VHF Signals Research Articles

A comparative study of VHF scintillation and spread F events over Natal and Fortaleza in Brazil

A comparative study is carried out of the equatorial ionospheric irregularity occurrences, as observed by range spread F events over Fortaleza (4°S, 38°W, dip latitude 1.8°) and satellite VHF signal scintillation event over Natal (5.8°S, 33.23°W, dip latitude 4.8°S) for a one‐year period 1978. Close association is observed in the occurrences and the durations of the irregularities at the two stations. Marked differences in the seasonal behavior of the occurrences of the irregularity events over these two stations are noted with respect to that over Huancayo, most important of them being that while the latter station presents equinoctial maxima in the occurrence rates (as over some African stations), the Brazilian stations show an additional and pronounced peak in December. This result is explained as possible evidence of magnetic declination control of the spread F irregularity generation and its seasonal dependence. The present results have important implications also on the coexistence of different irregularity scale sizes during equatorial plasma bubble irregularity events.

Population exposure to VHF and UHF broadcast radiation in the United States

The U.S. Environmental Protection Agency has been collecting broadcast signal field intensity data for over 2 years to estimate population exposure to this form of nonionizing radiation. Measurement data have been obtained at 373 locations distributed throughout 12 large cities and collectively represent approximately 11,000 measurements of VHF and UHF signal field intensities. The VHF and UHF broadcast service is the main source of ambient radio frequency exposure in the United States. A computer algorithm has been developed which uses these measurement data to estimate the broadcast exposure at some 39,000 census enumeration districts within the metropolitan boundaries of these 12 cities. The results of the computations provide information on the fraction of the population that is potentially exposed to various intensities of radio frequency radiation. Special emphasis has been placed on determining the uncertainty inherent to the exposure estimation procedure, and details are provided on these techniques. A median exposure level (half of the population is exposed to greater than the median level) of 0.005‐μ W/cm2 time‐averaged power density has been determined for the population of the 12 cities studied, the cumulative population of which represents 18% of the total United States population. The data also suggest that approximately 1% of the population studied, or about 380,000, is potentially exposed to levels greater than 1 μ W/cm2, the suggested safety guide for the population in the USSR. Alternative techniques of using the measurement data to estimate population exposure are examined, and future extensions of this work are discussed.

Spatial and temporal distributions of midlatitude ionospheric scintillations observed by low-altitude satellites

Spatial and temporal distributions of ionospheric scintillations have been observed at Kashima (36.0°N, 140.7°E) using VHF and UHF signals from low-altitude satellites. From these observations, three different types of prevailing ionospheric scintillations seen from Japan are identified. Scintillations of type I are rather weak scintillations, occur most frequently during the daytime in summer and are primarily associated with the sporadic E-layer. However, considerable occurrences of type I scintillations are also observed during the night in summer and autumn, not necessarily due to the sporadic E-layer but occasionally due to F-layer irregularities which originate from localized midlatitude processes. Type II scintillations are much stronger than type I and occur near the equatorward horizon during spring, summer and autumn. Their occurrences start after sunset, reach a maximum before midnight and decrease subsequently, with a tendency for negative and positive correlations with the magnetic and solar activities, respectively. It is concluded that type II scintillations are the midlatitude aftermath of equatorial plume-associated irregularities and cause trans-equatorial propagation of VHF waves. From observations of type I and II scintillations, the boundary between midlatitude and equatorial scintillations is clearly identified. Type III scintillations are as strong as type II and appear only during magnetically active periods. They can be regarded as another aspect of the severe scintillation events observed on gigahertz waves from geostationary satellites as reported by Tanaka (1981).

Short radio-wave propagation in magnetospheric channels

We consider the problem of a beam of radio waves propagating in a magnetospheric chanfiel. We establish a relationship between the parameters of the magnetospheric channel and the range of frequencies which it transmits. We show that at low geomagnetic latitudes (~o ~< 30~ it is possible for the magnetosphere to act as a waveguide for the propagation of sw and vhf signals.

