UNITED STATES DEPARTMENT OF AGRICULTURE

**Rural Electrification Administration**

**BULLETIN 1751H-701**

**SUBJECT: [Radio System Fundamentals and Point-to-Point]{.underline}**

**[Digital Microwave Radio Systems]{.underline}**

**TO:** Telephone Borrowers

REA Telephone Staff

**EFFECTIVE DATE:** Date of Approval

**EXPIRATION DATE:** Three years from the effective date.

**OFFICE OF PRIMARY INTEREST:** Transmission Branch,

Telecommunications Standards Division

**PREVIOUS INSTRUCTIONS:** This bulletin replaces TE&CM Sections 930,
931, and 932 which are to be rescinded.

**FILING INSTRUCTIONS:** Discard existing TE&CM Sections 930, 931,

and 932 and place this bulletin in Section 1751H, Transmission Design
Bulletins.

**PURPOSE:** To provide basic information on radio telecommunications
and specific information on general point-to-point microwave radio
systems and on microwave radio propagation. Major emphasis is placed on
digital systems, but some information on the carriage of analog
television signals is included. This bulletin provides information on
FCC frequency allocations and FCC application forms required from system
operators. All information in this bulletin is advisory.

Michael M.F. Liu 4-7-92

\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_       \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_

Acting Administrator Date

**REA Bulletin 1751H-701**

Page 2

**RADIO SYSTEM FUNDAMENTALS AND**

**POINT-TO-POINT DIGITAL MICROWAVE RADIO SYSTEMS**

**TABLE OF CONTENTS**

**1. POINT-TO-POINT MICROWAVE RADIO SYSTEMS**

1.1 GENERAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-1

1.2 FEDERAL COMMUNICATIONS COMMISSION REQUIREMENTS . . . . . . . . . .
1-2

1.3 CAPABILITIES OF MICROWAVE . . . . . . . . . . . . . . . . . . . . .
1-6

1.4 SITUATIONS WHERE MICROWAVE MAY BE DESIRABLE . . . . . . . . . . . .
1-6

1.5 COMPONENTS OF A MICROWAVE SYSTEM . . . . . . . . . . . . . . . . .
1-7

1.6 MICROWAVE EQUIPMENT ARRANGEMENTS . . . . . . . . . . . . . . . . .
1-10

1.7 EQUIPMENT PRESENTLY AVAILABLE FOR COMMON CARRIERS . . . . . . . . .
1-12

1.8 MICROWAVE SYSTEM CONSIDERATIONS . . . . . . . . . . . . . . . . . .
1-14

**2. MICROWAVE RADIO PROPAGATION AND PATH SURVEYS**

2.1 CHARACTERISTICS OF MICROWAVE TRANSMISSION . . . . . . . . . . . . .
2-1

2.2 LOCATION OF STATIONS AND PATH SURVEYS . . . . . . . . . . . . . . .
2-1

2.3 LOCATION OF ANTENNA SITES . . . . . . . . . . . . . . . . . . . . .
2-1

2.4 PATH PROFILE. . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-2

2.5 REFRACTION. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-4

2.6 DIFFRACTION AND WAVE INTERFERENCE . . . . . . . . . . . . . . . . .
2-6

2.7 REFLECTION. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-8

2.8 FADING. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-9

2.9 MICROWAVE PATH LOSSES AND GAINS . . . . . . . . . . . . . . . . . .
2-11

2.10 ILLUSTRATIVE EXAMPLE OF CALCULATION OF MICROWAVE ANTENNA HEIGHT . .
2-13

**3. RADIO SYSTEM GUIDE TO FORMS REQUIRED BY**

**THE FEDERAL COMMUNICATIONS COMMISSION**

3.1 GENERAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-1

3.2 POINT-TO-POINT MICROWAVE RADIO (FCC PART 21). . . . . . . . . . . .
3-2

3.3 PUBLIC LAND MOBILE RADIO, RURAL RADIO, DOMESTIC PUBLIC CELLULAR
RADIO

TELECOMMUNICATIONS, AND BETRS (FCC PART 22) . . . . . . . . . . . . 3-3

3.4 COMMON CARRIER LICENSE EXPIRATION DATES AND PROCESSING FEES . . . .
3-5

3.5 TELEPHONE MAINTENANCE RADIO (FCC PART 90) . . . . . . . . . . . . .
3-6

3.6 ANTENNA STRUCTURES (FCC PART 17) (FCC PART 1) . . . . . . . . . . .
3-8

3.7 EQUIPMENT AUTHORIZATION (FCC PART 2). . . . . . . . . . . . . . . .
3-10

3.8 REA REQUIREMENTS AND MISCELLANEOUS ITEMS. . . . . . . . . . . . . .
3-11

**REA Bulletin 1751H-701**

Page 3

**4. LIST OF FIGURES**

FIGURE A1 - REGENERATING REPEATER . . . . . . . . . . . . . . . . . . .
. 4-1

FIGURE A2 - BLOCK DIAGRAM OF TYPICAL MICROWAVE SYSTEM . . . . . . . . .
. 4-2

FIGURE A3 - BASIC RF EQUIPMENT ARRANGEMENT FOR A TWO HOP SYSTEM . . . .
. 4-3

FIGURE A4 - HOT STANDBY ARRANGEMENT FOR A TWO HOP SYSTEM . . . . . . . .
4-4

FIGURE A5 - SPACE DIVERSITY EQUIPMENT ARRANGEMENT . . . . . . . . . . .
. 4-5

FIGURE B1 - PATH PROFILE - RECTANGULAR GRAPH PAPER (Curved Microwave
Beam)4-6

FIGURE B2 - PATH PROFILE - RECTANGULAR GRAPH PAPER . . . . . . . . . . .
4-7

FIGURE B3 - PATH PROFILE - CURVED GRAPH PAPER . . . . . . . . . . . . .
. 4-8

FIGURE B4 - EXAMPLE OF MULTIPATH PROPAGATION BY REFRACTION . . . . . . .
4-9

FIGURE B5 - HUYGEN\'S WAVELETS . . . . . . . . . . . . . . . . . . . . .
. 4-9

FIGURE B6 - DIFFRACTION . . . . . . . . . . . . . . . . . . . . . . . .
. 4-10

FIGURE B7 - DIFFRACTION OVER A HILL . . . . . . . . . . . . . . . . . .
. 4-10

FIGURE B8 - FRESNEL ZONE . . . . . . . . . . . . . . . . . . . . . . . .
4-11

FIGURE B9 - CROSS-SECTION OF FRESNEL ZONE . . . . . . . . . . . . . . .
. 4-11

FIGURE B10 - FRESNEL ZONE ENVELOPES . . . . . . . . . . . . . . . . . .
. 4-11

**INDEX:** Microwave Radio Systems, Radio Propagation, Digital Radio
Systems,

FCC Radio Requirements, Point-to-Point Radio Systems

**ACRONYMS AND ABBREVIATIONS**

**(with Definitions)**

AC Alternating current

BER Bit error ratio or bit error rate

BETRS Basic Exchange Telecommunications Radio Service

CFR Code of Federal Regulations

CGSA Cellular Geographic Service Area

dB decibel - a power ratio commonly used by engineers. (Derived from

Bel which is the logarithm to the base 10 of the actual power ratio)

dBm decibels referenced to one milliwatt

DC Direct current

DS1, DS2, DS3 Designations used for digital carrier system line rates

FAA Federal Aviation Administration

FCC Federal Communications Commission

GHz Gigahertz (one billion Hertz)

log10 logarithm to the base 10

Mb/s Megabits (one million bits) per second

MHz Megahertz (one million Hertz)

NOAA National Oceanic and Atmospheric Administration

PCM Pulse Code Modulation

RF Radio frequency

SMRS Specialized Mobile Radio Service

USGS United States Geological Survey

ã Greek letter gamma used for wavelength in the text

Õ Greek letter theta defined in the text

Ð 3.1416, a mathematical constant

Note: There are more than 50 single letters and combinations of letters
used in [equations]{.underline} and [figures]{.underline} that are
explicitly defined in the text. They have no significance outside of
this bulletin, and have been arbitrarily chosen to explain some portion
of the text.

**1. POINT-TO-POINT MICROWAVE RADIO SYSTEMS**

**1.1 GENERAL**

1.1.1 This bulletin is intended to provide REA borrowers, consulting
engineers, and other interested parties with information on microwave
radio systems and other fundamental topics concerning radio. In
particular, it brings together in one place a large amount of basic
information on Federal Communications Commission (FCC) requirements. The
bulletin is divided into three parts so as to distinguish between three
basic topics. All information and recommendations provided in this
bulletin are [advisory]{.underline}. The bulletin is not intended to be
a tutorial. It is assumed that the reader already has a basic concept of
radio and its use in electrical communications.

1.1.1.1 The first part discusses the factors affecting the use of
microwave equipment currently available and should provide assistance in
determining the applicability of microwave for use in telephony.

1.1.1.2 The second part of the bulletin provides technical information
which can be used in making preliminary paper designs and economic
studies for microwave systems. It discusses in particular the theory of
radio path propagation and methods of making microwave path surveys.

1.1.1.3 The third part provides guidelines, information, and
recommendations on forms required by the Federal Communications
Commission (FCC).

1.1.2 Although emphasis is placed on general point-to-point microwave
service, this first part mentions three other fixed microwave services
that are licensed by the Federal Communications Commission (FCC) but not
normally provided by small independent telephone companies in a rural
environment.

Two of these provide for point-to-multipoint service but this should not
be confused with true area coverage.

1.1.3 Basic Exchange Telecommunications Radio Service (BETRS) and
Cellular Radio systems are examples of general subscriber service to a
large number of potential subscriber stations distributed throughout a
specified geographic area. These provide true area coverage but utilize
radio frequencies below microwave frequencies. Area coverage is not
included in this part of the bulletin.

1.1.4 The FCC has only recently started on the long process of revising
its extensive set of *Code of Federal Regulations* (CFR) volumes to
place all referenced terms in the metric system. This process does not
entail a simple mathematical conversion calculation. Rather, it involves
an adjustment of a regulatory quantity now expressed as a simple
(non-decimal) value in feet

or miles to a new arbitrary but approximate value in meters or
kilometers. Since a major portion of this bulletin discusses FCC
requirements it cannot at this time be fully converted to the metric
system. This will be accomplished through a revision at a later time.
This bulletin is not a specification; nor is it an REA regulatory
document.

1.1.5 Most manufacturers of microwave equipment conduct schools to
provide technical training for customer maintenance technicians. The
test equipment and methods recommended by the microwave supplier should
be used to make equipment measurements.

**1.2 FEDERAL COMMUNICATIONS COMMISSION REQUIREMENTS**

1.2.1 A microwave radio station authorization (license) must be obtained
from the FCC prior to commencement of any proposed station construction.
All construction must be completed within an 18 month construction
period extending from the date of the license grant. The license will
specify the termination date of the construction period. The license
will be issued for

a period not to exceed 10 years. It will normally expire on February 1
in the year of expiration. The FCC reserves the right to grant or renew
station licenses for a shorter period of time.

1.2.2 Microwave transmitting equipment must have an equipment
authorization from the FCC before a radio station authorization
(license) can be obtained. Either the manufacturer or the importer of
the transmitting equipment must apply for an equipment authorization
before it is offered for sale. Currently, the procedure to be followed
for point-to-point microwave radio transmitters is called notification.
In the past it was type acceptance. Either procedure will suffice. For
more information on FCC equipment authorization procedures see 47 CFR
2.901 through 47 CFR 2.1065. This citation covers subpart J of Part 2 of
the FCC Rules and Regulations. A brief discussion of this subpart can be
found in paragraph 3.7 of this bulletin.

1.2.3 Part 21 (or 47 CFR 21) of the FCC Rules and Regulations covers the
following four services:

\(a\) Digital Electronic Message Service

\(b\) Point-to-Point Microwave Radio Service

\(c\) Local Television Transmission Service

\(d\) Multipoint Distribution Service

1.2.3.1 Digital electronic message service is a two-way domestic
end-to-end fixed radio service utilizing digital termination systems
(point-to-multipoint systems consisting of nodal stations and their
associated user stations) for the exchange of digital information.

1.2.3.2 Point-to-point microwave radio service is a
[general]{.underline} domestic public radio service between fixed
stations. This service is the major topic being discussed in this
bulletin. It is the only FCC Part 21 service currently being offered by
REA telephone borrowers.

1.2.3.3 Local television transmission service is a domestic public radio
communication service for the transmission of television and related
material.

1.2.3.4 Multipoint distribution service is a one-way domestic public
fixed radio service from a transmitting station to multiple receiving
facilities.

1.2.3.5 The above four services utilize microwave radio frequencies
which FCC Part 21 defines as frequencies of 890 MHz and above. They are
available to all communication common carriers. The first three may only
be used for common carrier purposes. The last may be used for both
common carrier and non-common carrier purposes.

1.2.4 An FCC radio station authorization for Point-to-Point Microwave
Radio Service may permit the transmission of broadcast television
signals, closed circuit television signals and other non-broadcast
television signals if the FCC Common Carrier Bureau determines that the
applicant does not originate or control the content of the transmission.
The FCC may, when certain conditions are met, authorize a radio station
that will be used primarily to relay broadcast television signals. The
FCC makes the above determinations, not REA.

