Library, N.W. Bldg

JUL 2 9 1980

neierence uook noi 10 De

taken from the library.

PB161562

< j3oulder laboratories

cjlo. 61

PROCEEDINGS OF THE I960 CONFERENCE

ON THE PROPAGATION OF E. L. F.

RADIO WAVES

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

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maintenance of the national standards of measurement and the provision of means and methods for making measurements consistent with these standards: the determination of physical constants and

properties of materials; the development of methods and instruments for testing materials, devices,

and structures; advisory services to government agencies on scientific and technical problems: in- vention and development of devices to serve special needs of the Government; and the development

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agencies when the work relates to and supplements the basic program of the Bureau or when the Bureau's unique competence is required. The scope of activities is suggested by the listing of

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the Government Printing Office: The Journal of Research, published in four separate sections,

presents complete scientific and technical papers; the Technical News Bulletin presents summary

and preliminary reports on work in progress; and Basic Radio Propagation Predictions provides

data for determining the best frequencies to use for radio communications throughout the world.

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of Documents, Government Printing Office, Washington 25, D.C.
NATIONAL BUREAU OF STANDARDS

technical *vtote

CI

PROCEEDINGS OF THE I960 CONFERENCE ON THE PROPAGATION OF

E. L. F. RADIO WAVES

National Bureau of Standards

Boulder, Colorado

January 26, I960

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

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PROCEEDINGS OF THE I960 CONFERENCE ON THE PROPAGATION OF

E.L.F. RADIO WAVES

Preface

On January 26, I960 a one -day conference on the propagation of

extremely low frequency radio waves was held. Papers based on

some of these oral presentations are being published in the July and

September (I960) issues of the JOURNAL OF RESEARCH of the

National Bureau of Standards, Section D. (Radio Propagation). The

conference was organized by the following:

A.D. Watt (Conference Chairman)

R.C. Kirby (Chief of the Radio Communication and Systems

Division)

J. A. Kemper, J. F. Brockman

Mrs. Winifred M. Mau, M.C Weeg (Local arrangements)

J.R. Wait, Mrs. Eileen Brackett (Technical program and

proceedings)

Mrs. Dorothea Lisle (Stenotypist)

For sake of completeness the titles and abstracts of these papers

and other relevant material are listed in this report. An edited

version of the oral discussions following the papers is also presented

here.

James R. Wait

National Bureau of Standards

Boulder, Colorado

June 6, I960

Contents Page

I. List of Titles and Abstracts 2

II. Discussions in Morning Session 11

III. Discussions in Afternoon Session 16

IV. List of Participants 23
-2-

I. List of Titles and Abstracts

W. L. Anderson and R. K. Moore, Frequency spectra of transient

e.m. pulses in a conducting medium. (To be published in Trans.

Inst. Radio Engineers, AP-8, I960.)

Abstract: The energy density spectra of transient electromagnetic fields generated by a pulsed ideal dipole source in an

infinite conducting medium have been investigated for various

distances from the source. A characteristic frequency cj , corresponding either to the peak of the spectrum or to its halfwidth, is defined and shown to vary inversely as the square of

distance at large distances. The behavior of co with distance is

a measure of the behavior of the pulse energy. Thus at large

distances it appears that the attenuation factor associated with

is independent of r, due to the constancy of the product r n/To . From this point of view, the transient fields do not decrease

exponentially as r, but as inverse powers of r. This should not be construed as meaning the transient

possesses an advantage over C.W. The attenuation for monochromatic components of the pulse is the same as for continuous

waves of the same frequency, and at large distances the energy

put into the high frequency components is wasted.

The phenomenon is illustrated by calculations that have been

carried out for the case of pulses in sea water.

M. Balser and C. A. Wagner, Measurements of the spectrum of

radio noise from 50 to 100 cycles per second, JOURNAL OF

RESEARCH of the National Bureau of Standards-D. Radio

Propagation, Vol. 64D, No. 4, pp. 415-418, July/August I960.

Abstract: Experimental spectra of radio noise in the band of

about 50 to 100 cycles per second have been obtained by means of

digital processing. Due to the long integration times which were

used, the statistical uncertainty in the estimates of power was

reduced to about 3 percent (0.13 decibel). It was hoped in this

way to observe maximums in the spectrum due to excitation of

higher resonant modes of the earth-ionosphere cavity (for the

accuracy of these data, such peaks should be observed if the Q

of the cavity were 10 or greater at these frequencies). No

statistically significant evidence of these cavity effects was

found.
-3-

Wallace H. Campbell, Natural electromagnetic energy in frequencies

below slow tail sferics band, JOURNAL OF RESEARCH of the

National Bureau of Standards -D. Radio Propagation, Vol. 64D,

No. 4, pp. 409-411, July/August I960.

Abstract: The transition of natural signals from sferics slow

tails to geomagnetic micropulsations was observed between 2.0

and 0.2 cycles per second. Micropulsations with periods of 5

to 30 seconds have characteristics which closely relate to solar

terrestrial disturbance phenomena. The low latitude diurnal

amplitude variation has maximums at 0945 and 1000 l.m.t.

Similar groups of oscillations appear in Alaska and California.

Simultaneous pulsation of X 3914 aurora and magnetic field micropulsations has been observed in Alaska.

G.D. Garland and T. F. Webster, Studies of natural electric and

magnetic fields, JOURNAL, OF RESEARCH of the National Bureau

of Standards-D. Radio Propagation, Vol. 64D, No. 4, pp. 405-

408, July/August i960.

Abstract: Simultaneous measurements of short-period natural

electric field variations across western Canada are reported.

From these it is indicated that the effect of the varying depth to

the Precambrian rocks is the dominant factor. Analysis of the

simultaneous magnetic and electric measurements gives a ,. resistivity for the Precambrian basement in excess of 30 x 10

ohm meters.

