Axial cross polarization in reflector antennas with surface imperfections

Expressions for the probability density function and mean of boresight cross polarization field intensity in terms of the rms surface errorepsilonare derived. It is shown that for small errors the mean on-axis cross polarization field intensity is directly proportional to the rms surface error. For relatively large errors (epsilon > 0.04 lambda) forward cross polarization is proportional to the square root of (1 - exp (-2sigma^{2})) wheresigmais the rms phase error. It is also shown that forward cross polarization is proportional to the correlation diameter and is a function of the polarization efficiency. Experimental measurements are revealed and results are compared with theoretical findings.     

Effect of profile errors with large correlation diameter on axisymmetric antenna crosspolarisation

The effect of profile errors with large correlation diameter on the boresight crosspolarisation of axisymmetric antennas is investigated. An expression for boresight isolation as a function of the correlation diameter, the r.m.s. surface error and the polarisation efficiency of the antenna is given. Curves of isolation as a function of the r.m.s. surface error and the correlation diameter are given for polarisation efficiencies of 0.99 and 0.999 (i.e. for antennas with peak off-axis cross-polarisation of ?25.3 dB and ?35.4 dB, respectively).     

Depolarization properties of offset reflector antennas

The cross polarized radiation for linearly polarized excitation and the beam displacement for circularly polarized excitation have been investigated for offset reflector antennas. Numerical calculations are given to illustrate the dependence upon the angletheta_{0}between the feed axis and the reflector axis as well as upon the half-angletheta_{c}subtended at the focus by the reflector. In the casetheta_{0} = theta_{c} = 45deg, measured results have been obtained for both linearly and circularly polarized excitations with a dual mode feed illuminating an offset paraboloid. The cross polarized radiation of horn reflector and open Cassegrainian antennas rises sharply to rather high values off the beam axis; however, in general, the maximum cross polarized radiation of offset reflector antennas can be made small by using a small angle between the feed and reflector axes. The cross polarization caused by offset is compared with that caused by an unbalanced feed pattern. The effect of the longitudinal current distribution and of departure of the surface from a paraboloid on cross polarization are also examined. The clarification of these cross polarization properties is found to be valuable in the design of reflector antennas.     

Dual offset reflector multibeam antenna for international communications satellite applications

Dual offset reflector antenna systems offer exciting possibilities for achieving both low scan losses and low cross polarization in geosynchronous communications satellite antennas providing narrow (100 leq D/lambda leq 400) and multiple beam frequency reuse coverages over an18degconical field of view. Novel geometrical configurations for the reflectors are characterized by simultaneously achieving: 1) blockage free apertures for all element beams within the18degconical field of view, 2) compatibility with large planar feed arrays, 3) additional degrees of design freedom by orientation and shaping of reflector surfaces for depolarization and scan loss optimization, and 4) large effectivef/Dratio achieved in compact and foldable geometries. A comparison of new front-fed offset Cassegrain (FFOC) and side-fed offset Cassegrain (SFOC) systems is made.     

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 sameD/lambdaratio, symmetrical antennas have larger off-axis angles (1.6 times larger) for peak cross polarization in comparison with offset antennas.     

Multichannel coherent optical communications systems

Balanced coherent receivers perform substantially better than single-detector receivers in multichannel optical fiber FDM communications systems since the balanced approach eliminates the direct-detection and signal-cross-signal interference. The permissible channel spacingDdepends on the intermediate frequency fIF, on the bit rate Rb, and on the modulation/demodulation format. In particular,Dincreases by 2 Hz for every 1-Hz increase of the fIF. The signal-to-interference ratio SIR, as defined in the text, provides a simple measure of the amount of the interference generated by undesired channels. The criterion SIR = 30 dB is selected in this paper and leads to the following minimum channel spacings: for heterodyne systems,3.8R_{b}for FSK,9.5R_{b}for ASK, and12.4R_{b}for PSK; for homodyne systems,7.5R_{b}for ASK and10.5R_{b}for PSK. Simultaneous transmission of several channels generates an excess shot noise studied here for the first time. If the local oscillator power is 40 dB above the received signal power and 2000 channels are transmitted without optical prefiltering, the excess shot noise power penalty is less than 1 dB.     

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.     

Subreflector shaping to improve multiple-beam performance of a Cassegrain antenna

A method of designing multiple-beam antennas based on shaping the subreflector of an offset Cassegrain antenna is described. It is applied to a compact system having a diameter of 300? that is required to produce beams up to 4-6°either side of boresight. The performance is shown to be only slightly less than that obtainable by also shaping the main reflector, indicating that efficient multiple-satellite-access antennas can be designed without specially shaped main reflectors.     

