national Bureau of Standard

library, H.W.

OCT 8 1964

NBS MONOGRAPH 85

A Study of Lunar Surface

Radio Communication

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
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UNITED STATES DEPARTMENT OF COMMERCE • Luther H. Hodges, Secretary

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

A Study of Lunar Surface

Radio Communication

L. E. Vogler

National Bureau of Standards Monograph 85

Issued September 14, 1964

For sale by the Superintendent of Documents. U.S. Government Printing Office Washington. D.C., 20402 - Price 70 cents
Library of Congress Catalog Card Number: 64-60070
CONTENTS

PAGE

1. INTRODUCTION. 1

2. SYSTEM LOSS 2

3. GROUND WAVE ATTENUATION A 3

4. ANTENNA EFFECTS. 19

5. NOISE EFFECTS 25

6. POSSIBLE IONOSPHERIC EFFECTS. 29

7. CALCULATION OF REQUIRED POWER .................. 36

8. CONCLUSIONS 48

9. ACKNOWLEDGMENTS 48

10. APPENDIX: INPUT IMPEDANCE. ..................... 49

11. REFERENCES 55

12. GRAPHS OF GROUND LOSS 57

13. GRAPHS OF INPUT IMPEDANCE ...................... 87

iii

A STUDY OF LUNAR SURFACE RADIO COMMUNICATION

L. E. Vogler

The problem of point-to-point radio communication on the moon is discussed,

and equations and curves are presented to estimate power requirements in lunar communication systems. Assuming a smooth surface, consideration is given to ground wave attenuation over both layered and non-layered grounds, antenna ground losses in situations where ground screens are impractical, noise level estimates in the receiving system, and the effects on propagation of possible lunar ionospheres. An example

of the calculation of required power for a particular communication system is given,

and further studies are suggested.

1. INTRODUCTION

This monograph presents the results of a study investigating the problem of point-to-point

radio communication on the surface of the moon. It is intended as a survey of all the work accomplished in a project sponsored by Jet Propulsion Laboratory and, consequently, much of the material

has appeared in previous papers concerned with various phases of the study [Vogler, 1963a, 1963b;

Vogler and Noble, 1963, 1964] ; the rest of the material, notably the discussions on the effects of layered grounds and possible lunar ionospheres, has not been published before.

No attempt is made here to analyze in detail one particular type of communication system;

rather, it is hoped that this study may provide part of the general information necessary to the design

of any future lunar surface communication link. For example, the antenna used in a permanently

based installation would most likely be much more elaborate than the one needed for communication

by a mobile exploring party. Once the purpose of the system is determined, the curves herein may

be used to help estimate the frequency, range, and power requirements.

Most of the curves, aside from those figures describing illustrative examples, are plotted in parametric terms for the simple reason that very little information exists at the present time concerning the exact nature of the lunar environment; however, as more and more data become available,

the curves, using the better estimates of parametric values, will still be useful in the analysis of communication requirements.

The study is primarily concerned with the propagation aspect of lunar surface communication,

although considerable emphasis has been placed on the evaluation of antenna power losses due to the proximity of the ground. For low antenna heights and poorly conducting soils, these losses may be

far from negligible, especially where ground screens are impractical. The effects of noise and its relationship to the desired signal is discussed briefly, consideration being given to galactic and

receiving system component noise.

The lunar model assumed is that of a smooth sphere of radius r = 1738 km. The effects of o rough terrain on signal propagation are not included in this analysis, although it is recognized that such things as knife-edge diffraction, "obstacle gain", etc. , might be important at some locations on the moon's surface; a study including these mechanisms could be undertaken for particular surface areas of interest. The present curves are expected to be most useful for applications in the frequency

range from about 1 kc/s to 10 Mc/s. Throughout the present analysis, antenna heights are assumed

to be low enough such that height gain effects are negligible.

1
2. SYSTEM LOSS

The concept of system loss [Norton, 1959; Wait, 1959; CCIR, 1963 ] will be used to describe the effects of the various system parameters. System loss L is defined as the ratio,

expressed in decibels, of the radio frequency power input to the terminals of the transmitting antenna

p and the resultant radio frequency signal power available at the terminals of the receiving antenna p : r

L

s = 10 log(p

t /pr

) = P

t - Pr

. (2. 1)

Unless otherwise specified, capital letters will be used to designate the decibel equivalent of the corresponding lower case letters; e.g., P^ = 10 log^p^ , The system loss may be divided further into a free space component that would be the loss expected between isotropic antennas situated in free space, an attenuation relative to a free space inverse distance field which accounts for the effects on propagation of the intervening terrain,

and the transmitting and receiving antenna components L^, L^, and describing the effects of the particular antennas used. Thus, (2. 1) may be written as

, 2 Ptsr = L + P = 10 log6 (4rrd /X.) + A - G + L + L + P , (2. 2) o t p t r r

x

'

where d^ is the arc distance separating the two antennas and \ is the free space wavelength

expressed in the same units as d . The term G denotes the path antenna gain which is a function

o p

° of the transmitting and receiving antenna maximum directivity gains relative to isotropic antennas,

G and G , and any aperture -to-medium coupling loss or polarization coupling loss. For the

t r purposes of this analysis, the latter two losses will be assumed negligible such that G = G^_ + G ; however, it should be noticed that, in general, the path antenna gain will be less than or equal to the sum of the maximum directivity gains. The symbols L and L,^ denote antenna losses arising

from the antenna's proximity to the ground: ^= 10 log(r/r

£ ), where r is the actual input

resistance and r

£ the radiation r;sistance in free space. Notice that, in order to calculate transmitter power, any transmission line loss and appropriate circuit losses should be added to the right

side of (2.2).

The power available at the receiver consists not only of the desired signal but also of radio

noise arising both from within and from without the receiving system. If r (s/n) denotes the desired

minimum signal-to-noise power ratio that will provide a given grade of reception as measured at the receiver predetection output, and an operating noise factor f^ is defined as the ratio of (1) the signal to a reference Johnson-noise power ratio that is available from a loss-free receiving antenna

to (2) the receiver predetection output signal-to-noise ratio r(s/n), then p may be expressed as

r

p = f k t b r(s/n)/4 , (2. 3) r a o c

'The relationship between f and the effective "noise temperature" T used in some discussions

is given by T

g£f

= t Q

f.
where k t b is the reference Johns on -noise power. The symbol k denotes Boltzmann's B ° -23 B

constant (= 1.38054 X 10 joules /degree) , t is a reference temperature in degrees Kelvin,

b is the effective noise bandwidth in cycles per second, and is the ratio of the antenna input

resistance to its radiation resistance.

Using the above definitions and assuming i ^ S.^ , (2. 2) may now be expressed as

P. = 32.45 + 20 log d (km) + 20 log f + A - (G + G ) + L, + R + F + B + 10 log (k t ) , (2.4)

t o mc t t r t Bo 1 '

where d^ is measured in kilometers, ^mc

denotes the radio frequency in megacycles per second,

and R = 10 logr(s/n). The following sections will discuss the various components of (2.4).

3. GROUND WAVE ATTENUATION A

Numerical procedures for the calculation of electromagnetic fields diffracted around a smooth

homogeneous sphere have been developed by various authors [Norton, 1941; Bremmer, 1949 ] . The ground wave may be expressed as a series of residues that depend on the radius of the sphere

r^, the arc distance d^, the antenna heights h^ and h^ , the free space wavelength X, the relative dielectric constant e and conductivity cr of the ground, and the polarization of the wave. As stated in section 1, the present analysis assumes negligible height gain effects corresponding to antenna heights near zero; for elevated antennas, estimates of height gain may be obtained for beyond

the horizon paths from another paper [Vogler, 1964] .

