The quadrature continuous-wave (QCW) radar has been extensively studied for small vibrational displacement detection such as non-contact sensing of human vital signals. One of the challenges of the QCW radar is the IQ-imbalance and DC-offset estimation by using curve fitting algorithms. Many algorithms have been proposed and have shown that the fitting error increases when the displacement length is small, in which case sufficient data is not provided to the algorithms. This paper presents a quadrature frequency-group (QFG) radar which utilizes a group of frequencies to enhance the fitting performance even with the small displacement. The grouped-frequencies in the QFG radar gives more data than the single-tone of the QCW radar under the same displacement condition. This paper presents the framework and properties of the QFG radar. Some fitting algorithms for the QFG radar are presented and the most adequate algorithm is suggested by simulation and experiments. Simulation and experimental results shows that the QFG radar outperforms the QCW radar. Specifically, it is shown that the fitting accuracy of the QFG radar is up to 100 times better than the QCW radar in the experiment.

The feasibility study of a 24 GHz industrial, scientific, and medical (ISM) band Doppler radar antenna in electromagnetic aspects is numerically performed for near-field sensing of human respiration. The Doppler radar antenna consists of a transmitting (Tx) antenna and a receiving (Rx) antenna close to the human body for a wearable device. The designed slot-type Doppler radar antenna is embedded between an RO4350B superstrate and an FR-4 substrate. To obtain the higher radiation pattern of the antenna towards the human body, a ground plane reflector is placed underneath the substrate. The measured −10 dB reflection coefficient (S₁₁) bandwidth is 23.74 to 25.56 GHz and the mutual coupling (S₂₁) between Tx and Rx antennas is lower than −30 dB at target frequencies. The Doppler radar performance of the proposed Doppler radar antenna is performed numerically by investigating the signal returned from the human body. The Doppler effect due to human respiration is investigated through the I/Q and arctangent demodulation of the returned signal. According to the results, the phase variation of the returned signal is proportional to the displacement of the body surface, which is about 0.8 rad in accordance with 1 mm displacement. The numerical experiments indicate that the proposed Doppler radar antenna can be used for near-field sensing of human respiration in electromagnetic aspects.