Due to the fast development of wireless communication technology, reconfigurable antennas with multimode and cognitive radio operation in modern wireless applications with a high-data rate have drawn very close attention from researchers. Reconfigurable antennas can provide various functions in operating frequency, beam pattern, polarization, etc. The dynamic tuning can be achieved by manipulating a certain switching mechanism through controlling electronic, mechanical, physical or optical switches. Among them, electronic switches are the most popular in constituting reconfigurable antennas due to their efficiency, reliability and ease of integrating with microwave circuitry. In this paper, we review different implementation techniques for reconfigurable antennas. Different types of effective implementation techniques have been investigated to be used in various wireless communication systems such as satellite, multiple-input multiple-output (MIMO), mobile terminals and cognitive radio communications. Characteristics and fundamental properties of the reconfigurable antennas are investigated.

Reconfigurable Antennas: Switching Techniques—A Survey

A compact planar ultrawideband antenna with continuously tunable, independent band notches for cognitive radio applications is presented. The antenna is fabricated using a copper-cladded substrate. A radiating patch with an inverted rectangular T-slot is etched on the top side of the substrate. A straight rectangular strip with a complete gap is embedded into the T-slot. By placing a single varactor diode across this gap, a frequency-agile band-notch function below 5 GHz is realized. On the bottom side of the substrate, a U-shaped parasitic element having an interdigitated-structure is placed beneath the radiating patch. The second narrow band notch is created by inserting a second varactor diode into the gap on one leg of the parasitic element. It has a frequency-agile performance above 5 GHz. The presence of the interdigitated structure suppresses higher order resonant modes and enhances the tunability of the notched bandwidth. Because these antenna structures naturally block dc, a very small number of lumped elements are required. The experimental results, which are in good agreement with their simulated values, demonstrate that both band notches can be independently controlled and the entire frequency-agile fractional bandwidth is as high as 74.5%, demonstrating a very wide notched frequency-agile coverage.

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Compact Planar Ultrawideband Antennas With Continuously Tunable, Independent Band-Notched Filters

In the last decade, the filter community has innovated both design techniques and the technology used for practical implementation. In design, the philosophy has become “if you can't avoid it, use it”, a very practical engineering approach. Modes previously deemed spurious are intentionally used to create in-line networks incorporating real or imaginary transmission zeros and also reduce the number of components and thus further miniaturize; spurious responses are re-routed to increase the passband width or stopband width, frequency variation in couplings is used to create complex transfer functions, with all of these developments using what was previously avoided. Clever implementations of baluns into passive and active networks is resulting in a new generation of noise-immune filters for 5G and beyond. Finally, the use of a diakoptic approach to synthesis has appeared an evolving approach in which small blocks (“singlets”, “doublets”, etc.) are cascaded to implement larger networks, (reducing the need for very complex synthesis), with this new approach promising a large impact on the implementation of practical structures. Filter technology has migrated towards “observe it and then adapt it”, pragmatically repurposing tools not specifically originally intended for the applications. Combinations of surface wave and bulk wave resonators with L-C networks are improving the loss characteristics of filters in the region below 2 GHz. Lightweight alloys and other materials designed for spacecraft are being used in filters intended for space, to provide temperature stability without the use of heavy alloys such as Invar. Fully-enclosed waveguide is being replaced in some cases by planar and quasiplanar structures propagating quasi-waveguide modes. This is generically referred to as SIW (Substrate Integrated Waveguide). Active filters trade noise figure for insertion loss but perhaps will offer advantage in terms of size and chip-level implementation. Finally, the era of reconfiguration might be approaching, as the basic networks are evolving, perhaps lacking only the appearance of lower-loss, higher-IP solid-state tuning elements.

Emerging Trends in Techniques and Technology as Applied to Filter Design

The paper presents a highly efficient, low cost, ultra-wideband, microstrip monopole antenna for microwave imaging and wireless communications applications. A new structure (z-shape, ultra-wideband (UWB) monopole) is designed, which consists of stepped meander lines to achieve super-wide bandwidth and high efficiency. Three steps are used to design the proposed structure for the purpose to achieve high efficiency and wide bandwidth. The antenna bandwidth is enhanced by varying the length of meander line slots, optimization of the feeding line and with the miniaturization of the ground width. The simulated and measured frequency bands are 2.7–22.5 GHz and 2.8–22.7 GHz (156% fractional bandwidth), respectively. The dimensions of the antenna are 38 mm × 35 mm × 1.57 mm, and its corresponding electrical size is 2.41 λg × 2.22 λg × 0.09 λg, where guided wavelength λg is at the center frequency (12.75 GHz). This antenna achieved a high bandwidth ratio (8.33:1). The realized gain is varying from 1.6–6.4 dBi, while that of efficiency is 70% to 93% for the whole band. Radiation patterns are measured at four operating frequencies. It has an acceptable group delay, fidelity factor, and phase variation results that satisfy the limit of ultra-wideband in the form of the time domain.

https://www.mdpi.com/1424-8220/20/1/115/pdf

High Efficient and Ultra Wide Band Monopole Antenna for Microwave Imaging and Communication Applications.