Impulse-radiating antenna with an offset geometry

An offset impulse-radiating antenna (IRA) is numerically analyzed and compared with a typical centered IRA. In the typical centered IRA, the transverse electromagnetic (TEM) feed arms block the aperture because they are located at the center of the aperture. This blockage causes multiple reflections inside the antenna and, thus, ripples in the tail of the radiated waveform. In the offset IRA, the TEM feed arms are removed from the aperture, lowering the tail ripples caused by multiple reflections between the TEM feed arms and the reflector. The boresight gains and the impulse amplitudes are seen to be essentially the same for both IRAs. The monostatic radar cross section of the offset IRA is significantly lower than that of the centered IRA for the plane wave incident from the boresight direction because the wave incident to the offset IRA is diverted toward the focal point of the reflector, which is away from the boresight direction. The offset IRA has a shadow behind the reflector. This feature can be useful in bistatic radar applications because the antennas can be placed in the shadows of each other.     

The statistics of HF sea-echo Doppler spectra

Several important statistical properties of the HF sea echo and its Doppler power spectrum, which are useful in optimizing the design of radar oceanographic experiments, are established. First-and second-order theories show that the echo signal (e.g., the voltage) should be Gaussian; this is confirmed with experimental surface-wave data i) by comparison of the normalized standard deviation of the power spectrum at a given frequency with its predicted value of unity, and ii) by cumulative distributiun plots of measured spectral amplitudes on Rayleigh probability charts. The normalized standard deviation of the dominant absolute peak amplitudes of the power spectrum (which wander slightly in frequency) are shown from experimental data to besim 0.7for the first-order peaks andsim 0.5for the second-order peaks. The autocorrelation coefficient of the power spectra is derived from measured data and interpreted in terms of the spectral peak widths; from this information, the correlation time (or time between independent power spectrum samples) is shown to besim 25-50s for radar frequencies above 7 MHz. All of these statistical quantities are observed to be independent of sea state, scattering cell size, and relatively independent of radar operating frequency. These quantities are then used to establish the statistical error (and confidence interval) for radar remote sensing of sea state, and it is shown, for example, that 14 power spectral samples result in a sample average whose rms error about the true mean is 1.0 dB.     

Optimizing multimode horn feed arrays for offset reflector antennasusing a constrained minimization algorithm to reduce cross polarization

This letter presents an algorithm for suppressing the cross polarization of single offset reflector antennas illuminated by a cluster of multimode horn feeds using a constrained minimization routine. The design variables are the mode ratios for the higher order modes excited in the feeds relative to the dominant TE₁₁ mode. The goal is to systematically determine the optimum mode ratios such that the overall crosspole performance of the reflector antenna improves relative to the single TE₁₁ mode excitation. The multimode horns considered are Potter-type (dual mode) horns and trimode horns     

基于BD-DFT扩展的大规模MIMO码本设计

考虑到大规模多输入多输出（Multiple Input Multiple Output, MIMO）阵列尺寸及外形等因素的限制,本文提出了一种适用于基站采用交叉极化天线面阵的大规模MIMO（Massive MIMO）码本设计方法。该方法综合考虑交叉极化信道的对角化特点和相邻天线之间的相关性,首先设计出与采用交叉极化线阵匹配的码本,进而利用垂直维度天线间的相关性对其进行扩展,最终生成与交叉极化面阵相匹配的码本。仿真结果表明该码本设计方法可使大规模MIMO系统的传输速率和误码率性能得到明显地提升。     

The polarization losses of offset paraboloid antennas

In this paper the electric field in the aperture of offset front-fed paraboloid antennas and open Cassegrainian antennas, excited by an electric dipole or Huygens source in the focus, is compared with the fields of front-fed circularly symmetrical paraboloid reflector antennas and classical Cassegrainian antennas. The aperture field forms the basis of expressions to calculate the polarization efficiency of all four types of antenna. Computed results are given, showing that offset antennas can compete with front-fed paraboloids if they are excited by an electric dipole; the classical Cassegrainian antenna, however, shows better results. If offset antennas are excited by a Huygens source, the result is very unfavorable compared with the symmetrical antennas which show no cross polarization.     

Design of multiple edge blinders for pyramidal horn reflector antennas

The design of multiple edge blinders for pyramidal horn reflector antennas is discussed. The blinders eliminate high sidelobes in the azimuthal plane near90degfor horizontal polarization at 4, 6, and 11 GHz. The successful design of multiple edge blinders for simultaneous operation in each of the three common carrier bands depends upon the suppression of an off-axis major lobe at 4 GHz and grating lobes which typically appear at 6 and 11 GHz. Using the blinder array factor and element pattern, it is shown that these lobes can be suppressed by properly choosing the blinder tilt anglebeta_{c}, the inclination of an individual blinder edgedelta, and an adequate number of edgesN.     