R. A. Helliwell, Whistler theory of ELF propagation.

Abstract: None.

A. G. Jean and W. L. Taylor, A proposed experiment to determine

ELF propagation characteristics.

Abstract: A plan for establishing an experiment at the Boulder

Laboratories to study the propagation of radio waves at extremely low frequencies (ELF) is described. The technique

consists of receiving the same atmospheric at two or more

stations situated approximately along a great-circle which

passes through the source. The attenuation suffered by the

atmospheric in propagating between two stations can be calculated from the spectra of the waveforms using methods developed at the Boulder Laboratories*. Recently, the analytical

1 W.L. Taylor, L.J. Lange, Some characteristics of VLF propagation using atmospheric waveforms, Recent Advances in AtmoS'

pheric Electricity, Pergamon Press, 1958.
-4-

techniques were extended to include calculations of relative

phase velocities . Recently, ELF recording stations were installed at

Fairbanks, Boulder and Maui. Preliminary results obtained

with this network confirm the feasibility of locating lightning

discharges in certain areas of the Pacific Ocean and of obtaining two station waveform observations of sufficient quality to

warrant performing detailed transient analyses.

G.V. Keller and F.C Frischknecht, Electrical resistivity studies

on the Athabasca Glacier, Alberta, Canada. (To be published

in JOURNAL OF RESEARCH of the National Bureau of StandardsD. Radio Propagation, Vol. 64D, No. 5, September-October

I960 issue.

Abstract: The use of electrical methods for measuring ice

thickness and properties on the Athabasca Glacier, Alberta,

Canada, has been studied by the U.S. Geological Survey. Two

methods for measuring resistivity were tried: one, a conventional resistivity method in which current was introduced galvanically

into the glacier through electrodes, and the other an electromagnetic method in which a wire loop on the ice was used to

induce current flow. Results of the galvanic measurements

showed large variations in the resistivity of the ice; in a surface

layer several tens of feet thick the resistivity is between 0.3

and 1.0 megohm-meters, and under this layer, the resistivity

of the ice is more than 10 megohm-meters. The resistivity of

the surface ice is determined by its water content rather than by

molecular resonance loss. The ice had no effect on the mutual

coupling measurements in the frequency range from 100 to

10,000 cycles per second. As a consequence, the electromagnetic

data could be interpreted simply in terms of ice thickness and

bedrock resistivity.

Ronold W.P. King and Charles W. Harrison, Jr. , Half wave

cylindrical antenna in a dissipative medium: current and

impedance, JOURNAL OF RESEARCH of the National Bureau of

Standards-D. Radio Propagation, Vol. 64D, No. 4, pp. 365-380,

July-August I960.

Abstract: An integral equation for the distribution of current

along a cylindrical antenna in a conducting dielectric is derived.

It is shown that the boundary conditions for an antenna in such a

medium are formally the same as for an antenna in free space.

2

A.G. Jean, W. L. Taylor and J. R. Wait, VLF phase characteristics

deduced from atmospheric waveforms, Journal of Geophysical Research, Vol. 65, No. 3, March I960.
-5-

The equation is solved for the current I and the driving point

impedance Z by means of a technique that achieves sufficientlyhigh accuracy in the leading terms of an iteration procedure so

that the higher-order terms do not need to be evaluated. Moreover, these leading terms cousist only of trigonometric functions

with complex coefficients. The electromagnetic field in the infinite dissipative medium may be computed easily and in closed

form since the current in the antenna is expressed in such simple

terms.

A numerical analysis is made to determine the properties of

an antenna with an electrical length of one -half wavelength in the

medium with conductivity o- and relative dielectric constant e . Universal curves are given of Itye~~ with <r/u>€ e r as the parameter

and of Z \le T with a-/cue e as the variable in the range

= o"/co€ e =0.4. A table of numerical values of the impedance

is given lor media such as an isotropic ionosphere, dry salt,

dry earth, wet earth, and lake water.

R. S. Macmillan, W. V.T. Rusch, andR.M. Golden, A very-low

frequency antenna for investigating the ionosphere with

horizontally polarized radio waves, JOURNAL OF RESEARCH

of the National Bureau of Standards -D. Radio Propagation, Vol.

64D, No. 1, pp. 27-35, January-February I960.

Abstract: The advantages of a horizontal half-wave resonant

antenna for very-low frequency propagation experiments lie in

its relatively simple and inexpensive construction and in its

radiation pattern which is maximum in the vertical direction.

The radiation fields of this type of antenna located at the surface

of a condcuting earth consist of: 1, A horizontally polarized

space-wave field radiated in the perpendicular bisector plane of

the antenna; and 2, a vertically polarized groundwave field

radiated along the axis of the antenna. This vertically polarized

field is zero at right angles to the antenna. These fields have

been experimentally verified.

The use of a 50 -kilocycle horizontal half- wave antenna for

vertical-incidence ionospheric sounding experiments is described.

The radiation pattern of this antenna is well suited for ionospheric

soundings since a receiver located in the groundwave null receives

only the reflected skywave signal.

Ground- resistivity measurements made at a number of

locations in Central and Southern California were correlated with

the geology of the terrain. This correlation showed that the

ground resistivity is highest (a condition necessary for optimum

antenna efficiency) in areas where the underlying rock formations

are relatively unfractured. The amount of annual rainfall and

other climatic conditions have little or no effect on the resistivity.
-6-

Finally, a unique antenna system is presented which employs

resonant loading circuits to convert a section of an existing

power line into a horizontal half-wave very-low-frequency

transmitting antenna.

Elwood Maple, Sub-audio frequency (1 to 50 c/s) geomagnetic

fluctuations at Denver, Colorado.

