b. Most of these causes of magnetron pulling can be avoided by proper
care and preventive maintenance of the radar RF components.
30. Causes of faulty operation beyond the antenna.
a. Now, how about the causes of faulty magnetron operation from
something beyond the antenna?
At first you might think that all troubles
would have to be caused by something actually in the set. That's usually
true, but in the case of a radar transmitter, troubles can be caused by
something that is not a physical part of the equipment.
This happens
because anything that changes the impedance of the load also effects the
b. The usual cause of mismatch
from something beyond the antenna is a
nearby object.
The nearby object
may be a building, another radar, a
smokestack or anything else close
enough to the antenna to change its
impedance. These types of mismatch
can be avoided by selecting a good site
for the radar set.
c. Now, suppose there is no mismatch.
The load impedance remains
constant at its correct value, but the magnetron is not operating correctly.
Then, the trouble is probably magnetron pushing.
31. Magnetron pushing.
a. As stated before, magnetron pushing occurs when the frequency of
the
magnetron is changed (pushed off frequency) by something before
the
magnetron.
In other words, the trouble is something that changes
the
current of the magnetron even though the load and magnetic field are at
the
correct values.
b. The most likely spot to look for the cause of this trouble is in the
modulator.
Actually most magnetron pushing is caused by an improperly
shaped pulse coming from the modulator.
When the pulse is not the right
shape, the magnetron may oscillate in an undesired mode and at the wrong
frequency.
As with magnetron pulling, the output power drops and the
frequency may change enough to make the receiver useless.
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