In parametric form the ground wave attenuation relative to a free space inverse distance field

o

, A^ is conveniently plotted as a function of three parameters : K, b , and x . Through the recent work of Wait [ 1962 a ] both non-layered and layered grounds may be accounted for by generalizations

o of the definitions of K and b . Thus, with the subscripts h and v referring to horizontal and

vertical polarization respectively,

K, (2.r

o

/X) 3 |Th

-1 K :( 2lT r o /M*|T Q (3. 1)

where

\\\ =[<<r -D

2 + s 2 ]

, |T

v

2 1,1 1

(cr

- 1) + s /(c

r + s ) (3.2)

and

b

h

= 180 - tan {—-) - q

h , bv

= 2 tan ( — )

- tan J- %

(3.3)

with

-7 4

s = 2 X 10 c \ cr m 1.8 X 10 cr(mhos/m)/f

3
Both and s are dimensionless and refer to the top ground layer unless otherwise specified.

The parameters X and cr are in meters and mhos per meter respectively, and c is the velocity of light.

The symbols |Q| and q appearing in (3. 1) and (3, 3) denote the magnitude and phase

of a "stratification factor", Q = |q| exp(iq /2), that accounts for the effect of layering in the ground. In general Q is a complex quantity dependent on the heights and electromagnetic ground

constants of the various layers, whereas for a homogeneous (non-layered) ground, Q is equal to

unity, Wait [ 1962b ] has derived formulas for Q for the general case of n layers and has presented curves [Wait, 1962 a] for two and three layer situations; however, unless specific values

for the heights and ground constants of the various layers are given, a graphical representation

becomes quite complicated for a multi-layered ground. For the present discussion, restriction to vertical polarization over a two-layered model will be assumed.

Letting the subscripts 1 and 2 refer to the upper and lower layers respectively, Q may

be expressed as

i(b° -b° )/2 , i(270°-b° )/2^

T |/|T |)e Vvl V2/

+ tanhi(2ttI /\) | T I e hJ \ iq°/2

/

-N i(b°-b° )/Z , if 270°- b,° /2. ,, A

.

where i is the height of the upper layer measured in the same units as X. | T^ | and | T | are

given by (3.2) using the appropriate values of € and o- characterizing layer 1 or layer 2;

„ r

b, and b are obtained from (3. 3) after setting q = 0. As L becomes very large (correh v 1 sponding to homogeneous ground), approaches unity and | Q | = 1, q^ = 0 . Graphs of | T^ j and | T | as functions of and s are shown in figures 1 and 2 where the expressions for refer to a homogeneous ground ( |q| = 1); similar plots of b^

o ' o

"'' and b^ are given in figures 3 and 4, again for homogeneous ground (q = 0). It can be seen that b is always positive for either polarization as long as no layering occurs.

0 The variation of K and b^ for a two-layered ground is shown in figures 5 through 8 . The curves were calculated from (3. 1), (3. 3), and (3. 4) using the values of the ground constants and upper layer heights indicated. Only values of K for 0.01 < K < 10 are plotted since the

o variation of A as a function of b is negligible outside this range (see figures 9 - 14). The curves

o o of b^ indicate that, as the conductivity of the lower layer increases, b is more likely to become negative. Thus, if it is found that the moon consists of a highly conducting core underlying a

thin poorly conducting crust, ground wave communication could encounter "fade-outs" for certain

frequencies and ranges. This is indicated in figures 9 through 14 which show the ground wave atten- uation A^_ for zero antenna heights versus the distance parameter x^ , where

x' = f

3 d (km) . (3. 5) o mc o
5
V(€r -I)

2 + S: 1/2

Kv = [(2 IT r0 / X

)

1/3

1 Tv I]

"

' = 3.02 x I0" 2/ j

T

v | • fm,

r0

- MOON'S RADIUS = 1738 km

'/3

FOR LARGE S= |TV |~IA/S

0.05 0.1 0.5 1.0 5 10 S = 60Xo-(mhos/m) = i.8x I0

4 a (mhos/m)/f

50 100

mc

Figure 2. The parameter | T | for vertical polarization. The

formula for assumes a homogeneous ground.

6
7
8




13
1.0 2 3 4 5 7 10 20 30 40 50 70 100 200 300 500 1000 2000 5000

*o'= fMc

3 d0 (km)

Figure 10. Ground wave attenuation for zero antenna heights,

b = -45° .

5000

Figure 12. Ground wave attenuation for zero antenna heights,

b = 45°.

16

18
In figures 9 (b = -70 ) and 10 (b = -45 ) the curves show fading effects over short intervals of for certain values of K. As b becomes positive, these fade-outs disappear and is seen

to increase smoothly with distance.

The calculations of the attenuation curves were not done by numerically evaluating the complete

residue series, since a prohibitive number of terms are necessary for small values of x' . In this o region (the left hand portion of figures 9 - 14) use was made of the modified flat earth formula

developed by Wait [ 1962 b, see equation (108) ] in which the leading term is the plane earth attenuation function and successive terms account for the curvature of the sphere. The formula with two

curvature correction terms was employed wherever valid until x^ became too large, at which point

the first term of the residue series could be used from there on out. Equation (2.4) is strictly applicable only for the case of antennas separated a sufficient

distance apart (d^ >> \) such that the induction and static fields may be neglected. Because of

this restriction, A^ is not shown in the figures for values of x

1 < 1 . As d (or x' ) goes to zero, A^ approaches 20 log( 1/2) = -6.02, which would correspond to the surface wave field intensity

expected between short vertical electric dipoles situated near each other on a perfectly conducting

plane.

It is apparent from figures 9-14 that the attenuation is less for the larger values of K,

these values corresponding to vertically polarized waves at the lower frequencies. For example,

for K > 0.1 and f < 1.0, the attenuation is less than 50 db out to distances of 100 km. On mc

the other hand, with horizontal polarization, is usually much less than unity for all frequencies

and the attenuation is greater.

4. ANTENNA EFFECTS

The terms L, and G = G + G (the subscripts t, r referring to either transmitting

t, rptr ° b or receiving antenna) in (2.2) describe the effects of the particular transmitting and receiving antennas used in the communication system under consideration. L , called the ground proximity

t, r loss, is defined as the ratio expressed in decibels of the input resistance of the antenna r to its free space radiation resistance . As the height of the antenna above the surface is increased, the

ratio of the resistances approaches unity and L or L effectively become zero. For heights near the surface, the input resistance is a function of the electromagnetic ground constants « and

r

cr , the radio frequency £ c

» an-d the antenna height h above the surface.

G denotes the free space maximum directivity gain of an optimally oriented loss-free

t, r antenna above an isotropic antenna, e. g. , G = G =10 log (3/2) = 1.76 for elementary dipoles and

G = G =2.15 for half-wave dipoles.

t r Because present estimates indicate the conductivity of lunar surface material to be relatively poor, a numerical evaluation of input impedance was undertaken [Vogler and Noble, 1963, 1964]

considering four types of antennas: vertical and horizontal electric and magnetic dipoles. Short

i oo t dipoles of length di are assumed, the current in which varies as I e with I a constant

o o along the antenna; the magnetic dipoles are then equivalent to small circular loop antennas of area dA, the axis being in the direction of the dipole. The ground beneath the antenna is assumed to be

homogeneous and characterized by its relative dielectric constant e and conductivity o- .

19
The input impedance Z, obtained in terms of the field at the antenna, may be divided into a

free space component and a component AZ describing the effect of the presence of the ground:

Z = Z + A Z . (4. 1)

Explicit expressions for the impedance change due to ground normalized by the free space radiation

resistance, AZ/r^, where VED, HE!

of dipoles considered, are then given by

resistance, AZ/r^, where VED, HED, VMD, HMD refer to the initial letters of the four types

VED: AZ/r

f = i(3/2 a ) I^N ) + I 2 (N )

HED: AZ/r

f = i(3/4 a

3

) fl^l) + I 2 (N

2

) 1

VMD: A Z/r

f = i(3/2 a ) ^(1) + I 2 (l)

HMD: AZ/r

f = i(3/4 a

3

) ^(N2

) + I2

(l)

, r

f = 20 p

2 (di)

2

,

, r

f = 20 p

2 (di) 2

,

, r

f = 20 p 4(dA)

2

,

, r

f = 20 (3 4(dA)

2

,

(4. 2a)

(4. 2b)

(4.2c)

(4. 2d)

where

I

l( 5) = / [

6x

2 2 2 x - a (N - 1)

6x +J x - a (N - 1)

> e dx , (4. 3a)

I 2 (6)

6x -

A 2 2,2

x - a (N -1 )

> e dx , 8x + | x

2

- «

2 (N2

- 1)

(4. 3b)

a = 2hp = 2h(2TT/\) , 2 N = € is, s = 60 \ cr(mhos /m) , (4.4)

and h denotes the antenna height in meters above the ground.