1.2.5 Frequencies in the following bands may be used by REA borrowers
for point-to-point microwave radio service:

\(a\) 932.5-935 MHz; 941.5-944 MHz

\(b\) 2110-2130 MHz; 2160 - 2180 MHz

\(c\) 3700 - 4200 MHz

\(d\) 5925 - 6425 MHz

\(e\) 10,550 - 10,565 MHz; 10,615 - 10,630 MHz

\(f\) 10,700 - 11,700 MHz

\(g\) 17,700 - 18,820 MHz; 18,920-19,160 MHz; 19,260-19,700 MHz

\(h\) 21,200 - 21,800 MHz; 22,400 - 23,000 MHz

The bands specified under (a) and (h) have little to offer REA borrowers
and, consequently, do not appear in the REA List of Materials. The FCC
lists four additional bands for point-to-point microwave radio service
but no manufacturing company offers equipment for use at the present
time. These bands are: 13,200 - 13,250 MHz; 27,500 - 29,500 MHz;
31,000 - 31,300 MHz and 38,600 - 40,000 MHz. The bands designated under
(b) through (h) above are quite often referenced as 2GHz, 4GHz, 6GHz, 10
GHz, 11GHz, 18GHz, and 23 GHz.

1.2.5.1 There are many footnotes associated with the frequency table in
47 CFR 21.701(a) that are too numerous to mention in this bulletin.
However, all telephone borrowers and their consulting engineers should
become familiar with these footnotes. Many of them are informational in
that they indicate other radio services that share the same frequency
band. A frequency search (which is always required) will detect possible
radio interference.

1.2.6 [Bandwidth Limitations for Point-to-Point Microwave
Radio]{.underline}. The maximum FCC authorized bandwidth for various
radio bands are set forth below:

[Frequency Band (MHz)]{.underline} [Maximum Bandwidth (MHz)]{.underline}

932.5 to 935; 941.5 to 944 0.2

2110 to 2130; 2160 to 2180 3.5

3700 to 4200 20.0

5925 to 6425 30.0

10,500 to 10,680 3.75

10,700 to 11,700 40.0

13,200 to 13,250 25.0

17,700 to 18,580; 19,260 to 19,700 220.0

18,580 to 18,820; 18,920 to 19,160 20.0

18,820 to 18,920; 19,160 to 19,260 10.0

21,200 to 22,000; 22,000 to 23,600 100.0

27,500 to 29,500 220.0

31,000 to 31,300 25.0 or 50.0

38,600 to 40,000 50.0

1.2.7 The FCC requires that microwave transmitters employing digital
modulation techniques and operating below 20 GHz shall have a bit rate
(in bits per second), equal to or greater than the occupied bandwidth in
Hertz. Currently available microwave radios usually have spectral
efficiencies of 2 to 5 bits per second per Hertz. As can be determined
from the preceding table, the bandwidth limitations for 2 GHz and 10 GHz
severely limit the capacity obtainable from these units. However, due to
new modulation techniques offering high spectral efficiency, 2 GHz
microwave radios have recently become available that permit bit streams
high enough to handle 288 encoded voice channels.

1.2.8 [Limitations on Path Lengths]{.underline}. Radio frequencies in
the following bands are not permitted by the FCC to be used on
transmission paths shorter than the indicated distances below:

2110 to 2130 MHz; 2160 to 2180 MHz 5km (3.1 miles)

3700 to 4200 MHz 17km (10.56 miles)

5925 to 6425 MHz 17km (10.56 miles)

10,700 to 11,700 MHz 5km (3.1 miles)

The FCC does grant exceptions to these limits if the rule would entail
excessive cost in construction or maintenance.

1.2.9 [Limitations on Channel Loading]{.underline}. Some of the commonly
used frequency bands available to telephone companies under Part 21
regulations have a minimum traffic loading restriction intended to
ensure that the radio spectrum is efficiently allocated among different
users. These traffic loading restrictions require the prospective
microwave user to demonstrate that within a reasonable growth period
after installation, the traffic requirements will reach the minimum
standard appropriate for licensing in that band.

1.2.9.1 There is one FCC requirement on initial channel loading that may
offer problems to telephone borrowers wishing to apply for an initial
working radio frequency allocation on the 4 GHz, 6 GHz or 11 GHz bands.
An application for an initial working channel over a given route will
not be accepted for filing where the anticipated loading (within five
years or other period subject to reasonable projection) is less than 900
voice channels. An exception is made, however, for an 11 GHz channel
radio that requires a band-width less than 20 MHz. The minimum for this
case is 240 voice channels. Digital microwave radio equipment is
available today at both 6 GHz and 11 GHz that provides 672 channels in a
10 MHz and a 20 MHz bandwidth, respectively. The above limitations were
established many years ago in the age of analog microwave.

1.2.10 [Digital Electronic Message Service]{.underline}. This is a
[two-way]{.underline} domestic end-to-end fixed radio service utilizing
digital termination systems for the exchange of digital information. It
may also make use of point-to-point microwave facilities, microwave
satellite facilities or other communications media to interconnect
digital termination systems to comprise a network. A digital termination
system (DTS) is a point-to-multipoint radio system consisting of digital
termination nodal stations and their associated digital termination user
stations. They normally exist only in Metropolitan Statistical Areas.
The user stations always have directive antennas. The nodal station
antenna may be omnidirectional or directional. Only the 10 GHz band
(10,550 - 10,680 MHz) and the 18 GHz band (17,700 - 19,700 MHz) are
available for use in this common carrier service.

1.2.11 [Local Television Transmission Service]{.underline}.
Point-to-Point microwave radio can be used to furnish service to radio
and television broadcasters as well as cable television system
operators. However, the Rural Electrification Act does not permit REA
funds to be used for radio broadcasting or commercial cable television.
The FCC Common Carrier Bureau permits the use of the 4 GHz, 6 GHz, 11
GHz and 23 GHz bands (listed previously for point-to-point microwave
stations) for studio to transmitter links. Separate sets of frequencies
are provided for television non-broadcast pickup mobile stations. These
are:

6,425 to 6,525 MHz\* 21,200 to 22,000 MHz

11,700 to 12,200 MHz\* 22,000 to 23,600 MHz

13,200 to 13,250 MHz 31,000 to 31,300 MHz

14,200 to 14,400 MHz\* (\* different from above point-to-point

microwave bands)

The FCC Mass Media Bureau will assign additional frequencies to common
carriers.

1.2.12 [Multipoint Distribution Service]{.underline}. This is a
[one-way]{.underline} domestic public fixed radio service from a station
transmitting (usually in an omnidirectional pattern) to
[multiple]{.underline} receiving facilities. It may be operated as a
common carrier or as a non-common carrier service. Although any kind of
communication service may be rendered,it is usually used in connection
with [Instructional Television Service]{.underline} which is licensed by
the Mass Media Bureau. Each channel may provide one standard television
signal with a bandwidth no greater than 6 MHz for both visual signal and
accompanying aural signal. It will normally employ vestigial sideband
amplitude modulation for the visual signal and frequency modulation for
the accompanying aural signal. Only microwave frequencies are employed
for transmission. If the licensee wishes to insure that transmissions
will not be received in intelligible form by unauthorized subscribers or
licensees, the standard television signal may be varied provided that
the encoded information is recoverable without perceptible degradation.
The tariff must clearly describe the degree of privacy or security a
subscriber can expect in ordinary service.

1.2.12.1 [Instructional Television Service]{.underline} utilizes eight 6
MHz channels (designated as E and F Channels) within the band from 2596
to 2644 MHz. The Common Carrier Bureau usually calls this multichannel
multipoint distribution service. There are some associated response
channels for the reverse direction located in the 18 GHz band. For other
types of service (e.g. non-broadcast omnidirectional radio) channels 1
and 2 located in the 2150-2162 MHz band may be used.

**1.3 CAPABILITIES OF MICROWAVE**

1.3.1 Microwave transmission is generally defined as the transmission of
electromagnetic waves whose frequency falls approximately in the range
between 1 Gigahertz and 50 Gigahertz (wavelengths of 30 cm to 6 mm). The
propagation through the atmosphere of signals in this frequency range
exhibits many of the properties of light, such as line-of-sight
transmission, reflection from smooth surfaces, etc. Microwave systems
have many applications in the telephone industry because high quality
circuits can be derived for intertoll trunks, toll connecting trunks,
extended area service trunks, subscriber service and special services.
Microwave is also suitable for transmission of black and white or color
television, data, and data under voice, with negligible impairment from
impulse noise, delay distortion, frequency error, frequency response, or
steady state noise.

1.3.2 Another attractive aspect of microwave is the ease with which
channels can be added or removed after the basic radio frequency (RF)
and carrier multiplex equipment is installed. Certain types of RF
equipment will carry up to 2000 or more voice channels without any
change in the basic RF equipment. The problems associated with cable
facilities such as physical damage, induction noise, right-of-way
problems, circuit expansion limitations and similar problems are reduced
with the use of microwave.

1.3.3 The initial cost of a microwave system depends on the type of
radio frequency and multiplex equipment used, the number of channels,
the number of hops in a system, the terrain, the type of antennas, the
cost of the necessary towers and other factors. In some cases microwave
will require a lower initial investment, provide greater reliability,
and have lower operating costs and maintenance than cable facilities.

1.3.4 It is highly desirable to use digital microwave equipment for all
new installations in order to eventually achieve a complete integrated
digital network. The only exception to this would be in the event that a
borrower wants to use the microwave equipment to carry television
signals. Analog equipment is the best choice for the current standard
television channel.

1.3.5 The input and output baseband signal for a digital microwave radio
is a single bit stream. This may range from approximately 1.544 Mb/s to
approximately 144 Mb/s. The baseband signal is used to modulate a radio
frequency carrier. The RF carriers used range from 2 GHz to 24 GHz.

**1.4 SITUATIONS WHERE MICROWAVE MAY BE DESIRABLE**

1.4.1 There are at least four conditions where microwave will often be
more desirable than other types of facilities. These situations are
discussed in the following paragraphs.

1.4.2 In some cases the length of a circuit route and the long range
estimate of channel requirements will result in the use of microwave
being more feasible than cable facilities. The expansion capabilities of
microwave are such that many channels can be added without any changes
or additional cost for the RF equipment. The cost per channel for
additional channels on a microwave system will sometimes be less than
for circuits derived on cable facilities, and as more channels are
required, the per channel cost will become less.

1.4.3 Another situation where microwave offers definite advantages is in
areas of difficult terrain. When communications must be provided in
mountainous areas, microwave can usually be provided at less cost than
cable facilities. In mountainous areas, the cable route will normally be
long since it usually does not take a straight-line path like microwave,
but follows a winding road. The necessity of crossing obstacles such as
lakes and swamps is made more practical and more economical with the use
of microwave, as well as other cases where topographical characteristics
make the construction of cable facilities expensive or impossible.

1.4.3.1 In areas where PCM cable carrier is advantageous with the
exception of an obstacle such as a lake or section of ground that is too
hard for buried cable, digital microwave radio can be used as an
\"aerial insert\" to bypass the obstacle. This application of digital
radio allows the PCM signals on the cable to be continued over microwave
radio without the need for any analog-to-digital or digital-to-analog
conversions.

1.4.4 In many instances it is necessary for one telephone company to
connect with another company\'s toll circuits. When the connecting
company derives its toll circuits using microwave, the other company may
be required to provide microwave radio and multiplex equipment such that
the two systems can meet \"mid-air\" on a microwave frequency basis; the
use of identical or closely similar radio equipment at both ends of such
a link is recommended for ease of maintenance and administration between
the connecting companies.

1.4.5 Microwave can also be used to provide video circuits. Most
microwave equipment has sufficient bandwidth for transmission of the
video portion of one television channel over each microwave RF channel.
The audio for the television channel can be transmitted over the same RF
channel using a subcarrier or it may be carried over a separate
facility. Most microwave systems of current design will permit the
application of several modulated subcarriers over the video channel
without significant degradation. This frequently permits the
transmission of FM stereo program material, voice and/or telemetry
circuits in addition to the video sound carrier over the

same RF channel as the video information.

**1.5 COMPONENTS OF A MICROWAVE SYSTEM**

1.5.1 [Transmitters and Receivers]{.underline}. The basic building
blocks of a microwave system are the radio frequency (RF) transmitters
and receivers. These units make it possible to send and receive
information at microwave frequencies. Most microwave transmitters are
capable of an output power of one watt or more. A transmitter used in a
terminal location has provisions for modulating the RF carrier with
baseband signals from the carrier multiplex equipment. Receivers are
capable of providing a useable baseband output with received microwave
signal levels as low as -80 dBm. A terminal receiver includes a
demodulator to provide the baseband output to the carrier multiplex.

1.5.2 [Carrier Multiplex]{.underline}. The microwave RF equipment has a
wide bandwidth which is capable of carrying many channels of
information. These channels are derived using multiplex equipment which
can combine several hundred channels for transmission over one RF
channel in a single bit stream.

1.5.3 [Active RF Repeaters]{.underline}. Active or regenerating
repeaters are used when the distance between terminal stations is too
great to allow a received signal of acceptable level and also when it is
necessary to insert and drop channels at points between terminal
stations. An active repeater can be used at one or more intermediate
points to regenerate the signal or to allow adding or dropping of
channels.

1.5.3.1 In the case of digital radio transmission, a digital repeater
can best be described as a [regenerating repeater]{.underline}, in that
the received pulses that are demodulated from the incoming RF frequency
trigger the generation of new pulses to be modulated onto the succeeding
transmitter, so that any pulse that has been degraded by noise is
completely renewed. Thus, even though this process is continued at each
repeater, noise and distortion is not added to the system, as in analog
radio transmission. The noise and distortion contributed by an analog
repeater can become serious in long-haul, high density toll routes where
many repeaters are required, and a digital radio system could be more
advantageous for this type of radio route. The receivers and
transmitters used in a regenerating repeater are the same type as those
used at a terminal station, and the repeater can be actually thought of
as two terminal stations connected back-to-back at the baseband level.
See Figure A1 for a block diagram of a regenerating repeater.

