Cross-polarized Antenna Research Articles

Measurement of satellite communication parameters: satellite antenna measurements

The in-orbit testing of a communication satellite involves the measurements of key communication parameters, which determine communication subsystem behavior and verify the design. Some of the tests, specifically those involving the satellite's antennas, are unique in that they require satellite antenna motion in both azimuth and elevation. They can only be conducted in the postlaunch period prior to the operational phase of the satellite. The focus is on the antenna tests, in order to verify antenna pattern design and put a bound on satellite antenna crosspolarization isolation, or lack of it, by comparing in-orbit test results with computer predictions and range measurements. Measurement procedures and instrumentation for the in orbit measurement of satellite antenna parameters are considered. The measurement procedures are reflected in equations in which the unknown parameter is expressed by measurable or calculatable quantities. The measurement error is evaluated on the basis of the parameter equation and component error estimates.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Axial cross polarization in reflector antennas with surface errors of large correlation diameter

Equations for boresight cross polarization and isolation of axisymmetric and offset antennas in the presence of surface errors are derived in terms of numerically computable integrals. Computations revealed that 1) for root mean square (rms) error cross polarization increases monotonically with increasing rms error; and 2) maximum cross polarization occurs when the correlation diameter is 0.5 D for axisymmetric antennas (0.8 D for offset antennas). For rms error >\lambda/4 boresight cross polarization remains almost constant. Furthermore, results revealed that for given correlation diameter and rms error, axisymmetric antennas offer better boresight isolation in comparison with offset antennas having the same polarization efficiency.

Ice depolarization of the COMSTAR beacon at 28.56 GHz during low fades and correlation with radar backscatter

Radar correlation with significant ice depolarization events accompanied by low copolarization fades of the 28.56-GHz COMSTAR beacon signal are described for an experimental program at Wallops Island, VA. Using a Faraday switch at the front end of the receiver, the copolarization and cross-polarization levels of the 28.56-GHz beacon signal are sequentially monitored. A nearby high resolution S -band radar pointing along the Earth-satellite path monitors the simultaneous ice and rain reflectivity. Excellent correlation is noted between the cross-polarization events and relatively large and extended ice reflectivities along a segment of the Earth-satellite path at altitudes near and above the 0°C isotherm. The radar and receiver data strongly suggest the cross-polarization mechanism is due to a hailshaft which intersects the path at altitudes well below the 0° isotherm. Since the intervening ice results in a cross-polarization signal which either adds or subtracts to the cross-polarization antenna residual, a method is described to remove the residual from the resultant measured cross-polarization level, without employing a phase measurement. Cumulative, month-of-year and time-of-day statistics associated with the depolarization signals are established.

Center-fed parabolic reflector antenna cross polarization due to slightly right-left asymmetric feed patterns

Slightly right-left (R-L) asymmetric feed pattern effects on a center-fed parabolic reflector antenna cross polarization have been described with some numerical computation examples, noting that measured feed patterns, both in the E plane and in the H plane, are generally not completely R-L symmetric and have a slightly different R-L field intensity which produces asymmetrical reflector surface currents. It is shown from the numerical computations that even a slight feed asymmetry, in both amplitude and phase, gives rise to a cross polarization in the secondary prinicpal planes and that, in particular, phase R-L asymmetry incurs two-dimensional movement of principal- and cross-polarized patterns resulting in the rapid increase of principal-plane cross polarization as well as the maximum cross polarization, principal-pattern beamshift, and the sidelobe variations.

The Use of Orthogonal Polarizations in Microwave Imagery of Isolated Canine Kidney

A method of imaging biological targets using microwave radiation at a frequency of 4 GHz is presented. Linearly polarized radiation is transmitted through an isolated canine kidney and received with co-polarized and cross-polarized antennas. Images are displayed as the spatial variation of the magnitude of the transmission scattering parameter S21 for each mode of polarization. The relationship between the spatial variation of the magnitude of S21 and canine renal anatomy is discussed. It is shown that within the kidney the cross-polarized image tends to emphasize linear or piecewise linear structures, whereas the co-polarized image balances renal cortical lobulations.

