摘要:

General formulations for the fluctuations of a beam wave propagating through a homogeneous or locally homogeneous medium are given in terms of the spectral density of the index of refraction. The amplitude and phase correlation functions and the mean‐square fluctuations are derived for a homogeneous medium showing the dependence on the radial distance in the transverse plane of the beam. The amplitude and phase structure functions are derived for a locally homogeneous medium. The correlation functions and the structure functions do not depend only on the difference coordinate; they are functions of the radial coordinates in the beam cross section. This particular inhomogeneity, however, is shown to be an analytic continuation of the homogeneous or locally homogeneous case. The meansquare amplitude fluctuation for the Kolmogorov's locally homogeneus medium is shown to behave as a plane wave for a short distance and then to become less than that of a spherical wave, and its spectrum is shown to behave asK−1for largeKin contrast with the plane and spherical waves. The spread of the beam radius is shown to be approximately the 8/3 powers of the distanceLfor small distance, and its, increase depends on the magnitude of the index of refraction fluctuat

ISSN:0048-6604    

DOI:10.1029/RS004i004p00295

年代:1969

数据来源: WILEY

摘要:

The radio loss between two small dipoles located in a forest is examined in the frequency range of 2–200 MHz by characterizing the forest in terms of a dissipative slab backed by an imperfectly conducting ground. A careful derivation of the total radio lossLshows that it consists of four distinct constituents: (1) a basic lossL0associated with the forest‐air interface, (2) a separation lossL8due to the vegetation above the antenna, (3) a wave‐interference loss Lidue to field reflections at the ground plane, and (4) an antenna input resistance loss Lrproduced by the ground proximity. The functional variation of these losses is consistent with previously obtained results that viewed the field in terms of a lateral wave, which now affords a simple physical interpretation for each of the separate factors enumerated above. By choosing parameters describing typical forests, we show that the variation of the four constituents leads to a total loss whose behavior agrees with available experimental data and with previous theoretical considerations pertinent to high antennas. In contrast to these, the present investigation predicts, for low antennas, that vertical polarization is preferable to the horizontal one and that communication conditions may be further improved if the operating frequency is increased rather than decreased. The antenna height gain effect is also examined in detail, and it is shown to be strongly affected by the ground proximity in a manner that had not been recognized hit

ISSN:0048-6604    

DOI:10.1029/RS004i004p00307

年代:1969

数据来源: WILEY

摘要:

The waveguide modes guided by a smooth concave tropospheric layer are considered. The model is chosen so that the effective dielectric susceptibility is a linear segmented function of height. The calculations are made explicitly for a model that is linear with height except for a single inversion layer. It is shown that such a layer will support whispering‐gallery or earth‐detached modes with exceedingly small attenuation even when the refractive index contrast is four or fiveNun

ISSN:0048-6604    

DOI:10.1029/RS004i004p00319

年代:1969

数据来源: WILEY

摘要:

Measurements of the phase difference between the signals from ionospherically propagated radio waves received on a vertical whip antenna and on a horizontal dipole at the same location are shown to lead to an estimate of the relative powers in the ordinary and extraordinary components when the propagation path is such that quasi‐longitudinal approximations may be used. The technique is valid for incoherent signals, where the incoming waves are considered to be made up of many individual rays. When spaced antennas are used, an estimate of the mean difference in bearing angle between the ordinary and extraordinary components is possibl

ISSN:0048-6604    

DOI:10.1029/RS004i004p00327

年代:1969

数据来源: WILEY

摘要:

An investigation of VLF radio‐wave propagation across a land‐sea boundary has been completed. The influence of the homogeneous ionosphere and the curvature of the earth, as well as the ground discontinuity, are considered. The problem of finding the field strength due to a horizontal line source is first formulated as a Wiener‐Hopf type problem via the concept of equivalent surface impedance and is subsequently solved in the Fourier‐transform plane. This solution has direct correspondence with that for dipole excitation and is expressed analytically in terms of the reflection and transmission coefficients of modes in the two sections of the earth‐ionosphere

ISSN:0048-6604    

DOI:10.1029/RS004i004p00335

年代:1969

数据来源: WILEY

摘要:

