## 1 Introduction  
High-voltage switchgears, which are widely used in power systems, directly provide power to distribution networks and consumers. However, potential insulation defects are inevitable in electrical equipment, and high-voltage switchgears are more likely to produce insulation defects than other types of electrical equipment used in power systems because of their narrow internal space, complicated structure, and small insulation distance [1]. In many cases, insulation defects can result in uneven distribution of the electric field inside electrical equipment, especially serious electric field distortions at the defect position, which easily lead to partial discharge (PD) phenomena. Under the deterioration effect of persistent PD, sudden insulation faults may eventually occur. PD is thus considered a characteristic of insulation deterioration, therefore, PD detection has become a significant and feasible means to determine the insulation condition of electrical equipment [2]. Based on various physical and chemical phenomena occurring along with the PD, the main methods for PD detection can be classified into two categories: non-electric and electric measurements [3; 4; 5; 6; 7]. Non-electric measurements, which mainly refer to ultrasonic measurements [8; 9], are generally used as an auxiliary detection method due to their low detection sensitivity. Electric measurements mainly include the transient earth voltage (TEV) [10], radio frequency (RF) [11] and ultrahigh frequency (UHF) methods [12]. Among these methods, the UHF one, which offers features of great anti-interference capability and high sensitivity as a noncontact measurement, has been extensively applied in PD detection. The UHF antenna sensors which are used to acquire the UHF signals caused by PD play the key role in UHF detection technology. According to the different installation locations of the sensors, UHF antenna sensors can be classified into built-in sensors and external sensors. Compared with the flexible and varied design of external sensors, the structures and sizes of built-in sensors are extremely limited in order to not affect the internal electrical equipment insulation environment. In general, built-in sensors have better PD detection performance than external sensors, but external antennas have other attractive characteristics such as the ability to monitor and locate PD over a much larger area. Currently, a variety of built-in and external UHF sensors with different performances and shapes have been designed, fabricated, and applied for PD detection in diverse electrical equipment such as transformers, and gas insulated switchgears (GIS) [13; 14; 15; 16; 17]. During field experiments, external UHF sensors are usually used to detect PD of high-voltage switchgears. However, their detection sensitivity and anti-interference capability are greatly limited because of the electromagnetic shielding effect of the all-metal switchgear casing and the electromagnetic interference from the surroundings. Furthermore, few specific built-in UHF sensors have been designed based on PD detection characteristics and the internal structure of high-voltage switchgears. In 2013, [PERSON] designed a built-in UHF sensor for high-voltage switchgear PD detection [1], and practical application confirmed the effectiveness of the sensor. However, with a complex structure and a large volume, the sensor cannot be easily installed. It can also inevitably affect the internal electrical insulation environment when installed in the high-voltage switchgear, which could potentially lead to new insulation defects. Therefore, it is necessary to design a new specific built-in UHF sensor for high-voltage switchgear PD detection. We observed the casings of high-voltage switchgear present large and flat features, although the high-voltage switchgear has a narrow interior space, complex components, and a compact structure.