Low-sidelobe Antenna Research Articles

Measurement distance effects on low sidelobe patterns

Because of the current strong emphasis on low sidelobe antennas, the effects of measurement distance in distorting patterns are reexamined. Previous calculations have used obsolete or suboptimum aperture distributions. The Taylor \bar{n} linear distribution is a versatile highly efficient and robust optimum distribution; its use here allows a single curve of sidelobe measurement error versus measurement distance (normalized to far field distance 2D^{2}/\lambda ) for a given sidelobe level. The calculations give data from a uniform distribution to a 60 dB Taylor. For example, the first sidelobe of a 40 dB Taylor pattern is in error 1 dB at a distance of 6 D^{2}/\lambda .

Range distance requirements for measuring low and ultralow sidelobe antenna patterns

The effects of quadratic phase errors due to finite length pattern ranges on the measurement of low and ultralow sidelobe antennas are considered. The well-known 2D^{2}/\lambda rule-of-thumb distance is shown to be adequate only for measuring patterns having moderately low sidelobes (i.e., down to about -30 dB). This distance is shown to be inadequate for measuring low (-30 to -40 dB) and ultralow (below -40 dB) sidelobe patterns if the near-in sidelobes are to be preserved within reasonable errors (e.g., less than 1 dB). A relationship between sidelobe errors and range distances is derived from calculated patterns for a number of one-dimensional amplitude distributions with various quadratic phase errors. This relationship is plotted on log-paper in terms of the change (error) in the highest sidelobe versus the range distances in multiples of D^{2}/\lambda . This plot clearly shows the range distances required for various low sidelobe patterns and measurement tolerances. A corresponding plot of mainlobe directivity errors versus finite range distances is also presented. The results show that the effects of quadratic phase error on near-in pattern sidelobes diminishes rapidly with angular distance from the main beam. Thus for a given range length ND^{2}/\lambda , the errors on the second and third sidelobes are significantly less than the error on the first sidelobe. This means that for many low and ultralow sidelobe antennas, practical range distances (such as 2D^{2}/\lambda .) can still be used to measure wide angle sidelobe levels accurately at the expense of losing the first one or two sidelobes.

Broad-band flared horn with low sidelobes

A circular horn antenna flared like a trumpet has been analyzed with the geometrical theory of diffraction and tested experimentally. Sidelobes are extremely low (-75 dB) and agree with theory. Low sidelobe performance is predicted to be broad-band and to improve with higher frequencies. The full aperture of the tested horn is about <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50 \lambda</tex> . Suggestions are made for even better low sidelobe antennas.

Effect of field amplitude taper on measured antenna gain and sidelobes

Amplitude taper of the field of a pattern measurement range produces gain and sidelobe values lower than the true values. A calculation is made using one-parameter distributions, for which the gain can be found in closed form, thereby avoiding the errors in previous calculations. For small tapers both gain errors and sidelobe level errors are linearly proportional to taper (dB). These results are useful in the design of pattern ranges, especially for very low sidelobe antennas, where the amplitude taper is more critical.

Adaptive Main-Beam Nulling for Narrow-Beam Antenna Arrays

Narrow-beam, low-sidelobe antennas may be used to enhance communication in the presence of sidelobe interferers. Protection against main-beam interferers as well can be obtained through the use of an adaptive multibeam antenna. Such an antenna, suitable for time-multiplexed, multichannel signals is described here. The objective is to permit successful communication and signal direction-of-arrival tracking in the presence of a large number of sidelobe interferers and a small number of main-beam interferers.

Further discrete optimizations for continuous aperture illuminations

Two applications of an iterative procedure to establish a means of optimizing theoretical low sidelobe antenna patterns are discussed. Examples described consider discrete element linear array antennas where the parameters involved in the optimization process are coefficients ordinarily associated with continuous aperture illuminations. One application uses the iterative procedure to control far-out sidelobe levels of the far-field pattern to establish array element excitations appropriate for low sidelobe behavior throughout the entire visible region of space. The other application uses the procedure to establish beam port amplitude weightings at a minimum number of beam ports in a multibeam feed network also suitable for low sidelobe antenna pattern behavior.

Pattern measurements of a low-sidelobe horn antenna

The power pattern of a corrugated horn antenna designed for low sidelobes was measured to levels 90 dB below the main beam maximum in both the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</tex> - and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</tex> -planes. The measured patterns were found to be in good agreement with theoretical predictions.

Effects of random phase errors at K&amp;lt;inf&amp;gt;a&amp;lt;/inf&amp;gt;band resulting from a composite material radome

The effects of random phase errors introduced by way of a composite material radome on the sidelobes of a low-sidelobe transmitting antenna are investigated. The nominal sidelobe level of interest is - 40 dB below maximum directive gain outside a minimum conical angle about boresight. The radome used was constructed using strips of fiberglass cloth pre-impregnated with epoxy resin with each strip overlapping adjacent strips. The overlapping is staggered so that the final thickness contains roughtly the same number of layers. The shape of the radome is cylindrical on the side, capped by a hemispherical sector. The radome curvature is large with respect to wavelength, and is several wavelengths thick at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K_{a}</tex> band (37 GHz). Measurements made on the radiation patterns of a large aperture low-sidelobe lens antenna enclosed by the radome indicate that the sidelobe degradation introduced by the radome can be directly related to the construction technique and materials used. Subsequent measurements on the insertion phase versus position on the radome support these conclusions. In particular, the correlation length of the random insertion phase introduced upon transmission through the radome was determined to be essentially that of the overlapping width used in the fiberglass layup. Furthermore, periodicities in insertion phase having the basic period of the overlap width cause grating lobes to occur in the radiation pattern, further degrading the sidelobes.

High-gain, very low side-lobe antenna with capability for beam slewing

High-gain, very low side-lobe antenna with capability for beam slewing