Coaxial inductive output tube

Abstract: An inductive output tube (32) where, in order to permit the use of coaxial output cavities, the electron beam propagates in first approximation in a radial direction from the cathode (34). The electron beam is generated by an in first approximation cylindrical cathode (34), and gated by a consequently in first approximation cylindrical grid (44). The required drive power is provided by a coaxial input circuit. Depending on the level of a bias voltage, V g , applied between grid (44) and cathode (34), the radial electron beam can optionally be operated in modulation classes A, AB, B or C. The modulated electron beam, accelerated by the beam voltage applied between cathode (34) and anode (52), passes through an in first approximation cylindrical output gap (54) where the modulation interacts with the electromagnetic field of a coaxial output circuit which is optionally connected to one or both ends of the gap (66) between anode (52) and collector (62). The spent beam is then collected by a radial collector (62). In this manner the desired use of coaxial cavities, operating in the suitable TE 011 coaxial mode, is achieved.

Description: This invention relates to the field of Inductive Output Tubes. More particularly, this invention relates to Inductive Output Tubes for use as amplifiers and oscillators having coaxial output circuits and therefore having an anode and a collector arranged radially about a central cathode. A major imitation to the power output obtainable from a conventional power grid tube is the power that can be dissipated by the grids, screens and anodes of such conventional tubes. Too much power dissipated into a wire grid can cause premature failure of the tube. A. V. Haeff, et al.'s Inductive Output Tube (IQT), developed in the 1930s and described in U.S. Patent No. 2,225,447, uses nonintercepting electrodes, such as apertures, rather than delicate wire grids by employing a magnetic field disposed coaxially with the electron beam. Power is removed from the bunched or density-modulated electron beam by passing the beam through a resonant cavity in which the kinetic energy of the electrons, previously accelerated to a high velocity, is converted to electromagnetic energy without the need to collect the electrons on the walls of the cavity. Inductive output tubes are thus a special family of tubes similar to tetrodes. They differ from conventional gridded tetrodes mainly by the way the radio frequency (RF) output power is extracted from the modulated electron beam inside the tube. While in the conventional tetrode both the screen grid and the anode form parts of the RF output circuit, the IOT features an output cavity separated from any beam current gating or collecting electrodes. The electron beam in the IOT interacts with the output cavity solely via electromagnetic field components, as in a klystron. Thus the amplitude of the RF output voltage is no longer limited to the DC potential difference between anode and screen grid, eliminating the typical tetrode compromise between gain and output power. As a result the IOT becomes an amplifier tube superior to the tetrode especially at UHF frequencies (300-3000MHz), providing higher gain, efficiency and output power in this frequency range. FIG. 1 is a schematic diagram of an IOT 10 according to the prior art. Electrons 12 from a thermionic cathode 14 are emitted and controlled by a grid 16 closely spaced from the emitting surface of cathode 14. A magnetic field 18 surrounds the linear electron beam 12. An RF signal to be amplified is introduced through input port 20 to input cavity 22. Interaction between the RF input signal in input cavity 22 and the electron beam 12 results in density modulation of the electron beam 12. Electrons are accelerated by a relatively high voltage on anode 25. In output cavity 26 the density modulated current induces an electromagnetic field resulting in output power available through output coupling 28 of output port 30. Accordingly, the IOT has been perceived as a linear electron beam tube. lOTs built to date are consequently all of the linear beam type, using electron guns, output cavities and collectors similar to those of klystrons. This linear structure creates certain disadvantages. The output cavities for such a linear beam design employ preferably the TE 101 mode (if rectangular) or the TM 011 mode (if circular), as in klystrons. This leads to fairly bulky amplifier assemblies, which become especially awkward in the case of IOT-equipped television transmitters, where two coupled output cavities are normally required in order to achieve the specified bandwidth (approximately 6 MHz). An IOT designed to operate in coaxial output cavities (like those commonly used for tetrodes operating in the same UHF frequency spectrum) would lead to an amplifier with a considerably smaller footprint, thereby reducing equipment and site costs. Another disadvantage linked with prior art lOTs is that in order to limit the space charge in the electron beam to values which still support a reasonable efficiency, and to extract output power at the desired levels despite limited availability of effective cathode surface area, the operating voltage of linear beam lOTs has to be even higher than that of klystrons of similar output power. Such IOTs typically operate in the