A patch is proposed to stack above a substrate integrated cavity (SIC) backed filtering slot antenna to enhance antenna frequency selectivity. The antenna is composed of a slot radiator backed by an SIC, a stacked patch, and a microstrip feed line. Two parallel slots are cut onto the top broad wall of the SIC, which is excited by a microstrip line through a nonresonant short slot on the center of the bottom broad wall. The stacked patch is positioned right above the SIC supported by a piece of substrate slab for gain enhancement. Compared with the traditional SIC-backed slot antenna, the proposed antenna achieves higher frequency selectivity as well as enhanced bandwidth and gain. The field distribution and parametric study are conducted to exhibit the operating mechanism of the antenna more clearly. Compared with the conventional SIC-backed slot antenna, the antenna prototype operating at 10 GHz, as example, achieves wider 10 dB impedance matching bandwidth of 8.67%, higher gain of 7.2 dBi, better frequency selectivity of 0.9/0.45 at the lower and upper edges of the operation band, and lower maximum cross-polarization levels of -23/-22 dB in E- and H-planes within the 3 dB beam range, respectively.

In this paper, a coupled-line balanced-to-single-ended out-of-phase power divider is proposed. Configured by three quarter-wavelength coupled lines, the proposed power divider is compact and able to enhance the bandwidth of differential-mode power dividing, isolation, common-mode suppression, and phase difference between the output ports. Furthermore, enhancing the bandwidth for common-mode suppression using additional quarter-wavelength open stubs is discussed. Analytical solution of the parameters for specified bandwidth can be achieved using oddand even-mode analysis. Analytical and practical bandwidth limitation has been analyzed, and it is shown that the bandwidths can be controlled by the oddand even-mode impedance of the coupled lines. For demonstration, a prototype of size 15 mm x 95 mm (0.15 λ _g x0.95 λ _g ) achieves an operating bandwidth (|S _ddAA | <; -20 dB) of 30% with the minimum insertion loss of 0.1 dB.

In this paper, a balanced branch-line coupler with an arbitrary power division ratio is proposed. Configured by four quarter- and half-wavelength microstrip lines, the proposed design achieved desired functions of branch-line coupler for differential-mode signal and common-mode suppression with the merits of wide bandwidth, a simple configuration and a design method, low insertion loss, and low cost. The even-and odd-mode equivalent circuits combining with the standard and mixed-mode S-parameters are adopted to derive the analytical equations. Three prototypes with the power division ratios of 1:1, 2:1, and 4:1 are designed to verify the theoretical prediction. The prototypes with the size of 0.52 λg X 0.5 λg (λ _g is the guided wavelength at 1.86 GHz) exhibits the operating bandwidths (IS _ddAA /DA|, |S _ccDA | <; -15 dB) of 4.3%, 7.3%, and 10.2% and the minimum insertion losses of 0.45, 0.25, and 0.1 dB, respectively, while the isolation and common-mode suppression are higher than 20 dB within the whole passband.

In this paper, an approach to 1-to-2 ⁿ (n = 2,3,4, ...) way microstrip balanced power divider is proposed for the first time. This approach is realized by one balanced-to-single-ended power divider, 2 ^n-1 single-ended-to-balanced power dividers, and 2 ^n-1 - 2 single-ended power dividers. The function of differential-mode power dividing and common-mode suppression can be ensured by the balanced-to-single-ended and single-ended-to-balanced power dividers. The advantage of this approach is that it can reduce the circuit size effectively when comparing with the approach using all the balanced-to-balanced power dividers. The analysis of the balanced-to-single-ended power divider and the single-ended-to-balanced power divider is realized by combining the standard S-parameters, the mixed-mode S-parameters, and the odd- and even-mode equivalent circuits. To verify the theoretical prediction, three prototypes (one balanced-to-single-ended power divider, one single-ended-to-balanced power divider, and one 1-to-4 way balanced power divider) are fabricated and measured. The fabricated 1-to-4 way balanced power divider with the size of 0.75λ _g × 2λ _g shows an insertion loss of 0.45 dB at 1.8 GHz, a fractional bandwidth for a 15-dB return loss of 29%, a 20-dB common-mode suppression of 74.6%, and a 20-dB differential-mode isolation (|S _dd32 | and |S _dd42 |) of 15.1% and 100%, respectively.

In this letter, the stacked dual-slotted substrate integrated cavities (SICs) are proposed to achieve a filtering antenna. The dual-slotted SIC shows dual-mode behavior (quasi dual-slot mode and quasi cavity mode), which can achieve multi order response with fewer SICs. Furthermore, the antenna exists two frequencies at which the electric field has opposite horizontal component (Ex) near each slot. Thus, two radiation nulls can be obtained to enhance the frequency selectivity. The frequencies of the radiation nulls and the dual modes can both be controlled to some extent by the distance between the slots. For demonstration, a prototype at 9.65 GHz is designed with the fractional bandwidth (FBW) of 9.1%, the gain of 7.2 dBi, and the frequency selectivity of 0.70.