dB Sidelobe Level Research Articles

Discrete amplitude shading for sidelobe suppression of sources

Amplitude shading techniques are used in arrays to achieve reduced sidelobe levels with reasonably small major lobe width. Most papers on shading consider arrays without restriction of amplitude and location of the shading function to discrete choices. For some arrays such as resonant sonar arrays, it is desirable to achieve the amplitude shading by series‐parallel combinations of discrete elements of the array. Such an arrangement forces a relationship between the width and amplitude of discrete zones. This problem is illustrated in the design of a linear array of 24 elements for which the sidelobe level must be less than −20 dB. One discrete shading configuration had −21.9 dB sidelobe level. The best choice of series‐parallel combinations achieved a narrower beamwidth and a −25.7 dB sidelobe level. [Work supported by Naval Sea Systems Command.]

Circular array providing fast 360° electronic beam rotation

The paper describes an experimental 16-element circular array and associated receiving system which is capable of forming a fan-beam directional pattern and electronically rotating this pattern through 360°. The receiver operates at a frequency of 427 MHz and a beam rotation rate of 400kHz. The system is designed to study the possible application of this technique to a 360° within-pulse scanning radar. Studies are made of the phase- and frequency-stability requirements of parts of the receiving system necessary to avoid distortion of the rotating directional pattern. Satisfactory results are obtained from the experimental receiver which employs a directional pattern in the form of a J0(x) function having an 8 dB sidelobe level; proposals are discussed for extending the system to handle patterns with lower sidelobe levels.

An investigation of concentric ring antennas with low sidelobes

An analysis is made of circular antenna arrays with diameters being 2 to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9\lambda</tex> (the minimum inner circle diameter being <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.4\lambda</tex> ) containing 3 to 18 concentric circles. For the purpose of computation of the array factor the elements of the array are assumed to be isotropic radiators. The elements of each circle have equal current amplitudes and are phased so that the contributions of all the elements add in phase in the direction of the main beam. The Chebyshev radiation pattern function is approximated by a truncated Fourier-Bessel series, from which the current amplitude of each circle is obtained. From these current amplitudes a method for computing the current amplitude to excite a new distribution of fewer circles is shown. Also, an empirical method is given for improving the sidelobe level of the radiation pattern by adding an element to the center of the array. A number of circles in the array sufficient to avoid pattern deterioration is found to be the integer nearest to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5d/4\lambda</tex> for -20 and -30 dB sidelobe level and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4d/3\lambda</tex> for -40 dB, where <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</tex> is the diameter of the array. This represents a large reduction in the number of circles needed over the Fourier-Bessel series representation in the case of large antennas. Experimental verification of the computed pattern is made for an array of two concentric circles with diameters of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.8\lambda</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.6</tex> at a frequency of 90 Mc/s. The elements of the array were vertical monopoles.

Effect of ionosphere on radar waveforms

Consideration is given to the determination of the distortion introduced by the ionosphere on radar waveforms from which we can in turn obtain the effect of the resolution capabilities, the measurement capabilities and the dynamic range of a radar system. The analysis was carried out for a variety of waveforms: A rectangular pulse; a Gaussian pulse; an unweighted chirped pulse; a Taylor weighted chirped pulse, weighted to provide 40 db sidelobe level; and finally, an unweighted and Taylor weighted step frequency coded pulse, the weighting being used to provide 40 db sidelobe level. It was found that the available bandwidth of the ionosphere was dependent to some extent on the receiver processing used. In particular, it was found that if an unweighted chirp pulse is transmitted, and if on reception, this signal is Taylor weighted with the weighting being adjusted to give 40 db skirt levels, then the available bandwidth can be increased by 50 per cent over that for the case where no weighting is used in the receiver. The available bandwidth was also found to be dependent on the waveform transmitted. For example, if a chirp signal is transmitted and Taylor weighting used in the receiver, then the available bandwidth is at least 50 per cent greater than if a Gaussian pulse is transmitted. For a system carrier frequency of 2 kmc and an uncompensated receiver, it was found that the available bandwidth is 120 mc for a system using either a Gaussian pulse, an unweighted chirped pulse, or an unweighted step frequency coded signal. For the system using a chirped signal or a step frequency coded signal and Taylor weighting on reception, the available bandwidth was found to be 180 mc at the 2 kmc carrier frequency. A more complete summary of the results obtained on the available bandwidth for the various signals considered is given in Table I. The results given in the table apply for an uncompensated receiver. The resolution achievable by the radar will most of the time be one over the maximum available bandwidth indicated in the table.