Radar System Engineering

SEC. 10.2] THE RESONANT SYSTEM 329
matters so that the electrons excite only the ir-mode. Adequate frequency
differences between the desired mode and all others favor stable
operation; in most magnetrons, the frequency of the mode nearest in
frequency is at least 10 per cent greater or less than that of the mmode.
Unfortunately it has been impossible to design magnetrons that do not
show multiple frequencies under some conditions and this phenomenon
remains as one of the most troublesome encountered in pulsed magnetron
operation.
F1~. 10.9.—Rising-sunmagnetron with waveguide output and radial cathode leads.
Power is extracted from only one of the oscillating cavities. This
may be done by means of either a coaxial line output or a waveguide
output. For the coaxial line output, it is usual to insert a loop L (Fig.
10.1) into the inductive portion of one of the oscillators in such a way
that it links the magnetic fhx. An r-f potential difference is thus
induced between the ends of the loop, which are connected to the inside
and outside conductors of the coaxial line. This type of output is satisfactory
for magnetrons whose frequency is 3000 Me/see or less. At
frequencies above 3000 Me/see a waveguide output is to be preferred as
small coaxial lines arc over and are unduly 10SSY. Power is extracted
from the back of the cavity by means of a slit which expands, usually
with discontinuities, until its dimensions correspond to those of the waveguide
(Figs. 109 and 10”10).
The magnetrons discussed above are referred to as “strapped”