Radar System Engineering

SEC. 105] MAGNETRON CHARACTERISTICS 349
Frequency Pulling by the R-j Load. -Magnetrons are self-excited
oscillators whose frequency depends on output loading. In the process
of scanning by the radar antenna, it is inevitable that variations in the
magnetron loading will occur as a result of changing reflections from
rotary joints, antenna housings, and large near-by reflecting objects.
Such changes in loading produce changes in frequency which may be
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Fm. 10.22.—Cutawayview of a 1000-Mc/sectunabl~magnetronshowingthe C-ring tuning
mechanismand the slot-type resonators. (Court.su of Bell Telephone 1.aboraiorie8.)
large, and this phenomenon is a source of considerable trouble. It
necessitates careful engineering of the entire r-f system from magnetron
to antenna, and also affects receiver design.
The problem is attacked in three ways: (1) the magnetron is designed
so that its frequency change with changing r-f loading is smalI; (2) the
variations in r-f loading are reduced by careful design and construction
of the r-f components; (3) the radar receiver is made insensitive to changes
in frequency by making its pass band broad or by incorporating autoxriatic
frequency control. It is the purpose of this subsection to discuss only
the first of these methods of attack, that is, magnetron design.
The effect of loading on the frequency of a magnetron is usually
expressed by what is called the pulling jigure, which is defined as the total
frequency excursion which results when a standing-wave ratio of 1.5 in voltage