Abstract: Magnetic-tape recording of three components of the

geomagnetic fluctuations in the 1 to 50 c/s range was begun at

five field stations under IGY Project 3.10. The present progress

report is based principally on six months of data from the Denver

station. At playback, the recorded waveforms are filtered,

rectified, and integrated to yield values of average magnetic -field

strength in six frequency bands. A single value is obtained for

the first fifteen minutes of each hour for each frequency band of

each component. At the middle latitude station of Denver, almost

all of the fluctuations throughout the 1 to 50 c/s range are judged

to be of thunderstorm origin during most of the six-month

interval. The wave-guide mode propagation theory is applicable

down to frequencies of about 10 c/s, the minimum attenuation

being reached at about 20 c/s. Below 10 c/s, the attenuation

again increases; at still lower frequencies, the induction component of the field of the lightning discharge, rather than the

radiation component, becomes the controlling factor. At frequencies below about 4 c/s, there are occasionally fluctuations

which appear to be geomagnetic in the sense that they are related

to the K indices. During winter, fluctuation levels at these frequencies are often near or below the instrument noise level.

Meteor-shower effects in this frequency range are discussed in

a separate paper.

R.K. Moore, Dipole radiation in a conducting half- space. (To be

published in JOURNAL OF RESEARCH of the National Bureau of

Standards-D. Radio Propagation.

Abstract: The problem of communication between a transmitting

and a receiving antenna, submerged in a conducting medium of

sea water is analyzed as a dipole radiating in a semi-infinite

conducting half- space, separated by a plane interface from a

semi-infinite dielectric half-space. The theory is discussed for

electric and magnetic dipole s for both horizontal and vertical

antennas.

After a discussion of electromagnetic waves in a conducting

medium, the Hertzian pot ential is derived for both electric and

magnetic dipole s. The appropriate boundary conditions at the

interface are applied such that the Hertzian pot.entials can be

reduced to the Sommerfeld case in which both the dipole and

point of observation lie in the interface. By investigation of
contours for the Hertzian potential integrals, the case of the

dipole in the conducting half- space can be reduced to that of the

Sommerfeld case of the antenna at the interface. Finally, the

Hertzian potentials are used to determine the electric and

magnetic field components for the four cases of the electric

and magnetic, horizontal and vertical dipoles.

This analysis shows that the main path of communication is

such that the transmitted wave travels perpendicularly to the

interface, attenuating rapidly and exponentially; travels as a vertically polarized wave almost unattenuated along the surface,

part of which refracts back into the conducting medium; and

travels perpendicularly from the surface to the receiver, again

attenuating rapidly and exponentially in the conducting medium.

K.A. Norton, Possible application of the system loss concept at

ELF, JOURNAL OF RESEARCH of the National Bureau of

Standards-D. Radio Propagation, Vol. 64D, No. 4, pp. 413-414,

July-August I960.

Abstract: A brief description is given of the possible application

of the system loss concept at ELF. A method for allowing for

the effect of external noise levels is outlined.

E.T. Pierce, Some ELF phenomena, JOURNAL OF RESEARCH of

the National Bureau of Standards-D. Radio Propagation, "Vol.

64D, No. 4, pp. 383-386, July-August I960.

Abstract: Properties of the electric and magnetic fields in

natural extremely low frequency (ELF phenomena) are briefly

discussed. The ELF fluctuations in the electric field are then

treated from two aspects; these are the electromagnetic changes

associated with atmospherics and the electrostatic variations in

atmospheric electricity. A final section attempts to integrate

the general subject of ELF effects of natural origin.

L. Rawls, A practical underground transmitting antenna.

Abstract: None.

Gurdip S. Saran and Gedalia Held, Field strength measurements in

fresh water. (To be published in JOURNAL OF RESEARCH of

the National Bureau of Standards-D. Radio Propagation, Vol.

64D, No. 5, September-October I960 issue.

Abstract: Experiments were performed to measure field

strength at a frequency of 18. 6 kc/s in fresh water of conductivity

2. 66 x 10 "3 mhos/meter down to depths of 1000 ft using monopole

and loop antennas. The experimental results verify the theoretical values of field strength attenuation with depth for all antennas

and of the ratio of vertical to horizontal field strength for the

monopole antennas.
-8-

Lee R. Tepley, Slow tails from intra-cloud lightning discharges.

Presented at Forty-First Annual Meeting of American Geophysical Union, Washington, D. C. , April 27-30, I960.

Abstract: It has been observed experimentally that a high percentage of slow tails (ELF sferics) are of negative polarity,

whereas signals of positive polarity would be expected from most

cloud-to-ground lightning discharges. It may be hypothesized

that intra-cloud strokes are a prolific source of negative slow

tails. An alternative hypothesis is that a polarity inversion

occurs during propagation. In order to test the latter hypothesis,

the polarity was measured for a large number of VLF groundwave transients and their corresponding slow tails. It was found

that the ground-wave and slow-tail polarities were almost always

the same. Since there is no reason to expect a polarity reversal

of the VLF ground wave, the latter hypothesis may be discarded.

(However, it is still conceivable that a reversal in slow-tail

polarity may occur if the wave propagates over a great distance.

This possibility cannot affect the present results since the sferics

considered here all originated at relatively short distances from

the receiver, as is evident from the appearance of the VLF ground

wave as a separate entity.) The wave form of the VLF sferic

associated with the negative slow tail supplies some evidence in

favor of the former hypothesis, since it is generally more ragged

in appearance than the VLF wave form associated with the positive slow tail. This implies that the spectral energy maximum

occurs at a relatively higher frequency as may be expected for

radiation from an intra-cloud discharge. However, the difference

in degree of raggedness for the two classes of VLF sferics is

not great and their peak amplitudes are comparable. Hence, it

appears that relatively more VLF energy is associated with

intra-cloud discharges than was previously suspected to be the

case. However, this last result must be considered as tentative

since the peak amplitude and appearance of the waveform may

lead to an incorrect estimate of both the energy distribution and

the total energy in the sferic.

