2-16
Since the strength of the magnet is critical, you should be careful when handling the magnet. Striking
the magnet, especially with a ferromagnetic object, will misalign the molecular structure of the magnet
and decrease the field strength.
Output Coupling
The OUTPUT COUPLING transfers the rf energy from the magnetron to the output transmission
line (coaxial line or waveguide). A number of considerations impose restrictions upon the output circuit.
The wavelength (frequency) and the power level of the magnetron output energy determine whether the
transmission line to the antenna will be waveguide or coaxial line.
The coaxial output circuit consists of a length of coaxial line in which the center conductor is shaped
into a loop and inserted into one of the magnetron cavities for magnetic coupling. The load side of the
coupling line may feed either an external coaxial line or a waveguide. If the external line is coaxial, the
connection may be direct or by means of choke joints. If the external line is a waveguide, the output
circuit must include a satisfactory junction from the coaxial line to the waveguide. One type of junction
used quite often is the PROBE COUPLER. The probe coupler acts as an antenna radiating into the
waveguide.
The waveguide output may be fed directly by an opening (slot) into one of the magnetron cavities, as
shown in figure 2-9. This opening must be covered by an iris window to maintain the vacuum seal.
The peak power ratings of magnetrons range from a few thousand watts (kilowatts) to several million
watts (megawatts). The average power ratings are much lower, however, and vary from a few watts to
several kilowatts. Additionally, many of the magnetrons used in modern radar systems are tunable in
frequency. Typically, a tunable magnetron can vary the output frequency 5 percent about the center of
its frequency band. Thus the carrier frequency of radar can be changed to obtain the best operation or
avoid electronic jamming on a particular frequency.
Modulator signals of many thousands of volts are applied to the magnetron cathode during operation.
These high voltage levels require large glass posts to insulate the cathode and filaments from the anode
block. In some high-power magnetrons, the cathode is completely enclosed in a container filled with
insulating oil.
WARNING
All radar transmitters contain lethal voltages. Extreme care and strict
observance of all posted safety precautions are essential when working on a radar
transmitter.
Q18. What is the frequency range of magnetron oscillators?
Q19. What two forms of instability are common in magnetrons?
Q20. What is the effect on magnetron operation if the magnetic field strength is too high?
Q21. What is the typical frequency range about the center frequency of a tunable magnetron?
POWER-AMPLIFIER TRANSMITTER
POWER-AMPLIFIER TRANSMITTERS are used in many recently developed radar sets. This type
of transmitter was developed because of the need for more stable operation of the moving target indicator
(mti). In a magnetron transmitting system, the high-power magnetron oscillator has a tendency to drift in
frequency because of temperature variations, changes in the modulating pulse, and various other effects.
