Input couplers for millimetre-wave gyrotron travelling wave amplifiers are presented in this study. A W-band input coupling system composed of a pillbox window, a smoothly curved waveguide bend, a T-junction and a broadband reflector was numerically optimised, constructed and measured. An average transmission coefficient of −2.0 dB over a designed operating frequency range was measured. Additionally, a higher-frequency input coupler for operation at a central frequency of 372 GHz was also designed based on a multiple-hole coupling configuration. The simulated transmission coefficient was −0.5 dB if the ohmic loss is not considered.

Design of a three-cavity Ka-band MW-level gyroklystron operating in the TE₀₂ mode in the output cavity while the input cavity and buncher are operating with the TE₀₁ mode is presented. The optimised design of the magnetron injection gun (MIG) and interaction circuit has been completed by particle-in-cell simulation and analytically using a MATLAB code. The simulations show the gyroklystron can deliver an output power of more than 1.5 MW with a gain of >35 dB at 36 GHz. The achieved efficiency exceeds 40% when driven by a 95 kV, 45 A beam. The optimised MIG has a transverse velocity spread of <3% when the velocity ratio is around 1.3.

This study presents a novel design of a wideband high-gain resonant cavity antenna (RCA) for millimetre and submillimetre wave bands, and its fabrication using additive manufacturing (AM). The proposed RCA antenna consists of a partially reflecting surface and three impedance matching layers fed by a waveguide. AM techniques are utilised to fabricate the design operating at 30 GHz. Two fabrication techniques are assessed for printing the antenna. The first technique is based on printing a dielectric material and fully coating the parts with a metallic layer, while the second technique involves printing the parts in a single process using metal three-dimensional printing. The first technique offers a lightweight solution while the second technique can print the whole model in one run. The antenna design is investigated by both simulations and experiments. The measured results show a 3 dB gain bandwidth of about 10%, and high gain over 15 dBi for all the three resulting antennas. Good agreement between simulation and measurement is obtained. The antenna is of low cost and achieved good performance in terms of wide bandwidth and high gain, thus it is potentially useful for high-speed wireless communications at millimetre-wave and sub-millimetre-wave frequencies.

This study presents a design of a tri-band dual polarisation common aperture feed horn used for the radiometer. The proposed feeding system is made up of a corrugated horn, an orthomode transducer (OMT) and two pairs of a high-pass filter (HPF) and a band-pass filter (BPF). The broadband profiled corrugated horn loaded with teeth (rings) is adopted as the feed of the radiometer. Wideband finline OMT is developed to realise a bandwidth ratio in excess of 2:1. Two different HPFs and BPFs are employed to block and pass waves of different frequencies. The feeding structure is then simulated, fabricated and tested. The measured results show that the radiation patterns possess good symmetry at all three working frequencies of interest (18.7, 23.8 and 37 GHz). Furthermore, the cross-polarisation levels are below −23 dB and voltage standing-wave ratios are lower than 1.5 throughout all working bands.

An extended interaction frequency-locking oscillator based on carbon nanotube (CNT) cold cathode is proposed to overcome locked-frequency limits of the conventional oscillator. Compared with the conventional oscillators, the oscillation frequency is locked by a modulation electron beam, which can be obtained in a field emission CNT cold cathode electron gun. The frequency-locking signal does not enter the high-frequency (HF) system but imposes an additional HF electric field on the cathode surface by a microstrip structure, which consumes considerably less power to lock the oscillation frequency. A ladder structure extended interaction oscillator operating in 2π mode is numerically investigated by three-dimensional Particle-In-Cell simulation code. By analysing the impacts of different frequency-locking power on the locked ranges, the results show that the average output power of 30.6 W is achieved at 35.11 GHz when the frequency-locking power consumption is 460 mW. The 3-dB bandwidth of a frequency-locking region reaches 100 MHz.

In recent years, terahertz in-line sensing has been used to monitor the film coating thickness of individual pharmaceutical tablets during the coating process. In the previous work, the in-line measurements were verified against off-line measurements of samples from the same population. Here, we report on our recent progress to further verify the validity of the terahertz in-line measurement modality using discrete element modelling for an artificial lab-scale tablet mixing process inside a tablet pan coater. By coupling discrete modelling with a ray-tracing method, it is then possible to estimate the cumulative measurements taken, which in turn can guide towards the fine-tuning of the selection criteria as part of measurement analysis.