The variable x in (4. 3) assumes purely imaginary values from the lower limit to zero, and

from there ranges over real values to the upper limit. The sign of the square root is chosen so that

the real part is greater than zero. A derivation of the dipole impedances together with asymptotic

expressions of (4.2) is contained in the appendix.

Ground proximity loss in terms of the input impedance as given by (4. 1) may be expressed as

L =10 log(r/r ) = 10 log I 1 + Re(AZ/rJ ] , (4. 5)

t, r i f J

20
! where Re(AZ/r ) denotes the real part of (AZ/r ). Using the relationships of (4.2), L, is

1 t t, r plotted versus

s = 60 \ cr(mhos/m)

-

j in figures A-l to A-28 for various values of and a. Notice that for a greater than about

5, i.e., h/\ ^, 0.4, the ground loss is always negligible; however, as the antenna height in wavelengths approaches zero, L may become quite large for certain combinations of ground cont» r ductivity and frequency. It is often possible to reduce the effect of the ground through the use of appropriate ground screens. Discussions of this subject are contained in papers by Wait and

Surtees [ 1954 ] and by Wait [ 1956 ]

.

To show the variation in the normalized input impedance change for both the resistance

(Re AZ/r^) and the reactance (ImAZ/r^), equations (4.2) are evaluated and plotted in figures

A-29 to A-64 as functions of |n| , (j>/2, and a where,

/ 2 2\

X/* 1

l N l = («r + s ) ' +/2 = (Vi) tan" (B/

<r

) , (4.6)

with a and s as defined in (4. 4). It should be emphasized that figures A-29 to A-64 do not show the total input impedance, but only the variation due to the presence of the ground. Impedance

curves such as those in the figures are useful not only in estimating antenna requirements for lunar

communication systems, but also may be used in determining the electromagnetic characteristics of material composing the moon.

This section will conclude with a brief discussion of the Beverage wave antenna [Beverage,

Rice, and Kellogg, 1923 ], If considered for semi-permanent installations on the moon, it has the advantage of relatively simply construction and operates most efficiently over poorly conducting soils. The wave antenna in its simplest form consists of a long horizontal wire situated a short height above

the ground and terminated at one end through its characteristic impedance. It is a uni-directional antenna with the maximum gain in the direction of the antenna axis and toward the terminated end. Wait [1954] has shown that the vertical electric field component of a wave antenna is proportional to a complex factor T , termed the "wave tilt" and defined by

T

v

(fr- 1) - is] /(«r- is > . < 4 - 7 )

and a function S' which depends on the wavelength \, antenna length i, an angle <|> measuring

the direction in which the antenna is pointing, and the propagation constant T defined as

r = a' + i(3m , (4. 8)

where a' and m are real and (3 = 2tt/X.

2 1
For low antenna heights h an approximate expression for the values of a' and m has

been given by Carson [1926 ] :

df'/p =a m - 1 2 3/2

(ph) N]e

r

2 + s 2 £n(4h/d

a

) (4.9)

where d is the diameter of the antenna wire. For an uninsulated infinite simally thin antenna

a actually lying on the ground, T may be expressed as [Coleman, 1950 ]

r is a + i pm = PnJ m

2

- + 1) + i(P/2) ( €r + l) 2 +s

2 +(€

r + l) (4.10)

The function S' , obtained by integrating the contributions of all the elements along the antenna, is given by

, _ 1 - exp [- (r - ipcos 4>')l] s " (r - ipcos^')£ • (4<11)

It can be seen from (4. 10) that the propagation constant of a wave antenna lying on the ground

approaches its free space value i p if the relative dielectric constant is very near unity and the conductivity is extremely small; in this case S' approaches unity.

The wave antenna power gain [Martin and Wickizer, 1949 ] referred to an isotropic antenna,

(p /p. ), may now be expressed as wa iso

(Pwa /piso ) = (|3iCOS *' )2

l S '| 2

* ^vl

2

^ lZQK

o

l \ Z

o\

2'

)

' (4' 12)

where Z = R + i X is the characteristic impedance of the wave antenna. The factor I T I , o o o

1 v

1 which was discussed in section 3 and defined by (3. 2), is shown in figure 2, and I S

1

| is graphed

in figure 15 as a function of the two parameters (a'l) and (pj?)(m-cos 4>'). An example of the wave

antenna pattern,

| S | = (picos <j>') •

| S' | ,

as a function of the direction angle tj>' is shown in figure 16 for several values of pi = 2tt(£/\) and

for m = 1, a' - 0. It is seen that the antenna is highly directional in the forward direction (cj)' = 0); other values of m and a' modify the pattern somewhat, but the general shape is the same. A modification of the wave antenna which is also discussed in the paper by Wait [ 1954 ] is

one in which the horizontal wire of length L = 2b is center-fed and the ends of the antenna are not

terminated with a load impedance. Assuming a current distribution along the wire of

I(y) = I [sinh(rb) ] _1

sinh Jr(b- |y| ) i , (4.13)

22
.23

where I and y are respectively the current at and the distance from the feed point, an integration of the elements over the length of the antenna yields for the antenna pattern | S j :

Ipbcos 4>'

| • \S±\

r 2i cosh x - cos (x 0 -x,

I 1 2 :

x, +(x

2 -x

3 y

'.\ cosh x^- cos (x

2 +x

3

)

x

i + ^

x

2 + x

:

2 2 x + x

1 2 cosh x - cos x

2

(4. 14)

where x„ or b, x^ = (3b m , and x^ = |3b cos <j> Graphs of (4= 14) are shown in figure 17 for the case of a = 0, which corresponds to very

poorly conducting ground. The values of | S | shown on the figure, however, should be used only

with antenna lengths I such that the quantity {a I) is very near zero; if this is not the case, | S | should be calculated from (4. 14), The center-fed wave antenna power gain corresponding to (4. 12)

is given by

P /p. wa iso

|S|

2

-

| T I 2

( 120R / | Z | 2 (4.15)

where T | , R and Z have been defined previously,

o The theoretical determination of L or L for a wave antenna in terms of its input

t r resistance is quite difficult. However, considering these quantities essentially as power ratios

measuring the effectiveness of the antenna, an estimate of their magnitude may be obtained from

(4. 12) by assuming that

L - G

t, r t, r 10 log (p /p. v wa iso (4. 16)

5. NOISE EFFECTS

The operating noise factor f [Norton, 1962 ] appearing in (2.4) provides a measure of

the effects of noise on the receiving system (in previous papers [Vogler, 1963a, 1963b] f is

called the effective noise figure). It is defined by

f =[Pa /% t Q

b)]/[r(s/n)] , (5.1)

where p_^ is the signal power available from an equivalent loss -free receiving antenna, (k^ t^ b)

is a reference Johnson-noise power [see (2.3)], and r(s/n) is the receiver predetection output

signal-to-noise ratio. Since p is the power available from a loss -free antenna, its relationship to the power

a available from the actual antenna, p , may be expressed as

r

p = I p , (5.2) a c r

25
26
where jf is the antenna circuit loss factor. Because the present study is primarily concerned with

low antennas over poorly conducting ground, the assumption is made throughout that £ I , c r where I is the ground proximity loss discussed in section 4. In terms of the noise factors of the receiving system's component parts, f can be expressed

in the following form:

f = f - l+ f + £ (f _ 1) + £ £ (f _ 1) e c c

x ti ' ctTr '

' (5. 3)

where f^, f^, f , and denote respectively the noise factors of the external noise, antenna

circuit, transmission line, and receiver. In terms of the loss factors £ and £ of the antenna

c t£ circuit and transmission line (i. e. , the ratio of the available input to output powers of the component),

f^ and f^ are given by

fc =l + (*c -l)(t

c

/to

), V= 1+^-1) (tu

/to

) , (5.4)

where t^ and t^ designate the absolute temperatures of the corresponding components and t is a reference temperature.