1.5.4 [Passive Repeaters]{.underline}. A passive repeater is sometimes
required when there is an obstacle such as a high mountain in the
line-of-sight microwave path, where the cost, maintenance and power
requirements for an active repeater would be prohibitive. The passive
repeater is located in such a position as to act as a microwave mirror,
reflecting the microwave signal as a mirror reflects a light beam, to
bypass the obstruction. A passive repeater may be a large reflecting
surface similar to a billboard, or it may be two parabolic antennas
connected back-to-back through waveguide. Both types are used to change
the direction of a microwave signal. The passive repeater does not use
transistors, or other active devices and therefore requires no power
source unless heating is necessary to prevent ice from accumulating on
the antenna surfaces. See Figure A2.

1.5.4.1 Another type of repeater that is in use is an RF repeater which
receives and amplifies a microwave signal and redirects it along another
route. It is essentially two parabolic antennas connected back-to-back
through a radio frequency amplifier, making this repeater arrangement
much more efficient than two parabolic antennas connected directly. The
power requirements of the RF repeater are such that solar energy can be
used to power the repeater efficiently and economically. In many
instances, the cost of this type of repeater is substantially less than
the billboard passive repeater.

1.5.5 [Antennas]{.underline}. A parabolic or a horn antenna is used in
microwave systems to concentrate radiated energy into a narrow beam for
transmission through the air. This results in the most efficient
transmission of radiated power with a minimum of interference. An
effective gain of 25 to 48 dB over an ommi-directional antenna is
possible depending upon the size of the antenna and the microwave
frequency used.

1.5.6 [Radomes]{.underline}. A radome is a protective covering used to
prevent snow, ice, water, or debris from accumulating on a microwave
antenna. Heated radomes are available for use in areas where severe ice
and snow conditions exist. The use of a radome results in lower antenna
gain.

1.5.7 [Transmission Lines]{.underline}. Transmission lines provide the
means of coupling the transmitter and receiver to the antenna. There are
two types currently available: waveguide and coaxial cable. The radiated
output power of the transmitter will be substantially reduced if the
transmission line is incorrectly used or if its length is too long, so
precautions should be taken to use the correct type of line for the
radio equipment used, and to keep all transmission line lengths short.

1.5.7.1 [Waveguide]{.underline}. A waveguide is a hollow metal duct
which conducts electo-magnetic energy. This type of transmission line
can be used for distances of a few feet up to several hundred feet. A
typical type of waveguide has a loss from about 1.7 dB per hundred feet
at 6 Gigahertz (GHz) to about 3.0 dB per hundred feet at 11 GHz. It is
used at microwave frequencies above 2 GHz and can have either a
rectangular, elliptical, or circular cross-section, depending upon the
system operation requirements. The length of a waveguide run is more
critical at higher frequencies since attenuation increases with
frequency. All waveguide runs are pressurized.

1.5.7.2 [Coaxial Cable]{.underline}. At low microwave frequencies, 2 Ghz
or less, coaxial cable can be used as the connecting facility between
the transmitter, receiver and antenna instead of waveguide. The loss of
coaxial cable depends on the type of conductor, the cable diameter, the
type of dielectric, and the operating frequency. Coaxial cable with a
diameter of one inch or more should be used for long cable runs; 7/8\"
diameter coax can be used satisfactorily for short runs. The coaxial
cable can have either a pressurized air or expanded polyethelyne (foam)
dielectric between conductors, however, the air dielectric coaxial cable
has less attenuation for a given diameter. In general, pressurized air
dielectric coaxial cable is used with higher capacity systems because
the return loss characteristics of foam dielectric lines may be a
significant distortion contributor in such systems. This is not usually
a consideration in systems of low channel capacity. The cost of coaxial
cable is less than waveguide and should be used when possible. Extreme
attenuation of radio signals above 2 GHz in the coaxial cable generally
prohibits its use at the higher microwave frequency bands.

1.5.8 [Reflectors]{.underline}. A passive reflector can sometimes be
used in systems operating near a power substation to avoid the
electromagnetic interference (EMI) potential in place of using long runs
of waveguide connected to a parabolic antenna at the top of the tower. A
reflector may be mounted at a 45 degree angle at the top of the tower,
while the antenna is mounted horizontally at the base of the tower,
aimed at the reflector. The microwave signal is radiated from the
antenna, reflected off the reflector, and sent in a direction of
propagation to the other end of the radio path, just as though the
antenna was radiating directly from the top of the tower. However, this
type \"periscope\" or \"fly swatter\" antenna system will not be
authorized by the FCC under ordinary circumstances because of its
interference potential with communications satellites. A waiver from the
FCC is required.

1.5.9 [Towers]{.underline}. The towers used in microwave systems must be
rigid to prevent antenna deflection during wind or ice loading
conditions. Guyed or self-supporting towers are available for use on
microwave systems. A guyed tower is about one-third the cost (per foot,
installed) of a self-supporting tower, but in some cases the difficulty
of acquiring enough land for guying prohibits the use of guyed towers.
The height of the tower is determined by the terrain, the microwave
frequency band used, the propagation characteristics, the distance
between the transmitting and receiving ends of a path, and the required
reliability. The tower must be high enough to provide a line-of-sight
path above any obstructions. If reflection interference is a problem,
the antenna mounting heights are critical and the optimum height may be
less than the maximum height available on the tower.

1.5.10 [Buildings]{.underline}. Microwave equipment should be located in
the central office equipment building when possible. There are some
situations, however, when RF equipment must be located remotely from a
central office building, as in the case of an active RF repeater. In
these situations some type of building must usually be provided for
equipment protection. Usually a simple prefabricated building is
sufficient. Where temperature and humidity variations exceed the
operating limits of the microwave equipment, a heater or air conditioner
is required to keep the equipment within its operating temperature
range.

1.5.11 [Primary and Standby Power Equipment]{.underline}. Primary power
sources for RF equipment may be DC or AC as specified by the purchaser.
Central office batteries or 117 volts AC commercial power may be used.
In some cases, thermoelectric generators or fuel cells can be used when
the power requirements of the microwave equipment are low. Standby power
equipment should be provided at microwave terminals or active repeater
locations to maintain system operation in the event of a commercial
power failure. Communication circuits are very important during times of
emergency such as storms, floods and other disasters which may cause
commercial power outages. Therefore, it is imperative that some type of
standby power source be available for circuits derived by microwave.
When microwave equipment is located in a central office building,
stand-by power is usually available from central office equipment
batteries or an engine-generator. However, at remote sites standby power
must be provided specifically for the microwave equipment. The stand-by
power source may be batteries, an engine-generator or in some cases a
thermoelectric generator, fuel cell or solar energy.

1.5.12 [Alarm Systems]{.underline}. When a microwave system has remote
unattended stations, it is desirable to have an alarm system which will
report faults from the remote location to an attended office via the
microwave signal. These alarms will expedite the maintenance of
microwave systems and reduce the circuit outage time. Where alarms from
a large number of unattended stations are reported to a central
maintenance control center, consideration is often given to a
computer-based alarm reporting system which prints out all changes in
status at each station with time and date information.

**1.6** **MICROWAVE EQUIPMENT ARRANGEMENTS**

1.6.1 [Basic Equipment Arrangement]{.underline}. A microwave terminal is
located at each end of a system and the basic arrangement consists of
one transmitter and one receiver. An active regenerating microwave
repeater can be thought of as two terminals back-to-back and a basic
repeater arrangement consists of two transmitters and two receivers. See
Figure A3.

1.6.1.1 The basic equipment arrangement is used when it is not necessary
to provide duplicate equipment for protection against path outgages.
This arrangement is not necessarily used since standby or diversity
equipment is desirable to provide increased system reliability.

1.6.2 [Hot Standby Arrangement]{.underline}. Hot standby is an
arrangement where two transmitters are on and operating at the same
radio frequency but only one is connected to the antenna. If the
operating transmitter fails, the second or standby unit is automatically
connected to the antenna and the defective transmitter is disconnected.
The switching of transmitters occurs in a few milliseconds after sensing
equipment has determined that a unit is defective. Two receivers are
used and both are permanently connected to the antenna. The two receiver
outputs are combined into one resultant signal. See Figure A4 for an
example of a hot-standby arrangement. Hot standby provides increased
equipment reliability, but does not increase propagation reliability. A
variation of the hot standby arrangement involves the main and standby
transmitters continuously feeding the antenna in a phase-locked
configuration; fault conditions then cause the disconnection of the
faulty transmitter. This arrangement has an RF power increase of 3 dB
under normal conditions over the single transmitter output.

1.6.3 [Space Diversity]{.underline}. For improvements in propagation
reliability, a space diversity antenna arrangement can be used. In a
space diversity system, one transmitter and its associated antenna
radiates on a transmit frequency. This signal is received by two
receivers which are tuned to the same frequency but connected to
separate antennas located at different positions on the tower. The
receiver output signals can be combined to give a composite output, or
switching can be done between receivers, keeping the receiver with the
best Bit Error Ratio (BER) (in the case of a digital radio system)
connected to the line. Vertical spacing between the two receiving
antennas should be approximately 60 to 80 feet at 2 GHz, 30 to 40 feet
at 6 GHz and 25 to 30 feet at 11 GHz. Space diversity provides a
substantial increase in reliability, especially over highly reflective
surfaces such as water or desert. The necessity of two receiving
antennas, two receiving waveguide runs, strong towers because of the two
antennas and a taller tower required to give the necessary antenna
spacing tends to make space diversity a more expensive means of
increased path reliability. See Figure A5 for a diagram of a space
diversity arrangement.

1.6.4 [Frequency Diversity]{.underline}. A frequency diversity
arrangement can be used at microwave frequencies above 2 GHz when
equipment and propagation reliability is desired and required
communications cannot practically be achieved by other means. This
method increases the total system reliability by providing both path and
equipment duplication. Two transmitters are on the air simultaneously
and both are modulated with the same baseband signal but are tuned to
different radio frequencies. The different frequencies can be either
within the same operating frequency band, or in two different operating
frequency bands. Both transmitters are connected to the same antenna
which radiates the signals to the far-end of the path. At the far-end of
the path there are two receivers and each receiver accepts the one
incoming signal to which it is tuned. Each receiver then provides as an
output the signal which modulated the transmitters. The two outputs are
then combined using a combiner device to provide one output signal to
the multiplex.

1.6.4.1 A frequency diversity system provides increased equipment
reliability as a result of two transmitter-receiver combinations and
increased propagation reliability as a result of using two different
microwave transmitting frequencies on the air simultaneously. Studies
and measurements have been made which indicate that increased
propagation reliability is realized when two transmitters with a
frequency separation of at least 2% to 5% are used. The amount of
frequency separation is mainly determined by the availability of
frequencies but the minimum separation for effective propagation
protection is generally regarded as 2%. It is difficult to find two
frequencies available giving the needed minimum separation.

1.6.4.2 In recent years the FCC has not permitted frequency diversity
for digital microwave systems. There are now some indications of a
possible change in attitude because of the increasing use of optical
fiber on high density routes, resulting in freeing of radio frequencies
now held. It is doubtful, however, that the high cost of frequency
diversity can be justified for the usual borrower\'s light rural routes.

**1.7** **Equipment Presently Available for Common Carriers**

1.7.1 [Radio Frequency Equipment]{.underline}. RF equipment is available
for the 2, 4, 6, 10, 11, 18 and 23 GHz common carrier microwave
frequency bands. This equipment is all solid-state (except for some
models that incorporate traveling wave tube final amplifiers for high
transmitter power output), has low power consumption, and requires
little maintenance. These features are particularly important when using
radio in remote areas. As a general guide for design purposes the
following path lengths are appropriate:

2 GHz 3 to 40 miles 10 GHz 3 to 20 miles

4 GHz 11 to 30+ miles 11 GHz 3 to 20 miles

6 GHz 11 to 30+ miles 18 GHz 2 to 10 miles

1.7.1.1 [2GHz Microwave]{.underline}. Current 2 GHz RF equipment is
capable of carrying up to 288 digital multiplex channels on a single
polarization. Propagation of radio waves at 2 GHz has higher reliability
against multipath and rain fading and is often less subject to ground
reflections than higher frequency bands; it is, therefore, preferable in
difficult propagation situations. The situations most appropriate for
the use of 2 GHz equipment are where only a relatively few trunks are
required to serve an exchange, where a few special circuits must be
provided for a customer, and where small groups of isolated subscribers
are to be provided service. The important point to remember when
considering the use of 2 GHz RF equipment is whether it is capable of
providing initial and future channel requirements. If a five year
forecast shows a greater requirement of circuits than the capacity of 2
GHz equipment, the use of a higher frequency band should be considered.
If the minimum five year channel capacity requirements of the higher
frequencies cannot be met as specified in the FCC Rules and Regulations,
a second RF channel at 2 GHz should be considered.

1.7.1.2 [4 GHz Microwave]{.underline}. This band was used primarily by
interexchange carriers for long-haul high density toll routes and has
not been used very much by independent telephone companies. However, 4
GHz equipment is available to independent telephone companies from some
microwave equipment manufacturers.

1.7.1.3 [6 GHz Microwave]{.underline}. 6 GHz RF equipment can be used to
carry a large number of channels for long distances. Radio paths at 6
GHz can be longer than paths at 11 Ghz because the 6 Ghz radio wave is
not affected as much by rain attenuation. High quality circuits for
short, as well as, long haul applications can be obtained. The capacity
of current 6 Ghz equipment is 672, 1344 and 2016 channels, using
bandwidths of 10 MHz, 20 MHz, and 30 MHz.

1.7.1.4 [10 GHz Microwave]{.underline}. In addition to point-to-point
microwave the same radio may be used for digital electronic message
service. At ideal sites it is possible to engineer path lengths up to 30
miles. However, it is affected by high rain rates. It is not difficult
to find available frequencies since they have only recently been made
available for point-to-point microwave service.