Television Service at a voltage potential of about 30 to 38 KV for a power output in the range of about 40 to 75 KW. This high voltage requirement results in increased equipment costs for power supplies due to a consequent requirement for higher voltage insulation and more X-ray shielding. Additional adverse effects of such high voltage operation include the difficulty in preventing high-voltage arcing across the DC insulation that is an integral part of the input circuit in lOTs and an increased danger of high voltage breakdown in the cavity due in part to the fact that the peak RF voltage in the output circuit is higher than the operating voltage of the tube, all of which limit both the useable output power of the tube and the physical elevation above sea level at which the tube can be operated (due to reduced air pressure and breakdown of air dielectrics at altitude), if external cavities are used as they are for television transmission. Current commercial television operators seek increased power output capabilities for television transmitters operating in the UHF frequency spectrum. Such transmitters are often operated on mountain tops and other high altitude locations having reduced air pressure and air dielectric breakdown voltages. Because power, P, voltage, V and current, I are related by the expression P=VI, more power can be obtained by operating a linear beam IOT at high voltage. However, as noted above, this apparently simple expedient, when implemented in reduced air pressure environments, requires substantial additional expense in power supplies, insulation, and the like, and is, as a practical matter, difficult and expensive to do. Similarly, more power can be obtained by increasing the electron beam current of the IOT, however, this is also difficult to achieve with current linear beam devices due to the space charge problems discussed above. Accordingly, there is a need for a higher power UHF electron device which can achieve such higher output power with higher currents rather than by resorting to increased voltage operation. In the prior art, EP-A-0 587 481 discloses a vacuum electron tube including a cathode emitting electrons to a collector, means for focusing the electrons in order to concentrate them into a beam and an output resonant cavity coupled with the beam for sampling the energy from the focused beam. However, there is no disclosure of the features set out in the characterising portion of claims 1 and 13 hereinbelow. Accordingly, it is an object and advantage of the present invention to provide an improved electron device especially adapted for operation in the 300 MHz to 3000 MHz frequency range. It is a further object and advantage of the present invention to provide an inductive output electron tube having a coaxial output. It is a further object and advantage of the present invention is to provide an inductive output tube having a radial electron beam at the anode. Yet a further object and advantage of the present invention is to provide an inductive output tube capable of higher current operation thus permitting high power operation at lower beam voltages. These and many other objects and advantages of the present invention will become apparent to those of ordinary skill in the art from a consideration of the drawings and ensuing description of the invention. According to the present invention, there is provided an inductive output tube as defined in the claims. One embodiment of the present invention is an inductive Output Tube where, in order to permit the use of coaxial output cavities, the electron beam propagates in first approximation in a radial direction from the cathode. For this purpose the electron beam is generated by an in first approximation cylindrical cathode, and grated by an : in first approximation cylindrical grid. The required drive power is provided by a coaxial input circuit. Depending on the level of a bias voltage, V g , applied between grid and cathode, the radial electron beam can optionally be operated in modulation classes A, AB, B or C. The modulated electron beam, accelerated by the beam voltage applied between cathode and anode, passes through an in first approximation cylindrical output gap where the modulation interacts with the electromagnetic field of a coaxial output circuit which is optionally connected to one or both ends of the gap between anode and collector. The spent beam is then collected by a radial collector. In this manner the desired use of coaxial cavities, operating in the suitable TE 011 coaxial mode, is achieved. Compared to a linear beam configuration, this solution provides a considerably larger cathode surface, permitting much higher beam currents at a given voltage, or vice versa, permitting much lower voltage at a given beam power value. This radial beam approach also provides low space charge values in the radial electron beam. It also offers low RF voltage in the output cavity and low specific thermal loading in output cavity and collector. In addition, the lower beam impedance offers the potential of increased bandwidth. For a better understanding of the present invention, embodiments will now be described by way of example, with reference to the accompanying drawings, in which: FIG. 1 is a schematic diagram of an inductive output tube in accordance with the background art. FIG. 2 is an electrical