A compact low-cost low-power smart antenna has been proposed in this paper. To reduce the cost and power consumption, it employs the structure of an Electronically Steerable Parasitic Array Radiator (ESPAR) antenna. To reduce the size of the antenna, a top-disk loaded monopole and six folded monopoles are employed as the driven element and parasitic elements, respectively. The proposed antenna is called “folded monopole ESPAR antenna”. The heights of the top-disk loaded monopole and folded monopoles are reduced to be less than . Furthermore, the radius of the folded monopole ESPAR antenna is reduced by using capacitive loading technique. An equivalent circuit model is proposed for analyzing the antenna. To validate the concept, a prototype is developed and the antenna operates from 2.3 GHz to 2.55 GHz. The measured results confirm that the folded monopole ESPAR antenna can achieve electronically beam scanning in horizontal plane over a 360 range. The prototype antenna achieves a gain of 4.0 dBi and a front-back ratio of 20 dB. The parasitic elements are loaded by varactors and beam forming is achieved by controlling the DC voltages applied to the varactors.

This paper presents a compact-size, low-cost smart antenna with electronically switchable radiation patterns, and reconfigurable polarizations. This antenna can be dynamically switched to realize three different polarizations including two orthogonal linear polarizations and one diagonally linear polarization. By closely placing several electronically reconfigurable parasitic elements around the driven antenna, the beam switching can be achieved in any of the three polarization states. In this design, a polarization reconfigurable square patch antenna with a simple feeding network is used as the driven element. The parasitic element is composed of a printed dipole with a PIN diode. Using different combinations of PIN diode on/off states, the radiation pattern can be switched toward different directions to cover an angle range of 0° to 360° in the azimuth plane. The concept is confirmed by a series of measurements. This smart antenna has the advantages of compact size, low cost, low power consumption, reconfigurable polarizations, and beams.

Gain reference antenna is an important part of over-the-air (OTA) measurement system using antenna gain substitution method. Without an accurate gain reference antenna, the measurement results of such system are envisaged to lead to a biased evaluation of the antenna gain. Furthermore, such antenna might be susceptible to undesirable currents on the coaxial cable during the measurement whereby the actual measured radiation pattern can be distorted causing significant differences from its expected radiation pattern. These undesirable currents are known as common-mode (CM) current. In this work, a novel gain reference dipole antenna (GRDA) with minimal CM current effects, has been proposed. It achieves less than 0.2 dB radiation pattern ripple from the ideal omni-directional radiation performance within the 4.2 GHz to 4.9 GHz frequency band. The GRDA achieves vertical polarization and omnidirectional radiation by employing an array of four folded wideband high-efficiency electromagnetic structure (WHEMS) antennas with dimensions of 22.1 mm × 22.1 mm × 46.2 mm. This proposed design overturns the classic design concept of the traditional dipole antenna. To suppress the CM current, a microstrip type-III balun with two ground points placed symmetrically around the middle of the radiating element has been designed. This design allows for the integration of a type-Ⅲ balun into the radiator, so as to achieve the effect of the balun while maintaining the desired GRDA pattern. The measurement results show that the GRDA proposed in this work achieves a symmetrical pattern, suppresses CM current, and attains a quantifiable gain of around 1.5 dBi within a 15% bandwidth, making it an ideal choice for gain calibration of OTA measurement systems.

This paper presents a comprehensive survey of state-of-the-art UAV–based antennas and propagation measurements. Unmanned aerial vehicles (UAVs) have emerged as powerful tools for in situ electromagnetic field assessments due to their flexibility, cost-effectiveness, and ability to operate in challenging environments. This paper highlights various UAV applications, from testing large–scale antenna arrays, such as those used in the square kilometer array (SKA), to evaluating channel models for 5G/6G networks. Additionally, the review discusses technical challenges, such as positioning accuracy and antenna alignment, and it provides insights into the latest advancements in portable measurement systems and antenna designs tailored for UAV use. During the UAV–based antenna measurements, key contributors to the relatively small inaccuracies of around 0.5 to 1 dB are identified. In addition to factors such as GPS positioning errors and UAV vibrations, ground reflections can significantly contribute to inaccuracies, leading to variations in the measured radiation patterns of the antenna. By minimizing ground reflections during UAV–based antenna measurements, errors in key measured antenna parameters, such as HPBW, realized gain, and the front-to-back ratio, can be effectively mitigated. To understand the source of propagation losses in a UAV to ground link, simulations were conducted in CST. These simulations identified scattering effects caused by surrounding buildings. Additionally, by simulating a UAV with a horn antenna, potential sources of electromagnetic coupling between the antenna and the UAV body were detected. The survey concludes by identifying key areas for future research and emphasizing the potential of UAVs to revolutionize antenna and propagation measurement practices to avoid the inaccuracies of the antenna parameters measured by the UAV.

A small director array (SDA) is an antenna gain enhancing section of the low-profile smart antenna which can be used as a fixed array or reconfigurable array using switched parasitic elements. Gain improvements up to +10 dBi are achieved in the SDA through a Yagi-Uda configuration using parasitic elements with a large diameter. The array height is reduced by 50% comparing with the standard Yagi-Uda antenna. The reconfigurable SDA used an electronically steerable switched parasitic arrangement so that the beam can be steered from 0° to 360° in the horizontal plane. The height of the reconfigurable SDA is 0.2 λ. The measurements proved that the reconfigurable SDA can increase the antenna gain by 3 dB. The front to back ratio showed significant improvement. The proposed SDA also had the advantages of low cost and low power consumption, thus promising for applications in small satellites, unmanned aerial vehicles and mobile terminals.