Communications: radio wave antennas – Antennas – Wave guide type
Patent
1991-05-23
1994-04-12
Hajec, Donald
Communications: radio wave antennas
Antennas
Wave guide type
343772, 343781CA, 333 21R, H01Q 1300
Patent
active
053029623
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to an antenna system producing a millimeter wave beam having a Gaussian-like distribution and using a high power source.
BACKGROUND OF THE INVENTION
Such sources, which are called cyclotron resonance masers, for example gyrotrons, have successfully been used for plasma build-up, profile control and resonance heating in the field of thermonuclear fusion research at power levels in the MW-range. The oscillation of these sources is in axially symmetric modes TE.sub.On or in high asymmetric modes (TE.sub.mn) with m>>1.
As future fusion devices are planned to be larger and to operate at higher magnetic fields, sources must be developed for higher frequencies (above 100 GHz) and higher microwave power. For these applications, it would be desirable to dispose of an axisymmetric narrow pencil-like beam with well defined linear polarization, as for example a TEM.sub.OO Gaussian beam. In fact, this kind of beam is not only required for efficient electron cyclotron resonant heating of a plasma, but also for low-loss high power millimeter wave transmission.
From IEEE Transactions Microw. Th. Techn., Vol. NTT-26 No. 5, 1978, page 332 to 334, a mode transducing antenna is known, having a corrugation by which a TE.sub.On mode is converted into a linearily polarized beam. However, the microwave radiation issuing from such an antenna still presents considerable sidelobes which reduce the energy concentration in the main lobe and increase the losses of the system.
The main object of the present invention is thus to reduce the energy dissipated in the sidelobes and to provide an axisymmetric, narrow, Gaussian-like main lobe.
According to a particular application of the present invention, the antenna system provides for an output radiation of the linear TEM.sub.OO -mode type.
These objects are achieved according to the invention by the antenna system as defined in the attached claims.
The invention will now be described in greater detail by means of several embodiments and in relation to the attached drawings.
BRIEF DESCRIPTION FOR THE DRAWINGS
FIG. 1 shows schematically a preferred embodiment of the invention in a cross-sectional view.
FIG. 2 represents a variant to the embodiment of FIG. 1.
FIG. 3 shows schematically a partial mode converter used in the embodiments according to FIGS. 1 and 2.
FIG. 4 is a detailed view on an enlarged scale of the corrugated surface of a reflector used in the embodiments of FIGS. 1 and 2.
FIG. 5 shows an alternative configuration of reflectors for an antenna system according to the invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
Although the invention applies indifferently to electric modes TE and magnetic modes TM, reference will be made hereafter in particular to TE modes.
The invention is based on the following common features for circular symmetric TE.sub.m,n modes, and in particular TE.sub.On modes:
a--all patterns have the same location of the minima between the sidelobes;
b--the maximum of the main lobe is close to the angular position corresponding to the n-th zero of the Bessel function J.sub.1 (x);
c--the phase of the main lobe in the TE.sub.On mode series is alternating.
These properties allow the reduction of the inner and outer sidelobes of a given dominant TE.sub.On mode pattern by superimposing a convenient percentage of power in the modes TE.sub.O,n-1 and TE.sub.O,n+1 (both with opposite phase with respect to TE.sub.On). The same rule applies also to any TE.sub.m,n mode. Experimentally, it has been shown that at the centre of the pattern, there is practically no radiative power (the main lobe is down by more than 30 dB) and that the sidelobe level is down also by more than 30 dB. It has further been observed that there is practically no cross-polarization which is a fundamental result, quite different from the starting rotating asymmetric modes.
Now reverting to FIG. 1, the system indicated generally at 20 comprises a high power microwave source (e.g. gyrotron) including a gun and cavity portion 1 able to release for
REFERENCES:
patent: 3235870 (1966-02-01), Hannan
patent: 4596047 (1986-06-01), Watanabe et al.
patent: 4697272 (1987-09-01), Harvey
Rebuffi Luigi
Thumm Manfred
European Atomic Energy Community (EURATOM)
Hajec Donald
Le Hoang-anh
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