schematic diagram of an inductive output tube in accordance with the background art. FIG. 3 is a cross sectional diagram of a further type of inductive output tube. FIG 4 is a cross sectional diagram taken along line 4 - 4 of FIG. 3. FIG 5 is a cross sectional diagram of a yet further type of inductive output tube. FIG 6 is a cross sectional diagram of a yet further type of inductive output tube, and FIG 7 is a cross sectional diagram of an embodiment of the present invention. Those of ordinary skill in the art will realize that the following description of the present invention is illustrative only and is not intended to be in any way limiting. Other embodiments of the invention will readily suggest themselves to such skilled persons from an examination of the within disclosure. The operation of the coaxial IOT is similar to that of the linear beam IOT in many ways. Turning to FIG. 2, an electrical schematic diagram of an IOT, drive power applied to the input circuit generates a radio frequency (RF) current in class A, AB, B or C, depending upon the value of the grid bias voltage, V g . This current is accelerated by the beam voltage, V b , and thereby induces an electromagnetic field in the output circuit. The spent beam is dissipated in the collector assembly. The principle presented in this disclosure can be used to design a variety of specialized tubes. The version shown in FIG. 3 is suitable for applications that require wide-band tunability at high frequencies. If tunability is not of the essence, a simplified version as shown in FIG. 5 can be used. FIG. 6 presents a version for lower frequencies, and FIG. 7 according to the invention shows a high-power variation of this tube, featuring access for a coaxial output coupler directly to the tube rather than to the output cavity. In any case, these are only examples for the variety of possible versions of a radial electron beam coaxial IOT. Many variations are likewise possible for details of each version, like grounded instead of insulated collectors, multi-stage collectors, means to suppress RF oscillation in or RF radiation from the grid/anode area, water- or air-cooling for collector or other parts of the tube, lay-out of the electrostatic focusing electrodes, possible electromagnetic or permanent magnetic focusing of the electron beam, position and connection of insulating ceramics and window ceramics, etc. Not shown are the tube-external parts of the required coaxial circuits. The technology for these elements is generally known to those of ordinary skill in the art from coaxial cavities for high-power tetrodes; the main difference being that the high-voltage choke, in tetrode amplifiers part of the output cavity, becomes part of the input circuit in an IOT amplifier. Turning now to FIG. 3, a metal ceramic coaxial inductive output tube 32 is depicted in cross section. Metal ceramic construction is presently preferred due to its ruggedness, relative replicability and high temperature capability. There is no requirement that the tube be built as a metal ceramic structure. As noted above, the inductive output tube 32 shown in FIG 3 is particularly well-suited to applications that require wide-band tunability at high frequencies (for instance, in a range of about 470 MHz to about 860 MHz as required for Television transmitters operating in the UHF Television Band). Thermionic cathode 34 is preferably a conventional, substantially cylindrical structure disposed about a central axis 35 of coaxial inductive output tube 32. Power is delivered to a heater (not shown -- but internal to the cathode in a preferred embodiment) for exciting thermionic cathode 34 into electron emission over wires 36, 38 which are, in turn, connected respectively to conductive elements 40, 42. A conventional substantially cylindrical grid structure 44 is disposed a distance from and coaxial with cathode 34. The cathode - grid gap or spacing follows conventional closely spaced design and is preferably in a range of about 0.15 mm to about 1.0 mm. Grid connections are made through conductor 46. For amplifier operation, a conventional coaxial RF input connection is made to the RF input port 48. This RF input is applied to the region 50 between the cathode and the grid, thus modulating the emission of electrons in the amplifier in accordance with the input signal as discussed above. An anode structure 52 is disposed radially about grid 44. In operation, anode 52 is held at a high potential. Electrons emitted from cathode 34 are accelerated in a direction substantially orthogonal (at right angles) to central axis 35 by the electric field caused by the high potential on anode 52 in the high voltage gap region 54. Gap region 54 is therefore radially disposed in anode 52. This causes an effect similar to that of conventional IOT electron "bunching" but it does so in a disk-shaped or radial beam form rather than in a linear beam form. Higher currents may thus be obtained without exceeding space charge limitations. In FIG. 3 element 56 is an insulator, preferably alumina, beryllium oxide or other brazeable ceramic vacuum material which retains the high vacuum of tube 32 while permitting RF input signals to pass through it. Element 58 is a similar insulator which stands off the voltage difference between anode 52 and grid 44. RF window 60 is also an insulator which stands