UHF television and the signal strength handicap

The disparate properties of the UHF and VHF bands have produced what has become known as the "UHF handicap" - UHF television (TV) signals are more difficult to receive than VHF signals and therefore are not as significantly viewed. There are at least three possible types of handicaps that contribute to the disadvantage faced by UHF. First, there is the picture quality handicap. Second, there is a channel selector handicap. Finally, there is a programming handicap. In this paper we will be discussing only the picture quality handicap. Furthermore, we will explore only one type of picture quality handicap-the "snow" that is produced by an insufficient signal-to-noise ratio (SNR) at the TV receiver picture tube. Available evidence suggests that this is the dominant difference between UHF and VHF signals. We will utilize a model that indicates the level of this signal strength handicap for several sets of assumptions, and determine the effects of improvements to the UHF service.

Ionospheric electron content observations during the total solar eclipse of February 16, 1980

Observations on the Faraday rotation of a transionospheric VHF signal obtained from a network of four stations near the path of totality during the total solar eclipse of 16 February 1980 are reported. A small decrease of 3–4% in the total ionization has been obtained around the time of totality. Absence of any periodic structure following the eclipse indicates that the TIDs are not of significant amplitude in the present case to be detected by the Faraday rotation technique.

Scattering of VHF and UHF radar signals from the turbulent air

In the derivation of Booker‐Gordon scattering cross sections in a turbulent atmosphere, certain assumptions have been made so that the power scattered within the scattering volume will add coherently. This results in a scattered power that is proportional to the effective scattering volume. As the scattering volume increases without bound, it leads to a physically unreasonable result of infinite scattered power. In this paper we examine these assumptions. In particular, we study the scattering cross sections, taking into account the possible phase incoherence of the scattered fields within the scattering volume. In addition, the effect of a thin turbulent layer on the complex autocorrelation function is investigated. The turbulence is assumed to be locally frozen. Expressions for both the Doppler shift and the correlation time have been derived.

Population exposure to VHF and UHF broadcast radiation in the united states

The U.S. Environmental Protection Agency (EPA) has been collecting broadcast signal field intensity data for over three years to estimate population exposure to this form of nonionizing radiation. Measurement data have been obtained at 486 locations distributed throughout 15 large cities and collectively represent approximately 14 000 measurements of VHF and UHF signal field intensities. The VHF and UHF broadcast service is the main source of ambient radio-frequency (RF) exposure in the United States. A computer algorithm has been developed which uses these measurement data to estimate the broadcast exposure at some 47 000 census enumeration districts within the metropolitan boundaries of these 15 cities. The results of computations provide information on the fraction of the population that is potentially exposed to various intensities of RF radiation. Special emphasis has been placed on determining the uncertainty inherent to the exposure estimation procedure and details are provided on these techniques. A median exposure level (that level to which half of the population is exposed greater than) of 0.005 µW/cm2time averaged power density has been determined for the population of the 15 cities studied, the cumulative population of which represents 20 percent of the total U.S. population. The data also suggest that approximately 1 percent of the population studied, or about 441 000, are potentially exposed to levels greater than 1 µW/cm2, the suggested safety guide for the population in the USSR. Alternative techniques of using the measurement data to estimate population exposure are examined and future extensions of this work are discussed.

The east-west asymmetry of the ionospheric electron content in south Atlantic geomagnetic anomaly region

The Ionospheric Electron Content (IEC) in the South Atlantic Geomagnetic Anomaly (SAGA) has been investigated by means of the Faraday rotation technique with the use of the VHF signal emitted from the ATS-6 radio beacon satellite during the east-west movement. The measurements were carried out at Atibaia, (46°33′W, 23°11′S) and show an increase of ~50% on the west side of the receiver in relation to the east side, principally during the night period. We suggest that this asymmetry is caused by low energy electrons ( E < 40 keV) precipitated during their longitudinal drift.