J.R. Wait, Mode theory and the propagation of E. L. F. radio waves,

JOURNAL OF RESEARCH of the National Bureau of Standards -D.

Radio Propagation, Vol. 64D, No. 4, pp. 387-404, July-August

I960.

Abstract: The mode theory of propagation of electromagnetic

waves at extremely low frequencies (1.0 to 3000 c/s) is treated

in this paper. Starting with the representation of the field as a sum of modes, approximate formulas are presented for the

attenuation and phase constants. Certain alternate representations of the individual modes are mentioned. These are used

as a basis for describing the physical behavior of the field at
-9-

large distances from the source, particularly near the antipode

of the source. At the shorter distances, where the range is

comparable to the wavelength, the spherical-earth mode series

is best transformed to a series involving cylindrical wave

functions. This latter form is used to evaluate the near field

behavior of the various field components.

The effect of the earth's magnetic field is also evaluated

using a quasi-longitudinal approximation. In general it is indicated that if the gyro frequency is less than the effective value

of the collision frequency, the presence of the earth's magnetic

field may be neglected for E. L. F. When this condition is not

met the attenuation may be increased somewhat. The influence

of an inhomogeneous ionosphere is also briefly considered and

finally, the propagation of E.L.F. pulses are treated. It is

suggested that certain observed characteristics of E. L. F.

waveforms may be attributed to the inclination of the current

channel in the lightning discharge.

A.D. Watt, ELF electric fields from thunderstorms. (To be published in JOURNAL OF RESEARCH of the National Bureau of

Standards-D. Radio Propagation, Vol. 64D, No. 5, SeptemberOctober I960 issue.)

Abstract: The varying electromagnetic fields produced by

thunderstorms and associated lightning discharges are examined.

Calculated field variations produced by an assumed typical cloudto-ground discharge model are found to agree well with observed

fields. The magnitude of these vertical electric field changes

are observed to decrease very slowly with distance from the

source for values comparable to discharge channel heights.

From 4 to 20 km a l/d^ relation is observed, and beyond 30 km

the field variations appeared to follow a l/d relation.

The expected radiation field frequency spectra from 1 c/s to

100 kc/s are calculated employing models assumed to be typical

of "long" and "short" discharges. The radiation spectra obtained from 1 to 100 kc/s for observed cloud-to-ground discharge

field variations normalized to 1 km are seen to agree within expected limits with calculated values.

The models employed indicate that below 300 c/s "long"

discharges produce much more energy than "short" discharges,

and that inter- and intra-cloud discharges may produce as much

energy as cloud-to-ground discharges. Anticipated variations of

total vertical electric field frequency spectra as a function of

distance based on the work of Wait, are shown for the frequency

range from 1 c/s to 100 kc/s.
-10-

H. A. Wheeler, Useful radiation from an underground antenna. (To

be published in JOURNAL OF RESEARCH of the National Bureau

of Standards-D. Radio Propagation, Vol, 65D, No. 1, JanuaryFebruary 1961 issue.)

Abstract: An underground antenna delivers power to the

surrounding conductive medium, and a fraction of the power goes

out as radiation above the surface. This fraction is denoted the

"radiation efficiency." It is expressed in simple terms for two

types of underground antennas. The first and simplest is a

vertical loop in a submerged spherical radome. The second is

a submerged horizontal insulated wire with each end connected

to a ground electrode. In each case, the efficiency is the product

of three simple factors: the first depending on the index of refraction between air and ground; the second proportional to the

size (radius of the radome or length of the wire); the third giving

the attenuation with depth. An example for 1 Mc gives an

efficiency of .0014 for an underground wire of specified dimensions,

The radiation efficiency is applicable to sender or receiver.
-11-

II. Discussions in Morning Session

R. C. Kirby opens session with welcoming address.

E.T. Pierce speaks.

Wait (moderator) Are there any questions on the paper by Pierce?

Wait: 1 would like to ask one question. Have you observed nonreciprocity in E. L. F. propagation?

Pierce: No.

Wait: Anymore questions? If not, we shall proceed.

Garland speaks

Wait: Any questions?

Wheeler: I didn't get it clear about how deep you had to go before

the conductivity starts to rise.

Garland: In that connection the change in conductivity suggested by

Niblett was at the base of the crust which in this part of Alberta

was at 40 kilometers.

Wheeler: Any idea of when it starts to increase?

Garland: Down through crust the resistivity seems to be on the

order of 10" ohm-meters. This seems to persist about 40

kilometers and then the decrease starts.

Wheeler: You don't associate this with temperature?

Garland: Undoubtedly this has some effect.

Wheeler: Suppose it could be caused by moisture or by nature or

what?

Garland: It is not clear whether the change in conductivity at the

base of the crust is due to a change in composition of the rock

or to other factors.

Wait: Have you tried using 3-layer correction curve rather than a

2-layer?

Garland: We didn't because of the lack of a broad enough spectrum

of disturbance to get enough character in the plot of the

apparent resistivity.
-12-

Wait: Are the range of the periods limited?

Garland: We think this is a limitation of the method of frequency

analysis. What is really needed is a complete spectral

analysis by digital recording and machine computation.

Pierce: I think you considered horizontal currents only. We know

also that an air earth current flows vertically at the earth's

surface.

Garland: I qualified that.

Pierce: It is a question of orders of magnitude?

Garland: The potential gradient of 100 volts per meter as measured

in the atmosphere would correspond to a vertical gradient below the limit of measurement in the earth because of the ratio

of conductivities.

