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Proceedings of the IRE

Issue 10 • Date Oct. 1955

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Displaying Results 1 - 25 of 49
  • Contents

    Page(s): 1171 - 1172
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  • Foreword

    Page(s): 1173
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  • Characteristics of Beyond-the-Horizon Radio Transmission

    Page(s): 1175 - 1180
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    This paper summarizes the principal characteristics of tropospheric transmission beyond the horizon and compares them with some of the properties of ionospheric scatter transmission. Quantitative results are given on the dependence of the average signal level on distance and frequency, fading phenomena, bandwidth capabilities and realizable antenna gain. A short historical summary of beyond-horizon tropospheric transmission is also included. View full abstract»

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  • Radio Transmission at VHF by Scattering and Other Processes in the Lower Ionosphere

    Page(s): 1181 - 1230
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    The results of a program extending over the last four and one-half years to investigate the nature and characteristics of high-loss regular VHF propagation by means of the lower ionosphere are presented. For the most part continuous-wave transmissions are employed, with the result that the effects of the different elements in the propagation are superposed. Three different ionizing agents operating in the lower ionosphere can be distinguished by means of their differing effects on the behavior of the signal, after the exclusion of behavior associated with sporadic-E propagation. They are solar radiation, corpuscular radiation presumably of solar origin, and meteors. The part played by each of these is discussed in the account and interpretation of the principal results obtained. Both short- and long-term characteristics of the observed composite signals are described, as well as the results of experiments with spaced and other antenna arrangements. The intensity and nature of the signals are discussed as a function of time of day, season, geographical position of the transmission path, and level of solar activity. The dependence of the strength of the signals on path length, on frequency, and on scattering angle has been studied. In the latter connection, pulse experiments were performed with equipment permitting fairly direct height determinations, which are found to be in satisfactory agreement with earlier less direct height determinations. Attempts to compare the observations with various existing theories designed to account for the propagation as a scattering process have been made. The results are not entirely satisfactory. View full abstract»

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  • A Message to the Readers

    Page(s): 1231
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  • On the Scattering of Radio Waves by Turbulent Fluctuations of the Atmosphere

    Page(s): 1232 - 1239
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    This paper presents a theoretical analysis of the mechanism that enables transmission of vhf-signals over distances of the order of 103 km. It is found that turbulent mixing, operating at the lower edge of the E-layer (h=80-90 km) produces fluctuations in electron density of sufficient intensity to account for the observed signals. The basic assumptions are the existence of a sufficiently strong gradient of electron density (dN/dh > 103 cm-3/km) and a reasonable level of turbulent activity. View full abstract»

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  • Aerodynamical Mechanisms Producing Electronic Density Fluctuations in Turbulent Ionized Layers

    Page(s): 1240 - 1252
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    Various radio effects of the turbulence in the low ionosphere are discussed with a view toward determining the order of magnitude of the turbulence parameters in this region. These effects include vhf scattering, sporadic E-layer phenomena, and diffraction patterns in reflection. The required electron density fluctuations (¿N/N)2 fluctuate around 10-4 and the scale of the turbulence I is roughly 100 to 200 meters. The latter quantity measured directly in the troposphere is of about the same order of magnitude. The underlying purpose of the paper is to study the aerodynamical mechanisms of turbulence. Pressure fluctuations result from the "collisions and extensions between turbulent eddies" in a uniform gas, or from vertical transport in the atmosphere with its varying pressure. It is shown that such pressure fluctuations are too small, by a factor of about 10-4, to produce the observed density fluctuations. However, the vertical transport mechanism produces two other effects which are independent of the energy of the turbulence and are of the right order of intensity. First, in a nonadiabatic atmosphere the vertical transport of air masses produces fluctuations of temperature. These, in turn, give rise to air density fluctuations, and proportional electron density fluctuations. Second, fluctuations of electron density result directly from transport in the presence of a gradient of electron density. These two effects, for the same uniform turbulence, occur at different levels in an ionospheric layer. They are sufficient to explain the stratification properties of sporadic E. View full abstract»

