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The Microwave Magnetron Tube Structure and OperationThe Magnetron Tube

Discussion in 'General Electronics' started by Br Dan Izzo, Sep 7, 2004.

  1. Br Dan Izzo

    Br Dan Izzo Guest

    The Microwave Magnetron Tube
    Structure and Operation



    The heart of every microwave oven is the high voltage system . Its
    purpose is to generate microwave energy. The high-voltage components
    accomplish this by stepping up AC line voltage to high voltage, which
    is then changed to an even higher DC voltage. This DC power is then
    converted to the RF energy that cooks the food.

    Basic Magnetron Structure
    The nucleus of the high-voltage system is the magnetron tube . The
    magnetron is a diode-type electron tube which is used to produce the
    required 2450 MHz of microwave energy. It is classed as a diode
    because it has no grid as does an ordinary electron tube. A magnetic
    field imposed on the space between the anode (plate) and the cathode
    serves as the grid. While the external configurations of different
    magnetrons will vary, the basic internal structures are the same.
    These include the anode, the filament/cathode, the antenna, and the
    magnets
    The ANODE (or plate) is a hollow cylinder of iron from which an even
    number of anode vanes extend inward (see Fig. 2). The open trapezoidal
    shaped areas between each of the vanes are resonant cavities that
    serve as tuned circuits and determine the output frequency of the
    tube. The anode operates in such a way that alternate segments must be
    connected, or strapped, so that each segment is opposite in polarity
    to the segment on either side. In effect, the cavities are connected
    in parallel with regard to the output. This will become easier to
    understand as the description of operation is considered.


    The FILAMENT (also called heater), which also serves as the cathode of
    the tube, is located in the center of the magnetron, and is supported
    by the large and rigid filament leads, which are carefully sealed into
    the tube and shielded.

    The ANTENNA is a probe or loop that is connected to the anode and
    extends into one of the tuned cavities. The antenna is coupled to the
    waveguide , a hollow metal enclosure, into which the antenna transmits
    the RF energy.

    The MAGNETIC FIELD is provided by strong permanent magnets, which are
    mounted around the magnetron so that the magnetic field is parallel
    with the axis of the cathode.

    Basic Magnetron Operation
    The theory of magnetron operation is based on the motion of electrons
    under the combined influence of electric and magnetic fields. For the
    tube to operate, electrons must flow from the cathode to the anode.
    There are two fundamental laws that govern their trajectory:
    The force exerted by an electric field on an electron is proportional
    to the strength of the field. Electrons tend to move from a point of
    negative potential toward a positive potential. Figure 3-A shows the
    uniform and direct movement of the electrons in an electric field with
    no magnetic field present, from the negative cathode to the positive
    anode.
    The force exerted on an electron in a magnetic field is at right
    angles to both the field itself, and to the path of the electron. The
    direction of the force is such that the electron proceeds to the anode
    in a curve rather than a direct path.
    Effect of the Magnetic Field
    In Figure 3-B two permanent magnets are added above and below the tube
    structure. In Figure 3-C, assume the upper magnet is a north pole and
    you are viewing from that position. The lower, south pole magnet, is
    located underneath the page, so that the magnetic field appears to be
    coming right through the page. Just as electrons flowing through a
    conductor cause a magnetic field to build up around that conductor, so
    an electron moving through space tends to build up a magnetic field
    around itself. On one side (left) of the electron's path, this self
    induced magnetic field adds to the permanent magnetic field
    surrounding it. On the other side (right) of its path, it has the
    opposite effect of subtracting from the permanent magnetic field. The
    magnetic field on the right side is therefore weakened, and the
    electron's trajectory bends in that direction, resulting in a circular
    motion of travel to the anode.
    The process begins with a low voltage being applied to the filament,
    which causes it to heat up (filament voltage is usually 3 to 4 VAC,
    depending on the make and model). Remember, in a magnetron tube, the
    filament is also the cathode. The temperature rise causes increased
    molecular activity within the cathode, to the extent that it begins to
    "boil off" or emit electrons. Electrons leaving the surface of a
    heated filament wire might be compared to molecules that leave the
    surface of boiling water in the form of steam. Unlike steam, though,
    the electrons do not evaporate. They float, or hover, just off the
    surface of the cathode, waiting for some momentum.

    Electrons, being negative charges, are strongly repelled by other
    negative charges. So this floating cloud of electrons would be
    repelled away from a negatively charged cathode. The distance and
    velocity of their travel would increase with the intensity of the
    applied negative charge. Momentum is thus provided by a negative 4000
    volts DC, which is produced by means of the high-voltage transformer
    and the doubler action of the high-voltage diode and capacitor . (4000
    volts is an average. The actual voltage varies with make and model.) A
    negative 4000 volt potential on the cathode puts a corresponding
    positive 4000 volt potential on the anode. Needless to say, the
    electrons blast off from the cathode like tiny rockets. They
    accelerate straight toward the positive anode, or, at least they try
    to.

