.au or standards Mbrary, M. Bldg

iV 2 31962

X^ecknical vtete

PB 161640

739

SITING CRITERIA FOR

HF COMMUNICATION CENTERS

WILLIAM F. UTLAUT

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

The functions of the National Bureau of Standards are set forth in the Act of Congress, March 3, 1901, as amended by Congress in Public Law 619, 1950. These include the development and 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; invention and development of devices to serve special needs of the Government; and the development of standard practices, codes, and specifications. The work includes basic

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and various consultation and information services. Research projects are also performed for other government

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 divisions and sections on the inside of the back cover.

Publications

The results of the Bureau's research are published either in the Bureau's own series of publications or

in the journals of professional and scientific societies. The Bureau itself publishes three periodicals avail- able from 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 pre- liminary reports on work in progress; and Basic Radio Propagation Predictions provides data for determining

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and Technical Notes. A complete listing of the Bureau's publications can be found in National Bureau, of Standards Circular

460, Publications of the National Bureau of Standards, 1901 to June 1947 ($1.25), and the Supplement to National Bureau of Standards Circular 460, July 1947 to June 1957 ($1.50), and Miscellaneous Publication 240,

July 1957 to June 1960 (Includes Titles of Papers Published in Outside Journals 1950 to 1959) ($2.25); avail- able from the Superintendent of Documents, Government Printing Office, Washington 25, D. C.
NATIONAL BUREAU OF STANDARDS

technical ^tete

739

APRIL 1962

SITING CRITERIA FOR

HF COMMUNICATION CENTERS

WilUam F. Utlaut

NBS Boulder Laboratories

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

listed or referred to in the open literature. They are for sale by the Office of Technical Services, U. S. Department of Commerce, Washington 25, D. C.

DISTRIBUTED BY

UNITED STATES DEPARTMENT OF COMMERCE

OFFICE OF TECHNICAL SERVICES

WASHINGTON 25, D. C.

Price $1.25

Contents

1. Purpose 1

2. Nature of high frequency propagation 1

2. 1 Propagation path classification 1

2. 2 Ionospheric characteristics 2

3. Elements of siting 6

3. 1 Angles of departure and arrival «

3. 1. 1 Single-hop transmission 7

3. 1. 2 Transmission over distances greater than one-hop ?

3. 2 The effect of ground 8

3.3 Vertical angle of beam 12

3.4 The Fresnel zone 13

3. 5 Fresnel zone siting 15

3. 5. 1 Limited ground reflection areas 1°

3. 5. 2 Fresnel-zone roughness lo

3. 5. 3 Sloping sites IS

3. 5. 4 Elevated sites 19

3. 5. 5 Horizon 19

4. Radio noise and interference 20

4. 1 Types of radio noise 20

4. 1. 1 Atmospheric noise 20

4. 1. 2 Man-made noise 21

4. 1. 3 Galactic noise 21

4. 1. 4 Interference from nearby radio stations 21

4. 2 Site evaluation with regard to radio noise 22

4. 2. 1 Vehicular traffic 22

4. 2. 2 Industrial areas 23

iii
ABSTRACT

This note is intended to provide an engineering

guide for the siting of radio terminals for use in longrange HF communication systems. A brief summary

of ionospheric effects upon HF radio signals is presented.

Various factors required for an ideal site such as

Fresnel zone size and smoothness, noise, antenna

height, vertical radiation angle and the electrical

characteristics of the ground are discussed. Standards

to guide compromises for practical cases of siting are

considered. A partial bibliography which may be

useful for those desiring a greater depth of understanding

of the various subjects considered is provided.

iv
SITING CRITERIA FOR HF COMMUNICATION CENTERS

by

W. F. Utlaut

1. Purpose

Electrical and topographical characteristics of terminal sites

are of considerable importance in their effects on radio propagation,

and reception, for long-range ionospheric communication. It is the

purpose of this report to provide an engineering guide for the siting

of such radio terminals based on the current state of knowledge of

propagation factors and communication techniques. Aspects of siting

such as ground conductivity and dielectric constant; Fresnel zone

requirement, size and clearance; ambient noise level, vertical angle

of radiation; and other factors for an ideal site will be considered and

standards to guide compromises for practical circumstances will be

given.

2. Nature of high frequency propagation

2. 1 Propagation path classification

The energy radiated from a transmitting antenna may reach the

receiving antenna over any of several propagation paths. These are

generally divided into three classes; the space wave, the ground wave,

and the sky wave. The space wave generally consists of two components, one traveling the direct path from transmitter to receiver,

the direct wave, and the other, the ground-reflected wave, reaching

the receiver after one (or more) reflections from the ground.

Additional space -wave energy can reach the receiver through

scattering from various physical objects such as trees and hills and

due to diffraction around the earth' s surface and refraction in the

upper atmosphere. Space -wave communication accounts for most

communication above thirty megacycles, ionospheric scatter

propagation excepted.

- 1 -
- 2 -

The ground wave is guided along the earth' s surface, with the

earth carrying an induced current, much as an electromagnetic wave

is guided by a transmission line and energy is abstracted from the

ground wave to supply losses in the earth. Above 1600 kilocycles

(kc/s) the ground wave is attenuated to such an extent that its effect

is negligible at any appreciable distance from the transmitter.

