The terahertz (THz) regime of the electromagnetic spectrum is rich with the emerging possibilities in imaging applications with unique characteristics to screening for weapons, explosives and bio-hazards, imaging of concealed objects, water content, and skin, and these advantages can be harnessed by using the effective THz sources and detectors. In THz imaging systems, the pulsed THz sources and detectors find unique applications and thus we have emphasized on re-visiting these kinds of systems. Several novel imaging techniques which exploit the distinctive properties of the THz systems have been presented. Moreover, the THz antenna is one of the most important components of a THz imaging system as it plays a significant role in both impedance matching and power source. Therefore, the recent developments in THz antenna design for imaging applications are reviewed and the potential challenges of such THz systems are investigated. The photoconductive antennas form the basis of many THz imaging and spectroscopy systems and finds promising applications in various scientific fields. However, for the imaging applications, there is a requirement of planar and compact THz antenna sources with on-chip fabrication and high directivity in order to achieve large depth-of-field for better image resolution. Therefore, the key modalities of improving photoconductive dipole antennas performance are identified for imaging applications. Also, the ways to improve the directivity of the photoconductive dipole antenna are discussed. The main purpose of this review is to provide an assortment of all relevant literature to bring researchers up-to-date on the current state-of-the-art and potential challenges of THz antenna technology for imaging applications.

The detailed synthesis of a direct access ring filter topology fully controlled with the following targeted specification (center frequency, low transmission zero frequency, and matching level in the passband) is hereby presented. For this topology, the lowest achievable bandwidth is limited by technological constraints. Thereby a solution consisting in adding capacitive loads is proposed. The associated synthesis is also given and discussed. Both syntheses are illustrated with 60 GHz integrated planar filters implemented in the IHP 130 nm BiCMOS technology. Various 3 dB fractional bandwidths from 18 to 8% are targeted, some of them require the implementation of the capacitive loaded solution. The latter allows us to lower the bandwidth limit of the nominal topology as well as to get a high miniaturization, up to 3.4, depending on the capacitance value. Thanks to good measurement results, this implementation highlights the high efficiency, reliability, and versatility of the synthesis without the need of tuning simulations or post-simulations.

We present a new ultra-wideband (UWB) band-pass filter (BPF) configuration with a stepped impedance short-circuited stub (SISS)-loaded triple-mode resonator (TMR) and stepped impedance radial stub (SIRS). The novel SISS-loaded TMR and SIRS improve the roll-off ratio at both ends. Even though this UWB BPF is based on a single resonator, the measured results show excellent in-band performance with a small insertion loss less than 0.6 dB and return loss greater than −16 dB. The proposed UWB BPF has sharp roll-off ratio of 96 and 43 dB/GHz, respectively, at the lower and upper edges of the pass-band, and a wide stop-band from 10.8 to 15.6 GHz with the rejection greater than 20 dB.

A novel Gysel power divider with high power-handling capability based on half-mode substrate integrated waveguide (HMSIW) has been presented in this paper. A HMSIW ring is used and good input/output impedance matching is achieved based on HMSIW-microstrip taper transition. Two microstrip stubs are introduced in HMSIW ring to assemble two isolation resistors to improve the isolation between the output ports. The even- and odd-mode analysis method is used for the presented circuit. A prototype of the presented power divider is designed, fabricated, and measured. The measured results show a reasonable agreement with the simulated ones.

In this paper, a microstrip diplexer is designed based on a novel structure consisting of the meandrous and patch cells. It operates at 0.8 and 0.9 GHz for global system for mobile communications applications. The proposed diplexer is well miniaturized with an overall size of 0.01 λg2. It has the narrowband channels making it appropriate for the modern long-range communication systems, which are widely accepted by the industry. The introduced diplexer has the other advantages of low insertion and return losses and the good isolation between channels better than −30 dB. Moreover, there is a good upper stopband rejection so that the first and second harmonics are suppressed with a maximum level of −30 dB. The designing method is based on finding the effective parameters to control the resonance frequency, miniaturization and harmonic attenuation simultaneously. To validate the designing method and simulation results, the proposed diplexer is fabricated and measured. There is a good agreement between the simulated and measured results.