Generally speaking, the factors f and f will predominate in calculating values of f, especially at the lower frequencies [Crichlow, Smith, Morton, and Corliss, 1955 ]; at high frequencies f depends more on f and f , which are best obtained by direct measurement. By

arbitrarily assuming the transmission line loss to be negligible (i = 1) and the antenna circuit

temperature very near the reference temperature, ( 5. 1 ) may be written as

f-= f +1 f - 1 , i = 1 , t = t . (5. 5) e c r t£ co

With the approximation £ ^ i , this then becomes

c r

f .as ferr + i f - 1 , (5. 6)

where £ has been defined in section 4. rThe principal sources of external noise received by an antenna located on the moon are

(1) galactic or cosmic, (2) earth-based, and (3) solar. The amount of noise affecting the signal

depends on the location, orientation, and directional gain of the particular antenna under consideration. Some preliminary studies [Page, 1962 ] of earth-based and solar noise components indicate

that, in the case of a quiescent sun, solar noise is negligible compared with galactic noise unless

the antenna beam is oriented directly toward the sun. On the other hand terrestrial atmospheric

noise might become comparable to the galactic noise in the lower frequency range; however, this assumes a negligible ionospheric shielding effect. Galactic noise levels have been fairly well established at the higher frequencies through

earth-based measurements. Figure 18 shows an estimate of the external noise factor = lOlog f obtained from galactic noise data compiled by Menzel [ 1961 ] and by Hartz [ 1963 ]; the reference

27
UJ

i

NlZ J j /

£ /

/ 0

Co 9

/

/ <J /

//

//

/ y

/

—j-

/A/ h- —/ <r —7— / <

/ T -h-f j

1 / -1— T 1

1

i / —/

1 1i —f— —f—

/

/

/ i—

/

/

1 / —t

i / i

/

/

/

/

i /

/

9

j 601 01 = 8d

28
temperature assumed here is t Q

= 288. 37 K. Empirical expressions used to plot the galactic noise curve are

f = 6.467 X 10 f , f > 200 , e mc mc

(5.7a)

f = 1.585 X 10

5

f

~ Z ' 3

, 10 < f < 200 , e mc mc

from the Menzel data, and

f = 5.012 X 104£

" 1 ° 8

, 0.5 < £ < 10 , (5.7b)

e mc mc

from the Hartz data obtained from Topside Sounder measurements. The upper and lower dashed lines -2 3 -18

on the figure are extrapolations of the f " and f

° frequency variations.

mc mc

It is physically apparent that at some lower frequency the galactic noise curve will at least

level off. Until information from low frequency measurements are available, the extrapolations of figure 18 maybe used to provide (hopefully pessimistic) estimates of galactic noise.

6. POSSIBLE IONOSPHERIC EFFECTS

At the present time there is no indication that the moon possesses a neutral atmosphere of any significant magnitude. Because of this, it is unlikely that lunar surface propagation will be

affected by ionospheric considerations to the same extent as terrestrial propagation. However,

studies have been made showing the possible existence of a weak lunar ionosphere caused by solar

wind. This section will discuss briefly the effect on propagation of two forms of possible ionospheres.

If a negligible magnetic field is assumed and the motion of heavy ions is neglected, the ionosphere may be characterized by its index of refraction N. as follows :

(6.1)

where f denotes the signal frequency, f the collision frequency, and f the critical or mc b ^ ' v cr plasma frequency, all measured in Mc/s. In terms of the electron density/cm , n, the critical frequency is given by

f 2 8.1 X 10" 5 n, (6.2) cr

where, in general, n is a function of the height h above the ground: n = n(h)„

One form of ionosphere that may possibly occur extends from the moon's surface to some

distance above, the electron density decreasing with height. Elsmore [ 1957 ] , using studies of lunar occultation of the Crab Nebula, has hypothesized surface electron densities on the order of

3 4 3 10 to 10 electrons/cm . For signal frequencies near or less than the collision frequency, this ionospheric model presents a propagation problem which, as far as the author is aware, has never

29
been investigated and is beyond the scope of the present paper. Considerable theoretical study

would be necessary to determine propagation characteristics at these low frequencies.

For radio frequencies such that f /f < < 1, an estimate of the effect of a surface ionv mc

osphere on propagation at frequencies greater than the plasma frequency may be obtained through

the concept of effective radius [Burrows and Atwood, 1949 ]. The effective radius, r , introduces a correction to the actual radius, r , that allows for the refraction of the radio wave as it o travels through the medium (in this case the model ionosphere) above the ground. Expressed in terms of an effective radius factor k [Wait, 1961 ] :

r k= —r

o

1 +

r N. (h=0)

o 1 N (h=0)

(6.3)

where N. (h=0) denotes the gradient of the refractive index with respect to height evaluated at the surface.

With the restriction that f /f < < 1, (6. 1) may be rewritten as

v mc

NNl-f 2 2,2 /f = 1 - 8.1 X 10 -5,2 n(h)/f

l cr mc mc

(6.4)

and k then becomes

k -a 1 + 4.05XlO" 5 r {-n'(0)}

o

2 2 N. (0) f

l mc

1 +

0.07 { -n' (0) }

- 8. IX 10 _5

n(0)

(6.5)

where n(0) is the electron density/cm at the surface and n (0) is the electron density gradient

with respect to height in km evaluated at the surface. It can be seen that (6. 5) may be used only

for radio frequencies such that

-3

f > f = 9 X 10"" \l n(0) mc cr

thus, for an electron density at the surface of, say, n(0) =10 , the effective radius concept should

be restricted to frequencies of 1 Mc/s or greater.

The curves of figures 9-14 will now provide estimates of the attenuation if they are read as functions of a modified x' , x'(k), and a modified K, , K, (k) , where

o o h, v h, v

i A ? - -

(k) = k" 3 f

3 d (km) , K(k) = 3.02 X 10" k~ 3/| T I • I Q I • f

3

, mc o TT, <~ (6.6)

and k is given by (6.5). Notice that x (1) and K(l) are simply those parameters discussed l- n

section 3 and defined by (3. 5) and (3. 1); the effective radius factor k does not modify b

h, v

30
To show the variation of k with frequency, (6. 5) is plotted in figure 19 for several values

of the surface electron density gradient n'(0). The values chosen do not. represent any experimental

evidence but are used merely to show the variation of k with the gradient. Figure 19 shows that, as the gradient becomes more negative, k decreases and, in general, a higher attenuation of the signal is to be expected. Should a positive gradient exist, less attenuation of the signal would occur. Another possible lunar ionosphere that has recently been investigated [Weil and Barasch,

1963 ] is similar in form to the terrestrial ionosphere. Weil and Barasch, through theoretical

considerations of solar wind influence near the moon together with solar stream data, postulate an

3 ionized elevated layer with an electron density of about 350/cm at 0.6 lunar radii above the surface. The layer occurs only in the general direction of the sun, and their numerical values are based on quiet sun data. The theory of this form of ionospheric propagation is well established and, given the necessary data, will provide good estimates of propagation capabilities. The remainder of the section

will discuss briefly the simple geometrical-optics approximation to ionospheric propagation. A

detailed account of the subject is contained in a paper by Wait [ 1962 c ] .

