magnetron, a strong magnetic field creates circular paths of electron flow to excite cavities into oscillation.
34. A traveling-wave tube (TWT) is a microwave power amplifier with very wide bandwidth.
35. A microwave signal applied to a helix around the TWT produces velocity and density modulation of the electron beam over a long distance which induces a higher-power signal in the helix.
36. The most commonly used microwave antenna is the horn, which is essentially a rectangular waveguide with a flared end.
37. A pyramidal horn flares in both waveguide dimensions. A sectoral horn flares in only one dimension.
38. Horn antennas are directional and produce a beam width in the 10° to 60° range with a gain in the 10-to 20-dB range, depending upon dimensions.
39. A parabolic or dish-shaped reflector is used with most microwave antennas to focus the RF energy into a narrow beam and increase gain.
40. The parabolic reflector usually has a diameter that is no less than 10 wavelengths at the operating frequency.
41. The gain and directivity of a parabolic reflector antenna is directly proportional to its diameter.
42. Parabolic reflector antennas are fed by placing a horn antenna at the focal point or by placing the horn at the center of the reflector and placing a small reflector at the focal point. The latter is known as Cassegrain feed.
43. A helical antenna is made up of six to eight turns of heavy wire or tubing to form a coil or helix. It is fed with coax and is backed up with a reflector.
44. Helical antennas are used at UHF and microwave frequencies and have a gain in the 12- to 20-dB range and a beam width in the 12° to 45° range.
45. Helical antennas produce circular polarization where the electric and magnetic fields rotate. The polarization may be right-hand or left-hand depending upon the direction in which the helix is wound.
46. Helical antennas can receive either vertically or horizontally polarized signals but can only receive a circularly polarized signal of the same direction.
Good piece of information about waveguide components
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