Recent advances in antenna designs are aiming at smart antenna which is also known as adaptive array antennas, or Multiple-input multiple-output (MIMO) communications, of which a group of radiators or sensors are controlled by smart signal processing algorithms. They are used to track and locate the antenna beam on the mobile target by identifying spatial signal signature. Smart antennas are typically used in signal processing, RADAR tracking and scanning, and mostly in cellular systems like CDMA, GSM and UMTS.
Because of these new applications and design goals, the antenna simulation can no longer be simulated solely. It has to be co-simulated and co-designed together with the signal processing component which is often implemented through digital or analog circuitry.
This example is the simulation of a phased array antenna in Wavenology EM. The phased array consists of three main components, including a power divider, four digital phase shifters and four patch antennas. The Wilkinson power divider is used to distribute the input power to four output branches equally, as shown in Fig. 1. The digital phase shifter shown in Fig. 2 controls the signal phase shift by the digital switches. Four patch antennas shown in Fig. 4 forms the far field radiation beam and changes the main lobe direction according to the feed signal phase difference.
The center frequency of the whole system is 4GHz. The system is implemented by microstrips.
Fig. 1. Wilkinson power divider.
The phase shifter is a loaded microstrip. When both switches change their connects from C1 to C2, the equivalent electrical length changes but the impedances keep the same. The phase shifter in this system includes four loaded lines, corresponding to 45/8, 45/4, 45/2 and 45 four phase shift amounts. Four loaded lines are separated by standard 50 Ω microstrips.
Fig. 2. Phase shifter.
The phase delay caused by all switches to C1 changed to all switches to C2 is 45*(1+1/2+1/4+1/8)=84.37 degrees. At 4 GHz frequency, this corresponds to 0.5859 ns time delay.
Fig. 3 shows the simulated two 4 GHz sinusoidal signals. The time delay is accurately predicted when compared with calculated values.
Fig. 3. Validation of the phase shifter.
The antenna array is composed of 4 patch antennas. We feed the 4 antennas simultemiously but with 1/8 period time delay, i.e. 45 degrees phase difference, as is shown in Fig. 4(c). The main lobe approximately deviates to 40 degrees instead of in the positive Z direction.
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Fig. 4. (a) Patch antenna array; (b) Excitation signal; (c) Radiation pattern.
We simulated the whole system in different digital switch states. Totally there are 5 phase difference states, including 1/8*45，2/8*45，3/8*45，4/8*45，5/8*45 degrees. If we consider the opposite direction main lobe deviations as well as the zero phase shift, there are 11 phase shift states. The synthesized radiation main lobe cartoon is shown in Fig. 5.
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Fig. 5. (a) Modeling the whole system; (b) Radiation pattern controlled by the feeding circuitries.
Through this example, it is shown that Wavenology EM can co-simulate complicated EM fields and lumped circuits efficiently. In this example, the system contains 32 independent circuits. Each circuit contains 4 capacitors, 2 resistors and two digital controlling switches. The system contains 53 microstrips and two kinds of subtrates.