Pierce: It would seem to be legitimate to neglect air-earth currents

in fair weather by comparison with earth currents, but not

necessarily correct to do so under stormy conditions. In

amperes/cm the vertical earth currents are of the order of

lO""-^; fair-weather air-earth currents are 10"^^, increasing to

10 *"" during storms.

Garland: In fact, we hope to set up an atmospheric gradient recorder

and see if any of the pulsations are related to the horizontal

currents.

Wait: We should be getting on with the next series of papers.

Campbell gives paper.

Wait: Any questions?

Tepley: There was an article in Time magazine about some measurements at frequencies comparable to those considered in your

investigation. This had to do with a Signal Corps project during

the Argus series. They reported a large pulsation associated

with the arrival of a hydromagnetic wave and also a second and

third maximum at somewhat later times. These were supposedly

associated with waves moving back and forth in an ionospheric

duct. I wonder if this is the same type of thing that you have

indicated on your slides?
-13-

Campbell: Perhaps. It maybe a related type of phenomena. On

the first of August 1958, we did receive pulsations which we interpreted as due to the high altitude Johnston Island nuclear

explosion. There was no indication of signals from any of the

other shots.

Tepley: Referring to the Signal Corps project again. I believe that

the waves were reported to travel with something like sonic

velocity. Did the pulsation that you recorded appear to travel

with comparable speed?

Campbell: The velocity we are thinking of is in the 10 kilometers

per second range.

Wheeler: You didn't say very much about the maximum in 100 cycles.

Do you have any idea as to why 100 cycles?

Campbell: The loop antenna was used up to 20 c/s. A vertical

antenna was used from 3 c/s to 1, 000 c/s. The maximum at

100 c/s is typical of the sferics slow tails.

Taylor speaks.

Lehan: Are there any plans to extend your frequency range below

10 cycles down?

Taylor: Presently we only plan on going down to ten cycles per

second with vertical antennas. Trying to get much lower is

extremely difficult with vertical antennas.

Lehan: Trying to get much lower than that you would probably want

to use a horizontal antenna?

Taylor: Yes, another type, possibly a horizontal wire between

ground stakes.

Lehan: You people are not planning to do this?

Taylor: No, not below 10 cycles. One comment I would like to make.

I recently returned from talking with Dr. Mark Brooks at

Socorro, New Mexico. He has some beautiful lightning discharge

records separating individual lightning channels by moving the

camera as the flash occurs. It becomes obvious upon examining

these pictures that a cloud-to-ground stroke may not be a distinct discharge of only a few hundred microseconds. Sometimes

a continuous glow continues in the channel for as long as a halfsecond and often characterized with some minor pulsations in

illuminating during this period. With sufficient current flowing

in a discharge channel to produce this illumination means that
-14-

a very large total current was associated with this type of discharge. It may be that the longer duration discharges of this

general type are more efficient sources of ELF energy than the

conventional type of discharges that last for only a hundred

microseconds or so.

Pierce: May I comment on that last point? I do think it is rather

dangerous to consider the return stroke as the only thing in the

lightning flash. It is well established that about 100 microseconds is occupied by a return stroke. Then as Bill Taylor

pointed out, there normally is something like a quarter of a

second of electrostatic field change 'occurring afterwards,, so the

lower down in frequency we go, the more we have to take account

of the whole discharge; of the leader, of the following return

stroke, and of any continuing current afterwards. Equally, I think that a cloud discharge in which we have no return stroke at

all could be a very potent source of ELF of the order of cycles

per second.

Tepley speaks. (No questions after his talk.)

Maple speaks. (This is based on a paper, "Audio-Frequency

Fluctuations of the Earth's Magnetic Field, " to be published in

Journal of Geophysical Research.

Wait: Any questions?

Goldberg: Did you see anything like you have observed in the past,

such as a 7-hour precursor, in the 1 to 50 cycles per second

activity?

Maple: No, I haven't yet analyzed any data on our present project

from a high latitude station where I would expect to see the

precursor.

Campbell: Can't you explain the sudden rise the day before an

activity as being attributed to increase in E layer ionization and

still be recorded as spherics?

Maple: I don't think so. The diurnal variation of the precursor

signals doesn't suggest this. Also, decreasing the already low

attenuation rate over only a part of the propagation path

wouldn't raise the signal level by a large factor.

Campbell: Can you attribute the sudden increase in your activity

the day preceding magnetic storms to an increase in E layer

ionization affecting the sferics slow tails received?
-15--

Saran presents paper.

No questions.

Goldberg presents paper.

Wait: We have time for one question or so.

Tepley: You presented one slide in which the magnetic field intensity

was plotted against frequency. Campbell presented a slide in

which the same quantities were plotted. However, the two curves

were not at all similar. Could you comment on whether or not

there appears to be a discrepancy between the two curves?

Campbell: Yes. I recorded up to 20 c/s with the loop antenna and

then with vertical antenna I went from 1, 000 c/s down to 3 cycles

per second.
-16-

III. Discussions in Afternoon Session

Moderator: A.D. Watt

M. Balser (from MIT) speaks. (No questions.

)

Wait speaks.

Watt: Any short questions on the paper?

Tepley: Is there a dependency on the earth's magnetic field?

Wait: Of course, if you accept the quasi-longitudinal approximation,

in the normal sense it essentially means that the propagation

will be reciprocal. However, if you use this Q. L. approximation in a special way, the effective component of the gyrofrequency is slightly different for north-south and south-north

propagation. For propagation from east-to-west or west-toeast, the earth's magnetic field should produce the maximum

non-reciprocity, but even in this case it appears to be a very

small effect. It is possible, however, that the presence of

heavy ions could produce non-reciprocity at E.L.F. In fact

this would be a good way to test for their significance.

