Electric lamp and discharge devices: systems – Cathode ray tube circuits – Combined cathode ray tube and circuit element structure
Reexamination Certificate
1999-12-17
2003-12-16
Lee, Benny T. (Department: 2817)
Electric lamp and discharge devices: systems
Cathode ray tube circuits
Combined cathode ray tube and circuit element structure
C315S039300
Reexamination Certificate
active
06664734
ABSTRACT:
RIGHTS OF THE GOVERNMENT
The invention described herein may be manufactured, used, and licensed by or for the United States Government for governmental purposes without the payment to us of any royalty thereon.
BACKGROUND OF THE INVENTION
In the 1969 to 1975 time frame, the US Army had two Research and Development (R&D) efforts aimed at developing printed circuit Traveling Wave Tubes (TWTs). One effort utilized a meanderline as the slow-wave printed circuit on a dielectric substrate and a sheet electron beam to obtain amplification. The second effort utilized an equiangular spiral slow-wave printed circuit on a dielectric substrate and a radial traveling electron beam to obtain amplification. The primary goal of both R&D efforts was to demonstrate the feasibility of a TWT that was lower cost than a conventional TWT, and bridged the gap between solid state technology and vacuum technology for microwave oscillator/amplifier devices. The low cost of the TWT was achieved by printing on a pair of Ceramic substrates all of the internal tube parts except the cathode-grid assembly and spacers required to have a vacuum gap for beam flow. That is, the beam forming electrodes, collector, and microwave and electric connections are printed on a pair of ceramic substrates, which have two identical printed microwave slow-wave circuits. Amplification of a microwave signal propagating on the slow-wave circuits occurs by the well-known beam-wave, circuit-wave interaction. The amplification mechanism requires velocity synchronism between the space-charge wave on the beam and the electromagnetic (EM) wave on the circuit, where dc energy is extracted from the beam and converted to microwave energy. The electron beam is generated by a thermionic cathode (heated cathode) or field-emitter cathode (cold cathode), focused by beam forming electrodes (grid/anode) and magnet structure, and collected by the printed collector. For the equiangular spiral TWT, the sheet beam is a radial directed beam that travels outward from the cathode located on an innermost circumference to the collector located on an outermost circumference. The linear beam TWTs were designed and built to operate in S-band and the radial beam TWT was designed and built to operate in L hand from 0.5-1.5 GHZ. A C-band, linear beam TWT was designed and it is described in “A Design Study of C-band Printed Circuit TWT” an Army report dated May 1971.
Some technical problems were not solved in the 1970's, which adversely affected tube performance and thus were obstacles in achieving prototype production tubes. The ceramic substrates have a large dielectric loading effect, which lowered the interaction impedance, gain, and efficiency. Partial solutions to these problems compromised high-duty cycle operation. In order to achieve a higher gain and efficiency, air or low dielectric material gaps were placed between the ceramic substrates and metal tube housing. The gaps reduced the energy stored between the ceramic substrates and metal tube housing. This improved the beam interaction, gain, and efficiency at the expense of duty cycle, since the air gaps made it more difficult to transfer heat generated inside the tube to the outside environment. Also, the air gaps caused a more rapid gain roll-off over the frequency bandwidth of operation.
This invention replaces the ceramic substrates and metal ground planes with Photonic Band Gap (PBG) crystal structures. In particular the two- or three-dimensional Metallodielectric Photonic Crystals (MPCs) are used as the supporting structures for the printed slow-wave interaction circuits. This will significantly increase the interaction impedance, gain, and efficiency without compromising gain roll-off and duty cycle. The air or low dielectric material gaps are not needed between the PBG structures and tube housing to significantly improve the interaction impedance.
The two-dimensional MPCs (high-impedance surfaces) have surface band gaps that reduce EM propagation (typically −10 to −20 dB) through the crystal. They also forbid surface currents, unlike metals. The three-dimensional MPCs can be made to have both bulk and surface band gaps, and these two band gaps can be engineered to overlap. The bulk band gap forbids EM propagation (typically −40 to −60 dB) through the crystal. They also forbid surface currents, unlike metals. In addition, the MPCs are excellent heat sinks because they contain metal elements. In particular, the metal elements of the two-dimensional MPCs are attached to the ground plane. The excellent heat sink property allows high duty operation of the TWTs.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a TWT that is compact with a low-cost design.
It is another purpose of this invention to improve tube performance over prior art printed circuit TWTs.
It is also another purpose to reduce or eliminate oscillations in the TWT.
Another objective of this invention is to eliminate the air or low-dielectric material gaps between the substrates and tube housing that were used in prior art.
A further objective of this invention is to increase the critical frequency of printed circuit TWTs.
Another objective of this invention is to have multiple devices in one package.
The foregoing and other objects are achieved by an invention in which all of the tube's internal parts are printed on metallodielectric photonic crystal (MPC) structures except for the cathode-grid assembly and spacers required to maintain a vacuum gap for the electron beam propagation region.
This invention has higher duty cycle capability, higher interaction impedance, larger bandwidth, and higher critical frequency over prior art, which in turn gives higher gain, higher rated power and higher efficiency of the TWT.
These objectives are realized by using PBG crystals with one or more defects as the structures for the printed slow-wave interaction circuits. It is well known by tube designers that the radio frequency (RF)/microwave signal when coupled onto a slow-wave circuit decays approximately exponentially away from the circuit. If the circuit is on a dielectric substrate, dielectric loading further decreases the EM fields in the vicinity of the electron beam. It is highly desirable to have large EM fields in the direct vicinity of the electron beam to significantly increase the interaction impedance, gain, and efficiency. The PBG crystal accomplishes this because it is designed to have a forbidden band gap over the bandwidth that the TWT is designed to operate. An incoming EM signal whose carrier frequency is well within the forbidden band gap, and whose line width is finite, cannot penetrate (usually at least −20 dB) the crystal, and is reflected away from the crystal. For a PBG crystal composed of low-loss media, (loss-tangent <<1), large electric field oscillations, are built up in the direct vicinity of the beam, which causes the beam to bunch. Coupling of the beam with the EM circuit wave occurs when the beam velocity slightly exceeds the phase velocity of the circuit mode. Forward and backward operation of the TWT is possible. When the phase and group velocities are in the same direction, forward operation occurs. When the phase and group velocity are in the opposite direction, backward operation occurs. Operation in the forward mode gives higher power (amplification) and larger instantaneous bandwidth; operation in the backward mode gives voltage tunability.
The interaction impedance is furthered enhanced because the beam is sandwiched between two PBG crystal structures. The EM fields that decay away from the circuit on one PBG crystal structure in the direction of the other PBG crystal structure are also forbidden from entering that substrate which causes high EM fields to build up in direct vicinity of the electron beam.
Suppression of internal oscillations can be a serious problem, especially, for high-gain tubes. Techniques are needed to prevent high EM fields from existing in unwanted modes. PBG crystals that have induced defects c
Clohan, Jr. Paul S.
Lee Benny T.
Stolarun Edward L..
The United States of America as represented by the Secretary of
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