Microwave attenuation due to rain at 110 GHz in south-east England

Fading due to precipitation has been recorded at 110 GHz for a period of one year over a 2.65 km path near Slough, England. The results show that frequencies as high as this could be used for relay links over short distances with acceptable reliability, despite the large attenuation due to heavy rain.     

Measurements of rainfall attenuation at 8 and 15 GHz

Measurements of attenuation at frequencies of 8 and 15 GHz, and their correlation with rainfall rate, along a 15.78 km path near Ottawa, are described. Empirical expressions which relate the observed attenuation to the measured rainfall rate are derived and compared with similar expressions obtained on the basis of earlier theoretical studies of the problem. It is concluded that while the theoretical predictions of attenuation in rain are reasonably satisfactory, at least for rains observed at Ottawa, there is a definite tendency for observed attenuations at low rainfall rates to be somewhat higher than the expected values. The cumulative distribution of rainfall attenuation over a six-month period is compared with the attenuation predicted on the basis of average rainfall data obtained during the last five years. Although the agreement between observation and prediction, using the theoretical relation between rainfall rate and attenuation, is good for path attenuations greater than 2 dB, it can be greatly improved if the empirical expression relating rainfall rate to attenuation is used.     

The influence of heavy rainfall on attenuation at 18.5 and 30.9 GHz

Percent-of-time distributions of rain-induced fades obtained on a 6.4-km path in New Jersey operating at a frequency of 18.5 GHz are discussed for the period of 1968-1969; data obtained at 30.9 GHz on a 1.9-km path for the same period are also discussed. The attenuation distributions are compared with attenuations calculated from the distributions of average rain rates on the paths. With these data, it is found that distributions of attenuation can be predicted from the path-average rain-rate distributions for a given sample period. Detailed rain-rate and attenuation measurements at 18.5 GHz on the 6.4-km path are presented for the most intense storm observed in a three year recording period. Point rain rates in excess of 250 mm/h and path-average rates exceeding 180 mm/h were observed; the attenuation exceeded the 50-dB dynamic measuring range of the equipment for more than seven minutes.     

Rainfall attenuation at 36 GHz and 55 GHz

Simultaneous rainfall measurements at 36 GHz and 55 GHz on a common 4.1 km path show, under the conditions of these experiments, that the attenuation due to rainfall lies between the attenuation values predicted by the lognormal and the Best rain drop size distributions.     

Waveguide attenuation measurements at 890 GHz

A modified H guide has been proposed as a low-loss low-mode waveguide for the submillimetre-wave region, and is supported by theoretical studies and microwave model measurements. A resonance technique using guide-length variation with temperature change for attenuation and mode-characteristic measurement is shown to be feasible at 890 GHz with an HCN laser. Guide attenuations less than 8 dB/m are obtained in initial measurements on a submillimetre waveguide.     

Measurments of attenuation and refractive dispersion due to atmospheric water vapor at 80 and 240 GHz

Field line-of-sight propagation experiments were made at 80 and 240 GHz on a horizontal path of 810 m. The measured attenuations showed quadratic dependences on atmospheric water vapor density, and absorptions in excess of theoretical predictions were observed at both frequencies, while the measured refractive dispersion between these two frequencies showed a linear dependence on water vapor density and was in good agreement with theoretical prediction. Liebe's model for water vapor attenuation including empirical continuum absorption is confirmed to be effective for 80 and 240 GHz.     

Depolarisation due to snow at 11.6 GHz

Depolarisation events due to snow covering a period of three years on an 11.6 GHz, 18 km link employing linear and circular polarisation are presented. These are used to demonstrate the variability of wet and dry hydrometeor scattering. By deriving the equivalent differential phase-shift from the measurements it is possible to show the relative importance of differential phase and amplitude effects in depolarisation for the different hydrometeors.     

Rain attenuation at 15 and 35 GHz

In order to satisfy future earth-to-space communications needs, new regions of the electromagnetic spectrum must be exploited. A program to determine the feasibility of using millimeter waves for this application has been conducted at Air Force Cambridge Research Laboratories (AFCRL) for approximately 6 years and it has been shown that at frequencies of 15 GHz (lambda = 2.0cm) and 35 GHz (lambda = 8.6mm) atmospheric attenuation is relatively low except for conditions of heavy clouds and precipitation. A portable radiometric system designed to measure attenuation at 15 and 35 GHz under conditions of precipitation was constructed and located in Hilo, Hawaii, a region where it rains frequently thus making it possible to conduct many attenuation measurements for varying rainfall rates. Attenuation was determined from both extinction and emission measurements as a function of zenith angle and rain rate. On the basis of the results that were obtained, it is concluded that for orographic rain up to rates of 50 mm/h in Hawaii: 1) attenuations up to approximately 10 dB can be calculated quite accurately from an emission measurement; 2) zenith attenuations are well correlated with rain rate and can be estimated from the regression lines which have been obtained; 3) attenuations at angles off zenith are not as well correlated with rain rate and thus the values obtained from the regression lines are only approximate; 4) attenuations at 15 and 35 GHz are well correlated.     

Rainfall attenuation at 110 and 890 GHz

Attenuation due to rainfall has been compared at 110 and 890 GHz over a 180 m path in the free atmosphere. It was found that the attenuation at 890 GHz is, on average, 25% higher than that measured at 110 GHz.     

