2-15
The magnetron is an electron tube in which a magnetic (H) field between the cathode and plate is
perpendicular to an electric (E) field. Tuned circuits, in the form of cylindrical cavities in the plate,
produce rf electric fields. Electrons interact with these fields in the space between the cathode and plate to
produce an ac power output. Magnetrons function as self-excited microwave oscillators. These
multicavity devices may be used in radar transmitters as either pulsed or cw oscillators at frequencies
ranging from approximately 600 to 30,000 megahertz. (If you wish to review magnetron operation in
more detail, refer to NEETS, Module 11, Microwave Principles.)
Let’s examine the following characteristics of a magnetron used as a pulse radar transmitter oscillator
stage:
Stability
Pulse characteristics
The magnet
Output coupling
Stability
In speaking of a magnetron oscillator, STABILITY usually refers to the stability of the mode of
operation of the magnetron. The two main types of mode instability are MODE SKIPPING and MODE
SHIFTING.
Mode skipping (or misfiring) is a condition in which the magnetron fires randomly in an undesired,
interfering mode during some pulse times, but not in others. Pulse characteristics and tube noises are
factors in mode skipping.
Mode shifting is a condition in which the magnetron changes from one mode to another during pulse
time. This is highly undesirable and does not occur if the modulator pulse is of the proper shape, unless
the cathode of the magnetron is in very poor condition.
Pulse Characteristics
PULSE CHARACTERISTICS are the make up of the high-voltage modulator pulse that is applied to
the magnetron. The pulse should have a steep leading edge, a flat top, and a steep trailing edge. If the
leading edge is not steep, the magnetron may begin to oscillate before the pulse reaches its maximum
level. Since these low-power oscillations will occur in a different mode, the mode of the magnetron will
be shifted as the pulse reaches maximum power. This mode shifting will result in an undesirable
magnetron output. For the same reason (to prevent mode shifting), the top of the modulator pulse should
be as flat as possible. Variations in the applied operating power will cause variations in the mode of
operation. The trailing edge of the pulse should also be steep for the same reason--to prevent mode
shifting.
Magnet
The purpose of the MAGNET is to produce a fairly uniform magnetic field of the desired value over
the interaction space between the cathode and plate of the magnetron. The strength of the magnet is
critical for proper operation. If the magnetic field strength is too high, the magnetron will not oscillate. If
the magnetic field strength is too low, the plate current will be excessive and power output will be low.
Frequency of operation will also be affected.