For radio waves having frequencies between those propagated by

the space wave and the ground wave most propagation is via the sky

wave. The sky wave is one which leaves the transmitter at angles

above the horizon travelling outward from the earth until it reaches

the ionosphere at heights of 80-400 km (50-250 miles) where it is

reflected back toward the earth' s surface. It is this mode of propagation utilized for high frequency (3-30 Mc/s) communications between

terminal sites of which this report is concerned.

Z. 2 Ionospheric characteristics

A knowledge of some of the more important characteristics of

the ionosphere will aid in an intelligent over -all design of a communication system. The ionosphere is that region of the earth' s atmosphere

in which some of the constituent gases are ionized by various means

such as ultraviolet light from the sun, cosmic rays, meteors, and

perhaps other mechanisms. This ionized region extends from 50-500 km

or greater and although some ions and electrons are in existence to

some extent throughout the entire range the structure of the ionosphere

seems to be characterized by several layers, or areas, in which the

ion density is greater than it is immediately above or below this level.

The character of these layers although having diurnal, seasonal, and

geographical variations is well enough defined to be distinguishable

at most times and layers are classified according to ascending heights

as the D, E, Fl and F2 regions. Each of these regions is capable of
- 3

-

reflecting some frequency range of radio waves, however, for high

frequency considerations, the D region does not act as a reflecting

layer but rather as an ionized region below the E layer where its main

effect on HF communication is the daytime absorption, or attenuation,

of radio waves in the lower portion of the high-frequency spectrum.

If an HF radio wave leaves the earth at a small angle, a considerable distance can be covered before it comes down again after

reflection from E or F regions of the ionosphere. Under normal

conditions and with reflection from the F2 layer a radio wave emanating

from a transmitter at the earth' s surface can reach a point about

4000 km (2500 miles) away. With sufficient power much greater

distances may be achieved by a process of multiple reflections or "hops" between the earth and the ionosphere. Because of the lower

height of the E region the greatest distance a single hop E regionreflected radio wave can cover is about 2000 km (1250 miles).

Figure 1 shows a probable distribution of ion density as a function of

height along with some possible ray paths indicating reflections from

ionospheric layers.

The mechanism of reflection and refraction of radio waves by

the ionosphere is very dependent upon frequency. The highest frequency

which will be reflected at the ionosphere increases both with the angle

between the vertical and the ray path at the ionosphere and the ion

density of a layer. For any given frequency and ionization density

there is a maximum angle, above the horizon, at which a wave sent to

the ionosphere will be reflected back to the earth. Waves travelling

at greater angles above the horizon will not be reflected but continue

on into space. Ray path 3 in figure 1 represents this latter condition.

Because of this critical condition of reflection there exists then some

minimum ground distance -which may be covered by a sky wave of a
- 4 -

given frequency and this distance is known as the skip distance.

Correspondingly there is a maximum frequency which may be used

to send energy a certain distance away and this is known as the maximum

usable frequency (MUF). Thus the skip distance and the MUF are

inseparably related and dependent upon ionospheric conditions.

It has been mentioned that the D region primarily acts as an

attenuator for high frequencies which are reflected from higher levels.

Above the D region, at a height of 90 to 130 km (55-80 miles), is the

E layer in which the level of maximum ionization density remains

fairly constant throughout the day, as well as seasonally, but for the

most part disappears at night. Because of its comparatively low

height and maximum ion density, the transmission range which can be

achieved by reflection from the E layer is limited and useful propagation

is provided, mainly, for frequencies under ten megacycles.

In addition to the regular layer at E region heights it is found

that regions of much higher ion density exist at random times and

locations and are known as sporadic -E clouds. These over-dense

areas are at times capable of supporting propagation at frequencies

considerably greater than the normal E layer is, but because the

present state of knowledge does not allow prediction of their occurrence

at any particular place and time their potential usefulness in radio

communication is not realizable.

The F region exists above the E region, extending up to heights

in excess of 400 km (250 miles), in which the Fl and F2 layers are

either separate or merged into a single layer depending upon the time

of day. By convention when only a single layer is present it is

designated as the F2 layer. In general the F2 layer is the most

important layer for long distance high frequency communications
- 5 -

because of its height and greatest ion density which contribute to

higher MUFs and to greater transmission distances.

All of the layers exhibit variations, both regular and irregular

in nature, and unfortunately the F2 layer tends to exhibit a greater

variability than any of the others. The irregular variations are due

to many different causes which are essentially random in nature and

thus lead to a degree of unpredictability of ionospheric conditions.

Variations which tend to be more regular are of three types: diurnal,

seasonal and long-term.

These changes in all of the layers are related to the solar zenith

angle, and thus the amount of energy reaching the layers, but this

fact is not easily expressible by a simple law for all regions. In

particular the F2 layer diurnal maximum ionization lags behind the

altitude of the sun and ionization densities, and MUFs are higher

during the northern hemisphere winter than they are in summer. By

contrast, the D, E, and Fl layers have maximum ion densities in the

summer and the diurnal variations tend to relate to the solar zenith

angle more closely with ion density rising rapidly at sunrise, remaining

high through the midday, and declining again in the early evening.

