This paper proposes a robust combination of digital predistortion (DPD) and crest factor reduction (CFR) for radio frequency (RF) power amplifiers (PAs). It is constructed using the architecture of CFR-DPD connected with a compensation module (CM). The compensation module is introduced to achieve mutual compensation between the output signals of CFR and DPD, and this can reduce the damage to the signal by CFR. The combination of CFR-DPD-CM provides the means to exploit margins in the transmitter performance, allowing the tradeoff among peak-to-average power ratio (PAPR), adjacent channel power ratio (ACPR) and error vector magnitude (EVM). The proposed combination of CFR-DPD-CM is assessed using a GaN Class-F PA driven by two modulated signals (a 4-carrier OFDM signal and a WCDMA 1001 signal with 20-MHz bandwidth), and a GaN Doherty PA driven by a 15-MHz long-term evolution (LTE) signal. The experimental results show that when the CFR reduces the PAPR about 4 dB, applying the proposed combination of CFR-DPD-CM, the average reduction of ACPR is 5.12 dB, and the average reduction of EVM is 1.26% compared with the conventional architecture of CFR-DPD.

As we know, a moving target's azimuth shift in SAR image is proportional to the projected velocity of its across-track velocity in the slant-range plane. Therefore, we can relocate the moving target in SAR image after estimating its velocity. However, when Doppler ambiguity occurs due to the limitation of the SAR system's pulse repetition frequency (PRF), this relationship will not hold any more, in this case, we cannot relocate the moving target to the right position. The Doppler spectrum of a moving target with arbitrary velocity may entirely situate in a PRF band or spans in two neighboring PRF bands. In this paper, we conduct a detailed theoretical analysis on the moving target's azimuth shift for these two scenarios. According to the derived formulas, one can relocate a moving target with arbitrary velocity to the right position no matter Doppler ambiguity occurs or not. Simulated data are processed to validate the analysis.

A matrix method which takes into account the probe positioning errors in cylindrical and spherical near-field (NF) measurement techniques is proposed. The near-field irregularities made impossible the determination of the cylindrical or spherical wave expansion from the measured data using classical techniques based on 2D Discrete Fourier Transformation (2D-DFT) in cylindrical case (CC) and orthogonality properties in spherical case (SC). The irregularities can be randomly distributed but known and the matrix method expresses the linear relation between the measured near-field and the corresponding cylindrical or spherical modal expansion coefficients. Once the coefficients of the cylindrical and the spherical wave expansions are known the far-field of the antenna under test (AUT) is easily determined. Accuracy of the matrix method is numerically studied as a function of the irregularities magnitude and for different noise levels (data Signal to Noise Ratio). Also, experimental results have shown the efficiency of the proposed technique.

A dual-band and ±45° dual-polarized antenna is proposed for 2G/3G mobile communications in this paper. The proposed antenna consists of five elements for the lower band and ten elements for the upper band. Results for a single element and antenna array are presented. The front-to-back ratio is from 25 dB to 30 dB, and the axial cross polarization ratio is from 19 dB to 22 dB in a single low band unit. The dual-band array achieves a bandwidth of 15.7% (820-960 MHz) for the lower band and a bandwidth of 23.7% (1710-2170 MHz) for the upper band, covering the frequency bands required by 2G/3G systems. Simulation results show that the gains for antenna array are 15.7 dBi for the lower band and 19.1 dBi for the upper band, which is suitable for base stations applications.

A metamaterial inspired co-planar waveguide (CPW) fed compact low-pass filter is presented in this paper. The 3 dB cut off frequency of the filter is 1.4 GHz. The roll-off rate achieved for this filter is 47.4 dB/GHz. Sharp roll-off is obtained by introducing an additional resonance using an inductor in series with the shunt capacitor. The usage of chip inductor also results in a compact filter structure. The overall filter dimensions are 39 mm x 32 mm x 1.6 mm. The filter uses defected ground structure (DGS) for attaining stop band attenuation. The measured insertion loss of the filter in the pass band is less than 0.8 dB and average stop band attenuation is better than 23 dB. The equivalent circuit of the proposed filter is similar to that of a dual-CRLH (D-CRLH) transmission line.

