jjoulder laboratories

PB161634

*#©. 133

HISTORICAL SURVEY OF FADING

AT

MEDIUM AND HIGH RADIO FREQUENCIES

BY

*-

ROGER &. SALAIS4AN

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
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NATIONAL BUREAU OF STANDARDS

technical ^iote

No. 133

January 1962

HISTORICAL SURVEY

OF

FADING AT MEDIUM AND HIGH RADIO FREQUENCIES

by

K

Roger 45. Salaman

K

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-1-

HISTORICAL SURVEY OF FADING AT MEDIUM

AND HIGH RADIO FREQUENCIES

by

Roger K. Salaman

1. INTRODUCTION

This condensed historical survey contains information on

many of the articles concerned with HF and MF ionospheric fading,

which have appeared in the literature through 1960. The primary

emphasis is on an oblique incidence propagation, although many

articles pertaining to fading at vertical and near vertical incidence

(incorporating winds experiments) are also included. No effort was

made to include the fading and scintillation studies in the literature

of radio astronomy and satellite propagation, where they pertain to

determining the characteristics of the ionosphere, and not to MF and

HF communication.

Information is available on the origin of fading, the approximate dependence of fading rate on distance and frequency, and the

amplitude distributions for particular transmission paths. This

information is, however, not sufficient either for a realistic estimate

of the performance of communication systems or for signal design

consistent with the medium statistics.

With respect to communication systems at MF and HF, information which is needed for analysis and design includes statistics on the

amplitude distribution and the fade rate, depth, and duration. Such

information should be obtained as a function of propagation mode,

frequency relative to the predictable MUF, time, season, geographic

location, and sunspot cycle.
-2-

2. SURVEY

An early discussion of selective fading resulting from ground

sky wave interference over an MF oblique incidence path is given by

De Forest [1913]. He suggested that a change in height of an ionized

reflecting layer may cause interference fading.

An attempt was made by Dellinger, Whittemore, and Kruse

[ 1923] to correlate fading with meteorological data. They concluded

that fading is caused by variations of absorption in the ionosphere.

Pickard [ 1924] noted the variation in fading from day to night.

His experimentation at MF showed that the fading is essentially

uncorrelated for receiver separations of about 600 meters. Many

field strength records from quiet to highly disturbed conditions are

illustrated. He hypothesized that fading is caused by changing

absorption and multipath interference, and suggested an influence of

the earth's magnetic field.

Appleton and Barnett [1925a] also indicated that fading at MF

may be due to interference between ground and ionospheric rays.

They performed a frequency change experiment and concluded the

reflecting layer to be at about 80 km.

Eckersley [ 1925] suggested that more than two rays (i.e.

multiple ionosphere-surface reflections) must be considered.

The existence of the ionosphere was verified by Appleton and

Barnett [ 1925b] who showed that the components causing fading were

in a vertical plane. They also found that during a solar eclipse., fading

took on a nighttime character. They suggested magneto-ionic effects

in the ionosphere [ 1925c] .

Because of the difference in fading during the day and night,

Appleton and Barnett [ 1926] concluded that fading was caused by

variability of the rays deviated by the upper atmosphere, and not by

a change in the frequency of the transmitter. Their results indicated

a diurnal change in layer height.

Hulburt [ 1926] attributed fast fading to mode interference and

slow fading to cloud motion.
-3-

Appleton and Ratcliffe [1927] observed rapid fluctuations in

the angle of incidence of the down-coming wave, and suspected reflection at different points in the layer. They found that for wavelengths of about 400 meters and distances of about 130 km, the fading

was primarily due to intensity changes of the down-coming wave, and

not interference of the ground and sky waves or changes in angle of

incidence or polarization.

The relation of the earth's magnetic field to fading was again

suggested when Breit [ 1927] found that a change of 15^ in the field

would cause interference fading of the magneto- ionic components.

Appleton [1928] summarized his conclusions concerning

fading. He indicated that fading at frequencies around 1 Mc/s was

due to variations of rays deviated by the ionosphere. Most frequency,

fading could be attributed to the change in intensity of the down- coming

wave rather than the phase difference between the ground and sky waves.