Crombie: I have made a calculation, and it appears that in the ELF

region no non-reciprocity as far as attenuation is concerned to

first order of approximation. There is no effect whatsoever on

attenuation - but a slight effect on phase velocity.

Wait: That's an interesting point since it might affect cross-over

time on slow tails.

Helliwell speaks. (The paper dealt with the ELF spectrum of

whistlers and related sferics).

Watt: Any questions?

Goldberg: Do you notice much difference between day and night in

the lower cut-off region?

Helliwell: Yes, the day-to-night variation is quite marked and regular. The sharp cut-off is limited to nighttime. Let me qualify;

I never see it over a daytime path. You may see it around sunrise or sunset by looking at a source from the night side.

Campbell: For the cases in which you observed the whistlers

continuing down into the ELF range; did the accompanying sferics

also continue down into the ELF range?
-17-

Helliwell: No. A plausible explanation is that the whistler path

begins at the location of the originating discharge and ends at

the receiver, so that waveguide cut-off effects cannot develop.

The atmospheric, on the other hand, travels a considerable

distance in the waveguide, and hence will be modified by transmission properties of the guide.

Anderson speaks.

Watt: Any questions?

Wait: This is based on a plane wave and an infinite homogeneous

medium?

Anderson: Yes.

Wait: This is changed quite a bit in the presence of an interface?

Anderson: Yes, I am sure.

Norton speaks.

Watt: Any questions on Mr. Norton's paper?

Wheeler: In the case of the waveguide modes an interesting question

comes up. Should you regard the transmitter antenna as

radiating in half- space and then put the waveguide on it, or

should you go right to the transmission behavior in the waveguide mode, which of course would give a different radiation

resistance. Had you thought about that?

Norton: I think that you should determine the transmitting antenna

radiation resistance with the receiving antenna terminals open

and then separately calculate the mutual impedance which contains all of the propagation. It's as simple as that. All of the

propagation is in the mutual impedance but, of course, this

method of looking at the problem does not necessarily alleviate

any of its inherent difficulties.

Wheeler: The self- impedance of the antenna does reflect the waveguide boundaries, so you might be interested in the rule of

thumb that I have been using. If it is single-mode propagation

in waveguide, which in general means only one mode excited by

the antenna, then I say the radiation resistance is the resistance

that it has in that mode of propagation, and that cuts out one

factor. If, however, the waveguide is in a multi-mode propagation region - an example would be 15 kc - then I prefer to

write the radiation resistance of the antenna in half-space and

apply an ionospheric gain factor when we take the mode that is
-18-

actually the prevalent mode in propagation. Both of those do

work very well.

Wait: I think the simplicity of the original presentation is lost in

Dr. Wheeler's modification. The mutual impedance term

actually involves the complete mode series.

Wheeler: That is true if the waveguide is a lossy waveguide. When

you get down to below 10,000 cycles (1) the waveguide doesn't

have single mode propagation and (2) it is not a lossy waveguide

so it does affect the impedance of the transmitter antenna and

that's the reason for it.

Harrison speaks.

Watt: Any questions?

Lehan: Is the model antenna you are discussing actually in contact

with the dissipative medium?

Harrison: It is in contact with the medium; no insulation being

assumed in the theory. In practice the feed line and a portion

of the antenna in the immediate vicinity of the input terminals

would be insulated, I suppose.

Wait: I understand that the infinite susceptance across the drive

point that results from the assumption of a delta -function

generator is effectively subtracted out in the iterative scheme

you are using - - but how do you take into account the shorting

effect of the conducting dielectric across the slice generator?

Harrison: I would prefer not to discuss this point. Since the knife

edges between the idealized halves of the antenna are separated by

zero distance, and are in contact with the dissipative medium,

the conductance and capacitance are infinite. But if the integral

equation is solved in terms of continuous functions by the usual

method of iteration, the driving point current actually obtained

is essentially that maintained by the delta -function generator

minus this infinite gap current. Prof. Tai Wu of Harvard^,

and more recently Prof. R. H. Duncan of New Mexico State

University^, have discussed this point at some length as

I

Tai Wu and R. W.P. King, Driving point and input admittance of

linear antennas, Jour. Appl. Phys. , 30, 74 (1959).

2

R.H. Duncan and F.A. Hinchley, Cylindrical antenna theory,

Jour. Res. N.B.S., 64D, Sept./Oct. I960, (in press)
-19-

applied to antennas in free space.

Held: It appears to me that you are getting the familiar sinusoidal

current distribution, yet you say that the antenna is not insulated from the medium.

Harrison: It is true that the integral equation, which is valid in a

dissipative medium as well as in free space, yields sines and

cosines. But when the medium is dissipative, sines and cosines of complex arguments are involved, bringing in the

hyperbolic functions. In free space the sines and cosines are

of real arguments.

Bearce: Have you taken into consideration the effect of a magnetic

field on antennas in lossy media?

Harrison: No. We have only considered the case of an antenna

immersed in an infinite homogeneous dissipative medium.

Macmillan speaks.

Watt: Any questions?

Wheeler: Do you have any idea what modes are getting through to

Cambridge?

Macmillan: No, I would be interested.

Wheeler: I have a suspicion it was probably the TE mode which is

one not used normally at all and which takes advantage of the

oblique propagation upward which has a rather low attenuation.

Macmillan: It is very possible.

Wait: What direction was the launching plane relative to the path of

Cambridge?

Macmillan: I would say that it would be 30 or so off axis.

Wait: This would give fairly efficient launching into the TM mode,

although the maximum launching would be right off the end.

Helliwell: Weren't there some measurements of attenuation for the

experiment?