The sun as a test source for boresight calibration of microwave antennas

Although the diameter of the sun at microwavelengths is a little over half a degree, the sun can be used for establishing, say between 3 and 30 cm, the pointing corrections (boresight calibration) of microwave antennas, with high precision. There is, however, a discrepancy between the position of the microwave sun on any given day and the position of the center of the solar disc as tabulated in [16]. Over the period 1966-1969 this discrepancy had an rms value of 1.6' east-west and 1.2' north-south and a range of 11.4' east-west and 9.2' north-south. These conclusions are based on sun maps, made at 9.1 cm with an antenna having a beamwidth of approximately 3', which are published regularly. From such a map the microwave centroid can be determined with an error less than 1'. In the absence of such a map, the centroid can be estimated to within 2' or slightly better by taking account of optical data such as sunspots or plages. The correction on any one day is strongly correlated with that of the day before, but the correlation is small after about 5 days. The correlation returns in part after 27 days.     

Radome crosspolarisation on Intelsat standard 'A' communication satellite antennas

The reduction in size of Intelsat standard 'A' satellite communication antennas from 30.5 m to 15 m in diameter has necessitated the computation of depolarisation effects when the smaller antennas employ metal and dielectric space frame radomes for hardening and protection. Using the standard 15 m antennas enclosed by appropriately scaled-down metal and dielectric space frame radomes, the crosspolar discrimination (XPD) and the boresight shift (BS) have been computed close to antenna axis at 4 and 6 GHz. Comparison with the larger 30.5 m antennas enclosed by 45 m radomes have been made. It is reported that within 0.1 degrees of antenna boresight, the XPD introduced by the radomes does not exceed 33.0 dB as the worst case at 4 GHz. In particular, the Intelsat Specification of 30.7 dB for the XPD within the antenna's tracking beamwidth in clear weather conditions for frequency re-use satellite communication systems is satisfied. The BS is negligibly low.<>     

Antenna Effects in Depolarization Measurements

The depolarization of electromagnetic energy scattered from natural terrain has been of interest to experimenters and theoreticians for many years. However, the reported measurements have not agreed with theoreticaly predicted results. Boresight axial ratio has commonly been used as a figure of merit to describe the polarization isolation properties of the measurement system antennas. Since depolarization measurements from terrain involve extended targets which fill the full antenna beam, boresight axial ratio is often an inadequate measure of antenna polarization purity. This paper describes antenna isolation characteristics which impact the quality of the depolarized measurements. Representative calculations demonstrate the effects of nonideal antenna systems on the observed measurements. A criterion for antenna specification is given which will provide high-quality cross-polarized measurements.     

Design of 45-degree Linearly Polarized Substrate Integrated Waveguide-fed Slot Array Antennas

This paper presents a method of designing substrate integrated waveguide-fed (SIW-fed) slot array antennas. The design theory is based on the circuit model of slot and via as well as the reflection canceling. To prove the feasibility of this method, a 10-element K-band SIW-fed 45-degree linearly polarized slot array antenna with uniform power distribution is designed. By full-wave simulation, the antenna has a good impedance bandwidth of 7.5% and uniform power distribution. Besides, a maximum gain of 15.3dBi is obtained in the broadside and the cross polarization is suppressed below -23.5dB in the boresight. This type of SIW-fed slot array antennas can be a good candidate for microwave and millimeter-wave applications, especially for auto-motive collision-avoidance radar systems.     

Antenna-polarization and terrain-depolarization effects on pulse-radar return from extended areas at the near vertical

The methods now in the literature for calculating the terrain-backscattered radar-altimeter echo utilize the antenna power-gain patterns and the terrain radar cross section per unit area, usually designated σ0. In this paper, the polarization characteristics of the antennas and the echo depolarization by the terrain are taken into account for the first time. The formulas for the backscattered return are presented in terms of antenna and terrain parameters that can be measured directly or calculated from other measurements. These parameters are the four transmitting- and receiving-antenna gain patterns for the orthogonal polarization components, the two functions giving transmission- and reception-phase difference between the polarization components, the terrain-depolarization factor that accounts for the average backscattered power cross polarized with respect to the incident wave, and the radar cross section σ0per unit area. It is shown that the older formulas used for radar return from the near vertical are correct only when both antennas are linearly polarized and have the same direction of polarization. The errors that result from failure to consider polarization effects are illustrated by examples.     

Hemispherical power gain pattern measurements at 7.5 GHz

A technique is described for measuring the complete hemispherical power-gain radiation pattern of a 2.44-m diameter paraboloidal reflector antenna mounted atop a mobile small earth terminal operating at 7.5 GHz. Antenna power-gain data were measured versus azimuth and elevation angles with the earth terminal centered on a heavy-duty turntable flush with test range ground.Test site illumination was achieved with airborne transmitting antennas. Conventional and statistical power-gain patterns are presented for left-hand circular polarization and cross polarization. Results indicate that similar systems cannot rely upon orthogonal polarization to provide isolation or compatibility beyond the angular region of the main lobe.