In this study, a new straightforward method for construction of the printed quadrifilar helical antennas is presented. Folded helices with square shapes are used as the arms of the proposed antenna for satellite applications. The proposed structure is consisting of four printed flat walls with similar patterns of arms on each wall. Patterns of the folded arms are printed on four flat substrates with removed ground layers and the antenna structure is obtained by connecting these substrates. Two versions of the proposed antennas are designed for operating at 435 MHz with isolated and connected arms. Both antennas have minor responses at 145 MHz; however the large antenna with connected arms has better results. In addition, the large antenna has wider bandwidth and higher gain at both operating frequencies. Furthermore, the proposed antenna can be used as a wideband antenna with 34% bandwidth.

The problem of user scheduling with reduced overhead of channel estimation in the uplink of massive multiple-input multiple-output (MIMO) systems has been investigated. The authors consider the COST 2100 channel model. In this paper, they first propose a new user selection algorithm based on knowledge of the geometry of the service area and location of clusters, without having full channel state information at the BS. They then show that the correlation in geometry-based stochastic channel models (GSCMs) arises from the common clusters in the area. In addition, exploiting the closed-form Cramer–Rao lower bounds, the analysis for the robustness of the proposed scheme to cluster position errors is presented. It is shown by analysing the capacity upper bound that the capacity strongly depends on the position of clusters in the GSCMs and users in the system. Simulation results show that though the BS receiver does not require the channel information of all users, by the proposed geometry-based user scheduling algorithm the sum rate of the system is only slightly less than the well known greedy weight clique scheme.

In this study, a novel cross layout of polarisation conversion metamaterial (PCM) is investigated for the first time. Based on the principle of phase cancellation, the cross structure can reduce radar cross section (RCS) significantly over a wider band compared with the chessboard configuration. The low-profile metamaterial-based antenna is composed of the cross PCM structure and a driven patch sandwiched between the PCM patches and the ground plane. Simulated and measured results show that compared with the microstrip patch antenna, the proposed antenna can achieve a wider impedance bandwidth of 6.05–7.52 GHz (21.7%) and milder gain variation in the working band, while the monostatic RCS can be reduced over 5.8–18.0 GHz (102.5%).

A compact wideband coplanar waveguide (CPW)-fed antenna is designed for body implantable applications. The proposed antenna has an electrical dimension of only 0.42λ × 0.42λ × 0.0072λ at 2.3 GHz resonating frequency. Fractional bandwidth of 93.5% (1.35–3.5 GHz) is achieved with the low-profile antenna. To understand the extent of miniaturisation and bandwidth enhancement of the antenna, the characteristic modal analysis is performed. The wideband feature is achieved with the application of asymmetric complementary split-ring resonator structure through multiple mode excitations. Additional miniaturisation and impedance matching are obtained with a pair of asymmetrical arc-shaped annular ring slots within the main radiator. The substrate and radiator have been realised using silicon and gold due to their inherent property of excellent bio-compatibility with the human body. Single layer, as well as multi-layers phantom model, was used to assess the performance of the antenna. Owing to the inherent wideband feature of the designed antenna, it is able to cover the desired band efficiently even after detuning in different phantom models. The designed antenna is tested in vitro with the development of proper muscle mimicking liquid and the measured results are found to be in good agreement with the simulated ones.

This study presents an omnidirectional dipole antenna with wideband and low gain variation in the horizontal plane. The antenna is made up of two driven dipoles and two parasitic dipoles placed symmetrically on both sides of a central ground plane. The driven dipole is made up of two planar arms printed on both sides of the substrate. Two small slits on both sides of the driven dipole, a large centrally placed slit in the driven dipole, extensions on both sides of the driven dipole with different dimensions, and parasitic dipoles are introduced to greatly enhance the bandwidth and also attain a low gain variation. The fabricated antenna achieves a wide impedance bandwidth of 105.5% (1.36–4.4 GHz) for S ₁₁ ≤ −10 dB, and an average gain of 4.14 dBi with a low gain variation of 2 dB. The antenna may find applications in 2G/3G/LTE frequency bands.