1.7.1.5 [11 GHz Microwave]{.underline}. The capacities of current 11 GHz
equipment are identical to those obtainable for 6 Ghz; however, the
bandwidths are usually 20 Mhz, 30 MHz, and 40 MHz. The path lengths may
be as short as 3 miles. The maximum path length depends on the amount of
expected rainfall intensity over the area to be served. The worst areas
are those near the Gulf of Mexico and the Atlantic Ocean in the
Southeastern part of the United States. (See the discussion on Fading in
section 2.8 of this bulletin.) Propagation studies should be made to
determine the effect of rainfall over each contemplated path.
Satisfactory service is obtainable throughout most of the United States
if path lengths do not exceed 20 miles.

1.7.1.6 [18 GHz Microwave]{.underline}. When intense rainfall is
expected there will be a much greater amount of attenuation at 18 GHz
than at 11 Ghz. It is not recommended for the worst areas mentioned
above. However, since the required antennas are small and relatively
light in weight it is possible to engineer a considerably lower cost
system at these frequencies. Satisfactory operation may be obtained in
most areas of the United States if path lengths do not exceed 10 miles.

1.7.2 Digital Multiplexing Equipment is used to derive voice or
equivalent channels on the microwave baseband. The channels are
time-division multiplexed onto the baseband carrier. The equipment can
be used to derive a few channels or hundreds of channels. Some types of
equipment are used to derive toll, extended area service, data or
various special circuits while other types can be used for subscriber
service.

1.7.2.1 Pulse Code Modulation (PCM) carrier can be transmitted over
microwave radio at a DS1, DS2, DS3, or higher rate. The PCM carrier is
phase modulated onto the RF carrier. There are many techniques used with
the modulations of the radio to increase digital channel capacity. All
digital carrier multiplex used for trunk circuits are the T type, which
uses eight-bit pulse-code-modulation to convey signalling and message
information. A compatible end-to-end transmitted bit rate must be
realized in order to synchronize clock circuits that decode and encode
information in a timed manner. For T carrier, an increase of channel
capacity increases the transmitted bit rate, which in turn increases the
RF spectrum that is occupied by the carrier signal. The digital channel
is given a slot in time. For additional information see Bulletin
1751H-403.

**1.8** **Microwave System Considerations**

1.8.1 The design of a microwave system involves engineering for adequate
technical operating specifications at minimum cost. It is possible to
over-engineer a system at great expense, when the same operating results
could have been obtained without a lot of extras added and at less cost.
Several design approaches should be examined before making a final
decision. This can be done by examining different tower heights,
different antenna sizes, different repeater locations, and different
frequency bands.

1.8.2 Microwave system designs and costs are usually based on either
[ideal]{.underline} or [average]{.underline} radio frequency propagation
conditions. [Ideal]{.underline} and [average]{.underline} propagation
conditions refer to the atmospheric conditions which are prevalent in an
area. [Ideal]{.underline} propagation is generally prevalent in hilly
and mountainous areas where the air is in constant turbulence, ground
reflections of the radio waves are at a minimum, and the radio wave is
not diffracted from its line-of-sight path. This type path is economical
to design for acceptable reliability. Average propagation conditions
occur over plains and rolling hills where multipath reflections and
radio beam diffraction occur more often. System design considerations
must include the probability of these occurrences, increasing overall
system cost. In some cases, paths with ideal propagation conditions will
require a passive repeater to bypass obstacles such as mountains in the
path. The cost of these paths may be higher than for a system over a
path with average propagation which has no obstacles. Paths that go over
bodies of water or flat land, or are in areas of high humidity are in
the worst case propagation conditions and usually require special
considerations, such as the use of space diversity, for protection
against fading outages.

1.8.3 A digital radio system has three distinct advantages over an
analog radio system: less degradation of quality due to fades above
threshold, easy interface with PCM carrier and lower costs when adding
channels.

1.8.4 For actual up-to-date costs, the various suppliers of microwave
equipment should be consulted. To price a microwave system add the
individual system components cost plus 20% to cover installation and
contingencies. Costs for land, towers, passive repeaters, power,
generators or other components which will be included in the system must
also be added to estimate the complete system cost.

1.8.5 Reliability or availability of a digital microwave radio path
relates to the time a given microwave link is operational during a
specified period of time, typically a year. One should expect to achieve
a reliability (or availability) of 99.99% or better. The following table
illustrates the relationship between outage time and reliability:

OUTAGE TIME PER

[OUTAGE]{.underline} \| [3 MONTH]{.underline} \|

[RELIABILITY]{.underline} [TIME]{.underline} \| [YEAR]{.underline} [FADE
PERIOD]{.underline} [MONTH]{.underline} [DAY]{.underline} \|

\% % (Avg.) (Avg.)

99.9 0.1 8.8 hours 2.2 hours 43 minutes 1.44 minutes

99.99 0.01 53 minutes 13 minutes 4.3 minutes 8.6 seconds

99.999 0.001 5.3 minutes 1.3 minutes 26 seconds 0.86 seconds

99.9999 0.0001 32 seconds 8 seconds 2.6 seconds 0.086 seconds

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**2. MICROWAVE RADIO PROPAGATION AND PATH SURVEYS**

**2.1 CHARACTERISTICS OF MICROWAVE TRANSMISSION**

2.1.1 Microwave frequencies are generally defined as those frequencies
which have a wavelength short enough to display many of the properties
of light waves. A wavelength of 30 centimeters (approximately a foot) or
less is considered to be in the microwave region. Microwave energy may
be refracted, diffracted, reflected, or absorbed. The direct rays of the
radiated energy travel essentially in a straight line and there is
little reflection from the ionospheric layers in the upper atmosphere.
Because of the short wavelength of microwaves, the radiated energy can
be concentrated by relatively small antennas into a narrow beam similar
to that of a searchlight. Microwave energy can be obstructed or
attenuated by solid objects such as trees, buildings, and mountains. It
is for these reasons that microwave communication is almost always
limited to unobstructed line-of-sight paths.

**2.2 LOCATION OF STATIONS AND PATH SURVEYS**

2.2.1 As soon as the points are determined between which microwave
communication will be established, topographic maps should be obtained
that show the contour lines of the area over which the microwave path is
to traverse. Topographic maps can be ordered from the U.S. Geological
Survey at either of two locations: (a) USGS Map Sales, Box 25286,
Denver, CO 80225;

Telephone (303) 236-7477 or (b) USGS Map Sales, National Center, Reston,
VA 22092; Telephone (703) 648-6892 or (800) USA-MAPS \[800-872-6277\].
Free map indexes and information on computer generated topographic data
files are available on request at these locations.

2.2.2 A microwave system manufacturer has the capability of performing a
path survey. The survey for a [digital]{.underline} system is usually
based on the percentage of time that a given radio path is available for
satisfactory performance with a specified bit error ratio. There are
three or four contract service organizations in the United States that
specialize in frequency coordination matters. Their work is accepted by
the FCC.

**2.3 LOCATION OF ANTENNA SITES**

2.3.1 In general, the antenna sites should be located on high points so
that there will be a line-of-sight path with some clearance over the
intervening terrain between the transmitting and receiving antennas. The
clearance required at any point along the path will depend upon the
frequency used

and the distance of the point from the ends of the path. The method of
determining the clearance is discussed in paragraph 2.4.

2.3.2 The site\'s contour and soil characteristics, accessibility by
all- weather roads, and availability of primary power should be
determined and evaluated with respect to cost. In most cases it is less
expensive to provide a taller tower at a site that is easily accessible
than to build roads and power lines to a point of higher elevation.
Passive repeaters can be used to an advantage in many cases. The active
equipment can be placed near roads and power while at the same time
adequate clearance (for the proposed path) is provided by the passive
repeater. Passive repeaters are radio reflecting surfaces placed in a
microwave path to reflect the signal around or over some obstruction.
Passive repeaters are particularly effective in mountainous regions and
in urban areas where nearby obstructions such as a mountain or building
can be used to an advantage as a support for the passive repeater. The
total path loss when passive repeaters are used is a function of the
product of the individual leg lengths. Because of this, passive
repeaters are most often used near the end of the path. Application
manuals can be obtained from the suppliers of passive repeaters.

2.3.3 A site should be considered with respect to the availability and
cost of land and applicable zoning regulations. The proximity of a
potential site to an airport should be determined. Part 17 of the
Federal Communications Commission rules should be consulted to determine
the maximum tower height allowed at the location chosen.

2.3.4 Possible radio interference should also be a prime consideration
in the selection of microwave sites. Before any large amounts of money
are expended for the acquisition of sites or final path engineering, a
study should be made of existing and proposed microwave systems in the
area and alternate sites selected if a possibility of harmful
interference exists. After a study of existing microwave systems is
made, frequencies can be selected and sites located which will reduce
the interference from other stations.

**2.4 PATH PROFILE**

2.4.1 If the selected station sites appear desirable from the general
considerations under paragraph 2.3, the profile of the terrain between
the proposed tower sites should be plotted. The path profile may be
plotted on rectangular graph paper or on special curved coordinate graph
paper. The three basic profile plotting methods are discussed in
paragraph 2.4.3.

2.4.2 The potential tower sites for each microwave terminal station
should first be accurately located on a recent topographic map. A
straight line

path should be drawn between the two sites and marked off in increments
of one mile. The elevation points at the ends of the path and at each
one mile increment are then transferred to the graph paper on which the
profile is being plotted. If there are higher elevation points between
the one mile increment marks, these points should also be plotted on the
path profile. After the straight line path between the two sites has
been plotted, it can be determined if a line-of-sight path can be
achieved without requiring excessively tall towers. If the first direct
path profile reveals that a path is not possible using that route,
alternate path routes between the two sites should be plotted to
determine if a more feasible path is possible. If a direct path between
the sites cannot be provided, locations for active or passive repeater
stations should be determined at points of high elevation. Profile plots
can then be made between each end of the path and the potential repeater
points and the path selected which will provide adequate clearance with
minimum tower heights. The repeater location should be chosen on the
basis of path clearance and the considerations of paragraphs 2.3.2, and
2.3.3.

2.4.3 The three methods generally used when plotting microwave path
profiles are discussed in the following paragraphs:

2.4.3.1 The path profile can be plotted on transparent rectangular graph
paper which represents a flat earth. Templates with curves which take
into account the earth\'s curvature and the effects of refraction (refer
to paragraph 2.5) on the beam are available from most microwave
equipment manufacturers. It is extremely important that the vertical and
horizontal scales indicated on the templates not be changed since errors
are introduced if this is done. The line representing the beam would
have a curvature of KR, where K is an equivalent earth factor accounting
for the effects of refraction and R is the radius of the earth. The
transparent graph paper with the profile plotted to the same scale as
the template can be laid over the appropriate curve and critical points
marked on the path profile. Figure B1 shows an example of a profile plot
on rectangular paper with the microwave beam drawn with a true earth
curvature (K = 1). (The points A, B, C, D and the Fresnel zone
clearances shown on Figures B1, B2 and B3 will be discussed in
paragraphs 2.6 and 2.10.)

2.4.3.2 A second method also using rectangular graph paper to plot the
path profile is sometimes used. This method uses calculated corrections
to account for the earth\'s curvature and refraction rather than a
curved beam. Corrections are added above objects in the path to account
for the curvature

of the earth and refraction. The microwave beam is then drawn as a
straight line. The correction to be added above a point in the path may
be obtained from the following formula:

2d1d2

H = \_\_\_\_\_\_\_ (2:1)

3K

where the added height H is in feet, d1 and d2 (the distances from the
point to each end of the path) are in miles. K is the equivalent earth
radius factor. Therefore, trees and other high points of a path would be
increased by the amount of H to take into consideration the earth\'s
curvature and refraction. Corrections for the case of true earth
curvature (K = 1) are indicated on Figure B2 by the dashed line AB. The
earth profile is the same

as used in Figure B1.

2.4.3.3 A third method of plotting profiles is to use curved graph paper
which represents the appropriate KR curvature to account for refraction
and the earth\'s curvature. The microwave beam is drawn as a straight
line. This curved graph paper can be obtained from some suppliers of
microwave equipment. No corrections are necessary since the curved paper
has already accounted for refraction and the earth\'s curvature. Figure
B3 shows a typical path profile plotted with a true earth curvature (K =
1). The center of the path must be centered on the curved graph paper to
avoid errors. Figure B3 is the same earth profile as used for Figures B1
and B2.

2.4.4 The proposed path should be checked in the field since the contour
lines on the map could be in error or obstructions such as trees
(seasonal) or buildings not shown on the map may be in the path. Such
obstructions should be taken into consideration in determining the tower
locations and tower heights. The possibility of future building
construction should also be considered at this time. A sensitive
barometric altimeter may be used to take elevation reading along a
proposed path. When a barometric altimeter is used, the manufacturer\'s
instruction pertaining to the use of the instrument should be carefully
followed to reduce errors.

2.4.5 The path may also be checked at night by the use of lights mounted
on portable towers or balloons. Mirrors can be used to advantage during
daylight hours. If a long path is checked by optical means, it should be
done when the air is in motion rather than still since normal
propagation is prevalent on windy days when the atmosphere is thoroughly
mixed.

2.4.6 Microwave path planning kits can be obtained from most microwave
equipment suppliers at no charge. These kits contain the necessary graph
paper and information to plot a path profile. Not all microwave
suppliers use the same technique to account for earth curvature and
refraction, but one of the three procedures mentioned in the preceding
paragraphs is used by major equipment suppliers.

2.4.7 The methods discussed should be adequate for making a preliminary
survey to determine the feasibility of using microwave as well as to
locate potential station sites. If the preliminary survey does indicate
feasibility, it may be desirable to have an organization specializing in
laying out microwave systems make a more detailed survey. In most cases
the microwave equipment supplier will make a detailed survey before
installation of the system.