With a background of HF experiments, Eckersley [1929]

described multipath interference and magneto-ionic polarization

fading. He also related magnetic storms to increased fade rates.

Hafstad and Tuve [1929] examined multipath interference by

determining the change in phase of a pulsed HF signal at vertical

incidence. They found that phase of the second and third echoes varied

more rapidly than the first echo, but the phase changes were not

multiples of each other.

The cause of fading was further studied by Ratcliffe and

Pawsey [ 1933] who found that for wavelengths between 200 and 500

meters, and distances less than 200 km, fading of the ionospheric waves

was uncorrelated for receiver separation of about one wavelength.

They, therefore, concluded that there was considerable lateral

deviation, and suggested fading was caused by interference at the

ground of waves scattered by diffraction in the ionosphere over a

radius of at least 20 km.

Evidence of rapid fading caused by interference of several

waves rather than change of absorption was available from the

polarization measurements of Ratcliffe and White [ 1933] .
-4-

Appleton [ 1934] suggested a frequency change experiment to

deduce the equivalent magneto-ionic paths from interference fading.

By use of a lateral deviation experiment Pawsey [ 193 5]

concluded that an important cause of fading is horizontal movements

of ion clouds (winds) in the E region.

Colwell and Friend [ 1937] suggested that very rapid fading of

MF signals may be due to barometric changes in a C region at a height

of 1 to 12 km.

Further evidence of meteorological cause of fading was given

by Nasilov [ 1939] who suggested that fading was correlated with the

precipitation zone.

Khastgir and Ray [ 1940] showed that fading at a wavelength

of 370.4 meters propagated over

1 about 250 km follows a Rayleigh

distribution. They, therefore, gave further evidence that the fading

is caused by interference of waves scattered from diffraction centers

in the ionosphere.

Interference fading between the Pedersen (high) and low rays

near the MUF was discussed by Ladygin [ 1940] .

A brief history of fading up to 1945 was given by Green [ 1946] .

Fading at HF was related to weather forecasting by Gherzi

[1946] .

Appleton and Beynon [ 1947] presented an illustration and

explanation of magneto-ionic and high-low ray fading near the MUF.

Kelso [ 1948] showed that Appleton and Beynon' s relation of

phase path fading to group path was correct.

Ratcliffe [1948] suggested that since single component fading

was apparently uncorrelated at separations greater than a wavelength,

the roughness of the ionosphere may vary in a random manner. He

related ionospheric fading to noise passed through a band-pass filter,

and concluded that since the fading was Rayleigh, the velocity should

be Gaussian distributed. He also suggested that the scattering centers

may not all be the same, and therefore, the velocity distribution may

vary from Gaussian.
-5-

The fact that information was needed about oblique incidence

fading where the characteristics departed from Rayleigh was emphasized by Banerjee and Mukerjee [ 1948] . They attributed their periodic

fading results to multiple reflection interference and vertical movements of the ionosphere.

The cause of periodic fading was again concluded to be vertical

motion of the layer after a 678.4 km oblique incidence experiment was

performed by Banerjee and Singh [ 1948] .

Munro [ 1948] briefly described a three-spaced receiver method

of determining horizontal movements in the ionosphere by examining

correlated fading records.

Mitra [ 1949a] also used the three-spaced receiver method to

study horizontal movements by fading correlation.

After a statistical analysis of fading, Mitra [ 1949b] concluded

that fading was caused by the random movement of irregularities in the

ionosphere which, to a first approximation, have a Gaussian velocity

distribution. His evidence suggested that the irregularities responsible

for fading are situated below the point of reflection of the E region.

A study of fading at MF and HF up to 1500 km was made by

McNicol [ 1949] . He concluded that fading could be explained by the

addition of a specular and a random component, and the amplitude

distribution varied from Rayleigh to Gaussian.

Rao and Somayajulu [ 1949] observed flutter fading in India on

the 41 and 60 meter bands during the summer sunset period. The

fading rate varied from 20 to 30 cycles per minute before sunset to

120 to 140 cycles per minute at night.

The problem of ionospheric fading was examined by Booker,

Ratcliff, and Shinn [1949-50] who considered the amplitude autocorrelation function of a wave diffracted from an irregular screen.