In the geometrical-optics approximation, the field at a point is considered to be the sum of multiply -reflected radio rays arriving from the transmitting source, the reflections occurring

between the ground surface and the ionosphere. At distances less than its caustic, the single hop

component of the field, i. e. , the ray undergoing only one reflection from the ionosphere, will be

the most significant. However, the total field is given by the sum of all the rays added together in proper phase; at short distances from the transmitting antenna, the ground wave component must

also be included.

In order to gain some idea of the effect on lunar propagation of an elevated ionosphere, the attenuation of the single -hop sky wave is evaluated, assuming the Weil and Barasch model ionosphere with a sharp lower boundary. The assumption of a sharp boundary rather than a more

realistic one will give a lower estimate of the attenuation than is actually obtainable, but will still provide a reasonable estimate for the purposes of the present study. Letting 6( = d^/r^) ,

p( = h/r^), cj>^, and <j>^ denote the geometrical quantities indicated in figure 20, the sky wave

attenuation A

g may be expressed as

A ~ - 20 lof

s

e C sin2

4> \ 2 (6.7)

where R and R. are Fresnel reflection coefficients of the ground and ionosphere respectively : S i

N

2 cos (j> -J N

2

- sin

2

2 | I 2 cos (j>^ +^ N - sin 4>^

-, N

2

= e -is , (6.8)

31
o

o

ino

w "c

X

1

r n'(0)=

100

i \ 1

o

m

ii o

o

II

"c

i i

c

00

Io

COd

32
N?= €j- iSi

33
2 2 2 N. cos <j>. -

\J N. - sin <j>. R.

1 2 2 2 cos <j). + N. - sin (j>.

, N.li= e. - i s. , l (6.9

and the angles ejj and tj>. may be calculated from

<i> = tan

-1 (1+p) sin(9/2)

(1+p) cos (9/2) - 1 I ' vi

-1 sin tbJ

1+p

(6. 1

The quantity C in (6. 7) is a convergence coefficient defined by

1 + P cos (9/2)

1 +

sin (9/2)

(1+p) cos (9/2) - 1 (6. 1

and 9 j is related to the total distance traversed by the ray:

Ij =

\J p

2 + 4(1 +p) sin2 (9/4) (6. i;

Notice that (6. 11) [and therefore (6. 7) ] is valid only as long as

< 9

c

= 2 cos"

1

[ l/(l + p) ]

9 corresponds to the distance at which a caustic is encountered, and other formulas must be used

c to calculate the attenuation in this region. The distances considered here will be only those such

that 9 < < 9 . c At VLF and for the distances of interest, the reflection coefficient of the ground R is near

g unity even for the very poorly conducting ground expected on the moon. On the other hand, the ionospheric reflection coefficient R^ can have a pronounced effect on propagation estimates. Figure

21 shows a plot of | R^ | as a function of the ionosphere height p= h/r^ and

s. = f f 2 /f (f

2 + f 2

l v cr mc \ mc v

obtained from (6. 1). Notice that | R. | depends quite critically on the parameter and, hence, on estimates of n and f . If an elevated lunar ionosphere does exist, data concerning these quantities are necessary for VLF propagation predictions. The purely geometric factor , .1

34
IN- CO lO rO CM

35
C sin

2 *

g )y/(

2 9

1 / e )

appearing in (6.7) depends only on the distance d between the transmitting and receiving points

and on the height of the ionosphere; curves of this factor are presented in figure 22 for various

values of p . The attenuation A

g of a single -hop sky wave relative to a free space inverse distance field

is shown in figure 23 as calculated from (6.7). For these curves the ground is described by

N

2 = 2 - i 180, and the ionosphere at a height p = 0.62is described by N. = 1 - i s. , where s assumes

g the values 1 and 10. At a radio frequency of f = 0.01,

'ill for example, the curves mc

would be valid for a ground conductivity o- = 10 mhos/m and an ionosphere described by two

values of the ratio (n/f ) 125 and (n/f ) 1250 ; the latter values assume f >> f . The

v v v mc

dashed curves hi the figure show the vertically polarized ground wave attenuation for

f = 0.01 and 0.1; this component of the field is dominant at the shorter distances. It can be

mc

seen that over the range of distance shown, the difference in attenuation between the two sky wave

curves is about 10 db, this difference arising from the ionospheric reflection coefficient FL. A change in ground constants at VLF produces very little difference in attenuation for a given s.„ According to Weil and Barasch [ 1963 ] the ionosphere would exist (with any appreciable

magnitude) only within about 60 from the sun -moon line; thus, it would not be a dependable

mechanism for long-term propagation. It should be remembered, however, that it could occasion

interference effects in VLF ground wave communication circuits at those distances where the ground wave and sky wave components are comparable in magnitude.

7. CALCULATION OF REQUIRED POWER

Attempts have been made recently to deduce the electromagnetic properties of the moon's

surface material through the use of radar data [Senior and Siegel, I960 ]. Although differences

exist concerning the exact interpretation of the data [Brown, I960; Daniels, 1961 ], there is general agreement that the relative permittivity is not far above unity and that the conductivity is

quite low. In any cate, most of the graphs discussed in the preceding sections are applicable to a wide range of values of 6 and u . r Senior and Siegel estimate the relative dielectric constant and conductivity of lunar surface

_4 material to be e = 1. 1 and cr = 3.4 X 10 mhos/m. Materials constituting the earth's crust

r have been found with conductivities of this order, but not with such a low permittivity. For the purpose of arriving at some estimate of lunar propagation conditions, it will be assumed that e -3 -4

r ranges from 1.1 to 2.0 and o- lies between 10 and 10 mhos/m.

o By choosing a reference temperature t = 288. 37 K (as in section 5), the required power

given by (2. 4) may be rewritten as P=L -(G+G) + L+R + F+ B- 204 (7. 1) t p t r t

where the propagation loss L^ for ground wave attenuation between isotropic antennas is given by

36
37
38
L = 32.45 + 20 log d (km) + 20 log f + A

p o mc t (7.2)

For sky wave propagation, would be calculated by (7.2) after replacing by the A discussed in section 6. Figure 24 shows curves of for a vertically polarized ground wave over homogeneous

ground as a function of frequency and distance. Values of A^ were obtained from figures 11

o through 14 by linear interpolation in both the and b^ directions, being given by figure 2 and (3. 1) with I Q I = 1, and b being read from figure 4. Notice that L, varies inversely v 1 v f

. /' p

with conductivity at the lower frequencies, while at high frequencies the effect of cr variation

becomes negligible.

To show the use of (7. 1) in estimating required power for a particular communication

system, the following example will be considered. Assume a communication system consisting of a wave antenna lying on the moon's surface and transmitting towards a short vertical electric dipole

i 0 placed some distance away and in an optimum direction from the transmitter (cj) ~ 0 ). An estimate

of - may be obtained from (4. 16) and (4. 12) by assuming the free space value for the wave antenna characteristic impedance:

I Z I R ~ 120 tt .

1 o

1 o

For the case of an antenna of length I - X./4, - is plotted versus frequency in figure 2 5. Again, the antenna loss varies noticeably with conductivity at the lower frequencies, but not at the higher ones. A comparison of figure 18 and figures A-l and A-2 shows that for a dipole at a height of, say, h = X/16 the external noise tends to "blanket out" the effect of the antenna circuit loss. This

is shown in figure 26, which is a plot of the operating noise factor as given by (5. 6) with an assumed

receiver noise factor of f = 4. Now by using figures 24, 25, and 26, and the dipole gain of G = 1.76, the required power delivered to the transmitting antenna terminals for a given type of service may be estimated from (7. 1). Figure 27 shows values of P - (R+B) as a function of frequency for antenna separation distances of 10, 100, and 500 km.