**2.5 REFRACTION**

2.5.1 Refraction is one of the factors that must be considered when
determining microwave path clearance. Under normal propagation
conditions refraction results in the bending of the microwave beam
beyond the optical horizon in the direction of the earth\'s curvature.

2.5.2 As a radio wave front moves forward, it will travel in a straight
line if all points on the front travel at the same velocity. In air of
uniform pressure, temperature and relative humidity all points on a wave
front would travel at the same velocity. Since the pressure, temperature
and relative humidity of the atmosphere are not uniform, but normally
decrease with height, the upper portion of the wave front travels
slightly faster than the lower portion as it moves forward. The
difference in velocity causes the wave, under normal conditions, to be
bent or refracted toward the earth. This is the reason that when a path
profile is plotted, the radius of the earth must be corrected for
refraction by the appropriate \"K\" factor.

2.5.3 The greater the difference in velocity between the upper and lower
portions of the wave front, the more a wave will be bent toward the air
having the highest index of refraction. The amount of bending thus
depends upon the index of refraction of the air through which the wave
front passes. The index of refraction varies with relative humidity,
temperature, pressure, movement of air and other factors. The variation
of these factors from minute-to-minute and day-to-day causes the amount
of bending of a wave front to fluctuate.

2.5.4 Since normal atmospheric refraction results in the microwave beam
being bent downward, this effect is the same as a change in the earth\'s
radius and is expressed in terms of an equivalent earth radius factor K.
The actual earth\'s radius multiplied by the K factor represents a
fictitious earth with

a radius which accounts for the refractive index. A factor of K = 1
would be the case where the curvatures of the actual earth and the
effective earth are equal. A factor greater than K = 1, for example K =
4/3, would indicate that the effective earth has less curvature or is
flatter than the true earth. It is also possible when abnormal
propagation conditions exist for the beam to be bent upward, which would
indicate a K factor less than 1. The K factor varies for different
atmospheric conditions, but at microwave frequencies in the 4 GHz, 6
GHz, and 11 GHz common carrier bands a factor of K = 1 or K = 2/3 is
used for most areas of the United States. For frequencies in the 2 GHz
band

a factor of K = 4/3 is normally used. The selection of the K factor is
dependent on path location and path length.

2.5.5 Multipath propagation is a phenomenon that can be caused by
irregular changes in the index of refraction. This may occur on a still
night after

a hot, humid day when there are both temperature and humidity
inversions.

2.5.5.1 Temperature inversion (increase in temperature with height) is
caused by the earth and the air adjacent to it cooling faster than the
warm air above it. To cause a higher index of refraction in the air at
some height above the ground rather than near it, the absolute humidity
or vapor pressure at that point must be higher than near the ground.
This is called humidity inversion and usually occurs when the air is
super-saturated and the excess moisture appears as fog or dew.

2.5.5.2 Multipath propagation occurs when there exists a layer of air
some distance above the ground which has a higher index of refraction
than the air above or below it. Horizontal and vertical variations in
temperature, pressure, and humidity cause more than one propagation path
to exist between transmitter and receiver. For example, beam 1 in Figure
B4 might be close to

a layer with a high index of refraction. Another beam, such as 2, could
cross the layer at a greater angle and it would be bent enough by the
low density air near the earth to also arrive at the receiver.

2.5.5.3 Initially the separation would be slight and the electrical path
lengths equal. The signal via each path would arrive in phase and the
total signal received at the antenna would be doubled. However, as the
paths become more divergent with cooling in the lower atmosphere, the
signal will decrease until the energy received via both paths
practically cancel. Thus the signal will fluctuate depending upon the
difference in the electrical length of the paths. By morning, the
temperature in the layer will become cooler and the two paths will come
closer together until only one path remains. If the two paths still
exist by morning, the sun will warm the earth and air adjacent to it
faster than the air in the upper layer so that the temperature inversion
will soon cease to exist. Frequency diversity and fade margin may be
used to substantially reduce the adverse effects of multipath fading.
Space diversity may also be used to provide increased propagation
reliability. The FCC Common Carrier Bureau normally will not authorize
the use of frequency diversity for digital systems.

**2.6 DIFFRACTION AND WAVE INTERFERENCE**

2.6.1 After correcting the profile of the path to take into
consideration

the bending of the radio rays by refraction, it is generally necessary
that the rays clear the earth and obstacles along the path by a certain
amount

to prevent excessive attenuation of the signal by diffraction and wave
interference.

2.6.2 Diffraction may be considered as a modification which waves
undergo as they graze the surface of the earth, hills, or the edges of
any opaque body by which the rays are apparently deflected or bent.

2.6.3 As a wave front travels from a transmitting to a receiving
antenna, some portions of the wave front tend to cancel or reinforce
other portions, depending on the difference in path length traveled.
This is referred to as wave interference.

2.6.4 In order to understand the reason for apparent wave bending
(diffraction) and wave interference, it is helpful to refer to Huygen\'s
Wavelet Principle.

2.6.4.1 According to Huygen\'s Wavelet Principle, a wave front may be
considered to consist of an infinite number of individual secondary
radiators. Each of these secondary radiators sends out wavelets in
directions away from the original source radiator. For example, assume
in either Figure B5 or B6 that T is the primary source of radiation, and
that the front will have traveled to position AB. Consider then that
each vibrating particle such as 1, 2, and 3 in the wave front is a
secondary source of radiation from which spherical wavelets spread out.
Note that only a portion of a sphere ever reaches the desired
destination. After a period of time, the surface which envelops (i.e.,
tangent to) all these wavelets constitutes a new wave front such as CD.
The period of time between AB and CD is the time required for

the wave front to progress one wavelength.

2.6.4.2 The energy that is radiated from each secondary source does not
radiate equally in all directions as is the case with a nondirectional
primary radiator (isotropic antenna) but radiates in the forward
direction only. Also, the amount of energy radiated (from the secondary
source) is greatest in the direction of the direct source ray and
diminishes with the divergence from the direction of the primary wave.
In Figure B6 the vibrating particle 2 will radiate more energy in the
direction of 2P, the direction of the primary ray TR, than it will
radiate oblique to the primary ray in the direction 2N. Similarly,
particle 1 will radiate less in the direction 1N\' than in direction
1P\'. Thus the amount of energy in a wavelet decreases as the angle Õ
between it and the primary ray is increased.

2.6.5 An example of wave diffraction over a hill is shown in Figure B7.
In that figure waves are moving forward in the direction of the primary
ray TP. The numerals 1, 2, 3, and 4 represent the crests of the forward
moving waves. If A2C represents a hill which is tangent to the primary
ray at the grazing point 2, the disturbance at 2 will cause wavelets to
be sent out from that point so that some energy will be directed below
the horizon such as represented by the ray 2N. If the hill were broad in
relation to a wavelength, such as A\' 2 C\', the amount of energy
radiated below the horizon would be much less. On the other hand, if the
distance 1-4 were only a fraction of a wavelength, the hill A\' 2 C\'
may appear as sharp at that frequency as the hill A2C appears to the
wavelength represented. With other things being equal, the energy
diffracted beyond a given hill will increase

as the frequency is decreased.

2.6.6 In 2.6.4.1 it was stated that a wave front (such as AB in Figure
B8) may be considered to consist of an infinite number of secondary
radiators such as 1, 2, 3. Since the distances from points on the wave
front to a receiver R are not equal, the wavelets can arrive in- or
out-of-phase, in varying degrees, with the primary ray and result in
[wave interference]{.underline}. The areas or zones around the axial
between the transmitter and receiver that contribute energy either in-
or out-of-phase are called Fresnel zones.

2.6.6.1 The first Fresnel zone is bounded by points through which the
distance between the transmitter and receiver is 1/2 wavelength (1/2 ã)
longer than the direct ray. The second Fresnel zone is bounded by points
through which the distance is one wavelength (1 ã)longer than the direct
ray.

2.6.6.2 Point 1 in Figure B8 was so chosen that ray T1R is 1/2 ã longer
than the direct ray TOR; T2R is 1/2 ã longer than T1R; and T3R is 1/2 ã
longer than T2R. Figure B9 is a cross-section of the wave front AB in
which points on the circles traverse paths that are 1/2 ã, 1 ã, and 1
1/2 ã longer than the direct ray. The regions between these circles are
called Fresnel zones. The diameter of the circles will vary along the
radio path forming ellipsoidal envelopes as boundaries between the
Fresnel zones as indicated in Figure B10.

2.6.6.3 There is an unlimited number of Fresnel zones even though only
three are illustrated in the figures. The areas of all zones are equal.
The energy received from each zone, however, decreases with distance
from the primary ray as explained in paragraph 2.6.4.2. About one-fourth
of the energy received from an unobstructed wave front is in the first
Fresnel zone. The energy received from the second and other
even-numbered Fresnel zones is negative with respect to energy received
from the odd Fresnel zones. About half of

the total energy received from an unobstructed wave front is cancelled
by the waves received from the even numbered Fresnel zones. A sharp
obstruction such as a sharply pointed hill, which cuts off most of the
energy below the first Fresnel zone, would permit more energy to be
received than if the obstruction were not there since part of the
out-of-phase energy would be cut off by the obstruction.

2.6.6.4 If an obstacle cuts off the first Fresnel zone radius
(non-line-of -sight path), some energy will be diffracted around and
over the obstacle (paragraph 2.6.5) and will be received in the shadow
portion of the radio beam. It is for this reason that a certain amount
of radio energy is present beyond true radio (refracted) line-of-sight
when the path is intercepted by the earth. In general, the lower the
frequency, the farther the signal is diffracted beyond the point of
interception.

2.6.6.5 The radius of the first Fresnel zone varies along the radio
path.

It is maximum at the midpoint between the transmitter and receiver and
can be calculated by the formula:

Fm = 1140 ( [D]{.underline}) 1/2 (2:2)

f

where Fm is the radius in feet of the first Fresnel zone at the midpoint
of

the path, D is the distance in miles between the receiver and
transmitter,

and f is the frequency in MHz. The first Fresnel zone radius at any
point x miles from one end of the path is:

Fx = 2280 ([x (D-x)]{.underline}) 1/2 (2:3)

fD

2.6.6.6 Paragraph 2.10 discusses the application of the above formula in
determining the necessary tower height for proper Fresnel zone
clearance. It is desirable to receive as much of the first Fresnel zone
energy as possible and still keep the cost of towers as low as possible.
At frequencies of 4 GHz and higher, a clearance of 0.6 first Fresnel
zone radius with K = 1 is a good design objective for most areas of the
United States with hops of 25 miles or less. A more conservative design
for hops greater than 25 miles to account for possible upward beam
bending (earth bulging) would be 0.3 first Fresnel zone with K = 2/3. At
2 GHz, a value of K = 4/3 with a 0.6 first Fresnel zone clearance is a
typical design.

**2.7 REFLECTION**

2.7.1 If the terrain between the antennas reflects radio waves
efficiently, it is possible to receive strong reflected waves, either
in- or out-of-phase with the direct wave, depending on the difference in
the lengths of the direct and reflected wave paths. If we assume
complete reflection with the reflected wave equal in magnitude to the
direct wave, the resultant energy received would vary, depending on the
location of the point of reflection, between zero and twice that of the
direct path energy according to the following formula:

E = 2 Ed Sin [2Ð]{.underline} [d\']{.underline} (2:4)

ã 2

Ed is the direct ray field strength and d\' is the geometric length
difference

between the direct and reflected wave paths. E and Ed are in the same
units, such as microvolts/meter. Wavelength ã is in the same units as
d\'. Thus in Figure B10, the second Fresnel zone signal received from
the reflection path TPR would practically cancel the signal from the
direct path TR. If the

reflecting plane were moved upward to P\' so that it permits only the
first Fresnel zone to be received, the reflected signal TP\'R would then
be added to the direct signal TR. The path difference d\' is
approximately:

2 hrht (2:5)

d\' = \_\_\_\_\_\_

d

where hr and ht are the heights of the receiving and transmitting
antennas above the [reflecting plane]{.underline} and d is the distance
between transmitter and receiver in the same units as hr and ht.

2.7.2 Since the distance T1R in Figure B8 is 1/2 ã longer than TR, the
energy received from 1 as a secondary ray would be 180o out-of-phase
with the primary

ray TR. Also, the energy received would be less than that of the primary
ray, as discussed in Paragraph 2.6.4.2. If, on the other hand, 1 were on
a

reflection plane, a reflected primary ray 1R would arrive at R in phase
with the direct primary ray TR. The reason for its arrival in phase is
that there is approximately 180o phase shift when a wave is reflected.
If the reflecting plane were a perfect reflector, large enough, and of
the correct slope to reflect the energy in several Fresnel zones, almost
as much energy would be received at the receiver from the reflected wave
as from the direct wave.

2.7.3 If one of the heights such as hr is varied so that E goes through
a maximum and minimum, the difference in the two values of hr is
sometimes used as the spacing between two receiving antennas on a tower.

2.7.4 When two antennas are placed on a tower with this separation, the
reception of the two signals and the selection of the stronger of the
two

is called [space]{.underline} [diversity]{.underline}
[reception]{.underline}. If one antenna is receiving a minimum signal,
the other in all probability will be receiving a stronger signal.

A particularly difficult problem exists when the reflection point is
over tidewater which causes variations in the length of the reflected
wave path contingent on the tidal change in water level. The amount of
separation used on a space diversity system should be determined by
someone very familiar with this type of design.

