Mode transformer for microwave energy transmission circuit

Details Mode transformer for microwave energy transmission circuit Mode transformer for microwave energy transmission circuit

1990-07-13

1991-10-22

Laroche, Eugene R.

Wave transmission lines and networks

Wave mode converters

333 21A, H01P 116

Patent

active

050599283

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a mode transformer for a microwave energy transmission circuit.
2. Discussion of the Background
The technique of high power millimetric waves is now being developed due to generators and amplifiers such as gyrotrons, ubitrons, free electron lasers, etc. . .
Microwave energy is transmitted by waveguides. The cross-section of these guides have dimensions which must be chosen while complying with two contradictory restrictions: mode if possible.
The power to be transmitted imposes dimensions which are sufficiently large to avoid a breakdown due to electrical fields which are too intense; the electrical fields in fact vary in a way which is proportional to the square root of the power and is inversely proportional to the square root of the cross-section of the guide.
The propagation of a single mode in the guide is, on the contrary, ensured when the dimensions of the guide are smaller than a well determined threshold, becoming much smaller as the frequency becomes higher. This threshold is called the cut-off threshold.
These imperatives become contradictory if it is sought to transmit a high power at a high frequency.
There is therefore an obligation to use oversized guides in which several modes can propagate and to impose a single operating mode by a means other than the reduction of the dimensions above the threshold corresponding to the frequency to be transmitted.
Furthermore, microwave energy transmission circuits are generally constituted by devices operating in different electromagnetic modes, for example a generator in mode TE.sub.01, a transmission line in mode TE.sub.02 and an antenna excited in mode TE.sub.11.
In order to connect them a conversion must therefore take place from the output mode of an element to the mode of the following element.
A known solution to the problem of conversion between modes consists in using a waveguide having periodic disturbances in the geometry of the walls, in order to favour the conversion between two modes exhibiting beats having this periodicity along the guide. This solution generally leads to the use of waveguides of long length, for example having a length of several hundred wavelengths.
There is also known, by the application EP-0 171, 149, a mode conversion module using a conical tube whose large end constitutes the input for microwave energy in a first mode and whose small end constitutes the output for microwave energy in a second mode.


SUMMARY OF THE INVENTION

The present invention provides a particularly simple solution to the problem of the junction between two waveguides of different cross-section propagating different modes. It is particularly suited to the case in which one of the guides is operating in its fundamental mode and can only propagate that mode, and it proves to be entirely advantageous to impose a desired principal mode of propagation, in a guide whose dimensions allow, a priori, the propagation of several modes.
According to the invention, there is proposed a mode transformer comprising a conical tube, used in a microwave energy transmission circuit, interposed between, on the one hand, an electromagnetic wave source operating in a band of frequencies, in a first mode of propagation, and, on the other hand, a waveguide which must transmit energy in a second single mode of propagation, different from the first mode, to a circuit using electromagnetic energy, characterized in that:
the large end of the conical tube is connected to the waveguide,
the electromagnetic wave source is connected to the conical tube by a lateral opening in such a way that the conical tube has a smaller cross-section than that of the waveguide, in the region in which the excitation takes place, while, at the centre of the opening, its cross-section is that which would be that of a waveguide whose cut-off frequency, for the second mode, would be the central frequency of the working frequency band.
This transformer structure can be produced with particularly reduced dimensio

REFERENCES:
patent: 2769145 (1956-10-01), Zaleski
patent: 2782299 (1957-02-01), Whitehorn
patent: 4494039 (1985-01-01), Kim
patent: 4704611 (1987-11-01), Edwards et al.
D. R. Hamilton et al.: "Klystrons and Microwave Triodes", 1948, Mc-Graw-Hill Book Company, Inc.
Intermediate Electron Devices Meeting, 7-9, Dec. 1981, Washington, D.C. IEDM Technical Digest, IEEE (New York, US), H. R. Jory: "Gyro-Device Developments and Applications":, pp. 182-185.

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