## 1 Introduction  
As we rapidly become a super-aging society, the global medical device market is growing. Medical devices can improve the quality of life by providing a health status to assist a patient by diagnosing and treating the patient's health condition. The Doppler radar, in particular, has been widely used as a promising medical device for detecting vital signs [1, 2, 3, 4]. The Doppler radar can diagnose the health condition by detecting the movement of the human body surface. If the radiofrequency (RF) signal transmitted from the Doppler radar reaches the human body, displacement due to body surface movement results in the phase shift of the returned RF signal. The phase shift of the returned signal is linearly proportional to the displacement by the Doppler effect [5, 6]. Therefore, the Doppler radar can indicate cardiopulmonary function by analyzing the phase changes associated with displacement. Conventional 2.4 GHz Doppler radars have been studied for contactless applications and nowadays higher-frequency band Doppler radars are widely studied. The K-band Doppler radar can especially provide several advantages compared to low frequency bands. It can have high sensitivity to small displacements of the human body and can be miniaturized [7; 8; 9; 10]. Note that one of unlicensed radio bands is the 24 GHz industrial, scientific, and medical (ISM) band. However, conventional Doppler radars for sensing human vital signs are usually used in limited spaces because they are installed on fixed locations and operate remotely in the far field. Therefore, it is impossible to detect human vital signs when a person leaves a stationary test site in the conventional Doppler radar system. To overcome the limitation, the Doppler radar is necessary to be designed for wearable devices, which allows detection of human vital signs without space limitations. Note that the wearable device directly attached to the human body is not appropriate for near-field sensing of human vital signs due to zero relative displacement. The wearable device with a strap wrapping the human body (e.g., an ID badge holder or a wrist band) can be considered for near-field sensing of human vital signs, because the Doppler radar does not move in relation to the human cardiopulmonary motion or pulse. When the Doppler radar antenna is placed in the vicinity of the human body (whose electrical properties are high permittivity and conductivity at 24 GHz), the antenna performance may be degraded. In other words, the frequency response of the antenna may be distorted due to human body effects. Therefore, the Doppler radar antenna for wearable devices should be less affected by human body effects. In this paper, we study a 24 GHz ISM-band Doppler radar antenna suitable for near-field sensing of human vital signs. As a proof of concept, we present the feasibility study of the Doppler radar antenna close to the human body to sense human respiration in the viewpoint of antennas and propagation. The Doppler radar antenna should be designed to provide good matching performance and low mutual coupling for sufficient isolation between the transmitter and the receiver [11; 12]. The slot-type antenna is less affected by human body effects compared to other antennas [13]. Therefore, in this work, the slot-type Doppler radar antenna with good performance is designed and fabricated. To demonstrate the performance of the designed Doppler radar antenna, the Doppler radar antenna in front of the Duke phantom's abdomen [14] is simulated by using Sim4 life [15], which is based on the finite-difference time-domain (FDTD) method [16; 17; 18]. The performance is evaluated by analyzing the signal returned from the phantom. The phase variation of the received signal is converted to displacement due to human motion by using I/Q demodulation because quadrature receivers are commonly used in Doppler radar to detect vital signs.