This paper presents a frequency- and patternreconfigurable antenna designed for high-resolution multitarget detection. The proposed antenna is composed of 16 sector-shaped patches whose operating states are controlled by PIN diodes. To overcome the issue of correlated echoes originating from the same transmitting antenna, frequency reconfigurability is employed to achieve frequency-domain decorrelation. After decorrelation, the antenna can generate multiple distinct radiation patterns. Each pattern is treated as a virtual array element, replacing a conventional multi-element array for target localization using the MUSIC algorithm. By combining frequency-domain decorrelation with pattern-based virtual array synthesis, the proposed approach provides a compact sensing platform that achieves accurate multi-target localization while significantly reducing system size and complexity.

This work proposes a compact and hardware-efficient frequency- and radiation-pattern-reconfigurable antenna (FPRA) for passive multi-target direction-of-arrival (DOA) estimation using a single receive RF chain. The antenna consists of a sectorized circular patch loaded with 16 PIN diodes. By switching the diode states, the current-concentration boundary on the patch is shifted, enabling reconfiguration of both the operating frequency and radiation pattern. With a single receive RF chain, the proposed antenna achieves beam scanning from -40 degrees to 40 degrees and provides multiple operating frequencies across the S- and C-bands. Based on these reconfigurable observation states, radiation-pattern switching is used to emulate the spatial sampling of a conventional antenna array, while multi-frequency observations introduce phase diversity to reduce the correlation among echoes from multiple passive targets illuminated by the same transmitter. Experimental results demonstrate that the combined virtual spatial sampling and frequency diversity enable passive multi-target DOA estimation without a conventional antenna array or multiple receive RF chains. The proposed FPRA offers a compact and hardware-efficient sensing solution for future integrated sensing and communication (ISAC) systems.

By studying the impedance characteristics of the horizontal half-wave dipole near the metal reflector, we find that the antenna input impedance decreases when the distance between the dipole and the reflector is reduced. Therefore, a folded dipole with high input impedance is considered to replace the half-wave dipole to reduce the antenna profile. The folded dipole developed here occupies two connected dipoles with different widths, which is conducive to obtaining a higher input resistance, in other words, to obtaining a lower profile. In the rectangular area enclosed by the folded dipole, a coupling branch is added to extend the bandwidth. The antenna can cover the 2.3-2.62 GHz band, and provide stable directional radiation. Its size is only 0.35 x 0.2 x 0.08 lambda(3) (lambda is the free space wavelength corresponding to the center frequency of the dipole operating band). We made a prototype of the antenna and tested it. Both the measured and simulated results verify the effectiveness of the method of using such a dipole with high input impedance to reduce the antenna profile. The proposed antenna can be used in base station system.

This letter presents a dual-broadband antenna with three half-wave modes and one full-wave mode, which evolves from a traditional folded dipole. Initially, the author sets up a dual-branch impedance adjustment structure at the center of the traditional folded dipole. The impedance adjustment structure comprises two parallel microstrip lines, which compensate for the large capacitance of the folded dipole. Consequently, the frequency corresponding to the full-wave mode of the folded dipole shifts downward and merges with the half-wave mode of the dipole, achieving coverage of the 2.9 GHz to 4.2 GHz band. Subsequently, two rectangular branches of different lengths are added to the impedance adjustment structure, forming two equivalent dipoles that operate in the half-wave mode and generate two resonance points at 2.3 GHz and 2.7 GHz. Finally, the author extends the folded dipole along the narrow side to merge the aforementioned low-frequency resonance points. The antenna can cover the low-frequency band of 2.2 GHz to 2.7 GHz and the high-frequency band of 3.2 GHz to 3.9 GHz while maintaining high gain within the working frequency band. The antenna boasts a simple structure and wide bandwidth, making it suitable for miniaturized communication systems.

In this paper, a dual-polarized unidirectional antenna with extremely low profile is developed. The radiator of the antenna is a square patch with two stepped slots etched in the lower left corner and the upper right corner. A port is set at the end of the stepped slot, thereby stimulating the radiator to produce two vertically polarized waves. In addition, a square slot is set in the upper left corner of the square patch to adjust the current flow from the excited port into the non-excited port. Due to the special design mentioned above, an additional current peak appears on the edge of the radiator when either port is excited, which increases the input impedance of the antenna. In this way, the problem of decreasing input impedance caused by decreasing antenna height is solved. The transverse size of the proposed antenna is roughly the same as that of the ordinary dual-polarized base station antenna, whereas the height is reduced by 81%. In conclusion, the low-profile dual-polarized antenna proposed in this paper can be used in the base station system and has a good application prospect.