Long term variations in ionization density in the various layers

tend to rise and fall, for a given hour and season, with the mean annual

sunspot number. These sunspot numbers follow a cycle which averages

about eleven years in length and during years of sunspot maxima ion

densities are high and higher MUFs prevail. D-region absorption

also increases, but the net effect is to increase the usable range of

frequencies. The opposite effect is produced during periods of

sunspot minima and the useful high frequency radio spectrum is

reduced in width.
- 6 -

Prediction methods for obtaining the MUF existing between any

two points on the earth at any time of day are available and are based

upon a large accumulation of data from past years and a prediction of

the average sunspot number. One such prediction service is provided

by the National Bureau of Standards CRPL Series D frequency predictions. Bulletins of MUF prediction are available monthly three

months in advance and methods of their utilization are given in NBS

Circular 462 and Signal Corps Technical Report No. 9.

3. Elements of siting

Inasmuch as the high frequency band nominally covers the

frequency range of 3 to 30 Mc, a wide range of propagation path

lengths prevails at any given time if proper frequencies are utilized.

In general, for short distances the lower frequencies and high-angle

radiation from antennas is required; for long distance communications

higher frequencies and low-angle antenna radiation is needed. Site

requirements necessary to align properly and to form antenna lobes

are dependent upon the communication path length and radio frequency

used. From these, antenna beam elevation angle, height of antenna,

and ground reflection characteristics can be established.

3. 1 Angle of departure and arrival

In order to design the best antennas for use at a communication

center, knowledge concerning the angles of departure or arrival of a radio signal for the prevalent mode or modes of propagation is

necessary. The antenna radiation angle required for propagation

optimization then determines the required height of the antenna above

the reflecting surface as well as the amount of radio- smooth area

needed in front of the antenna.
3. 1. 1 Single -hop transmission

The angle of arrival or departure usually assumed for single

hop transmission is that obtained from the geometry of a triangular

path over a curved earth with the apex of the triangle placed at the

height at which it is assumed that reflection of the radio wave occurs.

This technique of course is only approximate and there are many

variable ionospheric conditions which cause the angle of arrival to

change even though the communication path terminals are fixed.

In order to estimate the desired radiation angle of antennas for

any path less than 4000 km in length figure 2 may be used. In this

figure, a straight line drawn between the point on the left hand

distance scale, corresponding to the path length, and a point on the

virtual height scale, corresponding to the assumed height of reflection,

is extended to the 'point of intersection with the vertical line corresponding to the path length. This point of intersection then lies on or between parametric angle curves from which the radiation angle may

be found. For propagation by E -layer reflection a height of around

110 km can be assumed for the virtual height. A virtual height of

from 300 - 350 km may usually be assumed when the propagation is

via the F2 layer.

3. 1. 2 Transmission over distances greater than one -hop

The maximum range for single -hop propagation by any ionospheric layer would appear to be determined by the earth' s curvatuve

and the height of the reflecting layer. For angles of arrival and

departure equal to zero this distance for F2 layer reflection, at

various frequencies, may range from 3500 to 5000 km or greater.

Beyond the single -hop range a simple, but incomplete, picture

involves propagation by a number of geometric hops as indicated in

figure 1. Empirically, however, it has been observed in a number of
- 8 -

cases of long -path multi-hop propagation that geometric modes are

not clearly defined. Horizontal gradients of ionization density

effectively create tilts in the ionosphere or cause ionospheric roughness

which contribute to scattering of the radio wave. These and other

phenomena which exist in the normal ionosphere provide reasons why

simple geometric modes may not exist for long-distance propagation.

Observations of long -path radio signals have shown that the

greatest number of hours of propagation over a given path is afforded

when low-radiation-angle antennas are used. In addition, frequently

greater signal intensity is received by low-angle antennas than by

antennas with radiation angles optimized for geometric modes.

Although signals via geometric modes may predominate at times,

most modern communication systems have adequate power available

so that it is most desirable to obtain the greatest number of hours of

system operation; and to provide antennas with the lowest possible

angles of radiation. Angles of 5° or less are recommended. In

designing low radiation angle antennas the effect of the ground reflection

in producing nulls and maxima in the antenna pattern is very important

and it is necessary to have the antenna at heights several wavelengths

above ground.

3. 2 The effect of ground

When an antenna is situated over flat ground, the field radiated

downward is reflected from the ground and combines in a certain

manner with the field from the direct wave forming an interference

pattern with the latter so that at certain angles field reinforcement

takes place while at other angles the combined fields cancel thus

forming lobes of the antenna radiation pattern. The effect of the

presence of the ground on the vertical radiation pattern may be
- 9

-

obtained by use of the image principle. An example of lobe formation

using the geometry of the image antenna concept is shown in figure 3.

The reinforcement of the direct wave by the ground is of great

importance. Under favorable conditions, the reflected ray may be of

comparable intensity to the direct ray and thus the received field

strength may be approximately doubled (6 db increase) in places where

the two rays have the same phase.

