On-axis Gain Research Articles

Corrugated conical horns with arbitrary corrugation depth

The form of fields in a conical horn with uniform circumferential corrugations, when the corrugation depth assumes arbitrary values in the interval 0.25 ≦ (h/γo0) ≦ 0.5 is investigated in this paper. Assuming the corrugations to be infinitely thin and sufficiently close-packed, an impedance boundary condition is imposed on the fields in the axial region. Subject to a far-field approximation, accurate expressions are derived for the aperture field through a hybrid-mode formalism, which are subsequently used to calculate numerically the diffracted far-field of the horn supporting the HE11 mode, using Silver's formula. Satisfactory agreement between calculated and measured values of the far-field pattern for a wide-flare horn having arbitrary values of h/γo supports the validity of the theory presented. When the half-flare angle (α2:) is less than 30°, a closed form expression is derived for the on-axis gain of the horn.

Errors in the predicted gain of pyramidal horns

The concepts of the geometrical theory of diffraction are used to derive the on-axis gain of two-dimensional <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</tex> plane sectoral horns. Geometrical optics and single (noninteraction) diffraction by the aperture edges yield essentially the Kirchhoff result-a monotonic gain versus wavelength curve. Reflection of diffracted fields from the horn interior and double diffraction at the aperture add an oscillation to this curve which is not significantly altered by further diffraction for moderate to large horns. Including these results approximately in Schelkunoff's equation for the pyramidal horn explains the gain variations observed in microwave gain standards and provides an error estimate in their predicted gain.

Radiation characteristics of waveguide-excited dielectric spheres with matched sphere-air boundary

The results of experiments on the radiation characteristics of circular-waveguide-excited dielectric spheres with matched sphere-air boundaries are reported. The test data have shown that a waveguide-excited dielectric sphere, backed by a metallic hemisphere and with simulated quarter-wavelength matching at the dielectric–air boundary through surface perturbations, produces a directive pattern with a low side-lobe level, increased on-axis gain and reduced 3 dB beam-width, as compared with a simple wavegudie-excited dielectric sphere of the same diameter and an optimum horn with an aperture of identical cross-sectional area. Also, it presents a low voltage standing-wave ratio over the waveguide band.

Aperture fields and gain of open-ended parallel-plate waveguides

The fields at the open end of a parallel-plate waveguide with a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TE_{10}</tex> mode incident are calculated. For plate spacings greater than about 0.6 wavelengths, these are well approximated by incident and first-order diffracted fields. Consequently single diffraction at the aperture alone is often adequate for pattern calculations and, although no more complicated, is superior to the Kirchhoff result, even for on-axis calculations. Numerical values for Kirchhoff, geometrical theory of diffraction and exact on-axis gain demonstrate this. In contrast, for TEM incidence, both methods yield the exact on-axis gain. Application of these results to the improvement of gain prediction of horns is discussed.

Radiation properties of conical scalar horns

It is shown that the eigenvalues, which are derived in a closed form from a simpler solution, associated with balanced hybrid modes in conical scalar horns are close (within 4%) to the exact values obtained numerically by Clarricoats for the first ten modes. It is verified that the simpler expressions obtained for the far-field radiation patterns of conical scalar horns, supporting the balanced HE11 mode, closely follow the available experimental results for several values of α0 (half the flare angle), including the case α0 = 90°. From a study of the variation of the on-axis gain with the significant dimensions of the horn, an empirical relation for determining the dimensions of optimum scalar horns is obtained.

Acoustic Antennas for Atmospheric Echo Sounding

The successful utilization of acoustic waves for atmospheric echo sounding requires an efficient antenna, with high on-axis gain and suppressed sidelobes. Existing microwave antennas and optical searchlights have been adapted for acoustic use, and their performance measured. A conical horn reflector antenna made of fiberglass has given excellent performance at 1–5 kHz after being coated with a viscous damping tar to keep reverberation time short for the reception of weak echoes with minimum delay following the transmission of 20 acoustic watts from the antenna. Septum dampers have been used successfully on parabolic dish antennas. Antenna beam patterns have been measured on an outdoor range, across a canyon, and the results agree closely with predictions from diffraction theory. Absorbing, cylindrical antenna enclosures have been found effective in reducing sidelobe reception by 10 to 20 dB, also in agreement with predictions. Attention to antenna design should provide unimpaired echo sounder operation in an urban environment, and keep noise pollution from the sounder pulses below the annoyance threshold.

New method of gain measurement using two identical antennas

A new method for the measurement of on-axis pattern vector and power gain using two identical antennas is described. The antennas must obey reciprocity, but may be otherwise arbitrary; the usual requirements in the conventional 2-antenna gain-measurement method, that the polarisation be known a priori and that the separation be large compared with the Rayleigh distance, are eliminated.

A high-performance line source feed for the AIO spherical reflector

An aberration-correcting line source feed has been designed, modeled, constructed, and tested in the Arecibo reflector. The feed is a linearly polarized flat wavegnide 40-foot-long array illuminating 700 feet of the 1000-foot-diameter spherical reflector at 318 MHz. The antenna, illuminated by the new feed, yields an aperture efficiency of 70 percent with a peak gain near 56 dB, half-power beamwidth of 16.2 minutes of arc, and sidelobe levels below 4 percent of the on-axis gain. Vignetting losses are approximately 30 percent at the highest zenith angle.

Finite-range gain of sectoral and pyramidal horns

Finite-range effects are incorporated into Schelkunoff&apos;s pyramidal-horn gain formula, which is arranged as the product of in-phase aperture gain and E and H plane range and slant-length gain-reduction factors. These factors, readily computed, permit the on-axis gain of a horn to be obtained from two single-line curves.

Gain of Antennas with Random Surface Deviations

On-axis gain of antennas with rough reflecting surfaces is computed as a function of rms surface deviation e, correlation distance c, antenna area A and wavelength δ. Gaussian stationary surface deviations, Gaussian correlation functions, and uniform illumination are assumed. Antennas with rectangular and circular apertures are considered. It is shown that a normalized gain can be defined which has the same functional form for both. A principal result of this work is quantitative calculation of the on-axis antenna gain when the normalized variance (4π∊/λ)2 of the rough surface is larger than 4. The off-axis gain is also considered, and it is shown that in the asymptotic limit (as λ → 0), the gain reduces to that obtained by using geometrical optics.

Optical Fresnel-Zone gain of a rectangular aperture

An equation for the on-axis gain of a uniformly illuminated rectangular aperture is derived which is valid in the Optical Fresnel Zone. This equation is formulated in terms of the ordinary radiation field gain multiplied by a correction factor which depends upon the aperture dimensions and the distance, R, from the aperture at which the gain is measured. A table of the function [C2(v)+S(v)]/v2 is given; C(v) and S(v) being the Fresnel integrals. The gain of a square aperture (LXL meters) in the Fresnel-zone region is compared with the gain of a circular aperture and it is shown that for apertures of equal Gsquare < Gcircle when (L2/?) <R ?(2L2 ?); it is also shown that Gsquare has minima, but not zeros at R=L2/7.30?, R=L2/5.24?. Furthermore it is shown that for R?L2/2? the first order gain correction factor of a square aperture is equal to the gain correction factor of a circular aperture having a diameter equal to 1.208 times the sidelength of the square. An approximate formula for the Fresnel-zone gain of a long and narrow aperture is also given.