Off-axis cross-polarization and polarization efficiencies of reflector antennas

Cross polarization in reflector antennas is related to the polarization efficiency. Simple approximate expressions for the peak cross polarization are derived in terms of the polarization efficiency. Front-fed, Cassegrainian, and offset antennas are considered. It is shown that symmetrical antennas have lower peak sidelobes than offset antennas with the same polarization efficiency by approximately 2.5 dB. Excellent agreement between the present method and other methods is achieved. Formulas for the off-axis angle at which peak cross polarization takes place are also given for symmetrical and offset reflectors. It is found that for reflectors with the same <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D/\lambda</tex> ratio, symmetrical antennas have larger off-axis angles (1.6 times larger) for peak cross polarization in comparison with offset antennas.

On the cross polarization of asymmetric reflector antennas for satellite applications

It is well known that focussed, axial symmetrical reflector antennas collimate the co- and cross-polar components of the primary field separately, i.e., the reflector does not create a contribution to the cross polarization of the far-field. By a simple extension of a classical physical argument it is demonstrated that this separability does not depend on the symmetry of the antenna, and that it, therefore, holds even for off-set fed reflectors. A new mathematical formulation of the collimation is derived in which this is shown. Yet the separability does depend on how the co- and cross-polar fields are defined, and the cross polarization of feeds for asymmetric reflectors is discussed in detail in the light of this. It is further suggested how to design low cross polarization feeds for off-set fed antennas. As a consequence of the separate collimation such feeds will lead to low cross-polarization of the secondary fields. Two simple examples are treated. The only limitations of the results are those due to the application of the aperture field version of the physical optics approximation.

Cross-polarization in satellite and earth-station antennas

Cross-plarization in axially symmetric reflector antennas can be reduced, theoretically, to zero by use of special feeds like the Huygens' source. Alternatively, paraboloidal reflectors with large f/D ratio do not deteriorate further the cross polarization level relative to the value due to the feed itself. The Cassegrainian optics is equivalent to a large f/D paraboloid. The reflector of linearly polarized off set fed antennas contribute more cross-polarization than symmetrical reflectors fed by the same feed. With symmetrical reflectors the cross-polarized component generated by the reflector vanishes in the principal planes and is confined to four main lobes that have peak values in planes at 45° to the principal planes. In the case of offset fed reflectors cross-polarization vanishes in the plane of symmetry and has its peak in the plane of asymmetry. The reflector generated cross-polarization with offset fed antennas may be reduced by use of small offset angles and large f/D ratios. Feed offsetting has but little effect on the peak level of cross-polarization. This is usually accompanied with an asymmetry in the cross-polarization radiation pattern. Feed offsetting also results in spatial tilt in the copolarized and cross-polarized lobes with the cross-polar minimum always coinciding with the main beam peak. The effect of surface errors on the antenna cross-polarization is to partly fill the cross-polar along boresight. The peak cross-polarization, however, changes but slightly.

Antenna temperature for a scanning microwave radiometer

Expressions for the antenna temperature are derived for a scanning Earth-viewing satellite-borne microwave radiometer. The result is in the form of an integral that includes the antenna gain function and brightness temperature. A composite emissivity term appears in the brightness temperature equation that contains the horizontal and vertical components of surface emissivity weighted by their respective antenna gains. Analysis is performed to obtain the antenna temperature components corresponding to the emission of horizontally and vertically polarized radiation. Calculations are performed showing the effects of beam width and scan angle on the two components of antenna temperature for a linearly polarized antenna. Effects resulting from antenna cross polarization are also analyzed.