    As the electrons hasten toward their objective, they encounter the
    powerful magnetic field of two permanent magnets . These are
    positioned so that their magnetic fields are applied parallel to the
    cathode. The effect of the magnetic fields tends to deflect the
    speeding electrons away from the anode, as described in page one . The
    illustration to the right shows the combined effect of the electric
    and the magnetic fields on the electrons' trajectory. Instead of
    traveling straight to the anode, they curve to a path at almost right
    angles to their previous direction, resulting in an expanding circular
    orbit around the cathode, which eventually reaches the anode.

    The whirling cloud of electrons, influenced by the high voltage and
    the strong magnetic field, form a rotating pattern that resembles the
    spokes in a spinning wheel, as shown in Figure 4 . The interaction of
    this rotating space-charge wheel with the configuration of the surface
    of the anode produces an alternating current flow in the resonant
    cavities of the anode. This is explained as follows. As a "spoke" of
    electrons approaches an anode vane (or the segment between the two
    cavities), it induces a positive charge in that segment. As the
    electrons pass, the positive charge diminishes in the first segment
    while another positive charge is being induced in the next segment.
    Current is induced because the physical structure of the anode forms
    the equivalent of a series of high-Q resonant inductive-capacitive
    (LC) circuits. The effect of the strapping of alternate segments is to
    connect the LC circuits in parallel.



    Next:Resonant Circuits...

















    The heart of every microwave oven is the high voltage system . Its
    purpose is to generate microwave energy. The high-voltage components
    accomplish this by stepping up AC line voltage to high voltage, which
    is then changed to an even higher DC voltage. This DC power is then
    converted to the RF energy that cooks the food.

    Basic Magnetron Structure
    The nucleus of the high-voltage system is the magnetron tube . The
    magnetron is a diode-type electron tube which is used to produce the
    required 2450 MHz of microwave energy. It is classed as a diode
    because it has no grid as does an ordinary electron tube. A magnetic
    field imposed on the space between the anode (plate) and the cathode
    serves as the grid. While the external configurations of different
    magnetrons will vary, the basic internal structures are the same.
    These include the anode, the filament/cathode, the antenna, and the
    magnets
    The ANODE (or plate) is a hollow cylinder of iron from which an even
    number of anode vanes extend inward (see Fig. 2). The open trapezoidal
    shaped areas between each of the vanes are resonant cavities that
    serve as tuned circuits and determine the output frequency of the
    tube. The anode operates in such a way that alternate segments must be
    connected, or strapped, so that each segment is opposite in polarity
    to the segment on either side. In effect, the cavities are connected
    in parallel with regard to the output. This will become easier to
    understand as the description of operation is considered.


    The FILAMENT (also called heater), which also serves as the cathode of
    the tube, is located in the center of the magnetron, and is supported
    by the large and rigid filament leads, which are carefully sealed into
    the tube and shielded.

    The ANTENNA is a probe or loop that is connected to the anode and
    extends into one of the tuned cavities. The antenna is coupled to the
    waveguide , a hollow metal enclosure, into which the antenna transmits
    the RF energy.

    The MAGNETIC FIELD is provided by strong permanent magnets, which are
    mounted around the magnetron so that the magnetic field is parallel
    with the axis of the cathode.

    Basic Magnetron Operation
    The theory of magnetron operation is based on the motion of electrons
    under the combined influence of electric and magnetic fields. For the
    tube to operate, electrons must flow from the cathode to the anode.
    There are two fundamental laws that govern their trajectory:
    The force exerted by an electric field on an electron is proportional
    to the strength of the field. Electrons tend to move from a point of
    negative potential toward a positive potential. Figure 3-A shows the
    uniform and direct movement of the electrons in an electric field with
    no magnetic field present, from the negative cathode to the positive
    anode.
    The force exerted on an electron in a magnetic field is at right
    angles to both the field itself, and to the path of the electron. The
    direction of the force is such that the electron proceeds to the anode
    in a curve rather than a direct path.
    Effect of the Magnetic Field
    In Figure 3-B two permanent magnets are added above and below the tube
    structure. In Figure 3-C, assume the upper magnet is a north pole and
    you are viewing from that position. The lower, south pole magnet, is
    located underneath the page, so that the magnetic field appears to be
    coming right through the page. Just as electrons flowing through a
    conductor cause a magnetic field to build up around that conductor, so
    an electron moving through space tends to build up a magnetic field
    around itself. On one side (left) of the electron's path, this self
    induced magnetic field adds to the permanent magnetic field
    surrounding it. On the other side (right) of its path, it has the
    opposite effect of subtracting from the permanent magnetic field. The
    magnetic field on the right side is therefore weakened, and the
    electron's trajectory bends in that direction, resulting in a circular
    motion of travel to the anode.
    The process begins with a low voltage being applied to the filament,
    which causes it to heat up (filament voltage is usually 3 to 4 VAC,
    depending on the make and model). Remember, in a magnetron tube, the
    filament is also the cathode. The temperature rise causes increased
    molecular activity within the cathode, to the extent that it begins to
    "boil off" or emit electrons. Electrons leaving the surface of a
    heated filament wire might be compared to molecules that leave the
    surface of boiling water in the form of steam. Unlike steam, though,
    the electrons do not evaporate. They float, or hover, just off the
    surface of the cathode, waiting for some momentum.