A planar periodic leaky-wave antenna with filtering capability, circular polarisation, and wideband performance is presented. Seamless beam scanning from backward to forward is provided by self-matching the unit cell of the antenna over a large frequency band from 22 to 28 GHz. A wide scanning range of 84° is achieved by using a relatively low-permittivity dielectric material. Filtering capability is embedded in the antenna structure which contributes to the minimisation of the overall circuit size which is aimed in recent millimetre-wave antenna in package systems. The proposed antenna has a circular polarisation with an axial ratio of <2 dB at broadside which makes it a good candidate for peer-to-peer and peer-to-multipeer communication systems.

Here, a wideband low-noise amplifier (LNA) based on the two-stage cascade configuration is presented to improve the radiofrequency (RF) performance. With the common gate (CG) input stage, the proposed LNA provides wideband input matching, while the wideband gain response was achieved using the peaking inductors inserted at the drain terminals of each stage. With a standard CMOS process, the chip area of the proposed wideband LNA is only 0.116 mm². However, it consumes a 5.4 mW power from a supply voltage of . From the post-layout simulation results, it achieves maximum power gain of 11.13 dB at 8.5 GHz, input return loss below −9.44 dB, reverse isolation less than −60 dB, and small group delay variation of ±97 ps across 8.5–20 GHz frequency range. Moreover, noise figure (NF) lies in the range of 2.19–3.23 dB, whereas the NF minimum () varies in the range of 1.55–2.91 dB for 8.5–20 GHz frequency range. Apart from this, the proposed LNA achieves an IIP3 of 0.96 dBm, when a two-tone test is performed with a frequency spacing of 50 MHz.

Here, a new strategy of ultra-broadband monostatic and bistatic radar cross-section reduction (RCSR) was theoretically investigated. A circular configuration of perforated dielectrics, as the coating layer, was designed to have full control on the bistatic RCS signatures of flat metallic objects. Using the proposed phase cancellation method, a customised level of bistatic RCSR (BRR) as well as operating frequency bandwidth were interpretively achieved. The presented study offered a robust and straightforward RCSR technique which does not rely on conventional blind optimisations. The coating layer was capable of reducing the monostatic and BRR of >10 dB in a broad frequency range from 8.3 to 23.1 GHz (BW = 94%). The results were significantly improved compared to those of preceding researches. Besides, unlike the preceding researches, the performance of the designed coating layer was analytically justified under TE and TM polarisations of oblique incidences. A good agreement was observed between numerical simulations and theoretical predictions, confirming the polarisation-independent feature of the designed stealth coating layer for incident angles up to . These superior performances and the flexibility of design guarantee the applicability of the proposed RCSR approach for various stealth applications.

A simple yet effective technique for emulating third generation partnership project spatial channel model extended (3GPP SCME) power-delay profiles in a reverberation chamber is investigated. Through use of a conventional vector signal generator and oscilloscope, urban macro- and micro-environments containing several strong reflections are replicated with <4% error. The generation of the input power delay profiles, the measurement set-ups in the reverberation chamber and the emulation optimisation process are described rigorously. In experiment, it is demonstrated that the averaging procedure is essential for acquisition of optimum results in the reverberation chamber. The radiation efficiency and correlation coefficient of a multiple-input–multiple-output (MIMO) antenna are also defined for different emulated multipath channels. The calculated results verify the efficacy of the proposed emulation method for MIMO antenna characterisation.

The radiation performance of the resonant cavity antenna (RCA) is mainly determined by the superstrate, and thereby various superstrates are designed for the RCA to obtain high performance. In this work, the comparative effects of the capacitive and inductive superstrates on the RCA's gain are investigated systematically. Three approaches including the leaky-wave model, full-wave simulations and prototype measurements are employed for the investigation. Using the leaky-wave model, the gains of the RCA with various superstrates are calculated and a difference between the gains with the capacitive and inductive superstrates is observed. Full-wave simulations and prototype measurements further prove the result. All the approaches demonstrate that the capacitive superstrate has the potential to obtain a higher gain, in comparison with the inductive superstrate having the same reflection magnitude and aperture size.