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  • Some Remarks on Scattering from Eddies

    Page(s): 1253 - 1254
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    The gaussian character of electromagnetic radiation scattered from turbulent atmosphere fluctuations is shown to follow from the Central Limit Theorem, even when the scattering is from only one macro-eddy and there is coupling between eddy motions of different sizes. It is suggested that including the effects of eddy coupling may increase the calculated average scattered power enough to give agreement with experiment. View full abstract»

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  • Investigations of Scattering and Multipath Properties of Ionospheric Propagation at Radio Frequencies Exceeding the MUF

    Page(s): 1255 - 1268
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    Some results are given of studies of ionospheric forward scatter propagation at frequencies exceeding normal muf in the high hf and low vhf range with the view of investigating potential point-to-point communications. Approximately 13,000 hours of data were analyzed from measurements conducted since late 1951, on several frequencies and paths, predominantly 1,000 to 1,100 miles in length, in mid-latitudes. Signal level characteristics are presented in summary form and discussed. Most of the work was conducted at 49.6 mc on the 1,066 mile Cedar Rapids, Ia. to Round Hill, Mass. path. Nominal 30 kw transmitters and large rhombic transmitting antennas were employed. The median received signal levels were relatively weak, of the order of 100 db below free space values. Fading during short periods in the absence of meteoric or sporadic E enhancements was found to be Rayleigh distributed. For longer periods, of the order of an hour, the fading was essentially Gaussian, although meteoric and sporadic E enhancements often changed the slope of the fading distribution curves at higher signal levels. The normalized median levels were generally lower and the diurnal variations less marked than those reported by CRPL for the shorter Cedar Rapids to Sterling path (773 miles). Seasonally, the median levels were higher in winter and summer and lower in spring and fall. There was an indication of lower levels with decreasing sunspot activity. View full abstract»

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  • UHF Long-Range Communication Systems

    Page(s): 1269 - 1281
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    Recent discoveries about long-range propagation of uhf radio waves, together with careful utilization of high-powered transmitters and large antennas, have made possible reliable multichannel point-to-point uhf radio communication systems which operate over distances of 200 miles or more beyond the horizon. Measurements by various investigators have shown that transmission losses on such paths have median values of the order of 80 db below free-space levels. A program of investigation at Lincoln Laboratory, MIT, has yielded detailed propagation data necessary for the design of long-distance multichannel uhf radio communication circuits. This has also led to the design of specialized equipment for this application. These circuits employ antennas having transmitting and receiving gains of 25-40 db; high-power FM transmitters of up to 10-kw output; sensitive FM receivers having low-noise input circuits, and excellent selectivity; and space-diversity reception. A procedure for system design is outlined. View full abstract»

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  • Diversity Reception in UHF Long-Range Communications

    Page(s): 1281 - 1289
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    Several diversity techniques employed in uhf beyond-the-horizon systems are discussed. Field experience is evaluated in terms of equipment reliability, flexibility and performance. A nonswitching parallel combiner has become the standard military diversity circuit for uhf long-range receivers. The circuit is described and analyzed. View full abstract»

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  • Factors Affecting Spacing of Radio Terminals in a UHF Link

    Page(s): 1290 - 1297
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    Measurements made by Central Radio Propagation Laboratory and others have established the feasibility of producing uhf fields at long distances which are sufficiently strong and consistent to be usable for communication purposes. This paper assembles the results of several investigators of the field strength-distance relation and shows that these results are in reasonable agreement in a range of about 75 to 250 miles. Statistical variations from the median due to fading are discussed. Graphical means are developed to facilitate the determination of the signal-to-noise ratio at the receiver for various ranges of distance, antenna gain, frequency, transmitter power, bandwidth, and receiver noise figure. Diversity systems are explored briefly. Estimates are made of the effect of the medium upon the antenna gain and the maximum bandwidth. View full abstract»

