How to implement SWaP-C satcom antenna arrays using SMD power dividers and directional couplers
The solution to this bulk and complexity has come from a focus on low SWaP-C that eliminates the brick-like structure resulting from chip-and- wire or hybrid fabrication techniques. Newer designs are made up of multiple
leg of the divider measures one quarter the wavelength of the incoming RF signal. For example, for an incoming signal with a center frequency of 15 GHz, each leg would be 5 millimeters (mm) in length. The legs operate as quarter-wavelength impedance transformers. An isolating resistor is used to match the output ports; because there is zero potential between the output ports, no current flows through the resistor so it doesn’t contribute to resistive losses. The resistor also provides excellent isolation, even when the device is used in reverse (as a power combiner), thereby limiting crosstalk between individual channels. To limit losses as the power is being split, the two output ports of the power divider must each appear as an impedance of 2 Zo. (The 2 Zo in parallel will present an overall impedance of Zo.) For an equal power distribution with R = 2 Z o , then: Z match = √2 Z˳ = 1.414 Z˳ Where: R = the value of the terminating resistor connected between the Figure 2: The basic Wilkinson power divider uses two quarter wavelength impedance transformers and an isolating resistor to match the output ports. Ports 2 and 3 each deliver half the Port 1 input power. Image source: Knowles DLI
microstrip 2D planar elements based on a pc board substrate using SMD packaging. This planar configuration removes the need for many connectors and cables, enhancing SWaP while increasing reliability and simplifying manufacturing (Figure 1). SMDs not only considerably reduce the bulk of the antenna array, but they also allow for the use of a single automated assembly line, dramatically reducing the cost of production compared to a conventional chip-and-wire or hybrid approach. SMD assembly also helps accelerate time to market. Such advances have been made possible because of a new generation of SMD components that can perform reliably in space at high operational frequencies. The devices feature innovative dielectrics, tight tolerance, thin- film manufacturing, and novel microstrip line topologies to Figure 1: The use of low SWaP-C SMD components (right) allows for a reduction in the bulk of satcom antenna arrays compared with a conventional 3D brick assembly (left). Image source: Knowles DLI
provide a high performance/ footprint ratio.
Key antenna array components: power divider A critical passive SMD in the antenna array is the power divider. Individual power dividers split an incoming signal into two or more signals to distribute across the antenna elements making up the array. In its simplest form, the power divider splits the input power (minus some circuit losses) evenly across each output leg, but other forms of power dividers enable the input power to be proportionally shared across the output legs. There are several power divider configurations, but for high- frequency applications, power dividers typically take the form of a microstrip line Wilkinson design (Figure 2). In the basic form, each
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