Macmillan: No! This was done in March just when the signal is not

sufficient to judge attenuation measurements. We are replacing

the crystal oscillator and receiving equipment so we hope to be

able to do that and operate at a higher power.

Keiser: Do you have any evidence of a second bounce transmission

for this?
-20-

Macmillan: We hope to investigate this.

Keller presenting Frischkneeht paper.

Watt: Any questions? I would like to make one comment, that is,

it is quite nice to sit here and look at these figures, but to stop

to think what went into getting them. We tried to make similar

measurements and at times the weather and other conditions

were quite adverse.

Moore speaks.

Watt: Any questions?

Wait: Regarding the approximation you made in the Hankel function.

Did you not estimate it by the first term of its asymptotic expansion? If so, this would require that the distance is quite

large compared to the wavelength.

Moore: Not quite true, p is the distance in (free space wavelengths)/^ it. For the frequencies I considered, and the conductivity of sea water, this approximation is valid down top

equals 0.01. The reason is that the asymptotic series for the

Hankel function is used in the integration along BLl where the

product i(jp is always large even for fairly small p. The

asymptotic expansion is not used close to BL2 and Pole 1. I did

not present the discussion of the part of the integration near the

pole and branch point number 2 here since it has been carried

out by Sommerfeld and others.

Wait: I believe your result is restricted to distances which are large

compared to the sum of the respective depths. However, in

certain cases of practical interest the horizontal range and the

depths are comparable. Furthermore, significant distances in

the conductor at extremely low frequency may be of the order of

the skin depth. Under these conditions your results are not

applicable.

Moore: That is true for some of the cases you have discussed. On

the other hand, this approximation is valid in many cases where

p is relatively small because the algebraic factor and the Hankel

function both tend to get small fairly rapidly along BL2 as we go

away from the axis of reals; thus one can make this statement as

long as a/a* is large so that i}j must be considered in the square

root only for large values. Since vjj appears in the Hankel function

multiplied by p the approximation is reasonable as long as the

p roduct vjjp is of fairly large size for a value of v(j equals the

"s/cr/W . Of course, if p is extremely small the approximation

is not valid.
-21-

Wait: I might mention at this stage that an almost exact treatment of

the problem may be carried out if the horizontal range is small

compared to a free space wavelength. In this case the integrals

may be evaluated in closed form (see for example, J.R. Wait,

Jour. Geophys., Res. 58, pp. 21-28 and pp. 167-168, 1953).

Wheeler and Rawls talk.

Watt: Any questions on both papers ? Moore: I would like to comment on three things. First, in Mr.

Rawls 1 paper he did not point out that there is some question

about using my approximations at frequencies as high as 1 megacycle for ground conductivities as small as he quotes. I am sure, of course, that he is aware of this, and I merely point

this out so that others will be cautious in using this approximation

under other conditions than the ones I used it for. Under some

conditions at frequencies of 1 megacycle, the conduction current

in the ground may actually be smaller than the displacement

current.

Another thing that might be worth mentioning is that, in the

case of a high conductivity medium such as sea water, there is

an effect of the high attenuation on the noise considerations. For

any significant depth it is necessary that the transmitter lay down

such a strong signal at the surface above the receiving submarine

antenna that it will always be stronger than any noise signal,

thus the effect of noise considerations in the case of antennas

under the sea might be quite different than in the case of the

antenna in the ground as considered by Mr. Rawls.

Wheeler: In the ground I assumed the conduction current is about

five times the dielectric current.

Moore: I am sure you checked them, but I would certainly think that

one would have to check the approximations quite carefully each

time he used them in such a different situation. The other point

I wish to discuss has to do with Dr. Wheeler's very interesting

paper. The picture he drew on the blackboard is essentially that

of Weyl's treatment of 1919. Unfortunately, for a conducting

medium complex angles of incidence, reflection, and transmission

occur which tend to confuse the issue. Asa matter of fact, Mr.

Williams is currently working on a physical interpretation of

this very picture for the case of the complex angles of incidence

that come up in resolution of the dipole source.

Wheeler: My study shows rather quickly that there isn't any additional

complication if you use the receiver method of analysis.
-22-

Wait: This is quite an interesting remark that Wheeler made. By

reciprocity I can see how we can get the mutual impedance in

the system by using this method, but I don't see how you get

the input impedance in the loop using a plane wave.

Wheeler; We do require a different method for the self impedance.
-23-

Name

Ames, L.E.

Anderson, W.L.

Ankers, R.E.

Bailey, D. K.

Balser, M.

Bearce, L. S.

Bishop, D. J. Blazedel, F.

Brown, F. W. , Dr

Brown, W. W.

IV. List of Participants

Organization

U.S. Army Signal Eng.

Agency

University of New Mexico

Developmental Engr. Co.

NBS

MIT, Lincoln Labs.

U.S. Naval Research Lab.

Boeing Airplane Co.

Raytheon Mfg. Co.

Director, NBS Boulder Labs.

NBS

Campbell, W. H., Dr. University of Alaska

Carr, L. H. Developmental Engr. Co.

Crichlow, W.Q. NBS

Location

Wash. 25, D.C.

Albuquerque, N. M.

Wash. 6, D.C.

Boulder, Colo.

Lexington, Mass.

Wash. 25, D.C.

Seattle, Wash.

Waltham 54, Mass.

Boulder, Colo.

Boulder, Colo.

College, Alaska

Wash. 6, D.C.

Boulder, Colo.

DeGregorio, J. F.

DeHart, W.

Disney, R.T.

NBS

Navy Dept.,Bureau of Ships

NBS

Boulder, Colo.

Wash. 25, D.C.

Boulder, Colo.

Forman, S. AVCO Research and Dev. Div. Wilmington, Mass.

Fri schknecht, F. N. U.S. Geological Survey Denver, Colo.

