national Bureau of Standards

Library, K.W. B14g

MAY 17 1965

^ecUnlcui ^lote

ATTENUATION OF THE GROUND WAVE OF

A LOW FREQUENCY ELECTROMAGNETIC PULSE

I. C. Morgenstern and J. R. Johler

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
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listing of its four Institutes and their organizational units. Institute for Basic Standards. Electricity. Metrology. Heat. Radiation Physics. Mechanics. Applied Mathematics. Atomic Physics. Physical Chemistry. Laboratory Astrophysics.* Radio Standards Laboratory: Radio Standards Physics; Radio Standards Engineering.** Office of Standard Ref- erence Data. Institute for Materials Research. Analytical Chemistry. Polymers. Metallurgy. Inorganic Materials. Reactor Radiations. Cryogenics.** Office of Standard Reference Materials. Central Radio Propagation Laboratory.** Ionosphere Research and Propagation. Troposphere

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NATIONAL BUREAU OF STANDARDS

technical ^ote 310

Issued April 1965

ATTENUATION OF THE GROUND WAVE OF A LOW FREQUENCY

ELECTROMAGNETIC PULSE

J. C. Morgen stern The Mitre Corporation

Bedford, Massachusetts

J. R. Johler

Central Radio Propagation Laboratory

National Bureau of Standards

Boulder, Colorado

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature.

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington D.C. 20402

Price: 30 cents

Contents

Page

Abstract 1

1

.

Introduction 1

2. Theory 2

3. Attenuation of a Peak of the Pulse 2

4. Illustration of Dispersive Effects 3

5. An Approximation to the Attenuation Rate 3

6. Sky -Wave Contamination 3

7. Experimental Verification 4

8. Acknowledgments 4

9. References 5

Figures 6 - 24

in

Attenuation of the Ground Wave of a Low

Frequency Electromagnetic Pulse

J. C. Morgenstern and J. R. Johler

The attenuation of the peak of the ground wave EM

pulse is presented as a function of distance. These attenuation functions are compared to combinations of single frequency attenuation functions and simple attenuation functions

based on inverse distance raised to integral and non-integral

exponents, (l/d) n

.

1. Introduction

A method of describing the propagation of an electromagnetic

pulse as a ground wave has been developed to define the effective propagation time of a pulse. The theory which describes propagation of the

ground wave of a pulse of low radio frequency energy around a finitely

conducting spherical earth has been treated in detail by Johler and

Walters [1959], Johler and Lilley [1961], and has been presented for

both ground and ionospheric wave by Johler [1962, 1963 a, b]. The

validity of the theory of the ground wave, in particular, has been carefully checked experimentally by Johler and Lilley [l96l]. The theory

describes the effects of physical constants and boundaries of the various

media on the propagation of the pulse. These effects manifest themselves in the apparent velocity of propagation and the distortion of the

waveform of the pulse as it is propagated. Since the effective values

ascribed to the physical constants in the theory are frequency dependent

and the pulse contains essentially a continuous frequency spectrum, it

is extremely difficult to evaluate these effects on the basis of tabulated

data such as NBS Circular 573 [johler, Kellar, and Walter, 1956] or

NBS Technical Note 60 [johler, Walters, and Lilley, I960]. Therefore,

the pulse propagation theory has been translated into a computer program to enable further study of the effects of these physical constants.
2. Theory

Johler and Morgenstern [1963] have utilized the theory and

computer methods to predict the effective propagation time of an electromagnetic pulse propagating in the ground wave mode. These mathematical methods simulated theoretically the propagation of a pulse

generated by a nuclear detonation as detected by a wide band receiver,

44 km from ground zero, to distances up to 2414 km (1500 statute miles)

for various values of conductivity.

An effective conductivity can be ascribed to the ground between a source and an observer for use in the theory of propagation (see Johler

and Lilley [1961]). The effective conductivity is not necessarily the

actual conductivity along the geodetic connecting source and observer,

but represents the integrated effect of a large volume of ground on each

side of the geodetic. Indeed, the rigorous calculations of the ground

wave used by Johler and Morgenstern [1963] is an integration throughout

all space.

The outputs of the computer program, which simulated the propagation of the pulse, included the waveform which would be expected at

various distances and effective conductivities. Attenuation rates of the

ground wave can therefore be obtained from these computer data.

3. Attenuation of a Peak of the Pulse

A sensing system generally acts on signals which either exceed a lower limit or which fall between two amplitude limits. Since it is

desirable, particularly in a system designed to detect an electromagnetic pulse, to observe the entire wavefore in the time domain, it is

important that one is able to predict the amplitude of an early peak of

the waveform. The computer data shown in figures 1 through 11 provide

information regarding the attenuation rate of the first peak of an observed

waveform, the complex spectrum of which is illustrated in figure 15.

This waveform has been propagated theoretically as a ground wave pulse.

From these data, curves are drawn (figures 1 through 7) showing the

attenuation rates vs. distance of the first peak of the propagated pulse

in terms of decibels (dB) below the amplitude of the pulse as detected at

44 km. These curves show that effective conductivity has a small effect

on the attenuation rate for the usual conductivities found in practice,

i. e. , a = . 001 to a = 5.