off the voltage difference between anode 52 and collector assembly 62 while permitting the output RF signal to pass through into an appropriate coaxial output interface (not shown). The region 66 between anode 52 and collector assembly 62 is known as the "interaction gap." It is in this region that the density modulated electron current may interact electromagnetically with the coaxial output through RF window 60. Collector assembly 62 may be a simple collector element held at a fixed potential, it may be a multi-stage collector of mere than one element, each held at a fixed potential, it may be a multi-stage depressed collector, or it may be of any convenient design as known to those of ordinary skill in the art. Collector assembly 62 as shown is a two-stage collector having a first element 68, corresponding to the "tailpipe" of a linear beam IOT, preferably held at a first fixed potential equal to that of anode 52 and a second element 70 preferably held at a second fixed potential lower than that of first element 68. Element 70 is preferably (but not necessarily) electrically insulated from element 68 with ceramic spacers 72, 74. Turning now to FIG. 4, a cross sectional view of coaxial IOT 32 is shown taken along line 4 - 4 of FIG. 3. As is clear from FIGS 3 and 4, anode straps 76, 78, 80 and tailpipe straps 82, 84, 86 which are preferably conductive members made of a material such as copper, are disposed so as to electrically connect or strap upper and lower elements of anode 52 and collector element 68. For example, anode 52 includes a top ring portion 88 and a bottom ring portion 90. These two elements are held apart yet are electrically connected to one another by anode straps 76, 78 and 80. By holding the two elements halves apart, a largely evacuated disk-shaped area is made available in which the radial beam of electrons may propagate relatively unimpeded from cathode 34 to second collector assembly element 70. Tailpipe straps 82, 84 and 86 perform a similar function with respect to elements of collector assembly 62, as shown. These stops between anode sections and between tailpipe sections also prevent the coupling of RF energy in the output circuits into the collector or grid/anode space of the tube and also provide mechanical support and stability to the tube. Turning now to FIG. 5, a version of the coaxial IOT is presented which is optimized for use with higher frequencies and where large range tuneability is not of the essence. In this version of the tube, the tube-internal part of the output cavity is short-circuited at conductive wall 89 a distance Z vertically from the horizontal plane at the center of the output gap. For optimal operation, z = λ/4 where λ is the wavelength corresponding to the desired center operating frequency of the tube. This modification ensures that the beam interacts exactly, or at least approximately, with the maximum RF voltage in the output gap, thereby providing a maximum of output power and efficiency. Turning now to FIG. 6, a coaxial IOT is presented which is tunable over a large frequency range while still maintaining the favorable condition of having only one short circuit at about λ/4 distance from the interaction gap. For this purpose this version permits the use of a 2-segment coaxial output cavity and includes first coaxial output port 92 and second coaxial output port 94. Turning now to FIG. 7, a relatively low frequency coaxial IOT according to the present invention possesses a cylindrical output window preferably formed of an insulator such as alumina which is gas tight to hold the vacuum of the tube, brazeable, and does not greatly attenuate the output frequency of the tube (in a frequency range where the distance between output coupler and interaction gap is considerably smaller than λ/4). Cylindrical output window 96 permits the use of variable coaxial output coupler 98. Output coupler 98 may be moved in or out of cavity 100 to adjust output coupling between the load and the amplifier as desired in a conventional manner. In this version, there is a single output window 96 but additional secondary circuits may be coupled coaxially to the IOT at, for example, port 102. Also note that instead of cylindrical output window 96, one could substitute a conventional disk-type output window disposed at plane 104. While illustrative embodiments and applications of this invention have been shown and described, it would be apparent to those skilled in the art that many more modifications than have been mentioned above are possible without departing from the inventive concepts set forth herein. Specifically, the collector assembly may be operated with or without insulation from the tailpipe and its own constituent pieces in a single or multi-stage configuration. Cooling elements have not been shown. Any kind of air, mixed phase or liquid type of cooling system may be used to carry away waste heat as required and well known to those of ordinary skill in the art. Likewise not shown are elements used to suppress RF generation if the grid/anode space. Such elements may be required in a particular tube design as is well known to those of ordinary skill in the art. Those of ordinary skill in the art will also realize that specific shapes and dimensions of tube parts will need to be adjusted to operate in a particular desired frequency and power range.