In order to obtain a better insight of the problems encountered

in reflection it is desirable to analyze this very complex phenomenon

in simpler constituents. First, the incident radiation field is resolved

into plane wave components of which there are two types,each forming

a ray which strikes the reflecting surface within some small area

called the reflection point. The manner in which such rays are

affected by the ground upon reflection is dependent upon the plane of

polarization relative to the reflecting surface. In radio field work the

direction of the electric vector is used to define this state of polarization.

When the electric vector of the electromagnetic wave is parallel to the

reflecting plane the wave is said to be horizontally polarized; if the

electric vector is in the plane of incidence the polarization is known

as vertical polarization. If the irregularities of the ground reflecting

surface are comparable to a wavelength, the reflected field may be

very complex even though the incident field is linearly polarized and

if the reflecting surface is sufficiently rough, diffuse reflection may

result which is ineffective in reinforcing the direct wave. Whether or

not diffuse reflection occurs depends primarily upon the size of

irregularities of the reflecting surface in comparison to the height of

the antenna above the reflecting surface. This problem will be

discussed in more detail in section 3. 5. 2.

k.
- 10 -

By definition the reflection coefficient for either horizontally or vertically polarized waves is the ratio of any field quantity in the

reflected -wave to the same quantity in the incident 'wave. By this

definition it may be shown that the complex reflection coefficient is

of the form

R= pe-j * where p is the amplitude and <\> is the lag in phase of the reflected

wave with respect to the incident wave at the point of reflection. The

parameters p and <)> are dependent upon the angle of incidence, a, i.e.

the angle between the incident ray and the reflecting surface, and

upon the ground constants, conductivity and dielectric constant, of

the reflecting body. They are quite different for the two types of

polarization.

Although extensive investigations of soil conductivity and

dielectric constant have been made it is quite difficult to catalog,

according to soil type, or other parameters, values of these constants

which might be universally applied throughout the world. Generally

speaking, however, moist soil, marshy land, or -water surfaces make

the best reflecting surfaces showing much higher values of conductivity

and dielectric constant than does a chalky or very dry type of soil.

Because of the dependence of the soil constants upon moisture content

marked seasonal changes may be anticipated for a given locality. For

those cases where horizontal polarization is utilized, the problem of

ground reflection from various types of ground is somewhat simplified.

This is so because the reflection coefficient is very nearly equal to

minus one for all types of soils and range of incident angles normally

encountered on long distance communication circuits. Figures 4 and

5 show the variation in magnitude and phase of the reflection coefficient
- 11 -

for a horizontally polarized wave as a function of the incident angle

at various frequencies for a relatively good conducting earth. For a

sea water reflecting surface, values of p would be greater and<f>

nearer 180° than the curves shown while for a poorly conducting earth

p would be somewhat smaller and$ further from the 180° phase angle.

The coefficients of reflection and phase angle for vertical

polarization vary rapidly with frequency and incident angle. These

variations are shown in figures 6 and 7 and it may be seen that the

results are quite different from those for horizontal polarization. As

in the case of horizontal polarization for angles of incidence near

grazing, a * 0, the magnitude of the reflected wave is equal to the

incident ray and 180 degrees out of phase with it. This is true for

all frequencies and ground conductivities. As the value of a increases

from zero, however, both the magnitude and phase of the reflected

wave rapidly decrease until the magnitude reaches a minimum value

and the phase angle is at -90 degrees. The incident angle associated

with this minimum magnitude is known as the Brewster angle. At

angles of incidence above the Brewster angle the reflection coefficient

magnitude increases and the phase angle approaches zero.

Frequently, although not always, elevated antennas horizontally

polarized are used for communication circuits in the high frequency

band. There are several advantages of utilizing such antennas among

which are (1) less response to locally generated man-made noise which

propagates as a predominately vertically polarized surface wave, (2)

convenience of antenna excitation, (3) less ground system loss than

with ground-based vertical antennas, and (4) some control over the

vertical radiation pattern by selection of the proper height of the

antenna above the reflection surface. Factors affecting the vertical

radiation pattern are discussed in the next section.
- 12 -

3. 3 Vertical angle of beam

The vertical pattern of an antenna, i. e. a relative plot of field

strength or power as a function of elevation angle in a vertical plane

passing through the antenna, in the absence of a reflecting surface is

referred to as a free -space pattern. The vertical directivity of an

actual antenna then is modified by the local terrain. In the discussion

to follow immediately assumptions of horizontal polarization and a

smooth, good reflecting local terrain are made. The effect of the

presence of the ground on the vertical radiation pattern can be

obtained by use of the image principle and the principle of pattern

multiplication. Briefly, this means that the ground is replaced by an

image antenna located a distance of 2h below the actual antenna where

h is the height above the ground at the reflecting point of the actual

antenna. The polarity of the image antenna is opposite to that of the

actual antenna. The field of this image antenna is then added to that

of the actual antenna to yield the resultant field. In this analysis the

relative horizontal pattern remains unchanged. For horizontal antennas,

in the vertical plane perpendicular to the axis of the antenna, the freespace pattern can be multiplied by the factor

, . /2TTh . \ (3.3-1)

2 sin ( —— sin a

J

to account for the effect of the ground where the symbols h and a have

the meaning indicated in figure 3. The height of antenna, h, and the

wavelength of the operating frequency, X, must be in identical units.