This paper presents a wideband dipole based on odd and even mode fusion. The wideband antenna evolves from a simple center-fed dipole that operates in the odd-mode. When the feed point of the center-fed dipole shifts by approximately 0.05λ, the dipole's even-mode is activated. Then, two offset-fed dipoles are placed across each other to form a dual-polarized antenna. Due to the coupling between the two orthogonal dipoles, the induced current on the non-excited dipole is large, and the port isolation is as low as 7 dB. To solve this problem, rectangular grooves are etched on the dipole arms, and decoupling branches are placed in the grooves, shifting the even mode resonance to a lower frequency, and crucially preventing the energy of the excited port from flowing into the non-excited port. As a result, the dual-polarized antenna can operate across the 1.7-2.7 GHz band with an in-band isolation larger than 33.2 dB. The proposed antenna offers great application potential in communication systems.

This paper proposes an integrated decoupling method that improves the flexibility of multiport, multiband antenna design and simplifies the antenna structure. First, a dual-band L-type MIMO antenna with center frequencies of 2.6 GHz and 5.6 GHz is created using the neutralization-line decoupling method and then a dual-band L-type MIMO antenna covering the WiFi spectrum (2.3–2.5 GHz and 4.6–5.6 GHz) is created using the type-T branch decoupling technique. After demonstrating the efficacy of neutralization-line and T-branch decoupling technology for L-type antennas, a four-port L-type MIMO antenna covering 4.4–4.9 GHz, 5.4–6.1 GHz, and 7.0–7.4 GHz is proposed by combining the methods of neutralization-line decoupling, T-branch decoupling, and defected ground structure decoupling. This antenna’s isolation is higher than 20 dB with ECC less than 0.04.

A low-profile patch antenna with dual-polarisation and high gain is proposed in this study. It contains a primary radiator, a secondary radiator, and a planar feed network. The primary and secondary radiators are square patches placed 5 mm from each other. Two planar feed networks are electrically connected with the primary radiator through the stepped impedance transformer, providing equal amplitude and 180 degrees phase shift signals. These two feed networks are settled in different layers to weaken the coupling effect. The resonance generated by the primary radiator is merged with the one by the secondary radiator, covering the 3.3-3.6 GHz band. Benefiting from the differential feed method, antenna specifications such as port isolation and cross-polarisation discrimination (XPD) are ideal. Moreover, as the primary radiator is fed by planar microstrip, the overall height can be reduced, and the gain can be enhanced. Finally, a 2 x 2 subarray is investigated. Desirable results show that the proposed antenna is suitable for application in Massive Multiple Input and Multiple Output (MIMO) systems.

This communication presents a new strategy to reduce antenna height. The basic idea is to replace the half-wave dipole whose input impedance will be greatly decreased due to the close proximity to the metal reflector with a full-wave dipole with a large input impedance. Furthermore, we move the current null of the full-wave dipole out of the central area and rotate one of the arms 180° to obtain a half-sized dipole with an input impedance of nearly $50 \Omega $ . Its transverse size is the same as the half-wave dipole, and its profile is only $0.05\lambda b _{0}$ ( $\lambda b _{0}$ is the free-space wavelength at the center frequency). Finally, we improve its structure, obtain a wideband counterpart, and expand the wideband element into a dual-polarized subarray. The height of the dual-polarized, wideband sub-array is only $0.08\lambda b _{0}$ . Simulated and measured results show that such low profile, a half-sized full-wave dipole has comprehensive application foreground and spread value in today’s communication system.

This communication presents a wideband directional antenna that consists of a hybrid mode dipole, a feed structure, and a reflector. The hybrid mode dipole is evolved from a conventional half-wave dipole resonating at 1.8 GHz. By adding two open-circuit stepped branches to one of the radiating arms of the half-wave dipole and full-wave modes at 2 and 2.15 GHz can be stimulated. The asymmetric structure of the hybrid mode dipole is beneficial to obtain a large input current near the feed point and good input impedance matching when the antenna works in the full-wave mode. The proposed antenna can provide a 24% bandwidth from 1.7 to 2.17 GHz with a compact size of $65\times 12\times31.5$ mm3 ( $0.41\times 0.08\times 0.2\lambda ^{3}$ ). To verify the design, the single antenna, and its $4\times4$ array is fabricated and tested. Results show that the antenna has a good application prospect in communication systems.