To estimate the required power delivered to the terminals of the transmitting antenna, the type of communication service desired must be designated, thus specifying R and B. For

example, with standard broadcast service and a bandwidth of 10 kc/s, R may have the value

39 db [CCIR, 1951 ] and B = 40 db so that 79 db should be added to the curves of figure 27 to obtain the required power in decibels above 1 watt. Thus the power required for this type of service

in the example system at a range of 10 km and for a frequency of 300 kc/s would be about 10 db

or 10 w. For a low-grade voice communication service and 6 kc/s bandwidth, R = 9 db [CCIR,

1951 ], B = 38 db, and 47 db would be added to the curves. The required power in this case at a distance of 100 km and for a frequency of 100 kc/s would be about 16 wb

If fixed wave antenna lengths and dipole heights are considered, rather than the variable

lengths and heights assumed for figure 27, ground proximity loss curves may be recalculated from

the equations and graphs of section 4 and required power estimates obtained from (7. 1). Figure 28

39
40
<3>

( OS!d/ DM d) 6o| Q| _

41

43
I s I §

o O <2 T—

( 0S 'd/ 0Md) 60 1 01-

44
shows a plot of the quantity L - G for fixed wave antenna lengths of f= 100 m and 1000 m

-3 -4

and ground conductivities of cr = 10 and 10 mhos/m with « = 2. It can be seen that, at

r the lower frequencies, the loss associated with the 1000 m antenna is about the same as that shown

in figure 25; however, a higher loss occurs at the lower frequencies if the 100 m length wave

antenna is used.

The ground proximity loss for a short vertical electric dipole at fixed heights h = 10 m

and 100 m is shown in figure 29 for the same ground constants assumed in figure 28. As before,

the extrapolated values of the external noise factor of figure 18 will tend to override the dipole loss

at the lower frequencies, but it should be emphasized that this loss will contribute significantly to

the calculation of f at low frequencies if future measurements show f to be considerably lower

e than the extrapolated values assumed here.

Using figures 24, 26, and 28, the quantity P - (R + B) is shown in figure 30 for the case of a wave antenna of length I = 100 m transmitting in the optimum direction towards a short vertical

electric dipole at a height h = 10 m. For the example of standard broadcast service used previously, the required power P^_ at a range of 10 km and for a frequency of 300 kc/s would be

about 12 db or 16 w. For the low-grade voice communication service the required power at a

distance of 100 km and for a frequency of 100 kc/p would be about 30 w. At high frequencies

the estimates of P^ shown in figure 30 will be somewhat pessimistic, since a small height gain

effect will occur due to the height of the dipole. Graphs for calculating this effect may be obtained

from another paper [Vogler, 1964].

The combination of a wave antenna and dipole and the particular lengths and heights used in

the preceding example were arbitrarily chosen. As pointed out in section 1, the purpose of the communication system together with the necessary restrictions placed on its physical components

must first be established before specific power requirements may be estimated. Combinations

of antennas other than the one assumed in the present example should be investigated before a decision is made as to what will constitute an efficient lunar communication system.

In the particular idealized system described by figures 27 and 30, a number of points should

be noted: (1) the required power does not vary appreciably over the range of assumed, but

does depend significantly on the conductivity at low frequencies; (2) propagation out to somewhat

beyond 100 km is practical for most types of systems and service, at least at MF or below;

(3) the curves indicate an optimum frequency exists, depending on the conductivity of the lunar

surface and the range of propagation; for the model assumed and for the conductivities and dis- tances shown, the optimum frequency lies in the LF band. It should be kept in mind that the operating noise factor of the receiving system was considered to be a function only of the galactic noise

(which, of course, is extrapolated at the lower frequencies) and a rather low receiver noise factor.

If the receiving antenna loss £ were large enough, it is apparent from (5.3) that F would have

higher values than those assumed. Also, during the lunar day, the antenna temperature t would increase, thus making the operating noise factor even higher.

45
46
0.01 0.02 0.03 0050.0701 0.2 0.3 0.5 0.7 ! 2 3 5 7 10 20 30 50 70 100 200300 500 700

f Mc

Figure 30. P^-^R+B) for fixed length wave antenna and fixed

height VED over homogeneous ground.

47
8. CONCLUSIONS

A study has been presented concerning various aspects of lunar surface communication; it

is hoped that these results may serve as a basis for future investigations of the subject. An attempt

has been made to present the main curves in a generalized form such that, as changes and refinements in our knowledge of the moon arise, the curves may still be useful in predicting lunar

communication requirements.

It is apparent that further studies, both theoretical and experimental, would be useful in the following areas:

1. A more precise evaluation of lunar ground conductivity, together with more information

concerning layering (if any) of the materials composing the moon,

2. Further investigation into the structure and magnitude of possible lunar ionospheres,

especially the determination as to whether an ionized layer exists at the surface, and if

so, its effects on propagation at the lower frequencies,

3. Further studies of noise effects including terrestrial noise sources, noise during high

solar activity, and low frequency galactic noise,

4. Investigations of various antenna combinations, taking into account antenna ground losses

for those situations in which a ground screen will be impractical.

5. A study of knife-edge diffraction and obstacle gain effects caused by irregularities in the moon's surface structure.

9, ACKNOWLEDGMENTS

The author gratefully acknowledges the assistance of J, L. Noble who programmed most

of the mathematical formulas used in the calculations of the many graphs appearing in this paper.

His thanks also go to K. A. Norton and J. R. Wait of the NBS Boulder Laboratories, and to Paul S. Goodwin of Jet Propulsion Laboratory for their suggestions and guidance.

48
10. APPENDIX: INPUT IMPEDANCE OF DIPOLE ANTENNAS

It is assumed that an elementary dipole of length di is located at the origin of a cylindrical

coordinate system (p , 9, z) at a height h above an isotropic and homogeneous conducting half- space characterized by a dielectric constant €^ and conductivity tr^. The interface between

the two media is the plane z = - h, and for z > -h the dielectric constant is assumed to be equal

to the free space value . The magnetic permeability of the whole space is taken as the free space

value |jl (MKS units are used here). In. the case of a vertical dipole, the antenna is oriented in

the z direction, whereas for a horizontal dipole the orientation is in the 0 = 0 direction. The

elementary magnetic dipole may be considered equivalent to a small circular loop with its axis in the direction of the dipole and with an area dA = di/p^, where |3 is the free space wave number

( = 2-rr/X). For practical short dipoles having a linear distribution of current, di should be replaced

by an effective length I = dj?/2„

The electromagnetic field of an elementary dipole above a conducting plane may be obtained

from the electric and magnetic Hertzian potentials, II and II , the components of which, if the vectors are referred to a cartesian

. coordinate system

em(x, y, z), satisfy the scalar wave equation

2 2 (V + (3 ) II = 0 . Using the cylindrical coordinate form of the wave equation, particular solutions are found by the method of separation of variables resulting in expressions for the II components

involving combinations of the eigenfunction

± ^X.

2

- p

2

z n

where X denotes the eigenvalue (following Sommerfeld's notation), J is a Bessel function of the

first kind and order n, and the C's are functions determined from the boundary conditions. These

conditions in the present instance require that the tangential components of the electric vector E

and the magnetic vector B be continuous at the interface between the two media (air and ground).

