Radar System Engineering

SEC. 10.8] LOAD REQUIREMENTS 365
totally wasted, but stray load capacity seriously affects time of fall and
back-swing. Short, well-spaced leads and a low-capacity magnetron filament
transformer are helpful in reducing load capacity.
The definition of pulse duration is intimately bound up with magnetron
behavior. From the radar system point of view it is the duration
of the r-f pulse which is of interest and not necessarily the duration of the
voltage pulse applied to the magnetron. Figure 10.30 shows the typical
appearance of the r-f pulse, of the current pulse through the magnetron,
and of the voltage pulse applied to the magnetron. Usually, the shape
lWII
— —_
——-
LA V*O
T
Magnetron conduction
current and R-F envelope
Voltage across magnetron
FIa. 10.30.—Time variation of voltage and currentof a pulsedcavity magnetron.
r, = time of rise of voltage WIV.2
7,= delay time for onset of osciilatione
ro = width of currentand r-f pulse
rf = time of fall of voltage wave.
and duration of the r-f pulse closely approximatee tb ose of tbe current
pulse; the voitage pulse is less steep and of longer duration.
The pulse length, r,, may be defined as the interval of time during
which the load current wave is within 50 per cent of peak value. In so
defining the pulse length, it is necessary to ignore a rather short-duration,
high-current spike often found on the leading edge of the current wave.
So defined, TOcan be used satisfactorily in calculations of average current
and average r-f power. Successful pulsers have been designed and put
into production for TOas short as 0.1 psec, and as long as 5 usec or a little
more. Practical design dificultiies increase at pulse Iengths outside this
range.
The voltage across the magnetron when it is oscillating vigorously in a
steady state is designated as V-o. In general, V~tiwill lie between a few
kilovolts and 30 to 40 kv, although it may be higher for magnetrons in the
1O-MW region. Most practical pulsers do not produce a perfectly uniform
flat-top wave because this is usually costly and unrewarding. A
drop of 2 to 5 per cent in voltage during the pulse, AV~,, can usually be
tolerated by the magnetron without harmful frequency modulation or
mode instability.