2.7.5 Reflections are greatest when the point of reflection is over calm
water, level moist earth, desert sand, and other types of smooth
terrain. It is desirable to adjust the tower heights or to reroute the
radio path so the reflection point will be over rough terrain. Radio
energy striking rough terrain will be either absorbed or scattered. Thus
the amount of reflected energy reaching the receiver will be only a
small percent of the total energy. The point of reflection can be
determined by trial and error from the path plot. At the point of
reflection the angle of incidence will equal the angle of reflection.

2.7.6 With zero clearance over a non-reflecting obstacle such as a hill
covered with trees or brush, the signal will be 6 or 7 dB below the loss
that would exist between two antennas in free space. If, however, the
top of the

hill were broad and barren and the soil had good reflection
characteristics, the loss could be more than 16 dB greater than free
space loss. Microwave systems should not be engineered for grazing paths
except by someone with extensive experience in the area of microwave
propagation and path design.

**2.8 FADING**

2.8.1 Fading is a condition which occurs during propagation of radio
frequency energy that causes a reduction in the power being received. It
may be caused by [refraction]{.underline}, [diffraction]{.underline} or
[reflection]{.underline} or by a combination of these conditions. This
combination is usually referred to as [multipath]{.underline}
[fading]{.underline}. In addition to multipath fading a very severe form
of fading may also be caused by rainfall. The effect on the 4 GHz and 6
GHz bands is quite small and is usually ignored. The attenuation of the
microwave radio signals increases substantially as the radio frequency
carrier is increased. This attenuation is definitely noticeable at the
10.5 GHz and 11 GHz bands. It becomes extremely severe at 18 GHz and 23
GHz.

2.8.2 In the absence of rain, variations in received signals due to
multipath conditions are greater in summer than in winter and greater
during nighttime than daytime. These variations are smallest when the
air is in a state of turbulence which prevents the formation of
stratified layers of air.

2.8.3 Radio energy is absorbed and scattered by rain drops. These
effects become more pronounced as the wavelength approaches the diameter
of the raindrops. When the size of the drops becomes large enough and
the drops are sufficiently concentrated, this scattering and absorption
will attenuate the signal appreciably. In addition, since the drops
represent a lossy dielectric, energy will be absorbed from the signal
and converted into heat. These phenomena are entirely negligible below 3
GHz but will place a limitation on transmission through rain
[over]{.underline} [appreciable]{.underline} [distances]{.underline} at
frequencies above 10 GHz.

2.8.4 The amount of attenuation caused by rain depends on the intensity
of the rainstorm. The [rate]{.underline} of rainfall and
[not]{.underline} total rainfall is the determining factor. Areas with
high annual rainfall accumulations may seldom experience rainfall of a
rate sufficient to interrupt service. Heavy rainfall rates are likely to
accompany thunderstorms which may be confined to an area with a diameter
of 1 to 2 miles. The fading margin may not be exceeded unless the heavy
rainfall extends a sufficient distance along the radio path. System
outages should be treated on a probabilistic basis by geographical area.
The worst areas occur near the Gulf of Mexico and the part of the
Atlantic Ocean adjacent to the southeastern area of the United States.

2.8.5 Major manufacturers of digital microwave equipment produce maps of
the United States depicting how a given product is affected by the
rainfall encountered in various geographical regions. Roughly, these
regions are separated by diagonal lines that show the Northwest as the
best area for radio signal propagation and the Southeast as the worst.
They also produce diagrams showing the variation of signal attenuation
with rain rate. The latter diagrams are quite accurate and reliable. The
former maps are approximations since they depend on the statistical
history of rainfall in all areas of the country. The usual source of
such information is the U.S. Weather Service. Reporting stations are
normally found only at airports in major metropolitan areas. These
companies also produce diagrams depicting the annual system outage to be
expected for given rain rates and given time durations of rainfall for
those rates above a certain value.

2.8.6 The attenuation produced by rain is polarization dependent. A
falling drop of rain is not spherical but flattened in the horizontal
plane. This results in a greater attenuation of horizontally polarized
waves. Rain attenuation over a given microwave band is almost
independent of frequency.

Since vertical space diversity and frequency diversity do not protect a
system against rain fading it is important that good fading margins be
obtained in the system design. Reducing the hop length not only reduces
the probability of severe rain fading but also increases the available
fade margin.

2.8.7 A digital microwave system always uses regeneration at each
intermediate terminal and each active repeater. Consequently, fading is
not accumulative over a multi-hop system. Each hop stands alone. This,
of course is true for noise, interference and other physical phenomena
as well. Adaptive equalization reduces the multipath dispersive effect.
(Dispersive means that it is frequency selective.) In the absence of
specific route information during design, one may allot 50% of the total
outage time to propagation fading (multipath, rain, upfades), 25% to
obstruction fading and 25% to equipment and man-made failures. Upfading
refers to the constructive (rather than destructive) effects of
multipath and introduces some additional problems. The newer modulation
technologies may have introduced more sensitivity to the dynamic
impairments of fading.

2.8.8 The magnitude of the received signal varies continuously. This
variation could cause a higher BER (see paragraphs 2.9.8 and 2.9.9);
however if the system is designed with an adequate fade margin, this
will be infrequent and not objectionable. Under extreme conditions a
fade can cause service failure, but transmission usually returns to
normal in a short time. The adverse effects of fading can be reduced
through the use of horizontal space diversity and by provision of
adequate fade margin to protect against both multipath and rain fades.

**2.9 MICROWAVE PATH LOSSES AND GAINS**

2.9.1 The system losses over a radio path can be calculated even though
some of the loss factors, particularly those caused by atmospheric
fades, may be difficult to obtain or estimate. Because of this, fade
margins are added to the calculated normal losses to provide adequate
reserve system gain to compensate for fluctuating atmospheric fades. If
losses are calculated beginning with the transmitter, the levels at
various points along the path can be obtained by the steps outlined in
the following paragraphs.

2.9.2 The power of most microwave transmitters is given in watts. This
can be converted to dB above or below a given reference. One milliwatt
is a commonly used reference in telephony. If the transmitter power (P)
is above one milliwatt, the power in dBm will be:

\+ dBm = 10 log10 [P]{.underline} (2:6)

.001

(i.e., dB above 1 milliwatt). If the power is less than one milliwatt,
then

\- dBm = 10 log10 [.001]{.underline} (2:7)

P

2.9.3 If the transmitter power is converted to dBm, the losses in the
transmission line between the transmitter and the antenna are
subtracted. The losses in dB per 100 feet of the most commonly used
transmission lines are given in the following table. Coaxial cable is
normally used for frequencies up to 2 GHz and in some cases a special
coaxial cable is used at frequencies above 2 GHz. Waveguide is generally
used at 4 GHz and above.

**[Typical Loss in dB per 100 Feet of Transmission Line]{.underline}**

[2 GHz]{.underline} [4 GHz]{.underline} [6 GHz]{.underline} [11
GHz]{.underline}

7/8\" Coaxial Cable

(Air dielectric) 2 dB \-\-\-\-- \-\-\-\-- \-\-\-\--

1 5/8\" Coaxial Cable

(Air dielectric) 1 dB \-\-\-\-- \-\-\-\-- \-\-\-\--

Flexible Elliptical

Waveguide \-\-\-\-- 1 dB 1.5 dB 3.5 dB

Rigid Rectangular

Copper Waveguide \-\-\-\-- 1 dB 2 dB 3.5 dB

2.9.4 Power is effectively increased at the antenna. Microwave antenna
gains are referred to a theoretical isotropic antenna which radiates
energy equally in all directions. The following table shows the typical
gain in dB of some commonly used parabolic antennas:

[Antenna Size]{.underline} [2 GHz]{.underline} [4 GHz]{.underline} [6
GHz]{.underline} [11 GHz]{.underline}

4 foot 25 31 35 40

6 foot 29 35 38 43

8 foot 31 37 41 46

10 foot 33 39 43 48

2.9.5 [Free Space Loss]{.underline} - The major loss between a
transmitter and receiver is through the space between the antennas. The
following formula gives the free space loss in dB:

A = 96.6 + 20 log10f + 20 log10d (2:8)

where f is the frequency in GHz and d is the distance between the
antennas in miles. This loss may be increased or decreased somewhat by
the type of path through which the radio waves travel. A free space loss
condition exists only when the antennas are high enough that the ground
and other obstacles and unusual air conditions do not affect the
received signal.

2.9.6 The signal will be increased at the receiving end of the radio
path by the gain of the receiving antenna. The transmission line loss
between the antenna and receiver should then be subtracted.

2.9.7 Miscellaneous losses from circulators, radomes, and antenna system
misalignment should be accounted for in making fade margin calculations.
The amount of miscellaneous loss will vary with systems but a value of 2
dB per [radio]{.underline} [channnel]{.underline} [end]{.underline} is
typical.

2.9.8 The result of subtracting the losses and adding the gains between
the transmitter and receiver will give the signal power in dBm at the
receiver input. [The fade margin is the difference between the received
power level and the power level required to produce a given Bit Error
Rate (BER)]{.underline}. Fade margins for digital radios are based on
Receiver Sensitivity specified in dBm for a particular BER; for example,
a certain radio may have a receiver sensitivity of -74 dBm at a BER of
10-6.

2.9.9 Finally, in digital radios a Composite Fade Margin is used for
calculations of availability. Composite Fade Margin is the power
addition of the Flat Fade Margin, typically calculated in analog
scenarios, and the Dispersive Fade Margin. Dispersive Fade Margins are
equipment dependent and are provided by the equipment manufacturer. The
above radio may have a Dispersive Fade Margin of \>49 dB at a BER of
10-6.

**2.10 ILLUSTRATIVE EXAMPLE OF CALCULATION OF MICROWAVE ANTENNA HEIGHT**

2.10.1 To illustrate the considerations involved in selecting a suitable
microwave path, assume that the profile of the earth is AB in Figures
B1, B2, and B3. All plots represent the same path profile. Figure B1 is
plotted on rectangular paper with the microwave beam drawn with a
curvature which represents the true earth\'s radius (K = 1). Figure B2
is plotted on rectangular paper with corrections made for true earth\'s
radius. Figure B3

is the same path plotted on curved graph paper.

2.10.2 To determine the required antenna heights at A and B, the profile
of the earth is first plotted directly on the graph paper. The profile
should be plotted with the midpoint of the path centered on the graph
paper.

2.10.3 When the path is plotted on rectangular paper with corrections
added for earth curvature and refraction, the corrections are calculated
from the formula in paragraph 2.4.3.2 and added to the profile plot. The
corrections are shown by the dotted line in Figure B2.

2.10.4 At the high points in the path, the height of any obstructions
such as trees or buildings should be added. The maximum height of trees
is assumed to be 40 feet in this example.

2.10.5 The next step is to calculate the first Fresnel zone clearance
for the high points in the path. In this example 0.6 of the first
Fresnel zone is used. The required clearance at the high points is
calculated from the formula in Paragraph 2.6.6.5, that is

.6Fx = .6 2280 [x(D - x)]{.underline} 1/2 (2:9) ( \[ fD \] )

For example, at point P on the profile the clearance required above the
tree would be

 

Fp = .6 2280 [6 (28 - 6)]{.underline} 1/2

( \[ 6000 (28)\] ) = .6 (63.5) = 38.1 ft.

  (2:10)

A safety factor of 10 feet or more is often added to the calculated
Fresnel zone clearance to account for future tree growth and survey
inaccuracies. Using a safety factor of 10 feet, the clearance at point P
would be 48 feet.

2.10.6 The corrections for earth curvature and refraction are drawn on
the profile as shown in Figure B2, and the Fresnel zone clearance is
calculated and shown with the safety factor on all profile plots as in
Figures B1, B2, and B3. The next step is to draw a line representing the
microwave beam between the ends of the path. The beam line is drawn so
that it clears all of the critical points marked on the profiles. When
drawing a curved beam line using a template, the grid lines of the
template and the grid lines of the profile graph paper must be parallel
at all times to avoid errors. The distance AC (135 ft.) is the antenna
height required at one end of the path while BD (70 ft.) is the antenna
height required at the other end. If it is assumed that the antennas are
mounted 5 feet below the top of the towers, the tower height at A and B
would be 140 feet and 75 feet, respectively. If it is more desirable to
have the antenna lower at A, the straight line DC in Figures B2 and B3
can be pivoted counterclockwise about point P to determine a lower tower
at A and a taller tower at B. When a template is used to draw the curved
beam line, the template can be moved horizontally to the left while
keeping the grid lines on the template and profile paper parallel and a
shorter tower determined at A.

(Intentionally left blank)

\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_

**3. RADIO SYSTEM GUIDE TO FORMS REQUIRED BY THE FEDERAL
COMMUNICATIONS**

**COMMISSION**

**3.1 GENERAL**

3.1.1 The FCC forms apply to the [construction]{.underline} and
[operation]{.underline} of the various radio services provided by REA
Telephone borrowers. All forms are not needed for a given radio service.
These forms are revised frequently and often replaced by others. A
consulting engineer or a telephone company borrower should check the
applicable rule or regulation applying to the given radio service.

3.1.2 Most of the material in this bulletin originates in the Rules and
Regulations of the FCC. These are printed in Title 47 of the *Code of
Federal Regulations* (*CFR*) which is revised once a year. Whenever the
FCC revises, creates or eliminates a rule or regulation the changed
version appears in

the *Federal Register* (published daily except on Saturdays, Sundays and
legal holidays). The *CFR* is an annual compilation of the current Rules
and Regulations. Publication in the *Federal Register* constitutes
official

notification to the public of the change and will contain an effective
date. Both the *Federal Register* and the *Code* are published by the
National Archives

and Records Administration.

3.1.3 To identify an FCC rule or regulation, one should use the title,
the part and the section number. For example, 47 *CFR* 21.2 refers to
Title 47, Part 21, Section 2. In this bulletin this reference will
appear as FCC 21.2. In like manner FCC 90.81 will indicate section 81 of
Part 90.