Claims: An inductive output tube (32) comprising: a cathode (34) disposed about a first axis (35) of the tube; a grid (44) disposed apart from said cathode and about said first axis; an anode (52) disposed apart from said grid and about said first axis, said anode having a radially disposed gap (54) for allowing electrons emitted from said cathode to travel in paths approximately orthogonal to said first axis; a collector assembly (62) disposed to receive electrons passing through said radially disposed gap; and an interaction gap (66) disposed between said collector assembly and said anode; characterised by a cavity (100) coaxial with said first axis, said cavity electromagnetically coupled to said interaction gap; an output coupling window (96) through which RF energy is electromagnetically coupled disposed at an inner portion of said cavity, said output coupling window coaxial with said first axis; and an output coupler (98) adjacent to said output coupling window disposed along said first axis for adjusting coupling between said inductive output tube and a load coupled to said inductive output tube. An inductive output tube according to claim 1, wherein said cathode (34) is approximately cylindrical in shape. An inductive output tube according to claim 1 or 2, further comprising: a hermetically sealed output port (102) through which RF energy is electromagnetically coupled, said output port coaxial with said first axis and electromagnetically coupled to said interaction gap (66). An inductive output tube according to any of claims 1 to 3, wherein said cavity (100) further comprises a conductive outer wall (89) located a fixed distance Z from a first orthogonal plane, said first orthogonal plane being orthogonal to said first axis and said distance Z being of a value which is substantially one quarter wavelength at a selected operating centre frequency of the tube. An inductive output tube according to any of claims 1 to 4, wherein said cavity (100) is approximately torroidal in shape. An inductive output tube according to any of claims 1 to 5, wherein said output coupling window (96) is approximately cylindrical in shape. An inductive output tube according to any of claims 1 to 6, wherein said output coupling window (96) is fabricated of an insulating material. An inductive output tube according to any one of claims 1 to 7, wherein said output coupling window (96) is fabricated from alumina. An inductive output tube according to any of claims 1 to 8, wherein said output coupler (98) is approximately cylindrical in shape. An inductive output tube according to any of claims 1 to 9, wherein said output coupler (98) is adjustable along said first axis. An inductive output tube according to any one of the preceding claims, wherein said output coupler (98) is disposed within a further cavity. An inductive output tube according to claim 11, wherein said output coupler (98) is adjustable along said first axis within said further cavity. An inductive output tube (32) comprising: a cathode disposed about a first axis (35) of the tube; a grid (44) disposed apart from said cathode and about said first axis; an anode (52) disposed apart from said grid and about said first axis, said anode having a radially disposed gap (54) for allowing electrons emitted from said cathode to travel in paths approximately orthogonal to said first axis; a collector assembly (62) disposed to receive electrons passing through said radially disposed gap; and an interaction gap (66) disposed between said collector assembly and said anode; characterised by a cavity (100) coaxial with said first axis, said cavity eletromagnetically coupled to said interaction gap; an output coupling window (96) through which RF energy is electromagnetically coupled disposed along an outer wall (89) of said cavity, said output coupling window approximately orthogonal to said first axis and intersecting said first axis; and an output coupler (98) adjacent to said output coupling window disposed along said first axis for adjusting coupling between said inductive output tube and a load coupled to said inductive output tube. An inductive output tube according to claim 13, wherein said cathode (35) is approximately cylindrical in shape. An inductive output tube according to claim 13 or 14, further comprising: a hermetically sealed output port (102) through which RF energy is electromagnetically coupled, said output port coaxial with said first axis and electromagnetically coupled to said interaction gap. An inductive output tube according to any of claims 13 to 15, wherein said cavity (102) is approximately torroidal in shape. An inductive output tube according to any of claims 13 to 16, wherein said outer wall of said cavity is located a fixed distance Z from a first orthogonal plane, said first orthogonal plane being orthogonal to said first axis and said distance Z being of a value which is substantially one quarter wavelength at a selected operating centre frequency of the tube. An inductive output tube according to any of claims 13 to 17, wherein said output coupling window (98) is approximately disk-like in shape. An inductive output tube according to any of claims 13 to 18, wherein said output coupling window (98) is fabricated of an insulating material. An inductive output tube according to any of claims 13 to 19, wherein said output coupling window (98) is fabricated from alumina.

Inventors: BOHLEN HEINZ, BOHLEN, HEINZ
Applicants: COMM & POWER IND INC, COMMUNICATIONS & POWER INDUSTRIES, INC.
Application year: 1998
Main classification: H01J25
IPC codes: H01J25