7.12 Generation of Microwaves
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value for a cell phone power within the brain tissue is less than 0.3 W/kg, but only
a fraction of this power is absorbed. (Cell phones use frequency bands within 800–
900 MHz and 1.8–1.9 GHz. The adult human brain has a mass of about 1.3–1.4 kg.)
7.12 Generation of Microwaves
Microwaves are usually generated in a resonant microwave cavity stimulated by
electrons. (A microwave cavity is an enclosure with conductive walls which can
support a standing electromagnetic wave with microwave frequency.) Within a
‘klystron’, a beam of electrons in a vacuum is accelerated and decelerated by a
cavity containing a weak microwave, generated by a stimulating electronic oscillator
or from positive feedback from the output. The electrons travel to a second cavity,
but in between, the difference in speed of the various electrons causes a bunching
of electron density which grows to a maximum in the length between the cavities.
These bunched electrons induce a strong microwave resonance in the second cavity,
which is used as a source of emission. Klystrons can have good control over the
frequency of the generated microwaves, but can only create a maximum of around
a factor of ten amplification from the original weak microwaves in the first cavity.
‘Magnetrons’ have an electron emitter (‘cathode’) constructed as a filament made
from a conductive wire, heated with an electric current. The cathode is placed at the
central axis of a cylindrical microwave cavity, with a conductive wall some distance
away acting as the anode. Electrons from the cathode must pass through a strong
magnetic field which is oriented axially. The magnetic field forces the electrons
to spiral outward rather than just accelerate radially. At a certain radius, the metal
anode has a series of axial cavities. The spiraling electrons passing by the holes of
these cavities get jiggled by the induced charges on the walls of the anode. This
jiggling motion has a period close to the resonant period of microwaves within the
cavity, so some kinetic energy from the electrons passes into microwave energy.
Magnetrons can have energy efficiencies in the 70% range (while klystrons are
typically in the 30% range) and generate powerful microwaves, but the spread and
variation of the microwave frequency is not as sharp as in a klystron.
Microwaves are ubiquitous in our technological world. They are used in communications (1–300 GHz), microwave ovens (2.45 GHz), cell phones (824–894 GHz
and 1850–1990 GHz), global positioning, electron paramagnetic resonance imaging, microwave diathermy, and power transmission via microwave antennae.
Microwaves are preferred over radio for communications for several reasons:
• Microwaves have a shorter wavelength than radio, and so, with parabolic
reflecting surfaces, they can be more directional between a transmitting antenna
and a receiver.
• Microwaves can carry more information than radio waves, being at a higher
frequency.
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value for a cell phone power within the brain tissue is less than 0.3 W/kg, but only
a fraction of this power is absorbed. (Cell phones use frequency bands within 800–
900 MHz and 1.8–1.9 GHz. The adult human brain has a mass of about 1.3–1.4 kg.)
7.12 Generation of Microwaves
Microwaves are usually generated in a resonant microwave cavity stimulated by
electrons. (A microwave cavity is an enclosure with conductive walls which can
support a standing electromagnetic wave with microwave frequency.) Within a
‘klystron’, a beam of electrons in a vacuum is accelerated and decelerated by a
cavity containing a weak microwave, generated by a stimulating electronic oscillator
or from positive feedback from the output. The electrons travel to a second cavity,
but in between, the difference in speed of the various electrons causes a bunching
of electron density which grows to a maximum in the length between the cavities.
These bunched electrons induce a strong microwave resonance in the second cavity,
which is used as a source of emission. Klystrons can have good control over the
frequency of the generated microwaves, but can only create a maximum of around
a factor of ten amplification from the original weak microwaves in the first cavity.
‘Magnetrons’ have an electron emitter (‘cathode’) constructed as a filament made
from a conductive wire, heated with an electric current. The cathode is placed at the
central axis of a cylindrical microwave cavity, with a conductive wall some distance
away acting as the anode. Electrons from the cathode must pass through a strong
magnetic field which is oriented axially. The magnetic field forces the electrons
to spiral outward rather than just accelerate radially. At a certain radius, the metal
anode has a series of axial cavities. The spiraling electrons passing by the holes of
these cavities get jiggled by the induced charges on the walls of the anode. This
jiggling motion has a period close to the resonant period of microwaves within the
cavity, so some kinetic energy from the electrons passes into microwave energy.
Magnetrons can have energy efficiencies in the 70% range (while klystrons are
typically in the 30% range) and generate powerful microwaves, but the spread and
variation of the microwave frequency is not as sharp as in a klystron.
Microwaves are ubiquitous in our technological world. They are used in communications (1–300 GHz), microwave ovens (2.45 GHz), cell phones (824–894 GHz
and 1850–1990 GHz), global positioning, electron paramagnetic resonance imaging, microwave diathermy, and power transmission via microwave antennae.
Microwaves are preferred over radio for communications for several reasons:
• Microwaves have a shorter wavelength than radio, and so, with parabolic
reflecting surfaces, they can be more directional between a transmitting antenna
and a receiver.
• Microwaves can carry more information than radio waves, being at a higher
frequency.