The recovery of polarized apparent temperature distributions of flat scenes from antenna temperature measurements

The reception theory for quasi-monochromatic partially polarized emissions is employed to describe the antenna temperature when observing a flat scene from within or above an absorbing/ emitting atmosphere. When the polarization aspects of the antenna and scene are carefully observed, the resulting integral expression for the antenna temperature differs significantly from that reported for polarization invariant temperature distributions. It is observed that the antenna couples to both emitted surface polarizations, and the phase character of the antenna introduces a term involving the product of the polarized and cross-polarized antenna patterns. A matrix approximation of the integral expression is suggested and developed as a means for inverting polarized antenna temperature observations for the underlying partially polarized apparent temperature distributions. The resulting matrix formulation is interpreted and special cases are treated. By means of sample cases, the cross-coupling effect between the antenna and the emitted field polarization vectors is illustrated. When the polarization aspects of the radiometer and the scene are ignored, inversions for the apparent temperature are shown to suffer appreciably even for a highly efficient antenna. The sensitivity of the matrix inversion technique to errors in the measurement vector is also examined.

Rocketsonde Transmission Link Characteristics

An improvement in high altitude communication with a small, rocket-borne sonde can be achieved by transmitting circularly polarized waves. Signal losses due to cross polarization of linear antennas can be reduced. Measured radiation patterns for instrument mounted linear and several helical antennas are presented. Flight data are discussed.

Antennas for Radio Astronomy in the Metre-Wavelength Region at Ahmedabad

The paper describes three antenna arrays used in conjunction with receiving and recording apparatus for some of the solar radio astronomy and ionospheric research problems at the Physical Research Laboratory, Ahmedabad.Three pairs of cross-polarized rhombic antennas have been constructed to record the dynamic spectra of solar radio bursts covering the frequency range of 40–240 MHz. These are equatorially mounted and driven by a common synchronous motor to follow the sun continuously for about ±4 hours around the local noon.A steerable broadside collinear array of 16 half wave dipoles backed by a reflector has been constructed for radio star scintillation studies at 110 MHz. This array is also equatorially mounted and can be used to follow a radio source continuously.A simple radio interferometer, operating at 110 MHz, consisting of two Yagi antennae spaced 10 wavelengths apart has been constructed. This will be used for both the solar as well as radio star scintillation work.The effective areas, matching ...

Radar Echoes from the Sun

The study of the sun by radar which was begun less than five years ago should become a valuable supplement to the study, by other methods, of the sun and interplanetary space. High powered transmitters and large antennas are required to detect a solar echo. Frequencies less than 50 Mc should be optimum, primarily because of increasing coronal absorption with increasing frequency. Routine observations were begun by the Lincoln Laboaratory of the Massachusetts Institute of Technology in April, 1961, at a site near El Campo, Texas. Observations since that time have been made on about 200 days per year. The transmitter has an average power output of 500 kw and operates at a frequency of 38.2 Mc. The system includes two cross-polarized antennas consisting of large arrays of dipoles. These antennas have maximum gains of 33 and 36 db. The received solar echo is usually 20 to 30 db below the solar noise and signal integration is required to detect the echo. The average measured solar radar cross section is approximately equal to that of the projected area of the photosphere although there are large fluctuations about the mean. Some possible reasons for these variations in cross section are discussed. The Doppler spreading of the solar echoes varies between 20 and 70 kc and is apparently due to mass motions on the sun. These indicated mass motions are large enough to affect the coronal temperature measurements made by the emission-line broadening method. The peaks of these spectra are usually shifted in the positive Doppler direction by about 4 kc. This shift implies a solar wind at the level of reflection. A theoretical solar model is developed to help explain some of the characteristics of the solar echo. Suggestions are made for improved equipment and procedures that should result in more significant solar radar results in the future. A system sensitivity increase of at least 10 db is needed in order to overcome the effects of non-Gaussian noise during times of solar activity, and an increase of at least 30 db is needed to study the echo from the quiet sun without signal integration.