Garland, G. , Dr. University of Alberta

Ghose, R. , Dr.

Goetter, W. F.

Goldberg, P. A.

Harmon, G.

Harrison, C. H. , Haydon, G. W.

Hedlund, D.A.

Hefley, G.

Held, G.

Helliwell,R.A. ,

Space Electronics Corp.

General Electric Co.

Rand Corp.

Raytheon Mfg. Co.

Dr. Sandia Corp.

NBS

Raytheon Mfg. Co.

NBS

University of Washington

Dr. Stanford University

Henry, Capt. J. W. U.S. Navy, Bureau of Ships

Jean, A.G.

Jenkins, A. W.

Johler, J.R.

Johnk, C. T. , Dr

NBS

Denver Research Institute,

University of Denver

NBS

Denver Research Institute,

University of Denver

Edmonton, Alberta,

Canada

Glendale, Calif.

Syracuse, N.Y.

Santa Monica, Calif.

Waltham 54, Mass.

Albuquerque, N. M.

Boulder, Colo.

Waltham 54, Mass.

Boulder, Colo.

Seattle 5, Wash.

Palo Alto, Calif.

Wash. 25, D.C.

Boulder, Colo.

Denver, Colo.

Boulder, Colo.

Denver, Colo.
24-

Keiser, BE.

Keller, G.V.

Kelso, J. M.

Kirby, R.C.

Lehan, F. W.

Leydorf, G.F.

Linfield, R. F.

Lunden, C.B.

Macmillan, R, S., Dr

Maple, E.

Martin, C.A.

McNish, A.G.

Maxwell, E. L.

Miller, W.A.

Moore, R. K. > Dr.

Norton, K.A.

Paroulek, E. F.

Pierce. E. T. , Dr.

Rawls, Li. Roubique, C. J.

Saran, G.

Shapley, A.H.

Slutz, R. J. , Dr.

Smith, C.E.

Still, Major W.

Storer, W.A.

Sullivan, J. A.

Taylor, W. L.

Tennyson, J. J.

Tepley, L.R. ,Dr.

Veazey, Capt.T.J.

Wait, J.R. , Dr.

Watt, A.D.

Watts, J. M.

RCA Laboratories

U.S. Geological Survey

Space Technology Labs.

NBS

Space Electronics Corp.

Developmental Engr. Corp.

NBS

Boeing Airplane Co.

. University of Southern Calif.

U.S.A.F., Cambridge

Research Center

RCA Laboratories

NBS

NBS

RCA Laboratories

University of New Mexico

NBS

Res. and Engr. Off. of Elect.

Pentagon

AVCO Research and Dev.Div.

Developmental Engr. Co.

NBS

Boeing Airplane Co.

NBS

NBS

Smith Electronic s, Inc.

U.S.A.F. Ballistics Miss.Div.

United Electrodynamics, Inc

A.F. Cambridge Res. Center

NBS

U.S. Navy Underwater

Sound Lab., Ft. Trumbill

Lockheed Aircraft Corp.

Special Projects, U. S. Navy

NBS

NBS

NBS

Princeton, N. J. Denver, Colo.

L.A. 45, Calif.

Boulder, Colo.

Glendale 1, Calif.

Wash. , D.C.

Boulder, Colo.

Seattle, Wash.

L.A. , Calif.

Bedford, Mass.

Rocky Pt. , N. Y.

Wash. 25, D.C.

Boulder, Colo.

Rocky Pt. , N.Y.

Albuquerque, N. M.

Boulder, Colo.

Wash. 25, D.C.

Wilmington, Mass.

Wash. 6, D.C.

Boulder, Colo.

Seattle, Wash.

Boulder, Colo.

Boulder, Colo.

Brecksville, Ohio

L.A. 45, Calif.

Bedford, Mass.

Boulder, Colo.

New London, Conn.

Palo Alto, Calif.

Wash. 25, D.C.

Boulder, Colo.

Boulder, Colo.

Boulder, Colo.
-25-

Weyl, T. J., Dr.

Wheeler, H.A., Dr.

Whitson, A. L.

Williams, R.H.

Office of Naval Research

Dept. of the NavyWheeler Laboratories, Inc.

Stanford Research Institute

University of New Mexico

Wheelon, A.D.,Dr. Space Technology Labs.

Zengel, Capt. J.

Zablocki, C. J.

U.S.A.F.

U.S. Geological Survey

Wash. 25, D.C.

Great Neck, N.Y.

Menlo Park, Calif.

Albuquerque, N. M.

L.A. 45, Calif.

Denver, Colo.

I .S. DEPARTMENT OF COMMERCE

Frederick H. Mueller, Secretary NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director THE NATIONAL BUREAU OF STANDARDS

The 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.

In general, each section carries out specialized research, development, and engineering in tin; field indicated In

its title. A brief description of the activities, and of the resultant publications, appears on the inside of the front cover. WASHINGTON, D.C.

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Optics and .Metrology. Photometry and Colorimetry. Photographic Technology. Length. Engineering

Metrology.

Heat. Temperature Physics. Thermodynamics. Cryogenic Physics. Rheology. Molecular Kinetics. Free

Radicals Research. Atomic and Radiation Physics. Spectroscopy. Radiometry. Mass Spectrometry. Solid State Physics. Electron Physics. Atomic Physics. Neutron Physics. Radiation Theory. Radioactivity. X-rays. High

Energy Radiation. Nucleonic Instrumentation. Radiological Equipment.

Chemistry. Organic Coatings. Surface Chemistry. Organic Chemistry. Analytical Chemistry. Inorganic

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Organic and Fibrous Materials. Rubber. Textiles. Paper. Leather. Testing and Specifications. Poly- mer Structure. Plastics. Dental Research.

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