Sheel Aditya

A dual polarized aperture coupled circular patch antenna using a C-shaped coupling slot

The design and development of a dual linearly polarized aperture coupled circular microstrip patch antenna at C-band are presented. The antenna uses a novel configuration of symmetric and asymmetric coupling slots. Variations in isolation between orthogonal feedlines and antenna axial ratio with the position of coupling slots are studied and broadband isolation and axial ratio are achieved. The prototype antenna yields 7.6 dBi peak gain, 70/spl deg/ 3-dB beam width, 25 dB cross-polarization levels and an isolation better than 28 dB between the two ports. With an external quadrature hybrid coupler connected to the two orthogonal feedlines, the antenna yields 3-dB axial ratio bandwidth of more than 30% at 5.8 GHz.

Instantaneous Microwave Frequency Measurement Using Photonic Technique

We propose a novel photonic technique for measuring microwave frequency instantaneously over a wide bandwidth. In our approach, an optical carrier is divided into two parts. Both parts are modulated by an unknown microwave signal; one part is phase modulated while the other one is intensity modulated. The two differently modulated optical signals are then launched into single-mode fibers with the same lengths to introduce microwave signal power fading. After photodetection, the radio-frequency powers of the two parts are used to generate an amplitude comparison function which provides a frequency-to-power mapping. The proposed scheme is simple and is experimentally verified over a frequency range of 13.5 GHz with a measurement error less than plusmn0.3 GHz.

Photonic measurement of microwave frequency based on phase modulation

A photonic approach for microwave frequency measurement is proposed. In this approach, an optical carrier is modulated by an unknown microwave signal through a phase modulator. The modulated optical signal is then split into two parts; one part passes through a spool of polarization maintaining fiber (PMF) and the other one, through a dispersion compensation fiber (DCF), to introduce different microwave power penalties. After the microwave powers of the two parts are measured by two photodetectors, a fixed frequency-to-power mapping is established by obtaining an amplitude comparison function (ACF). A proof-of-concept experiment demonstrates frequency measurement over a range of 10.5 GHz, with measurement error less than +/-0.07 GHz.

Planar Helix With Straight-Edge Connections in the Presence of Multilayer Dielectric Substrates

A planar slow-wave structure consisting of a planar helix with straight-edge connections has been studied in the context of application in traveling-wave tubes. The effects of several practical modifications to the basic structure are examined. These modifications comprise a vacuum tunnel, metal shield, and multilayer dielectric substrates. A modified effective dielectric constant method is proposed to obtain the dispersion characteristics for different possible configurations. Furthermore, coupling impedance for the different configurations has been calculated using the corresponding 2-D approximations. It is shown that, far from cutoff, the phase velocity and coupling impedance values calculated in this manner match very well with the simulation results obtained from CST Microwave Studio. The effects of variations in aspect ratio, metal shield distance, and dielectric constant of the substrates on phase velocity and coupling impedance are studied. A coplanar waveguide feed has been designed for one of the possible configurations. The measured S-parameters and phase velocity values for this proof-of-concept configuration agree well with the simulated results and confirm the ease of fabrication, low loss, and the wideband potential of the planar helix with straight-edge connections.

Instantaneous Microwave Frequency Measurement Using a Photonic Microwave Filter With an Infinite Impulse Response

A photonic technique for instantaneous microwave frequency measurement is proposed. In the proposed technique, a photonic microwave filter having a monotonic frequency response with the magnitude varying from positive infinity to negative infinity on a log scale, is constructed by cascading two photonic microwave filters with one having an infinite impulse response and the other having a finite impulse response. For a single-frequency microwave signal with a normalized magnitude, a unique relationship between the output response and the input frequency is established. Since the response extends from positive to negative infinity, for a given measurement range, a significantly increased measurement resolution is achieved. The proposed technique is verified by an experiment.