An improved meandered spiral antenna with a new semi-fractal reflector is designed and fabricated. It is considered as a unidirectional circular polarised broadband antenna having high gain. To improve the impedance bandwidth, the top layer of the antenna is shaped as equiangular Archimedean lines which its outer area meanders. The reflector consists of many fractal hexagonal shape cells. It is implemented at the back of the spiral antenna in order to improve the gain and circular polarisation. The simulated and measured results indicated that the antenna covers the impedance bandwidth of 446% within the frequency region of 0.8–4.37 GHz having the gain of 7.5–13 dBiC. The antenna is designed to be employed in positioning and short-range sensing systems.

A multibeam antenna consisting of a set of four L-shaped microstrip line sections is presented for pattern switchable applications. A planar electromagnetic band gap substrate is implemented to reduce the antenna profile to λ _2.45/35, where λ _2.45 is the free space wavelength at a design frequency of 2.45 GHz. An impedance matching network is integrated on the lowest layer below a ground plane. The microstrip lines have a resonant frequency of 2.45 GHz and are excited separately. When one of the strips is excited, the antenna generates a tilted beam with a gain of 8.6 dBi. By switching the excitation strip sequentially, a full azimuth beam steering is achieved. When the antenna is located above a large conducting reflector, the antenna maintains its performance. Thus, the antenna is a candidate for moving objects with high speed internet and vehicular tracking.

A novel wideband bandpass-to-all-stop reconfigurable filtering power divider is proposed in this study, which allows for four-order bandpass-to-all-stop reconfigurable operating function and equal power division. Its circuit configuration includes the cascaded coupled-line sections with tight coupling to extend the impedance transforming. Furthermore, with the introduction of the half-wavelength open-circuit stubs, which controls the bandwidth, extra transmission poles located at the cut-off frequency are generated, thus resulting in high frequency selectivity. Moreover, by using a single resistor between input coupled-lines, the high all-passband isolation can be achieved. The grounding are then loaded to the output coupled-lines to enable bandpass-to-all-stop operating functionality. For demonstration, a prototype operating at 2 GHz is designed, simulated, and measured with a 15 dB bandwidth of 51%, 19 dB stopband rejection up to 5 GHz, and 14.5 dB all-passband isolation, which shows a good agreement between the simulated and measured results.

The authors present the design and realisation of balanced antipodal tapered slot antenna (BATSA) with frequency notched ultra-wideband impedance characteristics, very high gain and extremely low cross-polarised radiation. The proposed antenna comprises a central elliptic conductor sandwiched between two elliptic conductors etched on two dielectric substrates both of which acts as the ground planes and thereby provide an electromagnetic balance for the radiating electric field. The proposed BATSA is fed by a microstrip line. The microstrip section is followed by the radiating flared section of the antenna. The notch frequency is achieved by introducing a quarter wavelength spur line in the feeding microstrip line of the antenna. The fabricated prototype of the antenna was characterised for impedance and radiation performance revealing a very good correspondence with simulation results. Radiation pattern characterisation confirms high gain in the end-fire direction and very low cross-polarisation compared to antipodal tapered slot antenna.

The authors propose the design of a compact mechanically frequency and pattern reconfigurable patch antenna. The frequency tuning is achieved by changing the positions of two adjustable shorting screws which are used to modify the length of the antenna radiating patch. Simulated and measured results are carried out to validate the proposed approach. When the screws are symmetrically positioned, the measured operating frequency can be tuned from 2.95 to 5.16 GHz without degrading the radiation characteristics over the entire comparatively wide measured tuning range of 54.5%. The antenna main radiating beam can be tilted from θ  = 0° to θ  = 30° when the screws are asymmetrically positioned. The measured gains and efficiencies range from 3.01 to 5.53 dBi and 68 to 74%, respectively. The overall antenna size is only 0.39λ ₀  × 0.39λ ₀  × 0.02λ ₀.