Phase Array Antenna

Antennas for phased arrays

Phase shifters are mostly used in phased array antennas (radar systems). It is well worth it to step a bit back to have a closer look on the antenna aspect.

An antenna should be viewed as a matching network that takes the power from a transmission line (50 impedance, for example), and matches it to the free space "impedance" of 377 . The most critical parameter is the change of VSWR (voltage standing-wave ratio) with frequency. The pattern usually does not vary much from acceptable to the start of unacceptable VSWRs (> 2:1). For a given physical antenna geometric size, the actual radiation pattern varies with frequency.

The antenna pattern depicted in Figure 1 is for a dipole. The maximum gain is normalized to the outside of the polar plot and the major divisions correspond to 10 dB change. In this example, the dipole length (in wavelengths) is varied, but the same result can be obtained by changing frequency with a fixed dipole length. From the figure, it can be seen that side lobes start to form at 1.25 and the side lobe actually has more gain than the main beam at 1.5. Since the radiation pattern changes with frequency, the gain also changes.

Figure 1. Frequency effects

How to avoid grating lobes in phase array

A grating lobe occurs when steer too far with a phased array and the main beam reappears on the wrong side. Elements must be spaced properly in order to avoid grating lobes. The equation for maximum spacing is a function of wavelength of operation and maximum look angle:

Thus for a 30 degree look angle, dmax is (2/3)xlambda, while for a 60 degree look angle, dmax is 0.54 lambda.

Calculating antenna gain in a phased array

Gain at broadside in a phased array is both a function of the individual element gain and the number of elements. The aperture gain is calculated by:

Advantages | Disadvantages |

* 1.high gain width los side lobes * 2.Ability to permit the beam to jump from one target to the next in a few...