d. Although magnetron pulling is more common than magnetron pushing,
either trouble is likely to occur because magnetron operating requirements
are so critical.
28. Magnetron pulling.
a. The trouble is called magnetron pulling when the frequency of the
magnetron is changed (pulled off frequency) by a mismatch somewhere past the
magnetron.
That is, the mismatch is either between the magnetron and the
antenna, or it's caused by something beyond the antenna.
You will recall
from a previous lesson, Transmission Lines, that a load must be matched to
its generator circuit in order to get maximum power out. This is true also
of magnetron loads. The load on a magnetron, of course, consists of all the
RF components that follow it, including the antenna. Magnetrons also pull
off frequency when the load is changed or mismatched.
The amount of
frequency change and loss of power depends upon how great is the mismatch.
b. Sometimes the mismatch is slight, and the frequency changes only
slightly, and the power output remains almost the same. The radar receiver
still picks up signals near the desired operating frequency because of its
wide frequency response. But if the mismatch is more than just slight, the
frequency will change more, and there will be even less power output. So
even if the receiver still picks up echoes, the echoes are weak.
In an
extreme case, the frequency change may be so great that the receiver will
not respond at all to the echoes.
c. You can see then, that anything that changes the magnetron output
impedance and causes a mismatch may be very harmful, and may even prevent
the radar from functioning.
29. Causes of impedance mismatch between magnetron and antenna.
a. The most common causes of mismatch between the magnetron and the
antenna are as follows:
(1) Dirt or moisture in the coaxial line or waveguide.
(2) Dirt or moisture on the antenna.
(3) Coaxial line or waveguide that is bent or dented.
(4) Joints that are not soldered properly.
(5) A defective rotating or coupling joint.
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