How to Design Multilayer Microstrip Array Antenna with Shaped Pattern

Microstrip Array Antenna and it’s Application

 Synthetic aperture radar (SAR) is widely used as an efficient tool for remote sensing and mapping by Aerospace industries. The desirable features of antenna for Airborne SAR applications include shaped radiation pattern and wide bandwidth capability and high power capability. Generally, planar antennas are well suited for SAR applications.

The most commonly used planar array is microstrip patch array antenna which is inherently low profile and light weight. Multilayer-stacked electromagnetically coupled printed antenna is selected, which overcomes the bandwidth limitation of the conventional microstrip antenna. There is a serious limitation associated with the power handling capability of microstrip patch antenna and cannot be directly used for SAR systems where a pulse peak power of several kilowatts is used. For such applications a hybrid antenna where the feeder incorporated in waveguide or square coaxial line (SCL) can be used. High input power level within feeder networks is brought to lower power levels by using cascaded power dividers. These lower power levels are then fed to microstrip patch antenna. The SCL technology has advantage over the waveguide feeder network in terms of volume and weight.

Design Mythology

 SAR array antenna design has been divided in two segments, design of multilayer planar radiating element and high power 3D feeder network.

a) Design of Microstrip Array
A single multilayer radiating patch is designed and optimizes using Momentum and FEM for 13% bandwidth, return loss better than -15 dB and gain 7.2 dB. Antenna configuration is shown in Figure-2. It has four layers. For this planar antenna momentum is almost took half time compare to 3D solver.After optimize design of single element, linear array design begins with design of unequal microstrip distributed network for each of 8 antenna element to generate shaped pattern .The computation of complex excitation distribution was carried out using Null Perturbation technique with element spacing 0.8 λ. Distribution coefficients are given in table-1.

Elements Amplitude distribution Phase (Degree
1 0.59 -53.7
2 0.38 -24.8
3 0.46 36
4 2.3 52.6
5 3.77 16
6 2.7 -19.7
7 0.91 -7.5
8 1 0

Table-1. Amplitude and phase distribution coefficient

The microstrip feed network was first simulated and optimized by ADS circuit simulator and optimizer by modeling asymmetrical coupled line to take into account the effect of the coupling between the lines. This results in fewer ripples in the shaped patterns because of better phase control of the order of 5 degrees. The same optimized distributed network along with patch radiator is simulated by momentum and minor adjustment in length was carried out for required phase matching. Gain of linear array is 13.1 dB with return loss better than -15 dB in band.
Using 8 optimized linear array a planar array of 8×8 element designed with inter array spacing 0.8 λ. Antenna shown in fig-4, has been simulated in Momentum for individual feed and pattern has been validated in 3D visualization by exciting all the ports. In ADS there is a single click button that complete translated momentum planar project to EMPro 3D projects. Now radiator design is over and ready for integrated simulation along with high power SCL feeder network with FEM.


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