Radar System Engineering

SEC. 10,10] LINE-TYPE PULSERS 375
impedance, and the capacity is chosen small enough to be almost completely
charged a very short time after the oscillator draws full load current.
A damping network may also be provided to help bring down the
trailing edge of the voltage pulse and prevent post-pulse oscillations.
The Pulse-jorming Network.—One of the possible lumped-constant
networks having electrical properties essentially equal to those of a
transmission line is given in Fig. 10.37, with the values of L and C necessary
for a line of given impedance and pulse length. If the inductance is
constructed by winding a uniform helix on an insulating cylinder, the
ratio of coil length to coil diameter should be that shown in Fig. 10.37, in
order to provide the right amount of coupling between coils. The appropriate
value of inductance at each end of the line differs slightly from that
in the middle sections. It has been found practically that the optimum
number of meshes is related to the desired pulse length as shown in the
following table.
TABLE 105.-RELATION OF PCLSELENGTHTO N’UMBEROF h’fESHES
Pulse length, Ksec Number of meshes
0,14.5 1–3
0.5-2.5 2-5
2,,%5 >8
The greatest obstacle to design of a compact high-power network is
the bulk of the necessary condensers. High-voltage — condensers are
inordinately large and expensive
Diameter
in comparison with the job they t —nl — /d
have to perform; all electronic
equipment would profit from a
major improvement in condenser
design. Though a few new dielectrics
have shown promise of
~lG. 1037.-Type E pulse-forming network,
increasing the energy stored per
unit volume, mica, paper, and oil
are still the primary dielectrics for
high-voltage con dens e rs. One f.
1.1to 1.2
new material of interest is Alsifilm To –
Lo = inductance per mesh
or Diaplex, an aluminum-silicate
L = end inductance
clay in an organic impregnate.
C = capacity per mesh
n = number of meshes
This material can be formed into
Zo = surge impedance
thin, homogeneous sheets which
TO = pulse length
1 = coil length of one meal]
have dielectric properties superior
d = coil diameter.
to mica or paper and oil. The
dielectric constant is 5 to 6 and the safe operating dielectric strength is 500
to 700 volts per thousandth of an inch. Greater uniformity would raise
the latter to 800 volts per thousandth. Most problems of fabrication have