To utilize the manipulation of wave properties by metamaterials, in this paper, a microstrip-fed patch antenna array, loaded with metamaterial superstrate, has been proposed. Under unloaded conditions, the conventional patch antenna array resonates at IEEE 802.16a 5.8 GHz Wi-MAX band with gain of 4.31 dBi and bandwidth of 425 MHz, whereas when each patch of this array is loaded with a metamaterial superstrate, composed with the pair of circular split ring resonators and wire strips, gain and bandwidth approaches to 11.9 dBi and 685 MHz, respectively, which corresponds to gain improvement by 7.6 dBi and bandwidth enhancement of 260 MHz. The proposed antenna is fabricated and tested to compare simulated and measured results. A good compliance is observed between these two results. Equivalent circuit model of the composite structure has been developed and analyzed.

Compact broadband circularly polarized elliptical slot antenna arrays (CPESAAs) are devised based on 2 × 2 and 4 × 4 circularly polarized (CP) elliptical slot antennas and a sequential phase feed network. To realize the circular polarization, eight rectangular stubs are embedded in the feed structure to excite two orthogonal E vectors in 2 × 2 CPESAA configuration. Moreover, by taking four rectangular slots out from each array element, return loss and axial-ratio bandwidths are noticeably increased. The investigated simple array element has a compact size of 20 × 20 × 1 mm3. As well, 2 × 2 CPESAA occupies a compact size of 55 × 60 × 1 mm3, which operates within the frequency band 4.18–9.35 GHz. Thus, 76.4% of S11 < −10 dB and 51.4% of CP bandwidth (4.43–7.5 GHz) are covered. Moreover, proposed 4 × 4 CPESAA is printed on 110 × 110 × 1 mm3 FR4 substrate and covers the frequency band of 1.15–12.98 GHz with CP at 2.95–8.82 GHz. Throughout the study, the design process of the proposed antenna arrays are presented and discussed in detail.

This article presents a compact novel quasi-self-complementary semi-octagonal-shaped antenna for ultra-wideband (UWB) application. The proposed novel structure is fed by a microstrip line where different rectangular truncation is etched to the ground plane as an impedance matching element, which results for much wider impedance bandwidth (VSWR<2) from 2.9 to 20 GHz. In order to obtain band-notched characteristics at 5.5 GHz, an open-ended, quarter wavelength, spiral-shaped stub is introduced in the vicinity of the truncated part of the ground plane. An equivalent circuit model is adopted to investigate the band rejection characteristics of the ground plane stub. Sharpness of the rejection band can be controlled by maintaining the gap between stub resonator and the slotted periphery of ground plane. The proposed antenna design is validated by experimental measurements.

A novel radio frequency identification antenna based on log-periodic dipole array (LPDA) antenna at 5.8 GHz is presented. The antenna design approach consists of two major steps. Firstly, the conventional primitive method is used to design a traditional LPDA antenna. The antenna structure is then miniaturized and its overall performance significantly improved. Dynamic scaling factor along with uniformly separated elements are introduced in this class of antenna, resulting in a 51.5% length reduction. To improve front-to-back (F/B) ratio a symmetrical folding technique is employed, leading to 21.5 dB F/B ratio for the fabricated prototype. An excellent level of input impedance matching (S11 = −28 dB) is achieved using a well-designed feeding system through a proximity coupling technique. The measured gain of the fabricated prototype is 6.2 dBi, and the overall dimension of the antenna substrate is only 30 × 45 mm2 (0.58λ0 × 0.87λ0).