A novel, compact asymmetric coplanar strip (ACS)-fed multi-band antenna for Bluetooth/WLAN/WiMAX applications is proposed and discussed in this paper. The proposed antenna is composed of a simple monopole structure with a mirror-L shaped branch and two rectangular radiating strips. It has a very small size of 13.75 x 26 mm² including the ground plane. The mirror-L shaped branch excites a resonant mode at 2.5 GHz, and on the other side, ACS-fed monopole structure with two rectangular strips (one horizontal and one vertical) excite the resonant modes at 3.3 GHz and 5.75 GHz respectively. By properly selecting the lengths and positions of these radiating branches, multiband operation with wider impedance bandwidth can be achieved. The measured and simulated results show that the antenna has impedance bandwidth of 200 MHz (2.40-2.60 GHz), and 2800 MHz (3.2-6.0 GHz), and it can cover the 2.4 GHz Bluetooth, 2.4/5.2/5.8 GHz WLAN and 3.5/5.5 GHz WiMAX bands. The resonances achieved with this technique can be tuned independently, and the equations governing the resonances are given and confirmed by parametric studies. The proposed technique is further validated by designing another antenna working at 1.8/1.9 GHz PCS, 3.5/5.5 GHz WiMAX, 5.2/5.8 GHz WLAN bands.

Satellites are the most important link in today's battle field, and with the advancement of anti-satellite technologies like anti-satellite missiles and directed energy weapons, satellites are becoming vulnerable to attack. The vulnerability of satellite depends highly on its probability of being detected and tracked, and optics or radars are the two major means of detection. To avoid detection, several suggestions have been made in the past to deflect ambient light and decrease the RCS (radar cross section) to avoid detection. The most notable RF stealth suggestion among them is the proposal of using an inflatable polymer cone to change its shape and reduce satellite's RCS. In this study we examine the RCS of this so-called stealth satellite in S-band with FDTD simulations, and analyze its frequency and radar incident angle dependence. Results indicate this shape is advantageous in bore sight monostatic backscatter RCS reduction, but in other directions the RCS increases due to sheer size effect, which makes it even more vulnerable to bi-static radar tracking. When it is slant illuminated, the RCS of the stealth satellite shows no RCS reduction effects. Such inflated device is susceptible to space debris damage and cumbersome to operate, and may interfere with the original mission of the satellite. Best strategy for satellite self-defense is orbit change.

A high selectivity wideband balun bandpass filter based on substrate integrated waveguide (SIW) and complementary split rings resonators (CSRRs) is proposed. 180° reverse phase characteristic between the two output ports can be easily realized by the multi-layer SIW power divider. Eight complementary split rings resonators are used to achieve the sharp rejection upper stopband. The proposed wideband balun filter exhibits a fractional bandwidth of 37% centered at 9.45 GHz and amplitude and phase imbalance less than 0.5 dB and 1°.

Many scientists do not believe that Maxwell's fish eye mirror (MFEM) can provide perfect imaging even if there is a drain array around the imaging points. However, one microwave experiment found a case where a 0.2λ resolution could be achieved in an MFEM experiment [New J. Phys. 13 (2011) 033016]. In this paper, we show that the MFEM cannot resolve two imaging points at such a subwavelength resolution in most cases even in the presence of a drain array, and an extraordinary case of subwavelength imaging requires a particular phase difference between two coherent sources. Both numerical simulations and experimental results show that the phase difference of two subwavelength-distanced coherent sources greatly influences the field distribution around the drain array. In very few cases (when the phase difference of the two sources is chosen to be a very specific value), we might resolve the image points in the drain array under the assumption that the power absorbed by the scanning cable on the left side of the drain array should be symmetric to that on the right side of the drain array [New J. Phys. 13 (2011) 033016]. However, in most cases, we cannot obtain a super-resolution imaging, as other drains around the image points will greatly influence the imaging. We also note that the experiment assumed that the power absorbed by the scanning cable on the left and the right sides of the drain array is symmetric is not correct for the experiment reported in [New J. Phys. 13 (2011) 033016], as the drain array itself is not symmetric. The highly non-symmetric distribution of the absorbed power is also verified by our simulation and experimental results. The experimental ``result'' of resolving two image peaks could potentially be recovered using only a single image peak, which demonstrates the wrong assumption of mirror symmetry. Comparisons and comments on perfect passive drains, ``super-resolution'' in a spherical geodesic waveguide, and time reverse imaging are also given.