    Electrons, being negative charges, are strongly repelled by other
    negative charges. So this floating cloud of electrons would be
    repelled away from a negatively charged cathode. The distance and
    velocity of their travel would increase with the intensity of the
    applied negative charge. Momentum is thus provided by a negative 4000
    volts DC, which is produced by means of the high-voltage transformer
    and the doubler action of the high-voltage diode and capacitor . (4000
    volts is an average. The actual voltage varies with make and model.) A
    negative 4000 volt potential on the cathode puts a corresponding
    positive 4000 volt potential on the anode. Needless to say, the
    electrons blast off from the cathode like tiny rockets. They
    accelerate straight toward the positive anode, or, at least they try
    to.

    As the electrons hasten toward their objective, they encounter the
    powerful magnetic field of two permanent magnets . These are
    positioned so that their magnetic fields are applied parallel to the
    cathode. The effect of the magnetic fields tends to deflect the
    speeding electrons away from the anode, as described in page one . The
    illustration to the right shows the combined effect of the electric
    and the magnetic fields on the electrons' trajectory. Instead of
    traveling straight to the anode, they curve to a path at almost right
    angles to their previous direction, resulting in an expanding circular
    orbit around the cathode, which eventually reaches the anode.

    The whirling cloud of electrons, influenced by the high voltage and
    the strong magnetic field, form a rotating pattern that resembles the
    spokes in a spinning wheel, as shown in Figure 4 . The interaction of
    this rotating space-charge wheel with the configuration of the surface
    of the anode produces an alternating current flow in the resonant
    cavities of the anode. This is explained as follows. As a "spoke" of
    electrons approaches an anode vane (or the segment between the two
    cavities), it induces a positive charge in that segment. As the
    electrons pass, the positive charge diminishes in the first segment
    while another positive charge is being induced in the next segment.
    Current is induced because the physical structure of the anode forms
    the equivalent of a series of high-Q resonant inductive-capacitive
    (LC) circuits. The effect of the strapping of alternate segments is to
    connect the LC circuits in parallel.



    Next:Resonant Circuits...
















    The heart of every microwave oven is the high voltage system . Its
    purpose is to generate microwave energy. The high-voltage components
    accomplish this by stepping up AC line voltage to high voltage, which
    is then changed to an even higher DC voltage. This DC power is then
    converted to the RF energy that cooks the food.

    Basic Magnetron Structure
    The nucleus of the high-voltage system is the magnetron tube . The
    magnetron is a diode-type electron tube which is used to produce the
    required 2450 MHz of microwave energy. It is classed as a diode
    because it has no grid as does an ordinary electron tube. A magnetic
    field imposed on the space between the anode (plate) and the cathode
    serves as the grid. While the external configurations of different
    magnetrons will vary, the basic internal structures are the same.
    These include the anode, the filament/cathode, the antenna, and the
    magnets
    The ANODE (or plate) is a hollow cylinder of iron from which an even
    number of anode vanes extend inward (see Fig. 2). The open trapezoidal
    shaped areas between each of the vanes are resonant cavities that
    serve as tuned circuits and determine the output frequency of the
    tube. The anode operates in such a way that alternate segments must be
    connected, or strapped, so that each segment is opposite in polarity
    to the segment on either side. In effect, the cavities are connected
    in parallel with regard to the output. This will become easier to
    understand as the description of operation is considered.


    The FILAMENT (also called heater), which also serves as the cathode of
    the tube, is located in the center of the magnetron, and is supported
    by the large and rigid filament leads, which are carefully sealed into
    the tube and shielded.

    The ANTENNA is a probe or loop that is connected to the anode and
    extends into one of the tuned cavities. The antenna is coupled to the
    waveguide , a hollow metal enclosure, into which the antenna transmits
    the RF energy.

    The MAGNETIC FIELD is provided by strong permanent magnets, which are
    mounted around the magnetron so that the magnetic field is parallel
    with the axis of the cathode.