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  • Demonstration of Bandwidth Capabilities of Beyond-Horizon Tropospheric Radio Propagation

    Page(s): 1297 - 1299
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    Tropospheric radio transmission beyond the horizon is characterized by rapid and selective fading, which suggests a possible limitation on the useful bandwidth. The tests discussed were made to explore the bandwidth capabilities of this medium. Bell Telephone Laboratories conducted these tests in cooperation with the M.I.T. Lincoln Laboratory on a 188-mile path at 5,050 mc with a power of 300w and 28-foot paraboloidal antennas. Two types of tests were made. First a 12-voice-channel multiplex system for intermodulation crosstalk. IF band was 1.3 mc. Then television tests were made with a deviation of ±4 mc and IF bandwidth of 30 mc. No significant impairment in system quality could be attributed to distortion in transmission medium in either test. View full abstract»

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  • Characteristics of Tropospheric Scattered Fields

    Page(s): 1300 - 1305
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    Experimental results obtained with transmissions at wavelengths of 3.2, 9.3, and 24 cms over a 46.3-mile path are presented. With low terminal heights the scattered field was dominant on this path. Tests with a narrow beam antenna indicate that the scattered field arrives at the receiver spread over an appreciable angle. This angle is some five to seven times as large as the Booker-Gordon theory predicts on the assumption that the scale of turbulence is large compared to the wavelength. Loss in ability to receive power in proportion to antenna gain was encountered for antennas with aperture diameters greater than about 20 wavelengths. This loss occurs for aperture sizes considerably smaller than the Booker-Gordon theory predicts. The speed of fading of the scattered field signal increases aimost linearly with frequency. This agrees fairly well with the concept (due to Ratcliffe and applied to tropospheric scattering by Booker and Gordon) that fading is due to beating between various scattered field components whose frequencies differ by a fractional Doppler shift due to motion of the scatterers. The speed of fading always increases, during the day, with time of day and does not correlate with mean upper wind speed. This increase with time of day is probably connected with the repetitive diurnal meteorological cycle prevalent in Arizona. View full abstract»

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  • Results of Propagation Tests at 505 mc and 4,090 mc on Beyond-Horizon Paths

    Page(s): 1306 - 1316
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    This paper gives the results of some radio propagation tests which were made on two beyond-horizon paths in New-foundland. During these tests simultaneous transmissions at 505 mc and 4,090 mc over a 150-nautical mile path were measured for a full year, and transmission at 505 mc over a 255 mile path was measured for a period of 5 months. Both of these paths were partly over land and partly over water. Long term median signal levels and fading statistics for the two paths are given, as well as the results of some measurements of the improvement obtained from space diversity reception and the gain realized from large (28 foot) antennas on the 150-mile path. The test results provide further evidence of the feasibility of using uhf beyond-horizon links in communication systems. View full abstract»

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  • Investigations of Angular Scattering and Multipath Properties of Tropospheric Propagation of Short Radio Waves beyond the Horizon

    Page(s): 1317 - 1335
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    Experiments designed to investigate the potential communications capacity of uhf and shf tropospheric propagation beyond the radio horizon were conducted by the MIT Lincoln Laboratory on circuits of from 161 to 188 miles in length along the coastal regions of the northeastern United States. Some of these tests were made in cooperation with the Bell Telephone Laboratories and the late Major E. H. Armstrong. A study of the following aspects of tropospheric propagation was considered necessary in order to gain a better understanding of the factors involved in design of communications systems: 1. The extent of useful communication bandwidths. 2. Variation of uhf and shf median signal levels over a full seasonal cycle. 3. The range and rates of fading. 4. The effective gain of highly directional antenna systems. 5. The effect of multipath propagation on modulated signals. 6. The polarization properties of scattered fields. 7. The angular dependence of scattered fields. Such experiments made from 1953 to 1955 have confirmed the utility of tropospheric circuits for wide-band communication systems and have provided information useful in the evaluation of tropospheric propagation mechanisms. View full abstract»