They applied these results to an ionosphere with irregularities which

had random velocities, and to one which moved with a steady velocity.

In the first case, at vertical incidence, the fading time (i.e. when the

auto- correlation function fell to 1/e) was directly proportional to wavelength, and inversely proportional to velocity. For oblique incidence,
-6-

the fading time was proportional to wavelength multiplied by the

secant of the angle of incidence, and inversely proportional to

velocity. Fur distances greater than about four times the reflection

height the fading was found to be proportional to the transmission

distance.

Briggs and Phillips [ 1950] indicated that at vertical incidence

the spread of the received signal which caused fading was about 5" for

the E and F layers. Briggs, Phillips, and Shinn [ 1950] also suggested a method of determining the drift velocity of the diffraction pattern,

the rate of change of the pattern, and the size of the irregularities by

examining fading using the three-spaced receiver method.

Galactic radio measurements of Smith, Little, and Lovell

[ 1950] indicated that fading could be caused by irregularities in the

F region.

Khastgir and Das [ 1950] showed that the periodic fading over

a 240 km, 4.84 Mc/s circuit was caused by magneto-ionic interference,

beating of the high and low rays, and Doppler variations due to the

vertical motion of the ionosphere. The frequency used was much lower

than the F-layer MUF, and slightly greater than the ordinary wave Elayer MUF, but lower than the E-layer extraordinary wave MUF.

Rapid periodic fading was attributed to interference of the high and

low extraordinary E-layer waves. A slower periodic fading was

attributed to Doppler variations due to vertical movements of the

layers when the singly and doubly reflected F2 waves, or singly

reflected E and F2 interfered.

Allcock [ 19 50] contended that the fading distribution of a

19. 15 Mc/s signal propagated over a 788 km path fit a long normal

distribution better than a Rayleigh distribution.

The variation with frequency of the amplitude of the echo from

a sweep frequency pulse transmitter is illustrated by Briggs [ 1951]

for F and E-layer propagation at near vertical incidence. The decrease

in amplitude as foF2 is approached, movements in the layers, and

magneto-ionic fading are noted.

Salzburg and Greenstone [1951] used three-spaced receiver

pulse fading to determine horizontal ionospheric movements.
-7-

The character of fading at frequencies well below the MUF

(one and multiple hop transmission), at frequencies between the

ordinary and extraordinary ray MUF's, and magneto-ionic fading

were illustrated by Srivastava and Rajan [1951] for HF signals.

Tantry and Khastgir [ 19 51] examined periodic and random

fading over 120 to 1145 km MF paths. The periodic fading was

attributed to magneto-ionic interference, high and low ray interference,

and E and F-layer simultaneous reflections with vertical motions of

the layer. The random fading distribution did not follow Rayleigh's

formula.

In a study of sporadic E fading, Banerji [ 19 51] found a

regular reflection component, and a scattered component. He

indicated that if the steady component was less than 70% of the r . m. s. scatter signal, the Rice amplitude distribution reduced to a Rayleigh

distribution. A Gaussian distribution resulted if the steady signal

was greater than twice the scattered signal.

Gerson [ 1951] attributed auroral flutter fading to either rapid

variations in density of incoming particles, or variation of absorption

produced by the particles.

Glaser and Van Wambeck [ 19 51] investigated fading under

diversity reception and determined the improvement for dual diversity

reception. The performance of the diversity receiving systems was

further analyzed by Van Wambeck and Ross [1951] .

Horizontal ionospheric movements were studied by Jones,

Millman, and Nertney [ 19 52] who found a good correlation between

amplitude and phase variations during the daytime.

Bowles [ 1952] found very rapid fading on 50 Mc/s signals

reflected from the aurora.

Banerji [ 19 53] stated that a Gaussian distribution of the velocity

of scattering centers could result in a non-Gaussian power spectrum.

He computed the power spectrum for steady drifting irregularities and

found it gave an oscillatory correlation function similar to that observed

by McNicol rather than the function given by Booker, Ratcliffe, and

Shinn

.
-8-

Chatterjee [ 1953] concluded, after studying sporadic E reflections/t hat the amplitude distribution followed the Rice distributions

and varied diurnally.