In the case of vertical dipoles, E and B may be expressed in terms of Hertzian potentials

having a z-component only. Thus in the upper medium

= n 7 = ( n

d

+ n

r ) 7, (a-2) vert z \ z z /

where the superscripts d and r refer to the direct or primary stimulation and the reflected or secondary stimulation, respectively. After solving for the coefficients C through the boundary

conditions, one finds that for

i K 00 VED: n

d

= —^ f J (Vp)e _ZU

°(X/u ) dX , ez B J o o

r o o

iK r N

2u_ - u, ^ -(z + 2h)u

, r

(A-3)

n ez TfM Jo^p>{-2-£—i

}

e °Wuo )dx,

P

o J

o <-N u + u J

49
VMD: n

j iK

d m

mz p

n -Z U

\ JQ

(\p)e °(k/u

o )d\,

(A-4)

mz p

iK p r u - u - r j (Xp) j «i u - u ~ -(z + 2h) u

(X/u )d\,

o

where

U

o

=

Ni x2 - P o' U

i = \

x2 -Pi

2

' N2 =(e l/

€ p)_i(<r

l /a>S o

) (A-5)

(3^ and p^ are propagation constants of the upper and lower media, respectively, and the K's are amplitude factors given by

P I (dl) p a I (dA)

K = -4-2 , K = 12-2-2

e 4ttw€ m 4ir (A-6)

For horizontal dipoles, the Hertzian potentials require both an x- and a z-component. Thus,

corresponding to (A-2),

n, =n xi-n z = (n +n )x + n z, nor x z x x z (A-7)

After solving for the C's through the boundary conditions, one obtains:

r

°° -zu

\ JJXp) e (X./u )d\,

u - u . -(z + 2h)

HED: n

d e

P J o- o o

iK

. r e c j (Xp ) (_2

1\

J o* K '

1 u + u , j o V. o 1 > (K/u )dX,

o

iK cos9 ~ , 2(u -uj^ -(z + 2h)u , , -\— ) V^-f-r

L h V/pjW 'o J

o v. n u +u, J o 1

(A-8)

HMD: fl iKm f J (\p) e (\/u )d\,

mx P J o

r o

iK , N u - u, -(z + 2h)u

"fl 3 f

J oW ( 2 —^

6

P o ^o N u +u

o 1

(X /P Q

)d\,

iK cos9 2(u -u,)-. -(z + 2h)u ?

r o o u +u

o 1 2 where u , u,, N , K , and K are defined in (A-5) and (A-6),

o 1 e m

(A-9)

50
With the Hertz vector n determined, the electromagnetic field in the upper medium is derived through the following relationships:

E = grad div n + (3 n , (electric dipoles)

B = grad div n mom + B fl (magnetic dipoles)

(A-lOa)

(A-lOb)

The components (in cylindrical coordinates) of interest in this paper are found to be

2 , 2 2,„ 2 _ Z V„d „r

o j \ e z e z

iK

P J o o o

J (\p) zu r Nu -u -(z+2h)u

o

e +

o 1 N u +u,

o 1

(X /u )dX,

o (A-lla)

HED: E (p, z) = (9/9p) div n + 3 cos 9 n

p 6 O 6 5t

iK cos9 „°° e r r / 2 2 r J

i Xp

j

, J (Xp) s

f u -u -(z+2h)u

2 2. J (XpK 2X u (u -u ) -(z + 2h)u

HJ»- -~-)^ , ° ° )e ° l(X/u )dX,

Xp J I 2, 2 H L 6 N u +u,

o o 1

(A-llb)

,2,2 2 > / 2, 2 2\ / d r VMD: B (p,z)= 3 /9z + B )n = 9 /9z + B ) i n +n

^o / mz \ o / \ mz mz

iK co zu . u -u -(z + 2h)u

° + j-i?_2le °l(X3/u)dX,

U

o +UJ J (A-llc)

HMD: B (p, z) = (9/9p) div n + B cos 6 n

p

r m r o mx

i K cos 6 m

CO

J o l v

2 2f , , J

l (Vp h

s„ J

o(x P )-x |jo(xP ) -

J(XP ) N N U

q -u -(z + 2h)u

o

o Ti

, J(XpK 2 x 2 u

2 (uo -u

i

) -(z + 2h)u

+ wxp ) - —;{ 2 2 - J

e (

W

K ^ V B ( N u + u, )

-J -1 dX

P (N .. o o 1

(A-lld)

51
To calculate the dipole input impedance using the emf method [Wait, 1953, 1962 d; King,

1956 (p. 258) ] requires a knowledge of the axial field at the antenna. This may be obtained by taking

the limits of (A- 11) as p and z approach zero (and with 9 = 0 in the case of horizontal

dipoles). The impedance Z is then given by

VED: Z = Lim -IE (p, z) di/I p-~o I z

iK dl » N

2 u -u -2hu

-0-4 \ H 2 ° 6 °I (X /u )dX, (A-12a)

P o

Ao J

o L L N u +u J J °

o 1

HED: Z = LimJE (p, z) dl/l L p-o I P °J z—

iK di

e

P I o o 1o

2 r uoun

~ Zhuo\

2 2

r x u^^ u^ -ui) " 2hu -i *{ z,z ', }• "l^V*. B N u +uj J J o ol

VMD: Z = Lim -| coB (p, z) d A/I

p —

z-*o

icoK dA n

°°' r ,u -u., -2hu m

P I o o

(A-12b)

o L o 1 J -I

HMD: Z= L,im-\ co B (p, z)dA/l

p—

z—

r-,-/ 2 2, \ / r N\- u

i-i - 2hu

oo*"oL ^ N u +u

o 1

o

e

2 2, \ u (u - u_ ) , -2hu _ o o 1 ] o 1 , ~ . 2

f e

I (X/uJ dX ° (A-12d)

(3 (N u +uj J J o o 1

52
Notice that the effect of the ground is contained in terms involving the factor exp(- 2h u ); thus, the

o change in impedance AZ due to the presence of the ground may be defined as

AZ = Z - Lim Z = Z - Z, (A-13)

and the free space antenna resistance r^ is obtained from the relationship:

r = Re -i Lim Z

Lh^oo

(A-14)

With the use of (A-13) and (A-14), one can now derive the expressions for the normalized

input impedance change AZ/ r^ given by equations (4. 2) of section 4; here, the change of variable

2 2 x = 2h u = 2h\ \ - R o N o

has been made and the following relationships used:

(\d \/u

o

) = (dx/2h), a=2hpo> ^

= \\x

Z

- <* 2 (N

2

- l)/2h, \

Z

= (c* 2 + x

2 )/(2h)2 (A-15)

Limiting expressions of (4.2), derived from integrations by parts of the integrals involved,

are given by the following equations. For aN < < 1 :

2 VED: AZ/R

£ « M^rFi(D)+ (Nfi F

2(D) 3(1+A)D 1 e -A (A- 16 a)

HED: AZ/R

f ^ ^-k(0) + N + 1 . f 3(1 + A)D 1 -A

F

2(D)

| +1|-L_L_ L e

L 2 a (A-I6b)

VMD: AZ/R ^ M+IL f 4 F

2

(0) + 1

. f 3(N + 1)D] -A

8a

(A-I6c)

HMD: AZ/R

f ^

3(N + 1) F^D) +

where

2. .2 F^X) = X+ {l-X(l+X)}d + j(l+X)(l-X)d , (A-17)

F

2 (X) = + {1 + X(l-X)}d +{1-X(l + X)(2-X)}d2 + |{1 + X(1-X)(2 -X

2 )}d 3

- 1 (1 - X) (1 - X

2

) d

4

,

(A-18)

(A-19) N - 1 „ N - 1 . 2 , ds nTT' Ds — ' ASQ"\| N "

1 N + 1

53
For a N > > 1

(A-20a)

HED: ~ tM^H'1 - A 2/N) +

- ^)} ie , (A-20b)-"

VMD: AZ/R

f ~ _3_ j N-l

3 I N+l

- i a

i e » (A-20c)

HMD: AZ/R V I SUc - «

2

- */») 4- £ (i - ^1 1 •

- M

f ,31 N+l . I^OI f! y

For the case of dipoles above a perfectly conducting plane (N—«>), equations (A-20) reduce to the well-known expressions :

VED: AZ/R

f — ( — {sin a - a cos a} + i {cos a + as in a} (A-21 a)

HED: AZ /R. — . f V

2a

3

f {(1 - a

2

) sin a - a cos a} + i {(1 - a; 2

) cos a + a sin a}

j

, (A-21 b)

VMD: AZ/R

f — - f y l~{sina - a cos a} + i {cos a + a sina} "j

, (A-21 c)

HMD: AZ/R

f ^-(^—

2 a

2 2 {(1-a ) sin a - a cos a} + i {(1 - a )cosa+asina} (A-21 d) i

54
11. REFERENCES

Beverage, H. H. , C. W. Rice, and E. W. Kellogg (1923), The wave antenna, Trans. AIEE 42, 215. Bremmer, H. (1949), Terrestrial radio waves, (Elsevier Publishing Co. , Amsterdam).

