High impedance circuit for injection locked magnetrons

A high impedance circuit has radially disposed first vanes and radially disposed second vanes interdigitating between the first vanes. The first vanes and the second vanes are each interconnected by a first toroidal strap and a second toroidal strap, respectively. The first strap and the second strap are dipsosed co-axially on opposite sides of the vane structure. The vanes and straps are dimensioned so that the circuit has a single cavity impedance commensurate with a predetermined interaction impedance for the oscillator which is sufficient to sustain oscillation for a preselected injection locking bandwidth of the oscillator.

FIELD OF INVENTION 
The present invention relates generally to injection locked magnetrons and 
more particularly to a high impedance circuit utilizing a novel vane 
structure. 
BACKGROUND OF THE INVENTION 
A study of injection locking of non-coherent oscillators is described in 
Adler, "A Study of Locking Phenomenon in Oscillators," Proceedings of the 
IRE, June, 1946, pages 351-357. As described therein, the coherent 
bandwidth, .DELTA.F, of an injection locked oscillator is substantially 
equal to the ratio given by (1) the product of twice the frequency of the 
oscillator and the square root of the ratio of the injected coherent power 
to the output power of the oscillator to (2) the external Q of the 
oscillator. 
The study of injection locking by Adler was further developed by others. 
For example, see Huntoon & Weiss, "Synchronization of Oscillators," 
Proceedings of the IRE, December, 1947, pages 1415-1423. The Huntoon 
reference provides a strong theoretical basis for injection locking 
regardless of circuit configuration. 
One of the earlier articles relating to the injection locking of magnetron 
oscillators is given in David, "R. F. Phase Control and Pulsed 
Magnetrons," Proceedings of the IRE, June, 1952, pages 669-685. Although 
the theoretical concept of injection locking of magnetrons is known, the 
practical reduction to practice in the prior art of injection locked 
magnetrons has not been realized until relatively recently. First, 
appropriate low cost coherent sources of RF energy with sufficient power 
to drive magnetrons have not been available. Secondly, the existing 
magnetron circuits have an apparent limitation which limit the obtainable 
circuit bandwidth. The disadvantage resulting from this limitation is that 
the known magnetron circuits were insufficient for commercial 
exploitation. 
Recent advances in solid state oscillators have all but eliminated the 
first limitation of the prior art noted above. Power levels for magnetrons 
are now aVailable in the 0.5 to 5.0 kilowatt level. With current devices, 
coherent gains of ten to thirteen dB are achievable over narrow 
bandwidths. The exploitation of these advances for magnetrons has, 
however, been limited by the ability of conventional magnetron circuits to 
present a sufficiently high impedance to the electron stream in the 
interaction region to sustain proper magnetron operation over a 
sufficiently wide bandwidth. 
In a known prior art magnetron with a conventional circuit configuration, 
manipulation of the coupling between the conventional circuit and its 
external load will reduce its external Q. The reduction of the external Q 
will achieve a wider injection locking bandwidth. Because of the 
fundamental relationship between the external Q and the loaded Q, this 
will cause the fields on the magnetron circuit to become lower and lower 
until a phenomenon called "sink" is reached. At this point the magnetron 
ceases to work. The reason is that the total RF impedance of the circuit 
becomes too low to sustain oscillation. 
The fundamental relationships which govern this sink phenomenon can be 
summarized as follows: 
EQU .DELTA.F=2F.sub.o (P.sub.i /P.sub.o).sup.1/2 /Q.sub.e 
EQU Z.sub.int =Q.sub.l (L/C).sup.1/2 
EQU 1/Q.sub.l =1/Q.sub.o +1/Q.sub.e 
wherein the locking bandwidth .DELTA.F is given by Adler's equation, 
Z.sub.int is the interaction impedance of the magnetron, Q.sub.o is the 
unloaded Q of the magnetron circuit and is a function of the frequency of 
the magnetron, Q.sub.l is the loaded Q of the circuit, Q.sub.e is the 
external Q of the circuit, and (L/C).sup.178 is the single cavity 
impedance of the magnetron and is a function of the configuration of the 
circuit. 
From the above equations, it can be seen that the interaction impedance is 
the product of the loaded Q, Q.sub.l, and the single cavity impedance of 
the magnetron. Because of the fundamental relationship between the loaded 
Q, which is related to the ability to maintain oscillation, and the 
external Q, which is related to the ability to obtain large injection 
bandwidth, decreasing the external Q for a fixed circuit decreases the 
loaded Q. As a consequence thereof, the interaction impedance Z.sub.int, 
is also decreased. 
SUMMARY OF THE INVENTION 
The present invention is directed to a novel high impedance circuit to 
satisfy the conflicting requirements of wide bandwidth and sufficient 
circuit impedance so as to increase the single cavity impedance of the 
magnetron. The novel circuit, in lumped constant terms is a very high 
inductive, very low capacitive circuit. 
According to the present invention, the high impedance circuit has radially 
disposed first vanes and radially disposed second vanes interdigitating 
between the first vanes. The first vanes and the second vanes are each 
interconnected by a first toroidal strap and a second toroidal strap, 
respectively. The first strap and the second strap are disposed co-axially 
on opposite sides of the vane structure. The vanes and straps are 
dimensioned so that the circuit has a single cavity impedance commensurate 
with a predetermined interaction impedance for the oscillator which is 
sufficient to sustain oscillation for a preselected injection locking 
bandwidth of the oscillator, in accordance with the above equations. 
In one embodiment of the present invention, each of the vanes is generally 
T-shaped. Each vane has a relatively wide high conductive first portion 
and a relatively high inductance second portion. The first portion is 
disposed proximate to an axis of the cavity with the second portion 
extending radially outward therefrom. 
Advantages of the present invention are the high-single cavity impedance of 
greater than 200 ohms in a 16 resonator configuration and a wide vane face 
which presents an adequate peak dissipation surface to the electron stream 
of the interaction space. This is an especially important advantage for 
high power applications. Other advantages of the present invention allow 
the independent control of the interaction impedance and the external Q by 
divorcing the single cavity impedance from the coupling circuit which 
controls the bandwidth. The simple shape of the vane allows it to be 
fabricated using conventional stamping operations. The toroidal strap can 
be easily made from available wire through a simple forming operation. The 
designs facilitate the manufacture of the circuit thereby reducing its 
cost. 
These and other objects, advantages and features of the present invention 
will be readily apparent to those skilled in the art from a study of the 
following description of an exemplary preferred embodiment when read in 
conjunction with the attached drawings and appended claims.

