A broadband compact-sized planar four-port multiple-input–multiple-output (MIMO) antenna with polarization diversity is presented. The proposed dual circularly polarized (CP) MIMO antenna consists of four G-shaped monopole elements, two of which are left-hand CP and the other two are right-hand CP. A vertical line strip in the G-shaped radiating element acts in balancing the vertical and horizontal electric field components to obtain 90° phase difference between them for circular polarization. Also, an I-shaped strip is incorporated between the ground planes of the G-shaped antenna elements to obtain equal voltage level in the proposed MIMO configuration. The dual circular polarization mechanism of the proposed MIMO/diversity antenna is analysed from the vector current distributions. The impedance bandwidth (S₁₁ ≤ –10 dB) of the MIMO antenna is 105.9% (4–13 GHz) and the 3 dB axial ratio bandwidth (ARBW) is 67.7% (4.2–8.5 GHz), which is suitable for C-band applications. The overall size of the MIMO antenna is 70 × 68 × 1.6 mm³, and the minimum isolation between the resonating elements is 18 dB. The envelope correlation coefficient is less than 0.25, and the peak gain within the resonating band is 6.4 dBi.

A narrowband dual-band bandpass filter (BPF) with independently tunable passbands is presented through a systematic design approach. A size-efficient coupling system is proposed with the capability of being integrated with additional resonators without increasing the size of the circuit. Two flag-shaped resonators along with two stepped-impedance resonators are integrated with the coupling system to firstly enhance the quality response of the filter, and secondly to add an independent adjustability feature to the filter. The dual passband of the filter is centered at 4.42 GHz and 7.2 GHz, respectively, with narrow passbands of 2.12% and 1.15%. The lower and upper passbands can be swept independently over 600 MHz and 1000 MHz by changing only one parameter of the filter without any destructive effects on the frequency response. According to United States frequency allocations, the first passband is convenient for mobile communications and the second passband can be used for satellite communications. The filter has very good in- and out-of-band performance with very small passband insertion losses of 0.5 dB and 0.86 dB as well as a relatively strong stopband attenuation of 30 dB and 25 dB, respectively, for the case of lower and upper bands. To verify the proposed approach, a prototype of the filter is fabricated and measured showing a good agreement between numerically calculated and measured results.

A dual-bowtie antenna loaded with a double-sided dielectric lens is presented for wideband and high gain performance. The dielectric lens is a combination of the elliptical and conical frustum lenses formed on two sides (top and bottom) of a rectangular dielectric block, respectively. The conical frustum lens is mounted at the backside of the bowtie radiator to collect the radiated energy towards the elliptical lens. The bowtie antenna is fed by a coplanar strip (CPS) taper connected to a CPS-to-MSL (microstrip line) transition. The proposed lens-integrated antenna is fabricated, and measured to have an operating bandwidth of 20.5 40.0 GHz. The antenna offers a peak gain of 15.1 dB. Also, the use of the double-sided dielectric lens helps to reduce the lens size of the equivalent gain by 31%. The overall size of the proposed lens antenna is 940 mil × 940 mil × 1008 mil.

A new design method for an asymmetric microstrip-to-coplanar stripline (CPS) transition based on the analytical electromagnetic model is presented. The cross section at each stage of the transition is analyzed through conformal transformation, and the corresponding line impedance is calculated. Then, the ground linewidth on the substrate bottom is adjusted to form an optimal Klopfenstein impedance taper. To demonstrate the design method, a transition between a 50-Ω microstrip line and a high-impedance CPS line (147 Ω) was implemented. Good transition performance with more than 10-dB return loss and less than 1-dB insertion loss was obtained for frequency ranges of 6-40 GHz.

A compact-sized planar super-wideband (SWB) monopole antenna with four notched bands is presented in this paper. The antenna consists of a rectangular ground plane and a circular radiator that is fed by a tapered microstrip feed line. The overall size of the antenna is 18 mm × 12 mm × 0.5 mm, and its impedance bandwidth (S₁₁ ≤ −10 dB) ranges from 2.5 GHz to 40 GHz (bandwidth ratio of 16:1). Four notched bands are obtained using two inverted U-shaped slots, a split-ring resonator (SRR), and a meandered slot. The notched frequency bands can be adjustable by changing the parameters of parasitic slot elements, and the realized notched bands in this paper are Wi-MAX band (3.5 GHz), WLAN band (5.5 GHz), satellite communication X-band (7.5 GHz), and amateur radio band (10.5 GHz). The simulated and experimental results show good agreement with each other. The antenna possesses a high gain, super-wide impedance bandwidth, and omni-directional radiation patterns.