High-power pulse generator using transmission line with spark discharge device

A high-frequency, high-power generator of the type in which energy is stored in the capacitance of a transmission line and then discharged into a load by the action of a spark discharge device. The load is connected between one end of a transmission line conductor and ground and the other end of the same conductor is grounded. The line is folded and both ends of the other transmission line conductor are connected to the spark discharge device. A charging circuit applies a voltage to the latter conductor and when it reaches the discharge potential of the spark discharge device, the latter discharges to connect both ends of that conductor to ground. This provides relatively well-defined positive and negative pulses to the load.

BACKGROUND OF THE INVENTION 
This invention relates to a high-power pulse generator of the type using a 
spark discharge device to initiate each pulse. More specifically it 
relates to a pulse generator which employs a transmission line both to 
provide capacitance for energy storage and to appropriately shape the 
output pulses. 
The invention is particularly useful in the generation of high-power RF 
signals in remote locations. For example, it is often desirable to deploy 
an RF generator from an aircraft. Since the generator is expendable in 
such cases, it should have a relatively low cost, and since a number of 
them may be carried in the aircraft, they should be light in weight. They 
should also be reliable, and since they are to operate from expendable, 
light-weight power sources, they should be efficient. 
Spark discharges have long been used to generate RF signals. In fact, so 
called "spark-gap" transmitters antedate vacuum tube RF sources by many 
years and, indeed, their use continued until long after the vacuum tube 
transmitters became available, largely because the spark-gap devices were 
both inexpensive and highly reliable. 
The principal object of the invention is to provide a spark discharge pulse 
generator meeting the above criteria and capable of multi-megawatt peak 
power in the VHF and UHF portions of the electromagnetic spectrum. 
SUMMARY OF THE INVENTION 
The pulse generator described herein is connected as a relaxation 
oscillator. That is, a capacitor is charged through a resistor and a spark 
discharge device connected across the capacitor discharges the capacitor 
when the voltage reaches the ignition point of the discharge device. Then 
the cycle repeats. Unlike conventional relaxation oscillators, however, 
the capacitor is physically embodied in a transmission line that is 
connected in such a way that the discharge device not only discharges the 
capacitance, but also causes waves to propagate along the transmission 
line to provide appropriate shaping of the output pulses. 
More specifically, assuming, for example, a coaxial transmission line, a 
length of line corresponding to the desired pulse width is looped so that 
both ends of the inner conductor are connected together to one terminal of 
the spark discharge device. The other terminal of the discharge device is 
connected to a common junction, e.g. ground. One end of the outer 
conductor of the transmission line is connected to ground; the other end 
of the outer conductor serves as an output terminal and thus is connected 
to the load. The capacitance between the inner and outer conductors of the 
transmission line charges until the voltage reaches the ignition of the 
spark discharge device. The discharge of this capacitance causes an 
immediate rise in voltage at the output terminal of the capacitance. If 
this were a conventional capacitor, the voltage would then gradually decay 
as the energy in the capacitor discharged through the output load. 
With present invention, however, the pulse continues with a relatively flat 
top and then sharply drops. The reason for this is two-fold. In the first 
place, the capacitor is elongated and time is required for energy from 
portions more and more remote from the output end to reach the output 
terminal. This has the effect of discharging a succession of capacitors, 
thereby maintaining the output voltage relatively constant. It corresponds 
with the propagation of voltage wavefronts along the transmission line, 
these wavefronts providing constant output voltages which are sharply 
terminated when the wavefronts reach the output end of the line. The pulse 
widths thus correspond to the length of time for an electromagnetic signal 
to propagate from one end of the transmission line to the other.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As seen in the drawing, the generator includes a high-voltage DC source 10, 
connected through a series resistor 12 to the parallel combination of a 
spark discharge device 14 and an energy storage line 18. The line 18 is a 
transmission line, shown here as a coaxial line, although other forms are 
also suitable. Thus it has an inner conductor 20 and an outer conductor 22 
with suitable insulation between them. Also, as will be apparent, the 
outer conductor is covered by an insulator 26. 