DESCRIPTION OF AN EXEMPLARY PREFERRED EMBODIMENT 
Referring now to FIG. 1, there is shown a schematic diagram illustrating 
the use of an injection lock magnetron 10. The source 12 of coherent 
microwave energy delivers low power energy to a circulator 14. The 
circulator injects the low power energy into the magnetron 10. The low 
power energy is amplified by the magnetron 10 as is well known in the art. 
The amplified energy developed by the magnetron 10 is redirected to the 
circulator 14. The high power microwave energy is then coupled to an 
antenna 16 to radiate the high power coherent output energy. 
Referring now to FIGS. 2-4, there is shown a high, impedance circuit 20 for 
an anode ring 22 in the magnetron 20. As is well known in the art, the 
circuit 20 is disposed within an inner cavity 24 of the anode ring 22. 
The high impedance circuit 20 includes a plurality of first radial vanes 
26.sup.1 and a plurality of second radial vanes 26.sup.2. The first radial 
vanes 26.sup.1 are coaxially positionable within the cavity 24. The second 
radial vanes 26.sup.2 are interdigital with the first vanes 26.sup.1 to 
form a vane structure 28. Each of the first vanes 26.sup.1 and second 
vanes 26.sup.2 has a relatively wide high conductance first portion 30 and 
a relatively narrow high inductance portion 32, as best seen in FIG. 4. 
The second portion 32 extends radially outward from the first portion 30. 
The first portion 30 is radially proximate to an axis 34 of the cavity 
about which the magnetron cathode is disposed. 
The circuit further includes a first toroidal strap 26 and a second 
toroidal strap 38. Each of the first strap 36 and the second strap 38 are 
coaxial with the axis 34. The first strap is disposed along the first side 
of the vane structure 28. The second strap is disposed along the second 
side of the vane structure 28. The first strap interconnects the first 
vanes 26.sup.1 and the second strap 38 interconnects the second vanes 
26.sup.2. 
According to the present invention, each of the vanes 26.sup.1, 26.sup.2, 
the first strap 36, and second strap 38 are dimensioned so that the 
circuit 20 has a single cavity impedance commensurate with a predetermined 
interaction impedance of the oscillator which is sufficient to sustain 
oscillation for a preselected injection locking bandwidth, as is derived 
from the above references. More particularly, the relatively narrow second 
portion 32 concentrates rings of magnetic field, B, around the vane 26, as 
best seen in FIG. 4, to create a high inductance. The electric field 
between the vanes reverses direction between each of the first vanes 
26.sup.1 and the second vanes 26.sup.2. The straps, being of circular 
cross-section, minimize capacitance of the circuit, while giving 
sufficient mode separation. Where the straps 36, 38 are connected to the 
appropriate one of the vanes 26.sup.1, 26.sup.2, a mounting portion 40 is 
provided therein with an annular channel 42. The second portion 32 of the 
vanes may be soldered to the anode ring 22. 
By the equations given above, for a given injection lock bandwidth, 
.DELTA.F, the interactive impedance, Z.sub.int, can be selected so that 
oscillation is maintained. It has been found that the interactive 
impedance, in the preferred embodiment, should be at least 5000 ohms. The 
shape of the vanes 26 are then structured so their inductance and 
capacitance satisfies the conditions set forth in the above equations. The 
T-shape of the vanes 26.sup.1, 26.sup.2, has been found to satisfy these 
conditions. 
There has been described hereinabove a novel high impedance circuit for use 
in the anode ring of a magnetron. It is obvious that those skilled in the 
art may make numerous uses of and departures from the preferred embodiment 
of the present invention without departing from the inventive concepts 
herein. Accordingly, the present invention is to be defined solely by the 
scope of the following claims.