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  • Some Tropospheric Scatter Propagation Measurements near the Radio Horizon

    Page(s): 1336 - 1340
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    Measurements of small variations in 100 mc field intensity within and just beyond the radio horizon are reported. The measured fields are assumed to be the resultant of two field components, one having a constant amplitude and the other being a rapidly-fading scattered component. The fading range of the resultant field intensity over a 10-minute period is used to determine K. Here, K is the ratio in decibels of the root-mean-square amplitude of the scattered component to the amplitude of the constant vector. Curves showing the measured median values of basic transmission loss, fading rate, and K plotted vs hour of the day are included for three of the Cheyenne Mountain transmission paths. Diurnal variation of these quantities is also discussed. The average basic transmission loss of the scattered component, Lba8, can be found if K and the resultant basic transmission loss, Lbm are known. Median values of Lba8 and Lbm are plotted vs the angular path distance, ¿. Measurement of the correlation of the resultant field strengths received on two horizontally-spaced antennas within the radio horizon is reported. When the spacing was varied from ¿ to 20 wavelengths and the correlation compared to other characteristics of the field, the correlation was found to be as much a function of Lba8 and fading rate as it was of antenna spacing. View full abstract»

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  • The Rate of Fading in Propagation through a Turbulent Atmosphere

    Page(s): 1341 - 1353
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    Fading rate is defined to be the number of times per minute that the envelope of the received field crosses its median level with a positive slope. This definition of fading rate is equally useful for ionospheric or tropospheric propagation studies. Furthermore, it may be used with equal facility on short transmission paths where the ground wave component of the received field predominates and on the longer transmission paths where the scattered component of the received field predominates. It is shown that this definition of fading rate provides a quantity which is numerically related to the parameters of the propagation medium under certain conditions which are normally satisfied in either ionospheric or tropospheric propagation studies. The pertinent parameters of the propagation medium in beyond-the-horizon transmission are the location and shape of the scattering volume and the turbulent and drift velocities of the scatterers. An extensive discussion is given of the shape of the tropospheric scattering volume for beyond-the-horizon transmission. An analysis is then given of some fading rate data obtained in the National Bureau of Standards tropospheric propagation program in the 92 to 1046 mc range of frequencies on transmission paths 70, 97, 226, and 394 miles in length. Finally an analysis is given for within-the-horizon propagation. In this case it is advantageous to define fading rate as the number of times per minute that the phase of the received field crosses its median level with a positive slope. View full abstract»

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  • The Probability Distribution of the Amplitude of a Constant Vector Plus a Rayleigh-Distributed Vector

    Page(s): 1354 - 1361
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    Formulas, tables, and graphs are given for the probability distribution of the instantaneous resultant amplitude of the sum of a constant vector and a Rayleigh-distributed vector. It is emphasized that two distributions are required to describe a Rayleigh-distributed vector: the distribution of its amplitude and the distribution of its phase. A summary is presented of physical conditions which must be satisfied for a given phenomenon to exhibit statistical properties of a Rayleigh-distributed vector. References are made to ways in which these distributions may be used to describe random variables occurring in ionospheric, tropospheric, and irregular terrain propagation problems. Finally, a discussion is given of amplitude and phase distributions of two other random vectors encountered in tropospheric propagation studies. View full abstract»

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  • Trans-Horizon Microwave Propagation over Hilly Terrain

    Page(s): 1362 - 1368
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    By introducing three path parameters into the geometry of scattered signal transmission, the spherical earth formulas of W. E. Gordon were extended to the nonspherical earth case. This paper describes the result of analysis. The nonspherical earth formulas were applied to the prediction of the signal power and characteristics of tropospheric scattered fields on the hilly 93-mile path between Ithaca and Wethersfield-Springs, N.Y., and on the 119-mile path between Ithaca and Buffalo, N.Y. The transmission experiments over these two paths were conducted at Cornell University, at frequencies of 2,780 mc and 9,150 mc. The results of the experiments are presented together with the predictions in this paper. View full abstract»

