by Peter Delos, Technical Lead, Bob Broughton, Director of Engineering, and Jon Kraft, Senior Staff Field Applications Engineer, Analog Devices, Inc.
This is the second article of our three-part series on phased array antenna patterns. In Part 1, we introduced the phased array steering concept and looked at the influencers on array gain. In Part 2, we’ll discuss grating lobes and beam squint. Grating lobes can be hard to visualize, so we’ll draw on their similarity with signal aliasing in digital converters, then use that to think of a grating lobe as a spatial alias. Next, we explore the issue of beam squint. Beam squint is an unfocusing of the antenna across frequency when we use phase shift, instead of a true time delay, to steer the beam. We’ll also discuss the trade-off between these two steering methods and understand the impact of beam squint on typical systems.
An Introduction to Grating Lobes
So far, we have only seen the case where the element spacing is d = λ/2. Figure 1 begins to illustrate why an element spacing of λ/2 is such a common metric in phased arrays. Two cases are shown. First, in blue, the same 30° plot from Figure 11 in Part 1 is repeated. Next, the d/λ spacing is increased to 0.7 to show how the antenna pattern changes. With this increase in spacing, note how the beamwidth reduces, which is a positive result. The decreased spacing of nulls brings them closer together, which is also an acceptable result. But now there is a second angle, in this case at –70°, where there is full array gain. This is a most unfortunate result. This replica of antenna gain is defined as a grating lobe and can be considered spatial aliasing.
Due to the precise placement of images and equations in the article, it is available as a pdf. Please click here to read the rest of the article.
To read part one of this article – click here.
Part three of this article will be in the November issue of Microwave Product Digest.