Microfabrication and Characterization of W-Band Planar Helix Slow-Wave Structure With Straight-Edge Connections

A slow-wave structure (SWS) consisting of a planar helix with straight-edge connections and incorporating a coplanar waveguide feed has been designed for operation at W-band and has been fabricated using microfabrication technique. On-wafer cold measurements have been carried out on a number of fabricated SWSs, and the results are reported here for the first time. The parameters measured are return loss, attenuation, and phase velocity, and the results cover a frequency range of 70-100 GHz. Cold-test parameters of the SWS have been also obtained using simulations, and the effects of fabrication, such as surface roughness, have been accounted for by estimating effective conductivity of different parts of the microfabricated structures. The measured and simulated results match well. The effects of silicon wafer resistivity have been also discussed. Planar helical SWSs fabricated in this manner have application in traveling-wave tubes operating at millimeter wave and higher frequencies.

Instantaneous Microwave Frequency Measurement Based on Amplified Fiber-Optic Recirculating Delay Loop and BroadBand Incoherent Light Source

A novel approach to implementing instantaneous frequency measurement (IFM) based on an amplified fiber-optic recirculating delay loop and a broadband incoherent light source (ILS) is proposed, analyzed, and experimentally demonstrated. Since the semiconductor optical amplifier-based fiber-optic delay loop has an infinite impulse response that varies from a large positive value to negative infinity on a log scale, a unique relationship between the output power, and the frequency of the input continuous-wave (CW) microwave signal is established. Meanwhile, it is experimentally shown that the use of the ILS can greatly improve the stability of the proposed IFM system. When the input power of CW microwave signal is within the range of -7 dBm to -16 dBm, the measured errors remain within ±400 KHz over a frequency range of 6.94-6.958 GHz. The measurement error, the complexity and cost of the proposed IFM system can be considerably reduced by only using one ILS, one modulator, and one photodetector. Since the proposed IFM system has a capability of optical integration, it is theoretically estimated that the measurement range can be extended to 20 GHz with a measurement resolution of 1.36 dB/GHz.

Wideband Microwave Absorber Based on a Two-Dimensional Periodic Array of Microstrip Lines

The modeling and design is presented of a new kind of circuit analogue absorber, which intercepts the electromagnetic waves through a two-dimensional periodic array of microstrip lines loaded with lumped circuit elements. For a plane wave incidence of polarization perpendicular to the strips, virtual magnetic walls are formed between the strips, and the geometry can be divided into many identical unit-cells. We first study the propagation characteristics of the unit-cell using the singular integral equation method. An RC network is then proposed to match this array with the free space and to dissipate the intercepted energy over a wide frequency range. The complete design procedure is explained through a design example that exhibits a bandwidth of 113% while the absorber thickness is less than 10% of the free space wavelength at the lowest operating frequency.

A new method for pHEMT noise-parameter determination based on 50-/spl Omega/ noise measurement system

A new method for determining the four noise parameters of pseudomorphic high electron-mobility transistors (pHEMTs) based on a 50-/spl Omega/ noise measurement system without a microwave tuner is presented. The noise parameters are determined based on the noise correlation matrix technique by fitting the measured noise figure of the active device. On-wafer experimental verification up to 26 GHz is presented and a comparison with a tuner-based method is given. The scaling rules for noise parameters have also been determined. Good agreement is obtained between simulated and measured results for 2/spl times/20 /spl mu/m, 2/spl times/40 /spl mu/m, and 2/spl times/60 /spl mu/m gatewidth (number of gate fingers /spl times/ unit gatewidth) 0.25-/spl mu/m double-heterojunction /spl delta/-doped pHEMTs.

An exact analytical model for dispersive transmission in microwave fiber-optic links using Mach-Zehnder external modulator

A new exact analytical model is presented to analyze the dispersive transmission in microwave fiber-optic links using a dual-drive Mach-Zehnder external modulator (DD-MZM). The model is very general and can be applied to almost all operating conditions of DD-MZM, such as bias point, drive level, phase shift, and modulation index difference between DD-MZM drives. The model results in simple new closed-form expressions for output power spectrum, permitting an accurate and fast analysis of such links. Expressions for two special cases, double sideband and single sideband modulation, are included, together with a novel presentation of some new results on power fading of fundamental and second harmonic.