3.1.4 In the past, the FCC required both an application for a
construction permit and an application for a radio station license. This
no longer applies. At present, the FCC only requires an application for
a [radio station]{.underline}

[authorization]{.underline}.

3.1.5 There are three different FCC forms for the application for radio
station authorization. Form 494 must be used for FCC Part 21 radio
services. Form 401 must be used for FCC Part 22 radio services. Form 574
must be used for FCC

Part 90 radio services. A Basic Exchange Telecommunications Radio
Service (BETRS) utilizing Specialized Mobile Radio Service (SMRS) radio
frequencies

(800 MHz) may be applied for under FCC Part 22 rules.

3.1.6 FCC Forms may be obtained by telephone. Call (202) 632-FORM

\[(202)-632-3676\]. A general form request may be made by calling
(202)-632-7000. The forms may also be obtained in Room B-10 at 1919 M
St. N.W., Washington, D.C. An alternate way is to contact any FCC Field
Operations Bureau office. The addresses of all offices appear in the
*U.S. Government Manual*. The above information is subject to change at
any time.

3.1.7 Copies of any volume of the *Code of Federal Regulations* as well
as the

*U.S. Government Manual* can be obtained at any Federal Government Book
Store.

Direct sales are handled by the Superintendent of Documents, Government
Printing

Office, Washington, D.C. 20402 (telephone 202-783-3238). Prices for
individual volumes can be obtained at the same telephone number. The FCC
Rules and Regulations appear in five different volumes under Chapter 1
of Title 47. These volumes are revised once a year and are current as of
October 1 of a given year. They are printed and available at a
considerably later date. Whenever the FCC revises a rule or regulation,
the new material appears in the daily *Federal Register* prior to
October 1 of the given year. There are no restrictions on the
republication of material appearing in the *Code of Federal
Regulations*.

3.1.8 For practical purposes, the FCC Rules and Regulations divide the
common carrier radio services that are of interest to REA borrowers into
two major Parts based on frequency. FCC Part 21 covers all radio
services that utilize [microwave]{.underline} radio frequencies (see
definition in FCC 21.2 and FCC 22.2). FCC

Part 22 covers all radio services that utilize radio frequencies
[below]{.underline} microwave. The exact dividing line (i.e. in the
range between 890 MHz and

1000 MHz) is no longer clearly defined. Furthermore, the titles
associated

with the above mentioned parts suggest a different method of division.

**3.2 POINT-TO-POINT MICROWAVE RADIO (FCC PART 21)**

3.2.1 All radio services covered in FCC Part 21 use the same set of
forms. The only radio service presently used by REA telephone borrowers
is Point-to-Point Microwave Radio Service (Subpart I of FCC Part 21).
Applications and licenses are covered in Subpart B of FCC Part 21. For a
detailed discussion of the forms used see FCC 21.7 and FCC 21.11.

3.2.2 [FCC Form 494]{.underline} (Application for a New or Modified
Microwave Radio Station License Under Part 21) must be submitted and a
license granted for each station prior to commencement of any proposed
station construction. There is a separate five sheet set of instructions
for this form. This same form may be used to amend an application and to
modify or delete a licensed facility.

3.2.2.1 [FCC Form 494A]{.underline} (Certification of Completion of
Construction) must be used to certify/notify completion of construction.

3.2.3 Two of the following five miscellaneous forms are also used for
FCC

Part 22:

3.2.3.1 [FCC Form 430]{.underline} (Common Carrier and Satellite Radio
Licensee Qualification Report) must be filed by the telephone company on
an annual basis by March 31 of each year. This will cover all changes of
information that occurred in the preceding calendar year.

3.2.3.2 [FCC Form 701]{.underline} (Application for Additional Time to
Construct a Radio Station) must be filed at least 30 days prior to the
expiration of the time for construction noted in the license.

3.2.3.3 [FCC Form 405]{.underline} (Application for Renewal of Radio
Station License in Specified Services) must be filed between 30 and 60
days prior to the

expiration date of the license.

3.2.3.4 [FCC Form 702]{.underline} (Application for Consent to
Assignment of Radio Station Construction Permit or License) must be
submitted to assign the station authorization. In addition, the
[assignee]{.underline} must submit an FCC Form 430

(mentioned above) unless a current report is already on file with the
FCC.

3.2.3.5 Partial Assignment. If only a portion of the facilities are to
be assigned both the assignee and the assignor must file an FCC Form 494
(described above). Another FCC Form 494 must be filed to return the
license to its original condition if assignment does not take place. The
FCC Form 702 is used only for assignment of the entire license.

3.2.3.6 [FCC Form 704]{.underline} (Application for Consent to Transfer
of Control of Corporation Holding Common Carrier Radio Station
Construction Permit or License) must be submitted in order to transfer
control of a corporation holding any licenses. In addition, the
[transferee]{.underline} must submit an FCC Form 430 (mentioned above)
unless a current form is already on file with the FCC.

3.2.3.7 The maximum construction period is 18 months from the date on
the license grant. The license period will extend for up to 10 years
depending on the renewal cycle. See paragraph 3.4.1.

**3.3 PUBLIC LAND MOBILE RADIO, RURAL RADIO, DOMESTIC PUBLIC CELLULAR
RADIO**

**TELECOMMUNICATIONS AND BETRS (FCC PART 22)**

3.3.1 All radio services covered in FCC Part 22 use the same set of
forms. All

of the radio frequencies used are below 1000 MHz. The services that
apply to

REA borrowers are described in subparts G, H, and K of FCC Part 22.
However,

the general parts of Part 22 [also]{.underline} apply unless they are in
conflict with the mentioned subparts. Applications and licenses are
covered in Subpart B of

FCC Part 22. For a detailed discussion of the various forms see FCC
22.9,

FCC 22.11, and FCC 22.39.

3.3.2 [FCC Form 401]{.underline} (Application for New or Modified Common
Carrier Radio Station Authorization Under Part 22) must be submitted for
each base station, rural central office station, rural interoffice
station and auxiliary test station. Fixed subscriber stations that
either exceed 60 watts effective radiated power or require notification
to the Federal Aviation Administration (see Antenna Structures in this
document) also require FCC Form 401. Mobile stations (except airborne)
and fixed subscriber stations (except those mentioned above) are
licensed as part of base station and central office authorizations. The
application form has two parts, Schedule A and Schedule B. One Schedule
A is required for each station. One Schedule (B) is required for each
antenna location. Separate Schedule (B)s may be obtained from the FCC.

3.3.3 [FCC Form 489]{.underline} (Notification of Status of Facilities
Under Part 22 of FCC Rules) is a general purpose form. It can be used
for the following eleven purposes:

3.3.3.1 Notification of completion of construction.

3.3.3.2 Request for extension of time to complete construction.

3.3.3.3 Request for reinstatement within 30 days after expiration of
authorization.

3.3.3.4 Notification that construction was completed with minor
modifications of authorization.

3.3.3.5 Request to delete or change antenna obstruction markings.

3.3.3.6 Notification of a change in points of communication (Rural Radio
Service).

3.3.3.7 Notification of the construction and operation of additional
transmitters on same frequency (include a completed Schedule B from Form
401). See FCC 22.117 (b).

3.3.3.8 Notification of the return of license to original condition if
partial assignment is not completed within 60 days.

3.3.3.9 Notification of a change in or addition of a radio frequency
(cellular radio).

3.3.3.10 Notification of a change to or addition of a cell site as long
as the composite 39 dBu contours remain totally within the cellular
geographic service area (CGSA).

3.3.3.11 Notification that the initial phase of a cellular system has
been completed and is ready to commence service to the public.

3.3.4 [FCC Form 405]{.underline} (Application for Renewal of Radio
Station License in Specified Services) must be filed between 30 and 60
days prior to the expiration date of the license.

3.3.5 [FCC Form 490]{.underline} (Application for Assignment or Transfer
of Control Under Part 22) must be completed for either assignment or
transfer of control. The assignee or transferee must submit FCC Form 430
unless an up to date form is on file with the FCC.

3.3.5.1 Partial Assignment. Request for Partial assignment must be made
on [FCC Form 401]{.underline} by the assignee and on [FCC Form
489]{.underline} by the assignor.

3.3.6 [FCC Form 430]{.underline} (Common Carrier and Satellite Radio
Licensee Qualification Report) must be filed by Mobile Service licensee
only as required by FCC Form 490.

3.3.7 [FCC Form 409]{.underline} (Airborne Mobile Radio Telephone
License Application) is for use by subscribers and may also be used for
modification and renewal. It may not be used for cellular radio.

3.3.8 A radio station authorization will specify the date of grant as
the earliest date for the commencement of construction. (See paragraph
3.3.8.4)

3.3.8.1 Construction must be completed within 12 months (except for
cellular radio) after the grant of authorization.

3.3.8.2 The construction of the initial phase of a cellular radio system
must be completed and service commenced within 18 months. The entire
CGSA (as specified in the existing Form 401) must be completed within 36
months.

3.3.8.3 Licenses will be granted for up to 10 years depending on the
renewal cycle. See paragraph 3.4.1.

3.3.8.4 The FCC has made a recent change in its rules that may or may
not apply to REA borrowers. (See paragraph 3.8.1 in this document.) It
concerns permission to commence construction [prior]{.underline} to the
grant of a radio station authorization and applies to [all Part 22
services]{.underline}. The general rule states that applicants must wait
90 days from the date of the public notice listing

the application as acceptable for filing; cellular tentative selectees
must wait 60 days from the date of the public notice announcing them as
tentative selectees. After the waiting period, certain FCC Form 401
applicants listed in FCC 22.43(d)(1) may commence construction provided
that the conditions listed in FCC 22.43(d)(3) have been met. This final
rule was published in the *Federal Register* on November 8, 1990 (page
46952).

**3.4 COMMON CARRIER LICENSE EXPIRATION DATES AND PROCESSING FEES**

3.4.1 All Common Carrier Bureau radio station authorizations (licenses)
expire on a calendar date specified in FCC 21.45, FCC 22.45 and FCC
23.29 or [on the authorization itself]{.underline} during the year of
expiration. If the Telephone Company does not renew its license, it will
be operating illegally. Common Carrier Bureau licenses are generally
granted for a period of up to 10 years. Most,

but not all, licenses for a given service expire in the [same
year]{.underline} of a 10-year cycle. The chief purpose of this
paragraph is to present a list of [expiration]{.underline}

[years]{.underline}. (This information cannot be found in the above
referenced *CFR*.) Recently, the Common Carrier Bureau resumed its
practice of renewing individual

licenses. Due to lack of manpower, this had not been done since 1978.
For a period of 10 years licenses were simply extended by a general FCC
public notice.

[Common Carrier and Fixed Satellite Licensing]{.underline}

License Duration - up to 10 years

Cellular Radio: October 1 of expiring year

Public Land Mobile:

Wireline Common Carriers July 1, 1998

Radio Common Carriers April 1, 1999

Air-ground base stations September 1, 1999

Rural Radio: November 1, 1998

Point-to-Point Microwave and

Local Television Transmission:

AT&T February 1, 2000

Bell Operating Companies August 1, 2000

Other Carriers February 1, 2001

(Table continued on next page)

[Common Carrier and Fixed Satellite Licensing]{.underline}

(Continued)

Digital Electronic Message Service: February 1, 2001

Multipoint Distribution Service: May 1, 2001

Offshore Radio Telephone: August 1, 2001

International Fixed: December 1, 3 a.m. E.S.T.,

of expiring year

Satellite Service: 10 years from effective date

Effective date:

Earth Station - Date of grant

Space Station - Date satellite was placed in operation after

a successful launch

As can be seen from the above table, Cellular Radio Service,
International Fixed Radio Services, and Satellite Services have no
specified year of termination. All the others do (these are Part 21 and
Part 22 services). If an individual license expires before the common
year, it will only be renewed for the remainder of the term. An
application for renewal must be sent to the FCC between 30 days and 60
days [prior]{.underline} to the expiration date.

3.4.2 Applications or filings requiring fees must be delivered or mailed
to

the FCC\'s lockbox bank in Pittsburgh, Pennsylvania with the correct fee
and a completed FCC Form 155 (Fee Processing Form) attached to the
application or filing. The lockbox bank is the Mellon Bank which
maintains separate post office boxes with appropriate zip codes for the
various forms being submitted.

The current fee, Fee Type Code, and mailing address for a given form can
be found in one of four lists contained in subpart G of FCC Part 1. The
Common Carrier list is FCC 1.1105; the Private Radio list is FCC 1.1102.
(A private radio application must be submitted to the appropriate
certified frequency

coordinator before filing.) Applications and filings not requiring fees
must

be sent directly to the FCC, Washington, DC 20554. Submitables without
the

correct address or the correct fee will be returned to the applicant
without processing. The Fee Processing Form 155 will no longer be
required when the various application forms have been revised to
incorporate the information requirements of the form. For the latest up
to date information call the

FCC hotline (202) 632-FEES \[(202) 632-3337\].

**3.5 TELEPHONE MAINTENANCE RADIO (FCC PART 90)**

3.5.1 All radio services covered in FCC Part 90 use the same set of
forms.

If an REA borrower desires to purchase a new mobile radio equipment that
would be used solely for telephone maintenance (at any part of the
spectrum) it must be handled through the Private Radio Bureau of the
FCC. There is at present

no other service that would require the borrower to utilize the Private
Radio Bureau. Applications and authorizations are covered in Subpart G
of FCC

Part 90. For a detailed discussion of forms see FCC 90.119 and FCC
90.135.

For discussion of Telephone Maintenance Radio Service see FCC 90.81. For
a list of radio frequencies see Table in FCC 90.81 paragraph (c).