This article proposes a high-isolated tri-port antenna group for 5G mobile terminals based on the shared radiator and mode orthogonality technology, which is realized by exciting the two orthogonal currents through two ports on a shared loop radiator and a patch antenna orthogonal to the loop on the back of the mobile terminal. The -6 dB impedance bandwidth of the three ports in the antenna group could cover the 3.4-3.6 GHz 5G band, while the isolation is above 20 dB within the antenna group, a very compact volume. In addition, a 12 x 12 MIMO antenna is proposed by integrating four antenna groups. The MIMO antenna still has measured isolation above 18 dB and efficiency above 50% over the operation band thanks to the orthogonal decoupling design. A good multiplexing performance is also achieved with the ECC of <0.08 and peak channel capacity of 60.29 bps/Hz at 20 dB SNR.

In the current 5G era, antennas with different functions are being assembled together to save tower space and facilitate subsequent maintenance. However, the assembly would deteriorate the antennas’ radiation pattern and isolation. In this letter, we propose an effective method for this typical application scenario. Considering the example of a tri-band antenna, a double-strip structure is first set around the arms of the low-band element to enhance the mutual isolation between the low- and middle-band elements. Then, a metamaterial surface is inserted between the low-/middle- and high-band elements to reduce the distortion of the radiation pattern. An embodiment is fabricated to verify the design. Simulation and test results show that the proposed combined decoupling method is effective for a hybrid antenna system.

A specific absorption rate (SAR) reduction approach is presented in this article, which is based on theories about electromagnetic properties and the boundary conditions of human tissue. The theory states that electric field components with different polarizations have different conditions for being absorbed by human tissue. A SAR reduction sticker made from a frequency selective surface (FSS) cell has been investigated in this article to reduce the energy absorbed by the human body from a mobile phone antenna. A dual-band IFA with a wireless local area network (WLAN) band is also proposed as the reference antenna. The antenna's integration with the sticker makes there more hard-absorbed components and less easy-absorbed components before the electric field gets into the human tissue. Numerical simulations on SAR display a 16.7% and 42% decrease in the peak value of 10 g spatial-average SAR at 2.4 and 5.5 GHz before and after the antenna was integrated with the sticker. The electric field has also been analyzed, showing great agreement with the theory. The analysis indicates that the proposed sticker would have a limited impact on the antenna's performance. The antenna and the sticker have been fabricated and measured, and the results agree well with the simulation.

Based on the traditional dielectric microstrip balun, a new-type coupling balun is presented in this study. The front side of this balun is composed of an open stub and two rectangular patches. One end of the rectangular patches is open-circuited, and the other end connects with the dipole. On the back side, there are two other rectangular patches with one end grounded and the other end open-circuited. When using such balun for feeding, one can still get a wide bandwidth, and more importantly, obtain higher port isolation. Compared with the dipole fed by the traditional balun, the maximum/average port isolation of the dipole fed by the new-type one is increased by 10.8 dB/7.9 dB. The new-type balun has a good application prospect in dual-polarized antenna.

5G antennas are usually integrated with the 2G/3G/4G antennas to save the tower space and cost. However, this will bring problems of mutual interference. In this letter, by setting a periodic coupling suppression structure near the radiating arms of the low-band element, the low-band element cannot perceive the energy of the high-band element, thus weakening the coupling between them. More importantly, the periodic coupling suppression structure has almost no effect on the radiation pattern of the antenna. The authors selected two different antennas for verification. The results show that the average/maximum value of the mutual isolation between the high- and low-band elements with the same polarization is increased by 12 and 17 dB with the adoption of the periodic coupling suppression structure. A robustness analysis of this decoupling method is given. The mutual isolation of different elements can keep at a high level, no matter where the low-band element is located. To sum up, the method to suppress the coupling of dual-frequency antennas has the characteristics of high integration, high space utilization, and no extra gain loss, and has a very important reference value for solving the coupling problem in 2G/3G/4G/5G array.