A brief discussion of scatter, short-period, and long-period

focus fading is given by Piggott [1953] . Chapman [1953] studied fading at near vertical incidence using

three spaced receivers. He concluded that the drift of the diffraction

pattern on the ground was consistent with the assumption of a drifting

ionosphere

.

Both the spaced receiver method and Doppler frequency shift

meteor reflections were discussed by Manning [1953] in a study of

ionospheric winds.

Somayajulu, Rao, and Rao [1953] noted fading of an HF oblique

incidence signal due to travelling disturbances.

Singh [ 1954] suggested that the nature of the fading over an

oblique incidence HF path could be at least partially explained by

variations in electron density.

Minozuma and Enomoto [ 19 54] found they were able to predict

fading rates by employing a factor of ionospheric turbulence.

Rapid fade rates of 50 to 200 cycles per second were noted by

Gerson [ 1954] on an auroral reflected signal. He suggested the cause

was Doppler shifts in reflections from incoming particles.

Ratcliffe [1954] and Newstead [1954] discussed the use of

fading records from three spaced receivers to determine ionospheric

winds.

The possibility of using the auto-correlogram of fading at one

point instead of the spaced receiver method of studying ionospheric

winds was suggested by Banerji [ 1955] .

Yerg [ 1955] indicated a method of determining correlation

from the fading records by obtaining six values of the correlation

coefficient. He was then able to deduce characteristics of ionospheric

winds.
-9-

Methods of using fading records from spaced receivers to

determine ionospheric winds were described by Court [ 1955] .

By consideration of the auto-correlation function, Booker

[ 1955] determined that fading at vertical incidence is imposed upon

the waves near the point of reflection.

Equipment for recording time between fades was described by

Phillips [1955] . A brief description of the effect of fading on communication

systems was given by Laver [1955] .

Meadows' [ 19 56] experimental results over a 5. 1 Mc/s 40 km

path indicated non-reciprocity of fading about one percent of the time.

A comparison of fading characteristics from VLF through HF

was given by Bowhill [19 56] .

Yerg [ 1956] , Chappell and Henderson [ 1956] , Barber [ 1956] , and Rao and Murty [1956] , continued to analyze ionospheric movements by use of fading records at spaced receivers.

A method of comparing phase and amplitude of an oblique

incidence signal was given by Price and Green [1957] .

Fading over long distances, and the use of diversity reception,

was discussed by Grisdale, Morris, and Palmer [1957] . They found

that the rapid fading component agreed closely with the Rayleigh

distribution. It was also noted that polarization diversity compared

favorably with space diversity.

Jacobs [1957] suggested that transequatorial flutter fading was

associated with a scatter mechanism.

Bowhill [ 1957] described the effect of the recorder time constant

on the recorded fading rate.

The performance of HF FSK signals in the presence of selective

fading was given by Allnatt, Jones, and Law [1957] .
-10-

A comparison of amplitude and phase of vertical incidence Elayer reflections was given by Landmark [ 1957] . An increase in

amplitude with rapid fading was associated with large scale phase

irregularities. The increase in amplitude was attributed to movement

of ionospheric irregularities as measured by Jones, Landmark, and

Setty ] 1957] using the spaced receiver method.

The inaccuracy of the three -spaced receiver method was noted

by Rao and Rao [ 19 57] who determined ionospheric winds by using

the fading records from four spaced receivers.

A study of the correlation of amplitude and bearing of fading

waves was made by Whale and Delves [19 58] .

King [ 1958] discussed the fading of waves from VLF to HF

which were reflected at oblique incidence. He found that, in general,

the fading rate was proportional to frequency and to the cosine of the

angle of incidence on the ionosphere. It was concluded that at MF,

the diffraction pattern on the ground was roughly circular, and

irregularities which caused fading were larger in the horizontal than

the vertical plane.

Reciprocity of pulse amplitude on a 168 5 km HF path was

investigated by Balser, Smith, and Warren [19 58] . They found that

although average conditions over the path were the same, the fine

detail amplitude fluctuations did not exactly correspond.

