Approximate boundary conditions are a means for simulating material and surface effects in scattering and propagation. A number of conditions are discussed, and criteria are given for their validity.

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Approximate boundary conditions

A hybrid technique is proposed to solve the entire problem of corrugated and noncorrugated rectangular horns without the usual approximations The horn antenna is simulated by a multistepped waveguide structure radiating into half-space The procedure is composed of three steps First, assuming the horn to be terminated by infinite metallic flange, the aperture problem is accurately solved by a hybrid modal-spectral method (HMSM) Second, the modal analysis (MA) method and scattering matrix concept of waveguide discontinuities are employed to solve the transverse multidiscontinuity problems Finally, the two methods are combined (MA-HMSM) to solve the entire problem of the horn antenna The proposed technique allows to obtain not only the radiation patterns, but also the reflection and near-field characteristics of the horn, without any restriction on the geometry (semiflare angles, profiled horn contours, corrugations) A good agreement between theoretical predictions and experimental results has been obtained for several horns

A hybrid technique for analyzing corrugated and noncorrugated rectangular horns

The radiation characteristics of a parabolic dish with a loaded peripheral flange are examined in detail in order to assess the effectiveness of such a loading in further reducing the backward scattered field. Uniformly valid diffraction coefficients are developed to deal with both isotropic and anisotropic surface impedances. It is shown that substantial improvement of the antenna performance can be obtained in a wide rear angular sector, and the optimal loading conditions are determined.

Parabolic antennas with a loaded flange

An analytical and experimental investigation of the radiation properties of wide-flare corrugated E -plane sectoral horns ( 10\deg , where \alpha_{0} is the half-flare angle) excited in the HE_{11} mode [1] with improved E -plane radiation patterns and suppressed side-lobe levels over a broad bandwidth is described in this paper. Given the flare length and half-flare angle of the horn, the far-field radiation pattern is determined by two methods, one of which involves aperture-field integrations based on vector diffraction formula over a constant phase surface at the mouth of the horn and the other of which employs an expansion of the aperture field in terms of free-space cylindrical TE and TM wave functions. Good agreement is obtained between the two methods with measured radiation patterns for several values of \alpha_{0} . Given the flare lengthy variation of the on-axis gain with \alpha_{0} of the horn is analytically investigated, and the design procedure for determining optimum \alpha{0} , in order to obtain maximum on-axis gain, is also indicated. Furthermore, design and experimental studies on wide-flare horns under consideration are also summarized.

Radiation from wide-flare corrugated E-plane sectoral horns

We describe a large (78 cm) diameter liquid‐helium‐cooled blackbody absolute reference cold load for the calibration of microwave radiometers. The load provides an absolute calibration near the liquid‐helium (LHe) boiling point, with total uncertainty in the radiometric temperature of less than 30 mK over the 2.5–23‐cm wavelength (12–1.3 GHz) operating range. Emission from those parts of the cold load not immersed in LHe is ≤25 mK and the reflection coefficient is ≤3.5×10−4. This cold load has been used at several wavelengths at the South Pole, Antarctica and at the White Mountain Research Station, California to calibrate spectral measurements of the cosmic microwave background radiation. For the instruments operated at 20‐, 12‐, 7.9‐, and 4.0‐cm wavelength at the South Pole, the total corrections to the LHe boiling‐point temperature (∼3.8 K) were 48±23, 18±10, 10±18, and 15±16 mK, respectively. In operation, the average LHe loss rate was ≤4.4 l/h, allowing day‐long periods of operation without a LHe fill. The boiloff rate is not strongly dependent on the radiative load at the aperture, yielding very stable operation and radiometric performance. Design considerations, radiometric and thermal performance, and operational aspects are discussed. A comparison with other LHe‐cooled reference loads including the predecessor of this cold load is given.

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A liquid‐helium‐cooled absolute reference cold load for long‐wavelength radiometric calibration

A study of the radiation characteristics of corrugated E -plane sectoral horns is described. Assuming the corrugations to be infinitely thin and sufficiently close packed, impedance boundary conditions are imposed on the fields in the axial region of the horn and it is established that a separable modal solution of fields in terms of TE to z modes is possible for this geometry. The horn aperture field is determined in terms of cylindrical wave functions and the vector diffraction formula is used to analyze the far-field radiation patterns. Excellent agreement is obtained between the theoretically derived pattern with experimental results for horns supporting the balanced HE_{11} mode with small flare angles ( 2_{\alpha 0} \leq 30\deg ), designed using the theory presented. Bandwidth properties of the horn have been studied which may be used to advantage in designing a corrugated E -plane sectoral horn without much degradation of its radiation characteristics corresponding to the balanced HE_{11} made over a wide frequency bandwidth (1.5:1).

V. Rao

Papers

Radiation characteristics of corrugated E -plane sectoral horns