Lobe maxima occur whenever the sum of the phase shifts

caused by reflection and path difference from actual and image antenna

points equal an even number of it radians and lobe minima or nulls

occur when the total phase shift is equal to an odd multiple of tt radians.
- 13 -

Lobe maxima and minima are found at angles corresponding to

a = sin -77- ,n = l,2,3... (3. 3-2) 4h

with n odd giving lobe maxima and n even the nulls.

For the case of vertical polarization no simple factor by which

the antenna free -space pattern can be multiplied to give the vertical

directivity pattern can be given because of the variability of reflection

coefficient with antenna height, however, lobe maxima and minima can

be determined 'approximately after determining the phase of reflection,

<j>, by the formula

a = sin"

1 n+ 4>/ ff - 1 (3. 3-3)

4h

The above formulas apply strictly only to the case of a plane

earth. The Reflection of a beam of radiation from a spherical earth

causes divergence or spreading of the wave and reduces the magnitude

of the reflected wave.

3.4 The Fresnel zone

A complete understanding of Fresnel-zone theory is not

essential to successful siting of ionospheric communication terminals

and therefore only salient results of the theory which are pertinent for

use in practical siting work are briefly presented below.

The concept of a first Fresnel zone is introduced to describe

an area required in front of an antenna in order to effectively form the

lowest ground reflection lobe. The ground for a considerable distance

in front of the antenna plays an important part in the formation of the

vertical plane radiation pattern; this distance increasing rapidly as

the angle of arrival or departure decreases.

With the aid of figure 8 the first Fresnel zone is defined with

respect to a reference ray as the area of ground, assumed to be a

—
- 14 -

smooth plane, in front of a point antenna A from within -which all

contributions of reflected energy in a plane -wave front, advancing

in* the direction of the reference ray, differ in phase by 180 degrees

or less from the phase of the reference ray. Horizontal polarization

only is considered in order that lobe formation not be complicated by

the large variations in ground-reflection coefficient associated with

low angle, vertical polarization transmissions. The antenna is

assumed to be a point source and the ground-reflected wave is

assumed to have the same amplitude as the incident wave but to

undergo a 180-degree phase change at reflection.

Utilizing geometric optics, with the origin of the coordinates,

O, coinciding with the ground reflection point for the reference ray

the following equations may be shown to express the important

dimensions of the first Fresnel zone.

The height of an antenna above the Fresnel zone for maximum

radiation in the direction of the ionospheric reflection point is, for

horizontal polarization,

h=—^— . (3.4-1)

4 sin a The distance from the antenna base, B,.to the geometric groundreflection point, O, is given by,

d = . (3.4-2)

tan a The distance from the antenna base B to the near edge of the Fresnel

zone may be calculated from the equation

d= -A-Cs-^M , (3.4-3) N tan a \ cos a J

and the distance to the far edge of the Fresnel zone is determined

from the relationship
- 15 -

d = -^— (3+ 2^y2

J

. (3.4-4) F tan a V J

cos a

' The maximum width of the zone is

w = 4n/2 h = 5. 66 h . (3.4-5)

The shape of the Fresnel zone, for a plane-earth case, is that

of an ellipse with the major axis oriented in the direction of propagation.

The effect of the curvature of the earth on the first Fresnel zone has

the quantitative result of reducing in size the elliptical area and altering

it into an elongated egg shape with the broad end near the antenna. The

value of cL is almost unaffected, while the distance d is reduced somewhat and the distance d^ reduced considerably. The maximum width F

is reduced only slightly but occurs somewhat nearer the antenna.

Earth curvature does have an important practical effect of reducing

the height, h, required for formation of the first lobe at a specified

angle. All of the effects of earth curvature are appreciable only -when

the angle of arrival or departure, a, is very small, 3 degrees or less.

The relationship between antenna radiation angle and the antenna height

and first Fresnel-zone dimensions for the plane -earth case are shown

in figures 9 through 13 for representative high frequencies. The

accuracy obtained from these curves is adequate for practical siting

problems.

3. 5 Fresnel zone siting

Because of the importance of the Fresnel zone in forming the

first lobe, particular attention must be given in a siting survey to this

zone for both transmitting and receiving antenna sites. In general

idealized conditions will not be found and thus compromises must be

made. The intent of this section is to offer guidance for acceptable

compromises but it should be well understood that compromises
- 16 -

will result in a degradation of service and that a compounding of

compromises may result in a site which would provide completely

unsatisfactory performance. Factors to be considered for each

antenna are the dimensions of the Fresnel zone, terrain irregularities

within this zone and the horizon clearance, in the propagation

direction, from all portions of the zone.

3. 5. 1 Limited ground reflection areas

As indicated earlier, figures 9 to 1 3 show the required

dimensions of the first Fresnel zone. In some cases it may be

impossible to obtain a site having dimensions as large as those

indicated but which may otherwise be desirable. The following factors

are suggested as a guide -when compromise is necessary and are

obtained by considering the dimensions, in limiting cases, of that

portion of the first Fresnel zone which includes phase shifts of the

reflected ray up to X/4, or 90°. While these factors may prove useful

where it is impossible to find an adequate Fresnel zone it should be

recognized that degradation in system performance by as much as 2 db

(37% reduction in power) is to be expected at each site where compromise is made. The quantities listed below indicate the range of

values by which the Fresnel zone dimensions may be altered in this

compromise.

