Details of the design of circumferentially corrugated conical horns are presented. Radiation pattern and phase-center characteristics for both "wide-band" and "narrow-band" horns are considered, and a comparison is made between the relative sizes of the two types of horns. The effects of horn aperture size, slot width-to-pitch ratio and frequency on the cross-polarization of the radiation pattern are included. The range of slot parameters which can be used and the design of the slots near the throat of the horn to achieve a low VSWR for bandwidths of 1.5 to 1 are also considered. The results of measurements on horns with both narrow and wide included angles are given.

Design of corrugated conical horns

Most of the present-day millimeter-wave gyrotrons developed for plasma experiments in controlled fusion reactors utilize cylindrical cavities operating in high-order modes. The choice of modes should obey certain restrictions dictated by the achievable mode selection and the maximum admissible level of the density of microwave ohmic losses in the cavity walls. Even with these restrictions, developers have successfully manufactured quasi-continuous-wave gyrotrons operating in the short millimeter wavelength bands that are capable of delivering microwave power on the order of 1 MW. To upgrade gyrotron power to the level of several megawatts, more complicated coaxial microwave circuits should be used. This statement is also valid for relativistic gyroklystrons, which are currently under development for driving future linear accelerators. This paper presents an overview of the history of the development of coaxial gyrodevices, a discussion of the physics-based issues which are the most important for their operation, a description of the state of the art in the development of coaxial gyrodevices for the above-mentioned applications, and a brief forecast for their future.

Coaxial gyrotrons: past, present, and future (review)

The HE11 mode in corrugated waveguide has a field distribution very close to that of an ideal gaussian mode. Its radiation pattern is symmetric about the waveguide axis and exhibits virtually no cross polarization. This work reports measurements on mode converters to transform the TE01 mode into HE11 for electron cyclotron heating (ECH) experiments. The first mode converter is a 28 degree bend in 1.094-inch I.D. circular waveguide which generates TM11 from TE01 with a measured efficiency of over 95% at 60 GHz. A second converter consists of a straight corrugated waveguide section of the same I.D. in which the corrugation depth increases gradually from zero to nominally a quarter wavelength. This section converts TM11 to HE11 with an efficiency of about 97%. The overall efficiency of conversion from TE01 to HE11 exceeds 91% over a measured range of 59.2 to 60.1 GHz.

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Mode converters for generating the HE 11 (gaussian like) mode from TE 01 in a circular waveguide

Low-loss circular waveguides will be needed for a large number of millimeter-wave transmission lines on ITER, including those transmitting electron cyclotron power and diagnostic signals. In order to provide low-loss transmission, the waveguides need to be several wavelengths in diameter. Corrugating the walls reduces the loss further not only in straight runs but also at bends, and makes the waveguide robust against small deformations. We present results of theoretical calculations showing that these properties can be maintained over very wide bandwidths suitable for ITER applications. The computer code used to make these calculations is based on a space-harmonic analysis of the fields. Measurements on waveguides are described that validate the theory for corrugated waveguides semiquantitatively. Tolerances on the corrugation geometry, waveguide bore, waveguide junctions, input Gaussian beam alignment, and waveguide support alignment are discussed. It is shown that the low-loss properties of corr...

Design of Circular Corrugated Waveguides to Transmit Millimeter Waves at ITER

The paper provides a review of current research on microwave reflector antennas with particular regard to those antennas which produce pencil-beam radiation patterns. After a preliminary examination of microwave antennas of different types attention is focused on circularly symmetric antennas with axially symmetric feed systems. This class of antenna accounts for the largest number of applications which include microwave point-to-point communication, satellite communication, and radio astronomy. The choice of optimum feed is considered in more detail and it is demonstrated that metallic feeds with corrugated walls represent the best choice in most cases. Brief reference is also made to dielectric cone feeds which may find a place in future systems. The design of dual-reflector systems is considered in depth and different computer optimisation techniques are reviewed. These include the latest methods of diffraction optimization in which more than one of the antenna specifications is involved. An example is given where efficiency and VSWR are simultaneously optimized.

High-efficiency microwave reflector antennas&#8212;A review

A study of the propagation behaviour of corrugated circular waveguides is described. The phase and attenuation characteristics are obtained using space harmonics in the interior and higher order modes in the slot region of the waveguide. It is found that only the first pair of harmonics and higher modes have a significant effect on the propagation behaviour. The HE11 mode, which is required in antenna applications, is shown to have an attenuation coefficient which is lower than that of any other mode, lower in fact than that of the TE010 mode in a smooth wall circular waveguide of comparable size. An investigation of the fields and currents in the waveguide aids a physical understanding of the phenomenon, which is shown to be only weakly sensitive to the waveguide tolerances. Experimental results to confirm the theory are given in Part 2 of the paper (see p. 1180).

Attenuation in corrugated circular waveguides. Part 1: Theory

Measurements of the attenuation characteristics of corrugated circular waveguides in the frequency range 8–11 GHz are described which support the theory given in Pt. 1 of the paper (see p. 1173). Both cavity techniques and waveguide measurements are deployed, and these lead to attenuation values which are about 40% above the predicted value for an ideal corrugated waveguide. However, it is believed that the discrepancy may be explained by the surface roughness of the metal and other imperfections. The actual measured value of the attenuation of the HE11 mode is just below the theoretical value for the TE010 mode in a comparable smooth-wall waveguide, emphasising the low values being measured. A new method is described for suppressing modes, which is based on the principle of the monomode optical waveguide.

S.L. Chong

Papers

Attenuation in corrugated circular waveguides. Part 1: Theory

Attenuation in corrugated circular waveguides. Part 2: Experiment