Brown, W. E. (I960), A lunar and planetary echo theory, J. Geophys. Res. 65, 3087.

Burrows, C. R. , and S. S. Atwood (1949), Radio wave propagation, (Academic Press Inc., New York, N. Y. ).

Carson, J. R. (1926), Wave propagation in overhead wires with ground return, Bell System

Tech. J. _5, 539.

CCIR (1951), Bandwidths and signal-to-noise ratios in complete systems, VI Plenary Assembly,

International Radio Consultative Committee 1, 30, Geneva.

CCIR (1963), The concept of transmission loss in studies of radio systems, Documents of the Xth

Plenary Assembly, Recommendation 341, ITU Vol. Ill, Geneva.

Coleman, B. L. (1950), Propagation of electromagnetic disturbances along a thin wire in a

horizontally stratified medium, Phil. Mag. 41, 276.

Crichlow, W. Q. , D. F. Smith, R. N. Morton, and W. R. Corliss (1955), Worldwide radio noise

levels expected in the frequency band 10 kc to 100 Mc, NBS Circ. 557.

Daniels, F. B. (1961), A theory of radar reflection from the moon and planets, J. Geophys.

Res. 66, 1781.

Elsmore, B. (1957), Radio observations of the lunar atmosphere, Phil. Mag. 2, Series 8, No. 20,

1040-1046.

Hartz, T. R. (1963), Satellite measurements of cosmic radio intensities at frequencies less than

12 Mc/s, talk given at the 1963 Spring URSI Meeting, Washington, D. C.

King, R. W. P. (1956), The theory of linear antennas, (Harvard University Press, Cambridge,

Mass. ) .

Martin, C. A. , and G. S. Wickizer (1949), Study of Beverage wave antenna for use with lowfrequency Loran, RCA, Final Engineering Report on Contract W-28-099-ac-315.

Menzel, D. H. (1961), Cosmic noise survey, Harvard College Observatory, Cambridge 38, Mass.

Norton, K. A. (1941), The calculation of ground-wave field intensity over a finitely conducting

spherical earth, Proc. IRE 29_, No. 12, 623-639.

Norton, K. A. (1959), System loss in radio wave propagation, J. Res. NBS 63D (Radio Prop.), No. 1, 53-73.

Norton, K. A. (1962), Efficient use of the radio spectrum, NBS Tech. Note No. 158.

Page (1962), Utility of lunar groundwave propagation, Report PCE -R-4541 -000 1 A, Page

Coirimunications Engineers, Inc. , Washington, D. C.

Senior. T. B. A., and K. M. Siegel (I960), A theory of radar scattering by the moon, J. Res. NBS 64D (Radio Prop. ), No. 3, 217.

Vogler, L. E. (1963a), Point-to-point communication on the moon, J. Res. NBS 67D (Radio

Prop.), No. 1, 5-21.

55
Vogler, Li, E. (1963b), Lunar point-to-point communication, chapter (pp. 533-559) in Technology of lunar exploration, edited by C, I. Cummings and H, R. Lawrence, (Academic

Press, New York, N. Y. ) ,

Vogler, L. E. (1964), The calculation of ground wave attenuation in the far diffraction region

Radio Sci. J. Res. NBS/ USNC-URSI 68D. No, 7, 819-826.

Vogler, L. E. , and J. L. Noble (1963), Curves of ground proximity loss for dipole antennas,

NBS Tech. Note No. 175.

Vogler, L, E. , and J. L. Noble (1964), Curves of input impedance change due to ground for dipole antennas, NBS Monograph 72,

Wait, J. R. (1953), Radiation resistance of a small circular loop in the presence of a conducting

ground, J. Appl. Phys. 24, No. 5 S 646-649.

Wait, J. R. (1954), Radiation from a ground antenna, Can. J. Technol, 32, 1.

Wait, J. R. (1956), Effect of the ground screen on the field radiated from a monopole, IRE Trans.

Ant. Prop. AP-4, 179.

Wait, J. R. (1959), Transmission cf power in radio propagation, Electronic and Radio Engineer 36, No. 4, 146.

Wait, J. R. (1961), Private communication.

Wait, J. R. (1962 a), Electromagnetic waves in stratified media, (Pergamon Press, Oxford).

Wait, J. R. (1962b), The propagation of electromagnetic waves along the earth's surface,

chapter (pp. 243-290) in Electromagnetic waves, edited by R. E. Langer, (University of Wisconsin Press, Madison, Wis.).

Wait, J. R. (1962 c), Introduction to the theory of VLF propagation, Proc. IRE 50, 1624-1647.

Wait, J, R. (1962 d), Possible influence of the ionosphere on the impedance of a ground-based

antenna, J. Res. NBS 66D (Radio Prop. ), No. 5, 563-569.

Wait, J. R. , and W. J. Surtees (1954), Impedance of a top-loaded antenna of arbitrary length over

a circular grounded screen, J. Appl. Phys, 25, 553,

Weil, H, , and M. L, Barasch (1963), A theoretical lunar ionosphere, Icarus 1, No. 4, 346-356.

56
12. GRAPHS OF GROUND PROXIMITY LOSS, ^ = 10 log{r/r

f

)

for

Vertical Electric Dipoles (VED)

Horizontal Electric Dipoles (HED)

Vertical Magnetic Dipoles (VMD)

Horizontal Magnetic Dipoles (HMD)

s = 60 \ (T^mhos/m), a - (2h) (2tt/X.) ,

e : relative dielectric constant of ground

rcr(mhos/m): conductivity of ground

h: height in meters of antenna above ground

X.: wavelength in meters

57
58
59
60
61
62
-100.001 0.01 0,1 I 10 100 S = 60Xa(mhos/m)= 1, 8 x I0

4 cr(m hos/m )/f

1000 10000

Mc

Figure A-6. Ground proximity loss L for VED, € = 50.

63
70

€r = 80

a = 477~h/\

64
f 65
66
67
68
69
70
71
72
73
74
75
76
77
70

78
79
80
TO

81
-100,001 0,01 0,1 I 10 100 S = 60 Xcr (mhos/m) = 1 .8 x I0

4 cr (mhos/m)/ f mc

1000 10000

Figure A-25. Ground proximity loss L, for HMD, € = 10 .

«2
83
70

S = 60Xcr (mhos/m)= l,8x l0

4 cr(mhos/m)/f Mc

Figure, A-27. Ground proximity loss L for HMD, € = 50 . t, r r
85

13. GRAPHS OF NORMALIZED INPUT IMPEDANCE CHANGE, AZ/r

for

Vertical Electric Dipoles (VED)

Horizontal Electric Dipoles (HED)

Vertical Magnetic Dipoles (VMD)

Horizontal Magnetic Dipoles (HMD)

|N| = 4»/2 = (^tan'^s/e ) , r

s = 60 \ cKmhos/m), a = (2h)(2irA.) >

e : relative dielectric constant of ground

r °

cr(mhos /m): conductivity of ground

h: height in meters of antenna above ground

\: wavelength in meters

87
Figure A«*29. Normalised input impedance change for VED,

[N] = 1.05 *

88
89
90
91

93





99
100
101
102
103

105


Figure A-47. Normalized input impedance change for VMD,

| N| = 1.05 .

108
109
-0.2 0 0.2 0.4 0.6 0.8 10 1.2 1.4 16 1.8 2.0 -(f)

Figure A-49. Normalized input impedance change for VMD,

|n! = 2.

no
Ill
112
5000

2000

gure A-52. Normalized input impedance change for VMD, J Nj =

113




Figure A- 56. Normalized input impedance change for HMD

|Nj = 1.05 .

118
119
120
0.1 0.2 0.5 0.7 I 2 5 7 10 20 50 70 100

Figure A-59. Normalized input impedance change for HMD,

|n| = 2 .

121
122
123

125
126

ft U. S. GOVERNMENT PRINTING OFFICE : 1964 O - 739-039