Gravity wave observations at a southern hemisphere mid-latitude station using the total electron content technique

The work reported here is based on the determination of the horizontal phase trace velocity of travelling ionospheric disturbances (TID's), by using observations recorded from December 1975 to October 1976 of the Faraday rotation of a VHF signal recorded on a spaced array of three receivers located at 37°S, 144°E. The speeds, azimuths, periods and times of occurrence of these disturbances were consistent with the observation of medium scale F-region TID's. Large numbers of daytime disturbances were seen moving zonally, suggesting the influence of the prevailing background winds. Directional filtering of TIDs caused by winds, which has been reported in the northern hemisphere, was observed for the southern hemisphere. As expected for southern hemispheric observations, the observed swing was anti-clockwise. This effect was maintained most obviously during the night, and was in evidence for individual seasons. TID's percentage and absolute amplitude were highly correlated with the disturbance period but had no significant relationship with either jetstream proximity or magnetic activity. Magnetic activity was however, positively correlated with TID speed. The wave periods observed tended to decrease with increasing distance from the jetstream. Disturbances observed when ƒoF2 was above its mean had larger amplitudes than TID's recorded under low ƒoF2 conditions.

Measured VHF and UHF Signal Strength and Spectrum Occupancy Versus Antenna Height

Results are presented of an antenna height-gain experiment performed by the FCC Spectrum Management Task Force in the Chicago Region at four different sites. Two bands, VHF and UHF, are analyzed for their signal strength and occupancy variation versus antenna height. These results are normalized to the system performance with a 12-ft-high antenna for comparison purposes. The normalized signal is compared with the signal predicted by the FCC propagation model. The results of the experiment indicate a reasonable agreement with the propagation-model. The results of the occupancy variation indicate that there is a definite increase of occupancy with height for VHF, and very slight increase of occupancy for UHF.

Ray Tracing Study of Effects of Ionospheric Irregularities on HF and VHF Radio Waves

A three-dimensional ray tracing programme is used to study the effects of ionospheric irregularities on radio waves in the HF and VHF range. The electron density model used is the alpha-Chapman electron density profile. A toroidal East-West irregularity with a 150 Km by 50 Km elliptical cross section, tilted 15° with respect to the horizontal, and having a peak electron density 10% higher than the background level is taken at various heights in the F-region. The following effects are observed for low latitude north-south propagation, (a) Ray paths are significantly affected by the irregularity, the strength of the effect depending upon where the ray meets the irregularity; the phase path and ground range can change by 100 Km at 20 MHz, (b) the effect is less if the irregularity is encountered on the return path, (c) there is a focussing effect of the irregularity which may enhance the signal in certain regions, and (d) the effect on VHF signals from satellites is small but significant for such applications as interferometric position determination and radio astronomy.

Comparison of ionospheric gravity wave periods as measured by different experimental techniques

A comparison is made of ionospheric gravity wave periods as measured by several different experimental techniques. These include high frequency CW doppler, vertical incidence ionosondes, Faraday rotation measurements of total electron content and refraction of VHF signals from a geostationary satellite. These observations are found to yield an increase in power at the longer periods as the height increases, consistent with gravity wave theory.

A sudden vanishing of the ionosphericFregion due to the launch of Skylab

Routine Faraday rotation observations of the VHF signal from the geostationary satellite ATS 3 made at Sagamore Hill (Massachusetts) revealed that an unusually large and rapid decay in the ionospheric total electron content (TEC) occurred near 1240 EST on May 14, 1973. The disturbance appeared as a dramatic ‘bite-out’ of substantial magnitude (≥50%) and duration (of the order of hours) in the expected diurnal TEC curve for that day. Observations from other sites revealed that a ‘hole’ in the ionospheric F region was created over a region approximately 1000 km in radius. The onset of the TEC disturbance occurred within 10 min of the launch of NASA's Skylab workshop by a Saturn 5 rocket. As the rocket moved at F region heights, the burning second-stage engines passed within 150 km of the Sagamore Hill ray path to ATS 3. A detailed analysis of the aeronomic reactions initiated by the constituents of the exhaust field revealed that the F2 region plasma experienced a devastating loss process as the plume expanded. The specific mechanism involved was the rapid ion-atom interchange reactions between the ionospheric O+ and the hydrogen and water vapor molecules in the plume, followed by dissociative recombination of the molecular ions. Model calculations of the diffusion of the plume in the ionosphere and its effect upon continuity equation calculations for TEC showed an excellent agreement with the observed onset and magnitude of the effect. The phenomenon has interesting astrophysical and geophysical implications.