A summary of the methods of determining ionospheric winds

from fading recordings at spaced receivers is given by Banerji [ 19 58]

A further study of winds using fading recordings was conducted by

Harnischmacher and Rawer [ 1958] . Rao and Rao [ 1958] used the

spaced receiver method to determine fading rate and drift velocity in

the E region. They concluded that the random fading is primarily due

to horizontal drifts, and that random variations are less important.

The accuracy of determining field strength when fading is

present was discussed by Meadows and Moorat [ 19 58]. It was concluded that the standard deviation of amplitude variation due to rapid

fading was greater at vertical than at oblique incidence.
-11-

Stein [ 1958] suggested that the severe post-sunset selective

ing on transequatorial paths may be attributable to interference

between the tilt-reflected and multiple-reflected modes. Yeh and

Villard [ 19 58] also reported very high fade rates over equatorial paths

In a crossed-dipole experiment Hedlund and Edwards [ 19 58]

found sinusoidal fading in phase quadrature on the two dipoles, which

they attributed to varying phase difference between the two elliptically

polarized magneto-ionic components of the sky wave.

Aggarwal [ 1959] found that on several frequencies and over

several oblique paths from 640 km to 1140 km the amplitude distributions of the apparent random fading were Rayleigh, Gaussian, or

log normal. He also compared oblique incidence and equivalent

vertical incidence, mid -point amplitude distributions, and found no

correlation. The auto- correlation function showed any periodicity

which was present in the received signal.

Thomas [ 1959] Tolstov [ 1959] , and Yerg [ 1959] , used three

spaced receivers to determine movements in the F region of the

ionosphere. A description of this method is given by Shimazak [19 59] .

Singh and Ram [1959a, 1959b] studied rhythmic fading. They

indicate that slow periodic fading may be due to interference of the

low ordinary and extraordinary waves. Rapid rhythmic fading superimposed on the slower periodic fading was attributed to interference

between the high and low ordinary waves. Fading of the high and low

extraordinary waves in also shown. Fading due to interference of

singly and doubly reflected F2 waves was illustrated.

The fading of a wave reflected at vertical incidence from the

F region was statistically analyzed by Dasgupta and Vij [1960] . It was found that the amplitude distribution is Rayleigh only for rapid

fading. Slow and quasi-periodic fading gave an M-type distribution.

A method of fading spectrum analysis of HF and VHF oblique

incidence waves was given by Watts and Davies [ 1960] .

Singh and Simha [ 1960] determined the constant of proportionality between fading rate and frequency for HF oblique incidence paths.
-12-

Examples of rapid fade rates on wavelengths of 19 to 280.4

meters was reported by Misra [ 1960] .

Yeh and Villard [ 1960] investigated fading over long auroral

paths. They found no diurnal variation in fading speed, except for a

minimum between 1740 and 2300 hours midpoint time which they

attributed to a special kind of propagation mode made possible by

ionospheric tilts. The fading speed increased with magnetic activity

except for the above period. The attenuation increased with magnetic

activity for all hours. High speed fading on a temperate latitude path

was attributed to movement of aurora to the path. Rapid fading on an

equatorial path was suggested to be connected with spread F. The

fading for long paths was found to be essentially Rayleigh distributed.

A correlation between flutter fading HF signals in the equatorial

region, with spread F, magnetic sotrms and the radiation belt was

made by Lol [ 1960] . Humbly [ 1960] discussed the equatorial sunset effect at HF

which causes very rapid fading.

Mitra and Vij [ 1960] , Khastjir and Singh [ 1960] , and Dougherty

[ 1960] described measurements of ionospheric winds by the spaced

receiver method.
-13-

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-14-

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-15-

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-16-

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study of radio wave paths, PIRE 17, 1786-1792 (1929).

Harnishmacher, E., and K. Rawer, Drift observations evaluated by the

method of "Similar Fades", JATP 13, 1-16 (1958).

Hedlund, D. A., and L. C. Edwards, Polarization fading over an

oblique incidence path, IRE AP-6, 21-25 (1958).

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V. S. DEPARTMENT OF COMMERCE

Luther H. Hodges, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

THE NATIONAL BUREAU OF STANDARDS

T!ie scope of activities of the National Bureau of Standards at its major laboratories in Washington, D.C., and

Boulder, Colorado, is suggested in the following listing of the divisions and sections engaged in technical work.

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