(a) d may be decreased in length from 1 to 0. 6 times full value F

(b) cL may be increased in length from 1 to 1.6 times full value

(c) w may be reduced from 1 to 0. 7 times full value

3. 5. 2 Fresnel zone roughness

The preceding discussion of Fresnel zones was based on

reflecting surfaces which were ideally smooth. Because the earth

is not necessarily radio-smooth, consideration to the effects of surface

irregularities upon the ground reflecting properties must be given. In

order to estimate how closely the ground approximates the condition of
- 17 -

an ideal reflecting surface for radio waves, guidance is obtained from

optical theory developed by Rayleigh. Rayleigh 1 s criterion indicates

that a transition region between specular reflection and scattered

reflection occurs when the height of an irregularity exceeds a value of

H.= t-£—.— (3.5.-1)

16 sin a

Hence a rule of thumb for determining maximum heights of deviations

(above or below) from the average terrain profile, which will still

allow specular reflection, may be shown to be equal to one -quarter of

the antenna height.

H

|

(3.5-2)

This is admittedly an approximation but does give an indication of the

size of tolerable irregularities of terrain.

"When the relative contribution of energy from the various

portions of the Fresnel zone and the fact that the angle at which a given

ray strikes the ground varies over the zone are taken into account, it

is clear that permissible deviations increase with distance from the

antenna. An exact solution to the problem, however, is difficult. The

following table is suggested as a guide when compromise must be made

with regard to terrain roughness and presents upper limits of departure

from the ideal smooth Fresnel zone. The Fresnel zone has been divided

into three regions determined approximately by the geometry reflection

point, the far X/4 point, and the zone extremities.

Compromise Roughness Criteria

Limit of departure from Region of Fresnel zone

average smooth terrain

h/4 <L_ to 0. 2 d_ N F

h/2 0. 2 d to 0.6 d_ F F

h 0.6d to d^ F F

I *
- 18 -

The amount of service degradation produced -when the above

compromise is made is not known but it is certain that there will be

some. This criterion ignores the surface shape of the obstacles and

while an obstacle may be within the height criteria given it may still

eliminate a major portion of the reflected energy, for example sharp

depressions or low hills. The criteria given above applies to cases

where the entire first Fresnel zone is rough. When a major portion

of the zone is smooth it is likely that somewhat larger obstacles may

be tolerated if they do not cover more than say 5 percent of the area.

Extreme caution should be used in selecting sites where compromise

in both area size and roughness must be made simultaneously.

3. 5. 3 Sloping sites

For very long distance communications it is most desirable to

use very low angle radiation, as has been pointed out, and thus it is

important to utilize terrain advantages, where possible, to minimize

antenna construction cost. Antenna tower heights may be substantially

reduced if antennas are located where the Fresnel zone area slopes

downward in the direction of the propagation path. The effect of sloping

terrain downward in front of the antenna is to give an apparent increase

in antenna height and resultant decrease in radiation angle. An additional

advantage is a reduction in Fresnel zone dimensions. Figure 14 represents the geometry of reflection from a sloping site where |3 is the angle

between the horizontal and the terrain. From this it is easily seen

that the effective antenna height above ground for sloping terrain, h , s is given for horizontal polarization by

h

a S 4sin\a+(3) (3 ' 5 " 3)

and the actual height above ground is
- 19 -

t h = h

S (3.5-4)

s cos (3

'

If the terrain slopes in the opposite direction, i. e. , upwar d in

the direction of propagation then the antithesis of the above arguments

is true and (3 is taken to be negative.

When sloping terrain is utilized equation 3. 5-3 is used to find

h and this value in turn is then used in conjunction with figures 9 to

s 13 to obtain values of cL , d and w, for the Fresnel zone dimensions.

3. 5. 4 Elevated sites

Another obvious method of reducing antenna construction cost

and still provide low angle radiation is to make use of elevated terrain

sites such as hills, mesas, or cliffs which overlook the Fresnel zone.

The effective height of the antenna is then the vertical distance

measured between the antenna and a horizontal line passing through

the geometric reflection point.

3. 5. 5 Horizon

In order effectively to form the lower portion of the first vertical

lobe the horizon in the direction of propagation as viewed from the

entire Fresnel zone ideally must be free of obstructions. Satisfactory

conditions will probably prevail if the horizon as seen from all parts

of the Fresnel zone does not extend up beyond angles of a/2 degrees.

If compromise in horizon clearance is required the criterion of

horizon less than a/ 2 degrees should be met for at least the X./4 phase

points of the zone while the remainder of the zone should have clearance

of horizon less than a degrees.

wmrr
- 20 -

4. Radio noise and interference

4. 1 Types of radio noise

Noise and interference from other communication systems are

two factors limiting the useful operating range of all radio equipment

and must be considered in selecting a receiver site. External radio

noise, i. e. , other than receiver set noise, consists of three types;

namely, atmospheric, man-made, and galactic. Each type has

different characteristics which are briefly discussed below.

