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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 ${4\times 4}$ -element radiometer phased array antenna for soil moisture measurement.

1.4 GHz Low-Cost PIN Diode Phase Shifter for ${L}$ -Band Radiometer Antenna