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  • VHF Tropospheric Overwater Measurements Far beyond the Radio Horizon

    Page(s): 1369 - 1373
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    Using a high-power radar-type transmitter near Boston, Massachusetts, point-to-point measurements have been made at a frequency of 220 mcps on 200- and 400-statute-mile overwater paths extending along the east coast. Comparisons are made with similar data available for overland paths. Correlations between the shorter path field-strength data and sea echo back-scatter near the transmitter site are indicated. Airborne field-strength measurements have also been made out across the North Atlantic Ocean to a distance in excess of 400 miles; the variation of field strength with distance is graphically displayed by this technique. View full abstract»

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  • Forward Scattering of Radio Waves by Anisotropic Turbulence

    Page(s): 1374 - 1380
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    This paper extends the theory of tropospheric scatter by deriving the appropriate formulas for the important radio system parameters under the assumption that the turbulence is anisotropic; i.e., that the scale of turbulence in the horizontal dimension is different from the scale of turbulence in the vertical dimension. The frequency dependence of the scattered radiation is the same for anisotropic large-scale turbulence as for isotropic. Furthermore, those radio systems parameters which depend only on the rate of decrease of scattered energy with elevation angle (such as the vertical correlation function and height gain) remain unchanged under the assumption of anisotropy while those parameters which depend on the energy coming out of the great circle plane (such as the horizontal correlation function) can be influenced quite substantially by anisotropy. Several other parameters such as the longitudinal correlation function, bandwidth of the medium and effective antenna gain may also be influenced by anisotropy but generally to a lesser extent. A comparison is made between our theory and some recent NBS data-indicating that anisotropy does exist. View full abstract»

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  • Note on Scatter Propagation with a Modified Exponential Correlation

    Page(s): 1381 - 1383
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    A phenomenological exponential space correlation of dielectric fluctuations is normally used to predict scatter field strengths beyond the horizon. This paper introduces a modified exponential model which includes effects of the smallest blob cutoff in the turbulent spectrum and rectifies the correlation's cusp at the origin. It is found that the present agreement of troposphere scatter experiments with the exponential function does not depend on this cusp. It is suggested that frequency-dependent tropospheric fields recently measured below 25 cm may indicate the influence of the correlation's fine structure. The same model is then applied to the ionosphere, where the extended range VHF scatter wavelengths just straddle the smallest blob size (~3 m) in the E layer. The turbulence fine structure is most important for this propagation and gives a qualitative explanation of the curious dualism in frequency scaling laws observed at opposite ends of the VHF band. Satisfactory variation of signal strength with scattering angle is also predicted by this model. It is shown that scatter measurements can provide valuable estimates of the atmosphere's fine structure at various heights. View full abstract»

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  • Propagation of Short Radio Waves in a Normally Stratified Troposphere

    Page(s): 1384 - 1390
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    Experiments of the past decade give stronger fields well beyond the horizon than are calculated by the 4/3 airless earth approximation. Post-war work on the theory of the WKB approximation for wave propagation in slowly varying inhomogeneous media, and the peculiar results for eigenvalues of the bilinear refractive index proffle offer valuable clues in the search for an oversight in conventional propagation theory. If the absolute value as well as the gradient of the refractive index at the earth's surface be specified, with a refractive index profile which tapers to vacuum at some arbitrarily large height, then allowed modes of the wave equation permit the field to be calculated within, just beyond, and well beyond the horizon, in agreement with many vhf and microwave experiments. Modes thus calculated are supported by ordinary coherent molecular scattering in normal air dielectric layer. At times, superrefraction and macroscopic turbulence are additional mechanisms for propagation deep into the shadow of the earth bulge. View full abstract»

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Aims & Scope

This Periodical ceased production in 1962. The current retitled publication is Proceedings of the IEEE.

Full Aims & Scope