## 1 Introduction  
Reconfigurable antennas have been increasingly used in wireless communication systems, owing to their selectivity for operating frequencies and radiation patterns [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]. Frequency-reconfigurable antennas can operate at different frequency bands during the operation of switches. By virtue of the use of only one element for a multi-standard system, frequency-reconfigurable antennas have the advantages of reducing the complexity and improving the size form factor for the overall system [1]. On the other hand, a beamwidth-reconfigurable antenna can help improve immunity against interference, such as noise sources and intentional jamming signals [5]. This antenna is suitable for base station applications in which interference sources and a dynamic number of subscribers exist in the area of concern simultaneously, requiring the real-time control of the beamwidth and power coverage of the antenna [5]. It can also be used for satellite communications from elliptical orbits or remote sensing from airborne platforms, in which the dynamic control of the radiation beamwidth is desired to achieve a radiation beam spot independent of the antenna altitude [12]. Furthermore, beamwidth-reconfigurable antennas are beneficial for the intersatellite link (ISL) of low earth orbit (LEO) formation flying and constellation, as they prevent unauthorized satellites from eavesdropping.  
Methods for antennas with beamwidth reconfigurations are classified into two categories: mechanical approaches and electronic control methods. Mechanical approaches, such as defocusing the feed system [13] or the mechanical actuation of the director/reflector [14; 15], have been proposed. Structures configured by these methods are bulky and vulnerable to vibrations, misalignment, and hysteresis effects. Electric control methods have been studied using radio frequency (RF) switches, including field-effect transistors (FETs) [16], micro-electromechanical systems (MEMS) [17], PIN diodes [1; 2; 18], and varactor diodes [5; 12]. Figure 1 shows the antenna beam pattern according to an electrically controlled switch state.  
FET switches have the disadvantages of nonlinear distortions and high switching losses. MEMS switches have the disadvantages of a high actuation voltage and complicated fabrication process. PIN diodes in the ON state consume considerable DC power up to tens or even hundreds of milliwatts. Varactor diodes are not suitable for the K-band or millimeter-wave band applications due to their packaging parasitic inductance. In general, the PIN diode is used for connecting or disconnecting two transmission lines for RF signal path. Even when beam lead PIN diodes with low parasitic inductance are applied to the series type diode networks, excellent on/off characteristics related to the connection and disconnection operations are not to be expected for the K-band or millimeter-wave band. A new RF switch structure is required for an ISL with a K-band beamwidth-reconfigurable antenna to prohibit unauthorized satellites from eavesdropping. Furthermore, the RF switch with a low DC power consumption of a few milliwatts is beneficial for energy saving.  
This article presents a circularly polarized (CP) slot antenna for the K-band beamwidth reconfiguration introducing an RF switch with a T-junction power divider (PD), \(\lambda/4\) lines, beam lead PIN diodes, current control resistors, and no DC block circuit, and \(-45^{\circ}/+45^{\circ}\) linearly polarized (LP) half-mode substrate-integrated waveguide (HMSIW) slot arrays with short-circuited termination and 16 radiating slots. The overall CP slot antenna was designed on a microwave substrate RO4350B with a thickness of 10 mil and a relative dielectric constant \(\epsilon_{\mathrm{r}}\) of 3.48.