4. 1. 1 Atmospheric noise

Most of the atmospheric noise in the world originates in thunderstorms. Of the three noise types, atmospheric noise is the most

erratic in nature consisting of short, randomly occurring pulses

superimposed upon a background of random noise. At a given location

the atmospheric noise is made up of noise from nearby noise centers,

such as local thunderstorms, plus noise which has been propagated

over great distances from one of the principle noise centers, such as

the active thunderstorm areas in equatorial Africa, Central America,

and the East Indies. The activity of the various noise centers vary

with time of day and season and so also do propagation conditions which

transmit the noise. Major thunderstorm centers tend to shift above

and below the equator from summer to winter and even though ionospheric absorption is greater in the summer, increased storm activity

seems to predominate and as a result received noise tends to be highest

in the summer and lowest in the winter.

Received noise level varies with frequency, in general decreasing

with increasing frequency. There are variations also with geographical

location, highest noise levels being encountered in equatorial regions

and the lowest levels in polar regions.
- 21 -

4. 1. 2 Man-made noise

Man-made noise may arise from any number of sources such as

power lines, industrial machines, diathermy machines, ignition systems,

etc. , and thus its characteristics vary over a wide range of limits.

Much of the noise energy from sources of this type are propagated to

the receiver over power lines or by ground wave propagation. In

general the level of man-made noise decreases with increasing frequency

due in part to the source spectrum and in part to propagation factors.

4. 1. 3 Galactic noise

Galactic noise has a characteristic similar to thermal noise with

a Gaussian amplitude -time distribution. The intensity of galactic noise

decreases with increasing frequency. The amount of energy impinging

upon an antenna, in part, is dependent upon the portion of the sky seen

by the antenna lobes. Great noise sources are found in the direction

of the constellations Sagittarius and Cassiopeia. Since the galactic

noise source is external to the ionosphere the noise level received is

dependent upon ionospheric absorption characteristics and the relationship of the operating frequency to the critical frequency. In arctic

regions, where very low critical frequencies are often encountered

and noise from atmospheric and man-made sources is low it is

expected that at times galactic noise may be the principle external

noise source in the HF band. At temperate and tropical latitudes,

where critical frequencies are higher and atmospheric noise stronger,

galactic noise is usually negligible below 15 or 20 Mc.

4. 1 . 4 Interference from nearby radio stations

Radio transmitters located within several miles of a receiving

station may create serious interference due to harmonics, keying

transients, parasitic oscillation or co-channel operation. It is of

course also possible, when receiver sites are in strong radio frequency

1
- 22 -

fields, that cross -modulation products may be generated in receivers

even though transmitter frequencies are greatly different from the

normal receiving frequency. Existing, and even proposed, sources

of radio interference must be taken into account in the overall

evaluation of a site.

4. 2 Site evaluation with regard to radio noise

The above paragraphs have briefly indicated sources and

characteristics of noise and radio interference. Optimumly, the

noise levels at sites chosen for high frequency receiving terminals

should not exceed that due to natural causes of atmospheric or galactic

noise. The expected values of atmospheric or galactic noise levels

in the HF band on a world-wide basis for all times of day and seasons

of the year may be found in the International Radio Consultative

Committee (CCIR) report No. 65. While only through a noise

measurement program can it be ascertained whether natural or

man-made sources are the principle contributors of noise at a given

site there are obvious man-made environments near which a receiver

site should not be located. Some of these are enumerated below.

4. 2. 1 Vehicular traffic

Since this is a major source of radio noise it is most desirable

to choose receiving sites in locations where there is a minimum of

nearby vehicular traffic and it is especially important that no traffic

pass immediately in front of the receiving antennas. A reasonable

distance of clearance between a heavily travelled highway and a receiving antenna, in the direction of the main lobe, is probably

three miles or more. Somewhat smaller distances may be tolerated

in other directions as determined by the horizontal pattern of the

antennas used.
- 23 -

4. 2. 2 Industrial areas

Manufacturing areas, areas near power plants or power substations, medical facilities of diathermy and X-ray, street lighting

systems and high-voltage overhead power lines are examples of

environments likely to generate radio noise. This noise may be

radiated in a manner similar to that in which radio waves are radiated

from antennas or it may be conducted over an appreciable distance by

power lines and radiated by those which pass near the receiving site.

TT-
- 24 -

Bibliography for Section 2

1. Appleton, Sir Edward, Regularities and irregularities in. the

ionosphere, Proc. Royal Soc. of London, Ser. A, 162,

451-479 (Oct. 1937).

2. Appleton, Sir Edward and W. J.G. Beynon, The application of

ionospheric data to radio communication problems, Pt. 2,

Proc. Phys. Soc. of London, _59, 58-76 (1947).

3. Bennington, T. W. , Short-wave radio and the ionosphere,

seconded., (Wireless World, Iliffe and Sons, London, 1950).

4. Mitra, S.K. , The upper atmosphere, second ed. , Ch. VI,

The ionosphere (p. 176-353) (The Asiatic Society, Calcutta,

India, 1952).

5. Pfister, Wolfgang, et al, The ionosphere (Ch. 15, Handbook

of Geophysics for Air Force Designers, first ed. , Washington,

D. C. , 1957).

6. Ratcliffe, J. A. , The magneto-ionic theory and its application

to the ionosphere, (Cambridge University Press, London, 1959).

