32. Other magnetron troubles.
a. Mode skip.
Sometimes a magnetron does not fire (oscillate) every
time the pulse is applied to the cathode.
This happens when the pulse
applied from the modulator isn't powerful enough.
Of course when the
magnetron doesn't fire, there is no output from the set, so the set receives
fewer echoes. This means the set's average power output is reduced, and so
is its effectiveness. When the magnetron fails to fire on each pulse, we
call the trouble mode skip.
b. Mode jump. If each pulse applied to the magnetron isn't the same
shape and amplitude, the magnetron may oscillate in different modes with
different frequencies. This defect is called mode jump.
c. Mode shift.
Sometimes the magnetron may even change its mode of
operation during a pulse, and we call this trouble mode shift.
d. Mode jump and mode skip both result in less power being radiated
from the radar set and poor efficiency.
e. When a magnetron is not operating properly, it is not always easy to
determine the exact trouble. However, it is easy to determine if the output
of the magnetron is correct by using a piece of test equipment called a
a. A radar modulator generates a rectangular pulse that modulates the
RF carrier. The rectangular pulse is made up of the fundamental frequency
a sum and difference frequency.
These frequencies are called sidebands.
Frequencies above the carrier make up the upper sidebands, frequencies below
the carrier make up the lower sidebands. Now, if we plot the power of each
frequency, we have a spectrum.
b. A spectrum analyzer contains an oscilloscope that shows the
magnetron spectrum when the analyzer is connected to the output of a radar
transmitter. By checking the spectrum you tell whether or not the magnetron
is operating correctly. When the magnetron is operating as it should, and
the output of the transmitter is correct, the graph you see on the spectrum
analyzer scope looks like the one in Part A of Figure 94. So, if the output
of the transmitter looks just like the one shown in Part A or is very
similar to it, you know that the magnetron is operating correctly.