Satellite transmission studies of spread‐F produced by artificial heating of the ionosphere

Experiments are described to study the morphology of artificial spread‐F (ASF) irregularities by measuring the scintillation of VHF and UHF signals transmitted through the disturbed region from geostationary and orbital satellites. The ASF irregularities are found to be approximately field aligned with correlation distances of about 100 m transverse to the field direction; a smaller structure less than 10 m is also seen. The fluctuations seem to be distributed through the entire F layer, and are less than one percent of the ambient electron density at most times.

Radar cross section measurements and vertical incidence effects observed with Platteville at reduced power

During the past four years, experiments using high power, high frequency transmitters have demonstrated that significant modification of the F region of the ionosphere can be effected. This paper illustrates that relatively low power, creating power flux densities in the F region of the order of 1 μw m−2, can also produce significant modification in the ionosphere. Such low power illuminations create field‐aligned ionization irregularities that produce a significant scattering cross section in the modified ionosphere. It permits propagation of VHF signals over selected long distance paths that could not be used by natural means.

Nighttime fluctuations (scintillations) in Faraday rotation angle of VHF signals from geostationary satellites

Faraday rotation angle records of VHP (137.35 MHz) signals from ATS-3 received at São José dos Campos (Magnetic dip-23.7°) exhibit nighttime fluctuations during equinoctial and summer months. These fluctuations have periodicities ranging from several seconds to about an hour. We investigate the relationship between these fluctuations and the F-layer irregularities.

Dependency of scintillation fading of oppositely polarized VHF signals

Under the effects of ionospheric scintillations, the fading of RH and LH circularly polarized 136 MHz signals is observed to be perfectly correlated. Polarization diversity reception will not improve the reliability of VHF space communications through the turbulent ionosphere.

Day-to-day changes in the F-region in summer

Continuous Faraday rotation measurements of VHF signals from the synchronous satellite Early Bird were made from a Northern mid-latitude station during two months in the summer of 1965. The experimental technique and method of calculating the total electron content ( n t ) are described, and the main types of variation of n t during the daytime are classified. Using a continuous 42-day time series of measurements, cross correlation functions of several parameters with magnetic activity indices are calculated. A significant positive correlation at zero time shift is found between noon n t and K p , marginally significant positive correlations between n t at 20 hr and K p , between the time displacement from sunset of the diurnal peak in n t and ∑ 24 K p , And between noon slab thickness and K p , are also found. The apparent linear loss coefficient at midnight shows no significant correlation either with K p or with noon n t on the following day. The integrated electron production rate at dawn shows some long term correlation with both the decimetric solar flux and the sunspot number. The correlation between noon n t and K p can be explained as a natural consequence of shifts in the time of occurrence of the evening peak on moderately disturbed days. This behaviour is not observed in the maximum electron density, as it is probably masked by increases in slab thickness. Horizontal transport of plasma in the upper F-region must be occurring, this could be driven either by the neutral air wind or by electric fields.

Ionospheric Storms at Midlatitudes

The total electron content of the ionosphere often responds in a dramatic way to increases in geomagnetic activity. By monitoring the VHF signals from the geostationary satellite ATS 3, it has been possible to study in detail the very pronounced increases in total content often found during the afternoon hours on the day of the commencement of a magnetic storm. Comparisons with magnetic field data show that the enhancements in electron content coincide with increases in the total magnetic field. This simultaneity suggests that, when the magnetosphere is compressed during the initial phase of a storm, the ionization stored in the magnetic tubes of force may be dumped into the topside of the F region. Such a depletion of the protonosphere is in agreement with whistler measurements, which indicate that a contraction of the plasmasphere occurs during periods of increased magnetic activity.