7. Rawer, Karl, The ionosphere, (Frederick Ungar Publishing Co. , New York, N. Y. , 1957).

8. Smith, Newbern, The relation of radio sky-wave transmission

to ionospheric measurements, Proc. IRE 27, 332-347

(May 1939).

9. U.S. National Bureau of Standards, Circular 462, Ionospheric

radio propagation, (Gov. Printing Office, Washington, D. C. , 1948).
- 25 -

Bibliography for Section 3

1. Bailey, D. K. , R. Bateman, and R. C. Kirby, Radio transmission at VHF by scattering and other processes in the

lower ionosphere, Proc. IRE 43, 1181-1231 (Oct. 1955)

See pp. 1226-1229).

2. Burrows, C. R. , and S. S. Attwood (Eds. ), Radio wave

propagation, (Academic Press, Inc., New York, N. Y. , 1949).

3. Jordan, E. C. , Electromagnetic waves and radiating systems,

(Prentice -Hall, Inc., New York, N. Y. , 1950).

4. Utlaut, W. F. , Effect of antenna radiation angles upon HF

radio signals propagated over long distances, J. Research

NBS65D, 167-174 (Mar. -April 1961)
- 26 -

Bibliography for Section 4

1. Agger, C.V., D. E. Foster, and C. S. Young, Instruments

and methods of measuring radio noise, Electrical Engineering,

59, 178-192 (March 1940).

2. International Radio Consultative Committee, CCIR Report No.

65, Revision of atmospheric radio noise data (International

Telecommunication Union, Geneva, 1957).

3. Lovell, B. , and J. A. Clegg, Radio astronomy (John Wiley

and Sons, Inc. , New York, N. Y. , 1952).

4. Menzel, D. H. , The radio noise spectrum, (Harvard University

Press, Cambridge, Mass., I960).

5. Pawsey, J. L. and R. N. Bracewell, Radio astronomy

(Clarendon Press, Oxford, England, 1955).

6. Pawsey, J. L. , L. L. McCready, and F. F. Gardner,

Ionospheric thermal radiation at radio frequencies, J. A. T. P.

1_, 261-277, 1951).

7. Rawer, Karl, The ionosphere (Frederick Ungar Publishing Co. ), pp. 172-175 (1957).

8. U.S. National Bureau of Standards, Circular 462, Ionospheric

radio propagation, Ch. 8 (Gov. Printing Office, Washington,

D. C. 1948).

9. U.S. National Bureau of Standards, Circular 557, Worldwide

radio noise levels expected in the frequency band 10 kilocycles

to 100 megacycles, (Gov. Printing Office, Washington, D. C. , 1955).
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25 30
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Figure II
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U. S. DEPARTMENT OF COMMERCE

Luther H. Hodges, 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 the field indicated by

its title. A brief description of the activities, and of the resultant publications, appears on the inside of the

front cover. WASHINGTON, D. C.

Electricity. Resistance and Reactance. Electrochemistry. Electrical Instruments. Magnetic Measurements.

Dielectrics. High Voltage.

Metrology. Photometry and Colorimetry. Refractometry. Photographic Research. Length. Engineering Metrology.

Mass and Scale. Volumetry and Densimetry.

Heat. Temperature Physics. Heat Measurements. Cryogenic Physics. Equation of State. Statistical Physics.

Radiation Physics. X-ray. Radioactivity. Radiation Theory. High Energy Radiation. Radiological Equipment.

Nucleonic Instrumentation. Neutron Physics.

Analytical and Inorganic Chemistry. Piu-e Substances. Spectrochemistry. Solution Chemistry. Standard Refer- ence Materials. Applied Analytical Research. Crystal Chemistry.

Mechanics. Sound. Pressure and Vacuum. Fluid Mechanics. Engineering Mechanics. Rheology. Combustion

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and Metal Deposition.

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Crystallography.

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Engineering Applications. Systems Analysis.

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Instrumentation. Engineering Electronics. Electron Devices. Electronic Instrumentation. Mechanical Instru- ments. Basic Instrumentation. Physical Chemistry. Thermochemistry. Surface Chemistry. Organic Chemistry. Molecular Spectroscopy. Molecular Kinetics. Mass Spectrometry.

Office of Weights and Measures. BOULDER, COLO.

Cryogenic Engineering Laboratory. Cryogenic Equipment. Cryogenic Processes. Properties of Materials. Cryogenic Technical Services. CENTRAL RADIO PROPAGATION LABORATORY

Ionosphere Research and Propagation. Low Frequency and Very Low Frequency Research. Ionosphere Research. Prediction Services. Sun-Earth Relationships. Field Engineering. Radio Warning Services. Vertical Soundings Research.

Radio Propagation Engineering. Data Reduction Instrumentation. Radio Noise. Tropo spheric Measurements.

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Radio Systems. Applied Electromagnetic Theory. High Frequency and Very High Frequency Research. Modulaiation Research. Antenna Research. Navigation Systems.

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Standards. Millimeter-Wave Research.

Circuit Standards. High Frequency Electrical Standards. Microwave Circuit Standards. Electronic Calibration Center.
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