AbstractThe slot of the conventional longitudinal slot antenna is alternately opened on both sides of the broadside. A novel single‐sided slot array with a large‐declination (biased towards the load) is proposed, in which the slots are only biased to the same side of the waveguide broadside. The main beam declination of a single‐sided slot antenna is only biased towards the load direction and is only related to the broad‐side of the waveguide. To verify the theoretical analysis, a single‐sided slot array with 10‐slot was simulated, fabricated, and measured. According to the measured results, the antenna achieved a beam declination of 25° and a sidelobe level below −22 dB, which is in good agreement with the theoretical analysis. Furthermore, the main beam declination of the single‐sided slot and the traditional double‐sided slot array has been thoroughly studied and compared. When the main beam has a large declination angle (>30°) to the load, a single‐sided slot array is the only choice.

A dipole antenna based on a balun bandpass filter (BPF) is developed in this paper. The balun BPF employs two U-shaped resonators settled on the left side of the open-circuited transmission line and two L-shaped stubs to produce signals with equal amplitude and inverse phase. In this way, the volume of the balun BPF is reduced by half, and the distance between two output ports is dramatically decreased. Then, the balun BPF is integrated with a dipole. Instead of the traditional Γ-shaped line with a wide balun ground, two thin microstrip lines with width of 1 mm are adopted to connect the dipole and the balun BPF. The antenna bandwidth is further extended due to the fusion of the resonance of the dipole and balun BPF. As a result, the proposed antenna can operate from 4350 to 5025 MHz (covering the n79 band of 5G NR, 4400 MHz–5000 MHz), yielding a good filtering performance in the stopband. The measured half-power beamwidth is ranging from 61° to 63° and the measured gain is ranging from 7.95 to 8.5   dBi in the passband. This new balun BPF and the dual-polarized dipole based on it have great potential to be applied in 5G MIMO systems.

A novel, simple-structure, low-profile, wideband antenna with back cavity is proposed in this letter. The wideband antenna is derived from the planarization improvement of the sleeve dipole antenna. The antenna mainly consists of five parts, which are planar sleeve dipole, L-shaped feeding strip, substrate, coaxial feed, and back cavity. The planar sleeve dipole and the L-shaped strip are printed on both sides of the substrate plate. To verify the design, the antenna is processed and measured with the size of 0.5 lambda(L) x 0.5 lambda(L) x 0.16 lambda(L), where lambda(L) is the wavelength of the low frequency. From the measured results, the proposed antenna achieves a wide impedance bandwidth of 81.4% from 3 to 7.12 GHz with a return loss more than 10 dB. Moreover, the measured gain of the antenna is 6-9.5 dBi, and the radiation pattern is very stable over the entire frequency bandwidth.

A method to weaken the interference of antennas working in different frequencies is proposed in this paper. It uses a Frequency Selective Surface (FSS) placed between two different antennas. For one antenna, the FSS acts as an electromagnetically transparent structure, so the electromagnetic wave will penetrate the FSS and radiates into the free space. For the other one, the FSS acts as an artificial magnetic conductor, reflecting the incoming energy. With this design, two adjacent antennas could keep their individual performance especially radiation patterns normal. It is worth noting that although the FSS has been used in many aspects, such as reducing antenna's radar cross section or reducing the coupling between the identical antennas, its application in the field of 'different antennas decoupling' is the first time. An embodiment is displayed in this paper for verification. Two antennas worked at different bands of 5G are placed very close with an FSS structure inserted. It turns out that the whole antenna has good electrical and radiation properties over the working band. This decoupling strategy can be very useful in 5G Massive MIMO application.

A dual-broadband, dual-polarized base station antenna is demonstrated in this paper. The proposed antenna has five parts: the lower-band element, the upper-band element, the passive dipoles, the rectangle baffles, and the reflector. The lower-band element comprises of two full-wave dipoles to provide higher gain. The upper-band elements are located at the vacant area formed by the orthogonal arms of the lower-band element, like ikebana, to maintain the compact size unaltered. The lower-band element exhibits horizontal/vertical polarization, while the upper-band element exhibits +/- 45 degrees polarization. This multipolarization mode can improve the port isolation between the lower-band and upper-band elements, thus enhancing the effect of the receiving and transmitting diversity. To adjust the half-power beamwidths of the lower-band and upper-band elements, the passive dipoles and rectangle baffles are added to the original reflector. As a result, the antenna can cover 0.69 to 0.96 GHz for the lower band and 1.7 to 2.7 GHz for the upper band with high isolation. High gain over the entire working bands is obtained simultaneously. All the results reveal that the antenna has perfect performance and is suitable for the high-low frequency hybrid application scenario of base stations.