This paper presents a novel class of substrate integrated waveguide (SIW) filters, based on periodic perforations of the dielectric layer. The perforations allow to reduce the local effective dielectric permittivity, thus creating waveguide sections below cutoff. This effect is exploited to implement immittance inverters through analytical formulas, providing simple design rules for the direct synthesis of the filters. The proposed solution is demonstrated through the design and testing of several filters with different topologies (including half-mode SIW and folded structures). The comparison with classical iris-type SIW filters demonstrates that the proposed filters exhibit better performance in terms of sensitivity to fabrication inaccuracies and rejection bandwidth, at the cost of a slightly larger size.

Substrate Integrated Waveguide Filters Based on a Dielectric Layer With Periodic Perforations

A compact bandpass filter exhibiting an ultra-wide out-of-band rejection is studied and developed. A combination of electromagnetic bandgap (EBG)-loaded half-mode substrate integrated waveguide (HMSIW) and composite right/left-handed (CRLH) HMSIW is used in this development. The proposed filter operates below the characteristic cutoff frequency of HMSIW. The CRLH HMSIW loaded with EBG structures is investigated first. The filter with a miniaturized size is then implemented with two slots etched on HMSIW to reduce coupling effects between EBG structures and interdigital capacitors. Defected microstrip structure (DMS) as feed lines are used to obtain an ultra-wide out-of-band rejection. Measured results show that this effective stopband can cover up to 10.3 times the design center frequency for a rejection level of 20 dB. The filter exhibits high selectivity, good stopband performance, and compact size.

A Compact Half-Mode Substrate Integrated Waveguide Bandpass Filter With Wide Out-of-Band Rejection

By utilizing two optically controlled microwave switches (OCMSs) to control two independent narrow-band complementary split ring resonators, an optically controlled aperture ultrawideband antenna capable of generating reconfigurable multiband notches is proposed. Four reconfigurable states are achieved in the 3.1–10.6-GHz band: 1) a full FCC-allowed band; 2) a WiMax-notched band; 3) a WLAN-notched band; and 4) a WiMax-WLAN-notched band. These bands are useful to detect the available spectra or block the harmful interference from WiMax and/or WLAN systems by simply changing the ON/OFF states of the OCMSs with no more than 100-mW optical powers for each switch. A prototype of the proposed antenna is fabricated and measured. Both simulations and experiments demonstrate the proposed design.

Optically controlled reconfigurable band-notched UWB antenna for cognitive radio applications

The influence of laser wavelength on the insertion loss of the silicon-based optically controlled microwave switch (OCMS) operating at L-band is investigated.The OCMS is made of a section of microstrip transmission line by placing a silicon dice on a microstrip gap. Both numerical simulations and experiments are conducted in the range of the wavelength from 600 nm to 980 nm for this study. The results show that the 980 nm laser achieves the lowest insertion loss in this wavelength range if the OCMS is excited with the same optical power density. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:187–190, 2013; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27244

Influence of laser wavelength on insertion loss of silicon‐based optically controlled microwave switch

A reconfigurable dual-mode bandpass filter based on substrate integrated waveguide

The feasibility of utilizing small-size low-cost commercial light-emitting diodes (LEDs) to drive the optically controlled microwave switches (OCMS) is experimentally investigated. A series of experiments are performed within 0.5–5 GHz bandwidth by utilizing six kinds of LEDs and a 980 nm-laser to drive the OCMS. The results show that white, red, and violet LEDs are three competitive candidates to replace high-cost lasers as the controller of OCMS if their power density impinging on the silicon dice of the OCMS could be increased up to 100 . Additionally, the reason leading to high insertion loss of the OCMS driven by the LEDs is revealed. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:2768–2774, 2015

Qiaoli Zhang

Papers

A Compact Half-Mode Substrate Integrated Waveguide Bandpass Filter With Wide Out-of-Band Rejection

Optically controlled reconfigurable band-notched UWB antenna for cognitive radio applications

Influence of laser wavelength on insertion loss of silicon‐based optically controlled microwave switch

A reconfigurable dual-mode bandpass filter based on substrate integrated waveguide

Experimental study of silicon‐based microwave switches optically driven by LEDs