Forward scattering due to rain at 11 GHz

Forward scattering from rain was measured at 11 GHz over a 9-km line-of-sight path. For rain rates of 100-150 mm/h, the scattered signal was observed to be about 40 dB below the direct signal. Good overall agreement was obtained between theory and experiment.     

Attenuation due to rainfall on a 24 km microwave link working at 11, 18 and 36 GHz

The excess attenuation due to rainfall on a 24 km microwave path in Bedfordshire between Cranfield and Everton was measured over the period January 1967 to December 1968. A comparison is made between the fading at the three frequencies to determine the relationship between them. The results are compared with the theoretical work of Oguchi.     

Measurement of attenuation and change of refractive index due to water vapour in the atmosphere at 890 GHz

An open-resonator technique is used to measure the attenuation and change of refractive index at 890 GHz for a variable water-vapour content in the atmosphere, using an HCN laser as a source. The refractive index increases uniformly with water-vapour content, a change of relative humidity from 0 to 50% at 26°C resulting in an index change of 1.3 × 10?4. An attenuation of 85±20 dB/km was measured for 50% relative humidity at 26°C.     

Tropospheric emission and attenuation statistics at 110 GHz

The probability distribution of atmospheric emission noise from the zenith at 110 GHz has been measured by a radiometer at Slough, UK. The derived attenuations in the vertical direction exceed 13 and 6 dB for 0·5 and 5% of the time, respectively.     

Snow Property Measurements Correlative to Microwave Emission at 35 GHz

Snow microstructure, measured by plane section analysis, and snow wetness, measured by the dilution method, are used to calculate input parameters for a microwave emission model that uses the radiative transfer method. The scattering and absorbing properties are calculated by Mie theory. The effects of different equivalent sphere conversions, adjustments for near-field interference, and different snow wetness characterizations are compared for various snow conditions.     

Radiometric rain attenuation measurements at 11.6 GHz in Peru

Rain attenuation is one of the key design parameters when setting up satellite communications systems at frequencies above 10 GHz. Reliable and economical system design requires fairly accurate estimates of this parameter. Although rain attenuation prediction models are available for temperate climates, little has been reported for tropical regions where rain impairments can be very much more severe. This letter presents radiometrically determined rain attenuation results at 11.6 GHz in a tropical climate.<>     

Depolarization and attenuation effects of radomes at 20 GHz

The problem of describing the attenuation and depolarization effects of a wet radome on a transmitted signal is considered by experiments carried out with two commonly used radome materials, ESSCOLAM-6 and ESSCOLAM-8. The results suggest that a two-component model of depolarization is required to account for the observed results. Predictions for the behavior of a complete radome are obtained, but full-scale testing with an operating radome to compare predicted and actual results remains to be done. An important conclusion is that while highly water-repellent radome materials are desirable from the point of view of attenuation experienced, they are not so desirable when considered in terms of the degree of depolarization introduced between orthogonally polarized signal components in frequency reuse systems     

Cross polarization at 18 and 30 GHz due to rain

Analysis indicates that the canting angles of freely falling raindrops are of great importance in determining the magnitude of the cross polarization in higher frequency (18-30 GHz) radio relay systems. Since knowledge of these canting angles is sparse, the images of 463 raindrops obtained with a raindrop camera by personnel at the Illinois State Water Survey have been measured to obtain the distribution of canting angles during two rainstorms. The distributions for both storms are similar, with 40 percent of the drops having positive canting angles exceeding15degwith respect to the horizontal, and 25 percent having negative angles of less than-15deg. Calculations based on the measured distributions and representing upper bounds indicate that a relay system at both 18 and 30 GHz and with a repeater spacing of 3.5 km will experience cross polarization less than -20 dB at rain rates of 150 mm/hr.     

Atmospheric emission measurements of attenuation by microwaveradiometer at 19.4 GHz

Attenuation measurements on an Earth space path are presented using a passive microwave radiometer operating at 19.4 GHz in the emission mode. Attenuation measured under clear weather showed variation between 0.2 to 1.1 dB, whereas for cloud conditions attenuation as high as 1.0 dB have been recorded. Attenuation measurements for rain events have been correlated with rainfall rate using a fast-response 10 seconds opto-electronic rain gauge. The values of attenuation versus rainfall rate varied between A (dB)=0.01+0.18 R(mm/h) at the minimum and A (dB)=0.01+0.25 R(mm/h) at the maximum, showing considerable variability in the values of attenuation from year to year. The attenuation statistics for different seasons have also been computed and they show considerable changes from season to season-the largest attenuation in excess of 10 dB recorded in July-August-September, whereas minimum attenuation in excess of 2 dB recorded in December-March for nearly two years of data. Comparison of measurements made over New Delhi with those reported elsewhere show that for 0.02% of time attenuation values lie between those of Slough, England, and Crawford Hill, NJ. The concept of effective path length has been discussed based on the relationship between effective path length and the rain rate     

Rain attenuation on satellite-earth paths at 11.4 and 14 GHz

Results are given of the analysis of several gears of radiometric measurements in the European region, carried out by the European Space Agency. Special attention is given to the attenuation statistics of the worst month in a calendar year and its relation to the yearly statistics. A total of 30 independent worstmonth fading distributions were used in the analysis. Furthermore, fade duration, diurnal variation and frequency dependence of fades are analysed.