The effects of the auroral and polar zone ionosphere on forward propagation in the HF through low‐VHF (3 to ∼30 MHz) portion of the spectrum are reviewed. Phenomena that particularly affect the propagation of HF/VHF signals in this region include: the non‐great‐circle (NGC) mode, sporadic‐Eionization, F1‐layer effects,F‐region irregularities,D‐region absorption, and reflectivity properties of the polar ice caps. Specifically, we show that: (1) The NGC mode can cause deviations from the great‐circle path up to 90° and can carry the maximum frequency on a polar circuit. (2) Sporadic‐Eionization occurs primarily during the winter‐night period and is sometimes associated with the aurora. (3) F1‐layer effects are quite important and frequently determine the dominant propagation mode during summer mornings at sunspot minimum. (4) Auroral zone absorption is quite ‘patchy’ in spatial and temporal behavior and probably attenuates HF signals up to 60 db in severe cases. (5) Polar cap absorption, although a relatively rare event, can attenuate HF signals up to 100 db at times. (6) For multi‐hop modes, the relatively high attenuation for reflection on the ice caps should be considered in the transmission loss equation. Research performed during the decade ending in 1968 is emphasized, including some quite recent results of investigations conducted at the Geophysical Institute at the University of Alaska, not previou

ISSN:0048-6604    

DOI:10.1029/RS004i004p00347

年代:1969

数据来源: WILEY

摘要:

The phase and the field strength of VLF signals on 15.5 and 22.3 kHz from Australia have been observed in Japan, Some new features of the phase behavior during sunrise are pointed out, and the nighttime field strength relative to the daytime field strength is compared with results observed along another transequatorial path over the American continent.

ISSN:0048-6604    

DOI:10.1029/RS004i004p00367

年代:1969

数据来源: WILEY

摘要:

The University of Illinois and the GCA Corporation, supported by NASA, launched four Nike‐Apache rockets from Cassino, Brazil, during the solar eclipse of November 12, 1966. The measurement ofD‐region ionization loss rates was the principal aeronomic objective of the four rockets. Each rocket carried a 3385‐kHz radio propagation experiment and a Langmuir probe. The probe data were calibrated by the radio data to give electron number densities. At the mesopause (82 km) the electron densities were 800, 400, 300, and 100 cm‐3at full sun (2 hours after totality), at 2.5% visibility of the solar disk, at second contact and at third contact, respectively. The full‐sun measurement of electron density at 65 km was 100 cm−3. By the time of second contact, the electron density diminished to less than 10 cm−3at 70 km and below. At the time of third contact, the electron density was less than 10 cm−3at 75 km and below. No significant differences were visible between the electron densities observed at second and third contact and at 2.5% visibility for altitudes between 95 and 100 km. A well‐defined sporadic‐Elayer was

ISSN:0048-6604    

DOI:10.1029/RS004i004p00371

年代:1969

数据来源: WILEY

摘要:

A preliminary analysis suggests that the VLF signal strength minima (in decibels) observed during transition fading can be described by the First Asymptotic Distribution of the statistical theory of extreme values. In addition, the distribution of outage time relative to a fixed threshold level during transition fading seems to follow the Third Asymptotic Distribution.

ISSN:0048-6604    

DOI:10.1029/RS004i004p00377

年代:1969

数据来源: WILEY

摘要:

The distribution of the instantaneous frequency and power of the signal associated with a given Doppler spectrum is investigated for two cases: (case A) where the power at any frequency of the Doppler spectrum is exponentially distributed and (case B) where the power at each frequency is determinate. It is found that in case A the joint distribution of the instantaneous power and frequency depends only on (1) the total power, (2) the mean, and (3) the variance of the Doppler power spectrum. This result holds to a first approximation for case B also. The distributions of (1) instantaneous power, (2) instantaneous frequency, (3) average power as a function of instantaneous frequency, (4) average frequency as a function of instantaneous power, (5) variance of frequency as a function of instantaneous power, and (6) variance of power as a function of instantaneous frequency are also obtained. In particular, it is shown that the mean of the instantaneous frequency is equal to the mean power‐weighted frequency of the Doppler spectrum in case A. In case B, this result is only approximately vali

ISSN:0048-6604    

DOI:10.1029/RS004i004p00381

年代:1969

数据来源: WILEY