The inevitable increase in radio interference within microwave systems continue to be of major concern as more of radio communication services compete with bandwidth assigned to the fixed service, fixed satellite service and broadcasting satellite service. Interference hampers coverage and capacity of these services often lead to the reduction in the signal to noise ratio at the receiving terminals. The existing global hydrometeor scatter model proposed by the International Telecommunication Union, when applied to the tropical and subtropical location often leads to considerable inaccuracies due to the wide range of intense climatic and geographical nature of this region. In this study, the bistatic intersystem interference due to hydrometeors between satellite systems and terrestrial downlink receiver terminal systems in a subtropical station computation is based on the Awaka and Capsoni cell models. For the attenuation of both wanted and unwanted paths to the receiver, the existing model based on the specific attenuation has been modified to include the equivalent path length through rain in the estimation of the attenuation. Results obtained show that the Capsoni model exhibits the normal trend under a moist atmosphere with a gaseous attenuation more pronounced at frequencies greater than or equal to 30 GHz. Also at high rain rates greater than 70 mm/h and considering the rain with melting layer, up to about 70 dB difference was observed between transmission losses estimated using Awaka and Capsoni models at link probabilities ranging between 1-10^-3% unavailability of the time.

Cognitive radio is the enabling technology for license-exempt access to the TV White Spaces (TVWS). There is ever increasing demand of users in the broadcasting and communication services. Large portions of unused spectrum in the UHF/VHF bands exist in India which can be used on geographical basis. This paper describes a study on path loss variation in UHF/VHF bands in India. The aim of this study is to develop and optimize a path loss model based on Linear minimum mean square error estimation (LMMSE) for India. We propose the LMMSE based Optimized Perez-Vega Zamanillo propagation path loss model. The measured path loss values, collected across India, are compared with proposed Optimized Perez-Vega Zamanillo path loss model and other existing path loss models. It is found that Optimized Perez-Vega Zamanillo propagation path loss model has the least root mean square Error (RMSE) of 13.98 dB. Other existing path loss models have root mean square Error (RMSE) value greater than 24 dB. Therefore, Optimized Perez-Vega Zamanillo propagation path loss model is best suited for predicting coverage area, interference analysis in India for TVWS.

In this paper, we present an efficient formulation of the novel weakly conditionally stable finite-difference time-domain (NWCS-FDTD) method for the electromagnetic problems with very fine structures in one or two directions. The formulation is obtained by using only algebraic manipulation of the original method, and therefore the numerical stability and dispersion properties can be preserved. Moreover, due to its simpler right-hand sides of the updating equations, the proposed algorithm is more efficient than the existing WCS-FDTD methods, allowing a significant reduction in the cost of CPU time. Numerical experiments are finally given to verify the accuracy and efficiency of the proposed method.

A novel low-complexity dual-band digital predistortion (2D-LCMP) model for linearization of dual-band power amplifiers (PAs) is proposed in this paper. The in-band intermodulation (IM) and cross-band modulation (CM) distortion terms in the prior two-dimensional models have different impacts on the model performance. Therefore, they are considered respectively in the proposed model. Some redundant distortion terms are removed away to decrease the model complexity. In addition, the nonlinearity order and memory depth are frequency dependent for each band. Experimental measurements were performed on two types of wideband PAs. The results prove the superiority of the 2D-LCMP model.

A microstrip monopolar patch antenna with shorting vias in the circular patch and coupled ring for bandwidth enhancement is proposed. The bandwidth of the proposed antenna with shorting vias in the annular coupled ring is over 40% wider than that of the antenna without shorting vias in the annular ring. The proposed antenna provides a wide bandwidth because of four resonant modes, including the TM01 mode of the circular patch, TM01 mode of the coupled annular ring, TM02 mode of the circular patch, TM02 mode of the coupled annular ring. These modes can generate a omnidirectional pattern in the azimuth plane like a monopole antenna. A prototype was fabricated to confirm the simulation verdictions. Measured results show that 10-dB return loss bandwidth of 38.4% from 4.42 to 6.52 and average gain of 5 dBi acrossing the operating band are achieved for the proposed antenna with a low profile of 0.027 wavelength.

In this paper, a compact wideband planar monopole antenna suitable for slim mobile handsets applications is presented. The proposed antenna operates over LTE700/GSM800/900 (0.742 GHz-1.36 GHz), GPS L1/GSM1800/1900/UMTS/IMT2100/Wi-Fi/LTE2300/2500 (1.475 GHz-2.7 GHz), and WiMAX (3.4 GHz-3.72 GHz) based on reflection coefficient better than -6 dB. It consists of coupling strip, shorted radiating strip, and parasitic meandered lines. The wider impedance bandwidth is achieved by placing the meandered line as parasitic element on the back side of the coupling and shorted radiating elements. With this configuration, the antenna gives extremely wide impedance bandwidth which covers all the required frequency bands of the smart mobile phones. To investigate the proposed antenna, S-parameters, surface current distributions, and radiation performances are studied. To check the robustness of the proposed antenna, investigation is also carried out in the vicinity of the mobile environment. Further, specific absorption rate (SAR) is calculated on the human head and found to be below 0.535 W/kg. The simulated and measured results are found in close agreement.

