Tuesday, February 28, 2017

Printed Conformal Antenna Design

Conformal Antenna

In many applications pertaining to missile, satellite, spacecraft and aircraft a directive antenna mounted on a curved body is required. Conforming the antenna to the surface save space and is often essential for structural reasons. An antenna that conforms to a surface whose shape is determined by considerations other than electromagnetic; for example, aerodynamic or hydrodynamic called conformal Antenna. Microstrip antenna technology is most suitable for conformal applications because of their ability to conform to non-planar structures. Microstrip antenna patches are placed above what may be characterized as a conducting plane with a dielectric substrate separating the patch from the conducting planeThe properties of such an array depend strongly on whether it is small compared to the radius of curvature of the mounting body, in which case it behaves nearly like a planner array or whether it is comparable or large to the radius.
The conformal antenna may be put in the belly of aircraft then there is no need of radome. Besides this, shaping and resolution can be improved by using the conformal antenna as more elements can be accommodated. In conformal antenna, it is found that resonance frequency is same for various curvatures. However, as curvature increases, the pattern broadens.  The aperture design of the conformal array is predicted on the knowledge of the patterns and coupling coefficients in the mutual coupling environment. The analysis and synthesis of the conformal array are slightly more complicated than a linear array with uniform spacing because the array elements point in different directions.
Antenna Design
For the analysis of conformal array antennas, the knowledge of the mutual coupling among the radiating elements and the radiation patterns of individual elements are essential ingredients. The first step in the analysis of conformal antennas is to find the electromagnetic field on the surface or in space in the presence of an arbitrarily shaped body. If the geometry is complex there are several reliable numerical procedures, e.g. the moment method, that is available for solving the radiation problem.
conf2The best way to design conformal antenna is first to design rectangular patch antenna of an array of microstrip patch antenna, then conform this geometry over a curved surface and re-optimize design. As resonant frequency does not change with a curved surface, same dimensions of the patch of the planar surface are modeled over a cylindrical surface. Patch dimensions and width of the feed line are calculated is the same way as of microstrip antenna array. Final optimization is carried out using Electromagnetic CAD software.
conf3.jpgThis is example of one 220-element waveguide-fed microstrip patch array.To reduce the feed loss, a slotted waveguide is used instead of the main feed line to feed the microstrip patch array. To simplify the manufacturing procedure, the slots of the waveguide are etched on the ground plane of the dielectric substrate, and the etched ground plane serves as the upper wall of the waveguide. The cover of this planar array is made of the dielectric substrate and its relative dielectric constant is the same as that of the dielectric substrate under the patch array. The slotted waveguide is excited by a coaxial probe at the center of the bottom wall of the waveguide. The two ends of the waveguide are shorted.
After optimizing rectangular array, this is converted to conformal antenna array as shown below.
conf5.png
conf6.pngSimulated result of this conformal antenna using Finite Time domain Difference ( FDTD) is shown below. The main advantage of this center-fed structure is that it avoids beam squint with frequency. The slotted waveguide is regarded as a linear array, and each waveguide-fed subarray is supposed to be an element of this linear array. If this linear array is end-fed, confessedly, the beam squints with frequency. The center-fed slotted waveguide is equivalent to a two-element linear array, and each array element is an end-fed linear array whose array elements are waveguide-fed subarray.

Friday, February 17, 2017

Time Domain Reflectometry (TDR) Analysis of Transmission Lines

T
ime Domain Reflectometry is the analysis of a conductor lines (interconnects)by sending a pulsed signal into the conductor, and then examining the reflection of that pulse. A TDR transmits a short rise time pulse along the conductor. If the conductor is of a uniform impedance and is properly terminated, the entire transmitted pulse will be absorbed in the far-end termination and no signal will be reflected toward the TDR. Any impedance discontinuities will cause some of the incident signal to be sent back towards the source. By examining the polarity, amplitude, frequencies and other electrical signatures of all reflections; tampering or interconnect discontinuity may be precisely located

Time Domain Reflectometry is the analysis of a conductor lines (interconnects) by sending a pulsed signal into the conductor, and then examining the reflection of that pulse. A TDR transmits a short rise time pulse along the conductor. If the conductor is of a uniform impedance and is properly terminated, the entire transmitted pulse will be absorbed in the far-end termination and no signal will be reflected toward the TDR. Any impedance discontinuities will cause some of the incident signal to be sent back towards the source.
By examining the polarity, amplitude, frequencies and other electrical signatures of all reflections; tampering or interconnect discontinuity may be precisely located.


TDR Analysis Need in Signal Net

TDR : Working Principle

With TDR, step pulse voltage is input into the transmission circuit and the voltage reflected in the transmission circuit is measured over time. The characteristic impedance of the circuit board is then calculated from the voltage drop in the reflected voltage.


TDR Simulation Result of Transmission Line in Smart phone from USB to Chip

TDR Pulse (step waveform) is very important. Rise Time value specifies the rise time of incident TDR pulse. The resolution of a TDR measurement system is directly related to the rise/fall time of the incident pulse. Rise time is specified with either a 10%-90% or a 20%-80% definition.


For example, consider a 20* timestep rise time for a 10%-90% definition, there will be 20 time steps between the time where the waveform has a value of 0.1 to the time it reaches a value of 0.9.


Initial glitch on TDR response Since the instantaneous TDR response is directly calculated from V/I, it reveals the initial glitch on TDR response. It is due to the zero current flowing through at the time = 0