-2-
4. Illustration of Dispersive Effects

The waveforms given by Johler and Morgenstern [1963] show the

dispersive properties of the medium, that is, they show that different

frequency components of the pulse are affected differently by the propagation medium. The phenomenon is further illustrated in figures 8

and 9. NBS Technical Note 60 [johler, Walters, and Lilley, I960] gives

values for attenuation of the ground wave for specific conductivities.

Using these data, composite attenuation curves for a = . 005 and a = . 0005

have been constructed. The frequency component used in the construction of a particular portion of the composite curve was chosen to give

the best fit to the corresponding pulse attenuation curve. The composite

attenuation curves are given as overlays to both the corresponding pulse

attenuation curves in figures 8 and 9. For G = . 0005, it is seen that

the higher frequency components are attenuated more rapidly.

5. An Approximation to the Attenuation Rate

It would be convenient if a simple approximation could be developed

to describe the attenuation of the first peak of the propagated ground

wave pulse. An attempt has been made to evolve such an approximation.

Figure 10 shows a composite curve superimposed on the attenuation

curve for CJ = . 005. This composite curve is based on attenuation rates

proportional to l/dn where d is the distance from the pulse source. A

deviation of the fitted curve from the actual curve less than 1 decibel

has been obtained by letting n = 5/4 to 600 km, n = 3/2 to 1000 km, and

n = 5/2 from 1000 km out. A better fit is obtained by increasing the

number of divisions of the curve. In figure 11, five divisions with corresponding l/dn fits illustrate a closer approximation to the curve.

6. Sky -Wave Contamination

In practice, the detected pulse is made up of the ground wave

component plus a number of ionospheric wave components. The arrival

time of each ionospheric wave depends on the number of hops, altitude

of ionosphere at the reflection point, distance to the source, and several

other secondary factors.

-3-
The next propagation component to arrive after the ground wave,

at the distances under consideration, is the first ionospheric wave hop.

To utilize the attenuation curves given in section 3 for the first peak of

the ground wave, it is necessary to demonstrate that the first ionospheric wave hop does not contaminate the first peak of the ground wave.

The arrival time of the first ionospheric wave hop, parametric in ionospheric height, was given by J. R. Johler [1962] and is shown in

figures 12, 13, and 14.

The time of the peak of the ground wave must be compared to the

arrival time of the first ionospheric wave hop to determine the presence

of first ionospheric wave hop contamination at any distance for any given

ionospheric height. The time of the peak of the ground wave is plotted

on the ionospheric wave delay curves for a = 5, O = . 005, and a = . 0001

(figures 12 through 14). These figures show that ground wave attenuation rates presented earlier are valid to at least the following values of

distance for any ionospheric height.

a d(km)

5 1300

.005 1300

. 0001 800

7. Experimental Verification

The data presented in this paper has been compared with measurements of both man-made sferics and nuclear detonation waveforms

obtained at three distances. The two sets of data were found to be

within the estimated experimental error [Johler and Morgenstern,

1964].

8. Acknowledgments

The authors are indebted to Miss Mary McDonnell of MITRE for

her work with the computer data, Mr. Richard Millman of MITRE for

his review of the paper and work with the experimental data and

Miss Carlene Lilley of NBS for her computer work.

-4-
9. References

Johler, J. R. (Apr. 1962), Propagation of the low-frequency radio signal,

Proc. IRE 50, No. 4, 404 -427

.

Johler, J. R. (Nov. 1963 a), Propagation of a radio pulse, IEEE Trans,

on Ant. and Prop. AP-11 , No. 6, 661-668.

Johler, J. R. (Sept. -Oct. 1963b), Radio pulse propagation by a reflection process at the lower ionosphere, J. Res. NBS 67D, No. 5,

481-499.

Johler, J. R. , W. J. Kellar, and L. C. Walters (June 26, 1956), Phase

of the low radiofrequency ground wave, NBS Circular 573.

Johler, J. R. , and C. M. Lilley (Oct. 1961), Ground conductivity determinations at low radiofrequencies by an analysis of the sferic

signatures of thunderstorms, J. Geophys. Res. 66, 3233-3244.

Johler, J. R. , and J. C. Morgenstern (Aug. 1963), Propagation time of

the electromagnetic pulse, Private communication to Mitre

Corporation.

Johler, J. R. , and J. C. Morgenstern (Aug. 6, 1964) Attenuation of the

ground wave of LF electromagnetic pulse, Private communication

from Mitre Corporation, TM04067/S001.

Johler, J. R. , and L. C. Walters (Jan. 1959), Propagation of a ground

wave pulse around a finitely conducting spherical earth from a damped sinusoidal source current, IRE Trans, on Ant. and Prop.

AP-7 , 1-10.

Johler, J. R. , L. C. Walters, and C. M. Lilley (July I960), Amplitude

and phase of the low- and very low -radiofrequency ground wave,

NBS Technical Note 60.

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