A lossy metal-wall cavity resonator that extends well beyond perturbation theory limits is studied. An exact analytical solution is employed for the spherical cavity resonator, having walls transformed from being a perfect electrical conductor (PEC) to free space. This model then acts as an ideal benchmark reference standard. A plane-wave approximation is then derived. Independent full-wave numerical modeling of the spherical cavity resonator is undertaken using eigenmode solvers within two well-known commercial, industry-standard, simulation software packages (HFSS™ and COMSOL). It has been found that the plane-wave approximation model accurately characterizes the results generated by these solvers when equivalent finite conductivity boundary (FCB) and layered impedance boundary (LIB) conditions are used. However, the impedance boundary (IB) condition is accurately characterized by the exact model, but the precise value of complex wave impedance at the wall boundary for the specific resonance mode must first be known a priori. Our stress-testing results have profound implications on the usefulness of these commercial solvers for accurately predicting eigenfrequencies of lossy arbitrary 3D structures. For completeness, an exact series RLC equivalent circuit model is given specifically for a spherical cavity resonator having arbitrary wall losses, resulting in the derivation of an extended perturbation model.

In this paper, a compact dielectric resonator antenna (DRA) with bandnotched characteristics for ultra-wideband applications is presented. A comprehensive parametric study was carried out using CST Microwave Studio suite TM 2011 to analyze and optimize the characteristics of the proposed antenna. Three shapes for the coupling slot were investigated. Simulation results show that the proposed DRA had a -10 dB impedance bandwidth of 23% from 9.97 GHz to 12.558 GHz, and a maximum gain of 7.23 dBi. The antenna had a notched band centered at 10.57 GHz, which increased the reflection coefficient by 23.5 dB, and reduced the gain by 6.12 dB. The optimized designs were verified by experimental tests on fabricated samples.

A novel multi-band band-reject filter based on multi-ring complementary split-ring resonators (multi-ring CSRRs) is presented. The proposed filter is realized by etching the multi-ring CSRRs in the ground plane beneath a microstrip line. The multi-ring CSRR offers the possibility of designing multi-band filters with a small size and simple structure. To validate the proposed prototype of the multi-band filter, a dual-band and tri-band filters were fabricated and tested. The proposed filters show a good multi-band property to satisfy the requirement of WLAN in the 2.4/5.8 GHz bands and WiMAX in the 2.5/3.4 GHz bands. A good agreement between experimental and simulated results is obtained.

A high selectivity differential bandpass filter (BPF) using two pairs of dual-behavior resonators (DBRs) is proposed in this letter. A high selectivity passband for the differential mode with second harmonic suppression is achieved, by utilizing shorted coupled lines with two short stubs. For the common-mode (CM) circuit, the CM responses can be suppressed over a wide frequency band by the loaded open/shorted stubs. To validate the feasibility of the proposed filter, a planar differential BPF (3-dB fractional bandwidth 4.9%) with good CM suppression is designed and fabricated. The theoretical and measured results agree well and show good in-band filtering performances and out-of-band harmonic suppression performances.

This paper presents a new compact end-fire antenna for ultra-high frequency (UHF) radio frequency identification (RFID) applications. The antenna has two meandered dipole drivers. A folded reflector and a rectangular reflector are demonstrated. The advantage of the end-fire antenna with meander dipole drivers compared to the conventional quasi-Yagi antenna is a reduction in the length of the driver, which allows closer space for RFID reader. The end-fire antenna is fabricated on a FR4 printed circuit board (PCB), the dimension of the antenna is 81×58 mm². The measured bandwidth is around 25 MHz (905-930 MHz) under the condition of VSWR less than 2. The maximum gain of the end-fire antenna is 3.2 dB. The advantages of the new antenna element are that it is more compact than conventional design and it is suitable for fabrication on low-cost, low dielectric constant materials. The antenna configuration, design, simulated and measured results have been well discussed. A good agreement is obtained between the simulated and experimental results. This new compact end-fire antenna is desirable for RFID reader applications.