The two ends 20a and 20b of the inner conductor are connected together to 
the spark discharge device 14. One end 22a of the outer conductor is 
connected to ground and thus to the other terminal of the discharge device 
14. The other end, 22b, of the outer conductor is the output terminal of 
the generator and it is connected, therefore, to a suitable load 30. The 
connecting parts have exaggerated lengths in the drawing; they are 
preferably much shorter than the line 18. 
The capacitance betwen the inner and outer conductors of the line 18 is 
thus connected in parallel with the discharge device 14. This capacitance 
is charged from the source 10 through the resistor 12 until the voltage 
across the capacitance reaches a value, -V.sub.0, the ignition point of 
the discharge device 14. At that point the discharge device ignites, 
effectively connecting both ends of the inner conductor 20 to ground. 
The resulting operation of the circuit will best be understood by referring 
to the schematic diagram in FIG. 2. The transmission line is depicted as 
an unfolded line with the discharge device 14 shown as a pair of switches 
connected at opposite ends of the line to conform with its connection to 
both ends in FIG. 1. 
With reference to FIG. 2, the closing of the lefthand switch 14 connects 
the load 30 across the transmission line. Also, with the terminal 20b now 
grounded, its voltage rises by a value of V.sub.0 with respect to ground. 
The voltage at the terminal 22b also rises. However, assuming that the 
resistance of the load 30 equals the characteristic impedance of the 
transmission line 18, the voltage at the terminal 22b rises by a factor of 
V.sub.0 /2 and thus immediately attains a value of V.sub.0 /2. At the same 
time, energy flows along the transmission line 18 toward the load 30 so as 
to maintain this output voltage. This corresponds to a voltage wave front 
having a magnitude of +V.sub.0 /2, moving rightward along the line. 
At the same time, the closing of the righthand switch 14 provides a short 
circuit at the other end of the transmission line. The terminal 22a, which 
previously had a voltage of +V.sub.0 with respect to the terminal 20a now 
has the same voltage as the latter terminal and thus has undergone a 
voltage change of -V.sub.0 with respect to the terminal 20a. This change 
propagates leftward along the line as a voltage wave front having a value 
of -V.sub.0. When this wave front reaches the lefthand end of the 
transmission line 18, the voltage applied to the load 30 decreases by a 
corresponding amount and thus attains a value of -V.sub.0 /2. 
At the same time that the leftward travelling wave front reaches the load 
30, the wave front propogating to the right along the line 18 reaches the 
end 22a, 20a and because of the short circuit at that end, it is reflected 
with a reversal of polarity. This results in a wave front of +V.sub.0 /2 
travelling to the left along the waveguide. When this wave front reaches 
the end 20b, 22b the voltage applied to the load 30 is increased by 
V.sub.0 /2, resulting in a zero voltage across the load. Since the load is 
matched to the characteristic impedance of the transmission line 18, wave 
fronts travelling to the left on the line are not reflected and, 
accordingly, at this time the capacitance of the line has been fully 
discharged into the load. The source 10 then quickly recharges the line 
capacitance through the resistor 12 and when the voltage at the conductor 
20 reaches the breakdown level of the discharge device 14, the capacitance 
is discharged into the load once again in the manner described above. 
From the foregoing it will be apparent that each discharge of the line 
capacitance results in the application of first a positive and then a 
negative pulse to the load 30. The duration of each of these pulses is the 
length of time required for a wave to propagate from one end of the 
transmission line to the other and this in turn is equal to the product of 
the wave velocity along the line and the length of the line. 
The above-described operation provides sharply defined output pulses, this 
characteristic being enhanced by the folding of the transmission line so 
that both ends are connected to the same discharge device and thus are 
simultaneously grounded. 
The generator thus is capable of a rapid succession of high-power pulses 
and in fact is an efficient generator of high-power RF energy even at very 
high and ultra-high frequencies. Moreover, it has the reliability of a 
spark discharge device and as can be seen is inexpensive and light in 
weight.