Radar System Engineering

SEC. 10.1] CONSTRUCTION 321
of the order of hundreds of kilowatts at frequencies as high as 24,000
Me/see. The average power output of magnetrons at 3000 .Mc/sec is in
the neighborhood of hundreds of watts, or one hundred times that of a
triode operating at the same frequency. Klystrons are. very useful
sources of low c-w power at frequencies as high as 24,000 Me/see, but
cannot be considered high-power pulsed sources. Their pulse-power
output ranges from a few hundred watts at 3000 Me/see to a few milliwatts
at 24,000 Me/see.
Magnetrons are self-excited oscillators and their output does not have
the frequency stability possible at frequencies where power amplifiers
are available and the output frequency is established by crystal-controlled
oscillators. When properly designed and used, they exhibit stability
adequate to the demands of pulse radar.
10.1. Construction.-Microwave magnetrons, or cavity magnetrons
as they are frequently called, are basically self-excited oscillators whose
purpose is to convert the d-c input power into r-f output power. Figures
10.1 and 10.2 show a particular design of a 10-cm magnetron which is
typical of thk class of transmitting tubes. Between the cylindrical
cathode C and anode block A is an interaction space I in which the conversion
of d-c to r-f power takes place. A constant and nearly uniform
magnetic field is maintained in this interaction space in a direction parallel
to the axis of the tube. In operation, the cathode is maintained at a
negative potential while the anode block is usually at ground potential.
The anode block is pierced in a direction parallel to the axis by a number
of side cavities R which open into the interaction space so that the anode
surface consists of alternate segments and gaps. The ends of the resonating
cavities open into chambers which are called end spaces, through which
the lines of flux extending from one resonator to the next pass. The
coupling between the resonators is increased (in the design shown in
Figs. 10.1 and 10”2) by conducting bars called straps S which connect
alternate segments. Power is extracted from one resonator, one method
being the use of a coupling loop L which forms a part of the output circuit.
The combination of resonant cavities, end spaces, straps, and output
circuit is called the resonant system.
A more detailed discussion of these parts of a magnetron follows.
For pulsed operation, the cathode C is usually oxide-coated and heated
indirectly by an internal heating coif of tungsten or molybdenum. The
cathode structure is attached mechanically to two cathode stems supported
by glass to provide anode-to-cathode insulation. At each end of
the cathode there is an end shield, or hat H, whose purpose is to prevent
electrons from leaving the cathode structure in a direction parallel to the
axis of the magnetron. These end shields must be kept at a temperature
too low to cause the emission of electrons.