A 1.4 GHz 4-bit positive intrinsic negative (PIN) diode phase shifter with compact reflective loads for an L-band soil moisture measuring radiometer phased array antenna has been successfully designed. Unlike conventional PIN diode phase shifters using PIN diodes as switches, the designed PIN diode phase shifter adopting the low-cost PIN diodes with poor isolation as equivalent lumped components corresponding to the forward and reverse bias states. First, the PIN diode is fully characterized for its equivalent circuit models at on-and off-states, and, then, the reflective load is designed with only one inductance in parallel with the diode models to obtain the desired phase shift. The phase shifter consists of four individual bits in cascade, providing 16 states of phase shifts. Each bit is constructed with a meander branch-line 90 • coupler and two identical PIN diode reflective loads. Thus, the novel design yields a highly compact phase shifter at L-band. The phase shifter is theoretically analyzed, fabricated, and measured. The measured results show good agreement with analysis and calculations. The insertion loss of the 4-bit phase shifter is less than 2 dB within 25 MHz radiometer bandwidth and less than 2.3 dB from 1.37 to 1.43 GHz. The input and output return losses are larger than 16 dB within the 1.37 to 1.43 GHz frequency band. The root mean square (RMS) phase error is just less than 1 • at the center frequency of 1.4 GHz and less than 3 • within the radiometer bandwidth. The phase shifter is controlled with switching electronics and a microcontroller and integrated within the beamforming network of an L-band 4 × 4-element radiometer phased array antenna for soil moisture measurement.

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Substrate-integrated-waveguide (SIW) technology can eliminate the limitations associated with conventional rectangular waveguides, i.e., bulky structures and production complexity at higher frequencies. Moreover, SIW technology replaces microstrip structures eliminating the drawback of excessive radiation leakage at higher frequencies. SIW power dividers are key elements in SIW antenna array synthesis. A SIW power divider's design procedures are more complicated than conventional microstrip power dividers. However, no comprehensive review with design guidelines of substrate-integrated-waveguide (SIW) power dividers has been reported. In this paper, different types of SIW power dividers based on their configurations, functionality, and operating principles are analyzed and some recommendations are made for selection purposes. A comprehensive design guideline for SIW power dividers is given in this paper through a comparative study of T-and Y-type SIW junctions. These guidelines are based on key parameters, including insertion loss, isolation, co-phase bandwidth, return loss bandwidth, and phase balancing. This study provides valuable information for selecting and designing the best possible configuration of SIW power dividers for specific applications in microwave and millimeter-wave frequency ranges. A case study of a Ku-band SIW power divider is performed for a Ku-band soil moisture radiometer SIW antenna array.INDEX TERMS Insertion loss, isolation, millimeter-wave, power divider, phase balancing, RMS phase error, return-loss bandwidth, substrate integrated waveguide (SIW).

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