## 1 Introduction  
High-frequency (HF) radio waves with frequencies between 3 and 30 MHz are an important frequency band for radar to sense the Earth's environment [1]. Since the ionosphere can reflect HF radio waves, especially characteristics of its reflection characteristics are directly related to electron density in the ionosphere, HF radar sounding is one of the most important remote sensing means to study the ionosphere [2]. To date, a number of HF radars and ionospheric research programs have been developed and pursued for many applications, including the important contributions to validating the ionospheric models, investigating the HF radio communications, providing frequency management basis for over-the-horizon radars (OTHR) or tracking of earthquakes and so on [3; 4; 5; 6; 7]. These are all inseparable from the use of HF radar to detect the ionosphere. The type of HF radar dedicated to the ionospheric sounding is often called an ionosonde. According to the incident mode and propagation route of the sounding signal, ionosonde can be divided into the oblique backscatter sounder, vertical-incidence sounder and oblique-incidence sounder [8; 9; 10]. Among them, the oblique backscatter sounding is a very unique method. The HF sounding signals emitted by the transmitter can be reflected by the ionosphere to the distant ground or the water surface, and a part of the backscattered signals return along the original path to the transmitting place. By receiving and analyzing these scattered signals, the oblique backscatter ionogram can be obtained. Due to the time focusing effect, there is a steep leadingier on the frequency-group delay map of the backscatter ionogram. By extracting the traces of the leadingier and performing inversions, a wide range of ionospheric states can be monitored, which is not available in other detection methods. The conventional oblique backscatter sounder is mostly single channel, but the system with multi-channels has greater advantages. Employing the beam forming technology and the antenna array, directional detection can be realized and transmission power can be fully utilized. At present, some such systems have been built and widely recognized, such as Notradamus OTHR, Jindalee sky-wave OTHR, and Super Dual Auroral Radar Network (SuperDARN), etc. [11; 12]. Although they have good sounding performance, unfortunately, as OTHR, they are not dedicated oblique backscatter sounders. For ionospheric oblique backscatter sounding, it often exists only as a subsidiary function. To meet the capabilities required for OTHR tasks, they all rely on the large transmission power and antenna array. NOSTRADAMUS has a transmit power of 50 kW. The antenna array is Y-shaped and consists of 288 double-cone antennas. Each arm of the array is up to 400 m long [11]. Analogously, the total length of the 32 receiving sub-arrays of Jindalee is even 2.8 km long, and each of the 16 transmitting elements has a transmitting power of 20 kw [13]. Limited by the linear structure of the array, it can only perform azimuthal scanning. The main antenna array employed by SuperDARN also consists of 16 logperiodic antennas to scan at azimuth. The elevation information needs to be measured in conjunction with another subarray. Each transmitter has a peak output power ranging from 500-800 W [14]. The Ionospheric Laboratory of Wuhan University has conducted the research on the miniaturized but dedicated ionospheric oblique backscatter sounder system for a long time. Representatively, the Wuhan Ionospheric Oblique Backscatter Sounding System (WIOBSS) was developed in 2003 and constantly improved [15; 16]. In 2016, [PERSON] proposed a new generation of the WIOBSS system incorporating a logperiodic antenna array (WIOBSS-AA) [17]. However, because of the linear array, WIOBSS-AA does not support the two-dimensional (2-D) pointing in azimuth and elevation.