d. We call this manner of operation the pi mode because the RF fields
oscillate in such a way that each segment is 180 degrees out of phase. In
other words, every other segment is positive and the ones in between are
(That's why there are always an even number of cavities in a
magnetron.) One-half cycle later they all reverse their polarity.
17. Segments are strapped to ensure correct polarity.
a. It is because every other segment must be the same polarity that
magnetrons are sometimes strapped.
A strapped magnetron is one in which
every other segment is connected together with a wire or bar (called a
strap) as in Figure 93. The straps ensure that there is a good connection
(increased coupling) between alternate segments and thus help the magnetron
oscillate in the pi mode.
"Rising Sun" type magnetrons are not strapped
because the vanes are two different sizes. Every other vane is larger than
the ones in between.
This has the same effect as trapping the "hole and
slot" type magnetrons.
Figure 93. Straps Help Magnetron to
Oscillate in pi Mode.
b. The energy oscillating within the magnetron cavities must be
reinforced or the oscillations will die out. This brings us to the reason
why the electrons are made to curve and pass by some cavities on their way
to the anode block.