    Basic Magnetron Operation
    The theory of magnetron operation is based on the motion of electrons
    under the combined influence of electric and magnetic fields. For the
    tube to operate, electrons must flow from the cathode to the anode.
    There are two fundamental laws that govern their trajectory:
    The force exerted by an electric field on an electron is proportional
    to the strength of the field. Electrons tend to move from a point of
    negative potential toward a positive potential. Figure 3-A shows the
    uniform and direct movement of the electrons in an electric field with
    no magnetic field present, from the negative cathode to the positive
    anode.
    The force exerted on an electron in a magnetic field is at right
    angles to both the field itself, and to the path of the electron. The
    direction of the force is such that the electron proceeds to the anode
    in a curve rather than a direct path.
    Effect of the Magnetic Field
    In Figure 3-B two permanent magnets are added above and below the tube
    structure. In Figure 3-C, assume the upper magnet is a north pole and
    you are viewing from that position. The lower, south pole magnet, is
    located underneath the page, so that the magnetic field appears to be
    coming right through the page. Just as electrons flowing through a
    conductor cause a magnetic field to build up around that conductor, so
    an electron moving through space tends to build up a magnetic field
    around itself. On one side (left) of the electron's path, this self
    induced magnetic field adds to the permanent magnetic field
    surrounding it. On the other side (right) of its path, it has the
    opposite effect of subtracting from the permanent magnetic field. The
    magnetic field on the right side is therefore weakened, and the
    electron's trajectory bends in that direction, resulting in a circular
    motion of travel to the anode.
    The process begins with a low voltage being applied to the filament,
    which causes it to heat up (filament voltage is usually 3 to 4 VAC,
    depending on the make and model). Remember, in a magnetron tube, the
    filament is also the cathode. The temperature rise causes increased
    molecular activity within the cathode, to the extent that it begins to
    "boil off" or emit electrons. Electrons leaving the surface of a
    heated filament wire might be compared to molecules that leave the
    surface of boiling water in the form of steam. Unlike steam, though,
    the electrons do not evaporate. They float, or hover, just off the
    surface of the cathode, waiting for some momentum.

    Electrons, being negative charges, are strongly repelled by other
    negative charges. So this floating cloud of electrons would be
    repelled away from a negatively charged cathode. The distance and
    velocity of their travel would increase with the intensity of the
    applied negative charge. Momentum is thus provided by a negative 4000
    volts DC, which is produced by means of the high-voltage transformer
    and the doubler action of the high-voltage diode and capacitor . (4000
    volts is an average. The actual voltage varies with make and model.) A
    negative 4000 volt potential on the cathode puts a corresponding
    positive 4000 volt potential on the anode. Needless to say, the
    electrons blast off from the cathode like tiny rockets. They
    accelerate straight toward the positive anode, or, at least they try
    to.

    As the electrons hasten toward their objective, they encounter the
    powerful magnetic field of two permanent magnets . These are
    positioned so that their magnetic fields are applied parallel to the
    cathode. The effect of the magnetic fields tends to deflect the
    speeding electrons away from the anode, as described in page one . The
    illustration to the right shows the combined effect of the electric
    and the magnetic fields on the electrons' trajectory. Instead of
    traveling straight to the anode, they curve to a path at almost right
    angles to their previous direction, resulting in an expanding circular
    orbit around the cathode, which eventually reaches the anode.

    The whirling cloud of electrons, influenced by the high voltage and
    the strong magnetic field, form a rotating pattern that resembles the
    spokes in a spinning wheel, as shown in Figure 4 . The interaction of
    this rotating space-charge wheel with the configuration of the surface
    of the anode produces an alternating current flow in the resonant
    cavities of the anode. This is explained as follows. As a "spoke" of
    electrons approaches an anode vane (or the segment between the two
    cavities), it induces a positive charge in that segment. As the
    electrons pass, the positive charge diminishes in the first segment
    while another positive charge is being induced in the next segment.
    Current is induced because the physical structure of the anode forms
    the equivalent of a series of high-Q resonant inductive-capacitive
    (LC) circuits. The effect of the strapping of alternate segments is to
    connect the LC circuits in parallel.



    Next:Resonant Circuits...
     
  2. Dbowey

    Dbowey Guest

    Is there a question somewhere there or do you just have diarrhea of the brain
    and fingers?
     
  3. Julie

    Julie Guest

    What a moronic response. I think that your reply has much more to do w/ you
    than the op.
     
  4. Ken Taylor

    Ken Taylor Guest

    I think the response was quite on the mark - it was a lame-o troll post.
     
  5. Br Dan Izzo

    Br Dan Izzo Guest

    hii;

    i cut and pasted the microwave tube info from cd with the same title,
    the pasted
    page didn't list the autors, i paid $29 for this cd it is a great cd
    for people interested in ELECTRONICS of microwave generator like me
     
  6. i cut and pasted the microwave tube info from cd with the same title,
    I certainly found it interesting.Thanks for the post!

    Ewan
     
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