A novel Scarecrow-shaped patch antenna has been presented for broadband applications. The proposed antenna composed of an elliptical shape radiating patch, partial ground plane, and 50 Ω microstrip line which is Boolean added with the radiating patch. To achieve wide impedance bandwidth, an elliptical notch and two rectangular-shaped parasitic slots have incorporated on the ground plane. This antenna has printed on FR-4 substrate (tan(δ) = 0.02, εr = 4.3) with the thickness of 1.6 mm. It is energized by coaxial feed which is connected with a microstrip line through via. The proposed configuration has examined numerically with the help of CST Microwave Studio and experimentally also. The broadband property of this antenna has been achieved by adjusting the dimensions of an elliptical notch. The antenna exhibits the bandwidth of 116.32% from 1.23 to 4.65 GHz for S11 <−10 dB. Current distribution and far-field pattern at resonating frequencies 1.29, 1.77, 2.28, 2.94, and 3.72 GHz have been studied. It has been found that the far-field pattern is distorted at frequency 3.72 GHz due to higher order modes. Effect of parameters on frequency response of the antenna has also been investigated to know the behavior of the antenna.

A compact printed quadrant shaped monopole antenna is introduced in this paper as a good prospect for ultra wideband- multiple-input multiple-output (UWB-MIMO) system. The proposed MIMO antenna comprises two perpendicularly oriented monopoles to employ polarization diversity. An open circuit folded stub is extended from the ground plane of each radiating element to enhance the impedance bandwidth satisfying the UWB criteria. Two ‘L’ shaped slots are further etched on the radiator to provide good isolation performance between two radiators. The desirable radiator performances and diversity performances are ensured by simulation and/or measurement of the reflection coefficient, radiation pattern, realized peak gain, envelope correlation coefficient (ECC), diversity gain, mean effective gain (MEG) ratio and channel capacity loss (CCL). Results indicate that the proposed antenna exhibits 2.9–11 GHz 10 dB return loss bandwidth, mutual coupling <−20 dB, ECC < 0.003, MEG ratio ≈ 1, and CCL < 0.038 Bpsec/Hz, making it a good candidate for UWB and MIMO diversity application.

Synthesis of non-uniform circular antenna arrays using a hybrid invasive weed optimization is introduced in this paper. The excitation weights and positions of array elements are optimized to reduce the peak side lobe level (PSLL) of the radiation pattern. The method proposed in this paper can effectively constrain the number of the array elements, size of the array, and the angular spacing of the adjacent element simultaneously. The radiation pattern has the constraint of a fixed major lobe beamwidth. Synthesis results of three different circular antenna arrays are given. The results show the effectiveness and feasibility of the proposed synthesis method in non-uniform circular antenna arrays.

Millimeter-wave (mm-wave) beam-steering antennas are preferred for reducing the disruptive effects, such as those caused by high atmospheric debilitation in wireless communications systems. In this work, a compact broadband antenna array with a low loss feed network design is introduced. To overcome the short-range effects on mm-wave frequencies, a feed network – with a modified Butler matrix and a compact zeroth-order resonance antenna element – has been designed. Furthermore, the aperture feed technique has been utilized to provide a broadside stable pattern and improve the delivered gain. A Fabry-Perot layer without the height of the air layer is used. Taking advantage of this novel design, a broadband and compact beam-steering array antenna – capable of covering impedance bandwidths (from 33.84 to 36.59 GHz) and scanning a solid angle of about ~94°, with a peak gain of 17.6 dBi – is attained.

In this paper, multiple-input–multiple output (MIMO) antenna with dual-band frequency reconfiguration is presented. The proposed antenna consists of two symmetrical pentagon radiating elements. These radiating elements support bands of 1.5 GHz (GPS) and 2.4 GHz (Wi-Fi) frequency. The two PIN diodes are appropriately located on slot line in order to control the current flowing through the radiator. All simulated results are compared and confirmed with measured results. The antenna has VSWR ⩽1.8 and isolation of −28 dB. The advantage of this antenna is that bandwidth is increased by switching of PIN diode in the range from 80 MHz up to maximum 300 MHz. These characteristics demonstrate that proposed antenna is an attractive solution for a multimode application such as GPS, Wi-Fi routers, vehicular communication, etc. where wideband is required.