3.5.2 [FCC Form 574]{.underline} (Application for Private Land Mobile
and General Mobile Radio Services) must be used for a new or modified
Land Mobile Radio System authorization and for new components (such as,
base, fixed, or mobile stations). There is a 32-page booklet available
from the FCC which gives instructions for filling out the form. Most
Part 90 applications must be coordinated with the FCC\'s Certified
Frequency Coordinator, as described in the instruction booklet. Obtain a
Form dated 1990 or later.

3.5.3 The following minor FCC Forms needed for various reasons are:

3.5.3.1 [FCC Form 574B]{.underline} (Private Fixed, Mobile, and
Radiolocation Services) contains supplementary information to accompany
FCC Form 574. It is to

be used for frequencies below 27.5 MHz and for frequencies above 27.5
MHz in

counties bordering Canada and Mexico. It probably will be of limited use
to

most REA borrowers.

3.5.3.2 [FCC Form 574-R]{.underline}(Application for Renewal of Radio
Station License) must be used to apply for renewal of an existing
authorization without modification. It is usually mailed to the licensee
by the FCC prior to the expiration of the license.

3.5.3.3 [FCC Form 405A]{.underline} (Application for Renewal of Radio
Station License and/or Notification of Change to License Information)
must be used to apply for a renewal without modification of a station or
system license when the licensee

has not received renewal Form 574-R in the mail from the FCC within 60
days of license expiration. It may also be used to notify the FCC that
the licensee has discontinued station operation and wishes to cancel the
license.

3.5.3.4 [FCC Form 572]{.underline} (Temporary Permit to Operate a Part
90 Radio Station) should be properly executed if the applicant desires
to operate the station during the formal FCC processing of a Form 574.
It does not have to be submitted to the FCC and is only valid for a
period of 180 days from the date the required application forms are
mailed to the FCC.

3.5.3.5 [FCC Form 703]{.underline} (Transfer of Control) must be
submitted whenever it is proposed to change the control of a corporate
licensee.

3.5.3.6 [FCC Form 1046]{.underline} (Assignment of Authorization) may
accompany an FCC

Form 574 being used for assignment (to another person) in order to
inform the FCC that the assignor will submit the current station
authorization for cancellation upon completion of the assignment.

3.5.4 Stations authorized under FCC Part 90 must be placed in operation
within

8 months from the date of grant. Trunked stations on 800 MHz frequencies
have one year to construct. The license term will not exceed 5 years
from the date

of the original issuance, modification or renewal.

3.5.5 It is suggested that REA Borrowers specifically examine the
Specialized Mobile Radio service (SMRS) bands when they wish to purchase
a new telephone maintenance radio system since the newest and
most-up-to-date manufactured products will probably be for these bands.
Frequencies are available from

806 MHz to 821 MHz, and from 851 MHz to 866 MHz. These bands also
contain BETRS channel assignments. However, applications for BETRS must
follow FCC Part 22 rules.

**3.6 ANTENNA STRUCTURES (FCC PART 1, PART 17)**

3.6.1 Antenna structures include the radiating and/or receive system,
its supporting structures and any appurtenances mounted thereon.

3.6.2 The FCC developed standards for antenna structures in conjunction
with

the Federal Aviation Administration (FAA). The Communications Act of
1934, as amended, places full responsibility on the FCC to determine
whether any antenna structure constitutes a menace to air navigation.
The FCC will inform an applicant whether it is necessary to file a
[Notice of Proposed Construction or Alteration]{.underline} (FAA Form
7460-1) with the Federal Aviation Administration.

3.6.2.1 Antenna clearances are granted by the Antenna Survey Branch of
the Public Services Division in the Field Operations Bureau of the FCC.
If the Common Carrier Bureau receives either Form 494 or Form 401 from
an applicant or the Private Radio Bureau receives Form 574 from an
applicant, the antenna data is furnished to the Antenna Survey Branch if
either operating bureau determines that antenna consideration is
required.

3.6.2.2 Any communications between the FCC and the FAA are handled by
the

FCC Antenna Survey Branch and the FAA Airspace and Obstruction
Evaluation Branch for the appropriate FAA Region. The Airspace and
Obstruction Evaluation Branch of the FAA does not normally communicate
with the REA borrower but with the FCC, since the FCC has the major
responsibility.

3.6.2.3 If the FCC Antenna Survey Branch determines that an FAA Form
7460-1 is required the originating bureau will be so advised. If it
determines that FAA Form 7460-1 is [not]{.underline} required or if it
receives a favorable airspace analysis from

the FAA, the Antenna Survey Branch prescribes antenna tower painting and
lighting specifications or other conditions in accordance with FCC Part
17 and forwards this information to the originating bureau. When the FAA
disapproves

of a proposed tower, a report of the unfavorable recommendation is
forwarded to the originating bureau. (See FCC 1.61).

3.6.2.4 An REA borrower may submit an FAA Form 7460-1 to the FAA before
it submits an FCC Form 494, FCC Form 401 or FCC Form 574 to the FCC.
Each application form contains a place where the applicant can indicate
whether or not an FAA Form 7460-1 has been submitted to the FAA. Form
7460-1 must be sent to the Manager, Air Traffic Division, FAA Regional
Office having jurisdiction over the area within which the construction
or alteration will be located. Copies of this form may be obtained from
the regional offices or from FAA headquarters. In addition to FAA Form
7460-1, the telephone company must send FAA Form 7460-2 ([Notice of
Actual Construction or Alteration]{.underline}) to the above indicated
Regional office. The addresses of the Regional Offices may be found on
Form 7460-1.

3.6.2.5 If an antenna structure ceases to be licensed by the FCC, the
owner is required to maintain the prescribed painting and illumination
until the

structure is dismantled. The FCC may require the owner to dismantle and
remove the tower.

3.6.3 If the height of an antenna structure is 20 feet or less above the
ground the FAA does not have to be notified. Nor does it have to be
notified if it is shielded by an existing structure or by natural
terrain or topographic features of equal or greater height and is
located in the congested area of a city, town,

or settlement. If the height of an antenna structure is greater than 200
feet above the ground the FAA must [always]{.underline} be notified. If
the height of an antenna structure is between 20 feet and 200 feet above
the ground the criteria regarding notification depend on the nearness of
an airport runway of any kind. When the antenna structure is more than
3.8 miles from the end of the nearest

airport runway no notification is required. The details of the
notification requirements can be found in FCC 17.7 and FCC 17.14.
Consideration must always be given to the possibility of an airport
runway being extended at some time.

3.6.4 All FCC requirements concerning obstruction marking and lighting
appear in FCC Part 17 (Construction, Marking, and Lighting of Antenna
Structures). Subpart B of FCC Part 17 (Criteria for Determining Whether
Applications for Radio Towers Require Notification of Proposed
Construction to Federal Aviation Administration) covers in detail the
items mentioned above. Subpart C of

Part 17 (Specifications for Obstruction Marking and Lighting of Antenna
Structures) covers all painting and lighting specifications for antenna
structures that exceed 200 feet in height above the ground. The painting
requirements are relatively simple; however, nearly 20 different
lighting specifications have been specified for different antenna
heights. It is the responsibility of the FCC to see that all owners of
antenna towers maintain these standards.

3.6.4.1 The FCC has two additional forms that are not application forms
but rather concise specifications for marking and lighting. These are:
FCC

Form 715 ([Obstruction Marking and Lighting Specifications for Antenna
Structures]{.underline}) and FCC Form 715A ([High Intensity Obstruction
Lighting Specifications for Antenna Structures]{.underline}).

3.6.4.2 During construction, temporary warning lights must be installed
at top of the structure, and at each level where permanent lights will
be installed. The licensee of any radio station having antenna
structures requiring illumination must inspect the functioning of the
required lights daily and

report immediately any malfunctions of any top steady-burning light or
any

flashing light regardless of its position on the structure to the
nearest FAA Flight Service Station.

3.6.4.3 Whenever a licensee constructs or alters an antenna tower
requiring obstruction marking or illumination that will change the
antenna height or the location of the tower a National Oceanic and
Atmospheric Administration (NOAA) Form 76-10 must be filed with the
Aeronautical Chart Division of the National Ocean Service. NOAA Form
76-10 ([Report of Radio Transmitting Antenna Construction, Alteration
and/or Removal]{.underline}) must be submitted prior to the start

of construction and at the completion of such construction or change in
order that this information may be provided promptly for use on
aeronautical charts and related publications.

**3.7 EQUIPMENT AUTHORIZATION (FCC PART 2)**

3.7.1 The REA borrower should distinguish between two kinds of FCC
Authorization procedures. This bulletin has, up to this point, discussed
applications made by the borrower for a Radio Station Authorization or
License. The manufacturer or importer of radio equipment requests an
Equipment Authorization from the FCC. Subpart J of FCC Part 2 covers
Equipment Authorization Procedures.

3.7.2 Radio equipment must meet certain FCC technical standards. The FCC

requires that parties marketing certain radio frequency equipment,
subject to

its technical regulation in the United States, must perform measurements
to demonstrate compliance with the applicable technical standards. The
equipment authorization procedures by which the FCC authorizes equipment
are: type

approval, notification, type acceptance, and certification. Request for

authorization of equipment subject to these procedures is made on FCC
Form 731. Marketing in the United States of equipment subject to the
above-mentioned equipment authorization procedures is prohibited under
FCC 2.803 marketing regulations until the FCC has issued the
prerequisite Grant of Equipment Authorization.

3.7.2.1 [Verification]{.underline} is a procedure where the manufacturer
makes measurements or does whatever else is needed to ensure that the
FCC technical standards are met. No samples, test data, or forms are
sent to the FCC for verified equipment.

3.7.2.2 [Type Approval]{.underline} is an equipment authorization issued
by the FCC based

on examination and measurements of specific equipment by the FCC\'s
laboratory. New equipment and major modifications require testing, while
changes in identification of presently type approved equipment or minor
modifications do not.

3.7.2.3 [Notification]{.underline} is an equipment authorization issued
by the FCC for different classes of equipment operated without an
individual license under

Part 15, and certain transmitting equipment used pursuant to a [station
license]{.underline}. Notification is based on an applicant showing in
writing that the required measurements have been made to demonstrate
compliance with the applicable technical standards. Modifications and
permissive changes to authorized

equipment are normally allowed without the requirement to file an
application, unless these changes are so extensive as to cause the
equipment to be considered a new product, which would require a new
identification and requisite fees. The requisite fees are also charged
for abbreviated application procedures (changes in identification of
presently notified equipment).

3.7.2.4 [Type Acceptance]{.underline} is an equipment authorization
issued by the FCC for many categories of transmitting equipment used
pursuant to a station license, based on representations and test data
submitted by the applicant. The requisite fees are also charged for
abbreviated application procedures (changes in identification of
presently type accepted equipment).

3.7.2.5 [Certification]{.underline} is an equipment authorization issued
by the FCC for equipment designed to be operated without an individual
license under Part 15 and Part 18 of the Rules, based on representations
and test data submitted by the applicant. The requisite fees are also
charged for abbreviated application

procedures (changes in identification of presently certificated
equipment). Part 15 is entitled \"Radio Frequency Devices.\" Part 18 is
entitled \"Industrial, Scientific and Medical Equipment.\"

3.7.3 With the exception of Point-to-Point Microwave Radio, all radio
transmitters used by REA borrowers under FCC Parts 21, 22 and 90 require
[type acceptance]{.underline}. This includes signal boosters and
repeaters. Radio transmitters for Point-to-Point Microwave require
[notification]{.underline}. More specific information

on these equipment authorizations can be found in FCC 21.120, FCC
21.122,

FCC 22.120, and FCC 90.201. For a description of procedures see FCC
2.951 through FCC 2.1045 within subpart J of FCC Part 2.

3.7.4 Cellular radio transmitters require [type acceptance]{.underline}.
They must also conform to the [compatibility]{.underline} standard
(specification) for the air interface between base stations and
subscriber stations. The FCC requires that all conventional cellular
radio systems conform to American National Standard- ANSI/EIA/TIA
553-1989. The newer interim standard for Dual-Mode Operation EIA/TIA
IS-54 contains almost all of ANSI/EIA/TIA 553 but there are some minor
differences.

**3.8 REA REQUIREMENTS AND MISCELLANEOUS ITEMS**

3.8.1 An REA rule in the *Code of Federal Regulations*, 7 CFR 1753.68
(a) (10) states the following: The borrower must obtain authorization
from the Federal Communications Commission (FCC) to construct and
operate radio transmitting equipment. Evidence of FCC authorization is
required for REA contract approval.

3.8.1.1 This means that the borrower must have previously submitted an
FCC Form 401, an FCC Form 494 or an FCC Form 574 to the FCC and obtained
a grant of authorization (or a license) for a radio station. The
effective date on the document is the first day on which construction
may begin. (But see paragraph 3.3.8.4)

3.8.2 The FCC will not grant a Station Authorization (or License) unless
the radio equipments specified for use at the station already have
equipment authorization, if required. This implies that all transmission
equipments in the REA List of Materials or covered by a letter of
Technical Acceptance have equipment authorization, if required by the
FCC. This information should be obtained from the manufacturer when
request for product acceptance is received.

3.8.3 Most listed manufacturers have data sheets that state that a
certain product has an equipment authorization and give the number
identifying it (although not all do so). It may be difficult to obtain a
data sheet on a new product.

3.8.4 FCC Form 730 (Application for Registration of Equipment to be
Connected to the Telephone Network) is for use by any party wishing to
register equipment which connects to the public telephone network. (REA
does not make loan funds available for terminal equipment.) A general
discussion can be found in FCC

Part 68. Specific instructions can be obtained from the Domestic
Facilities Division of the FCC Common Carrier Bureau.

(Intentionally left blank)

**REA BULLETIN 1751H-701**

Page 4-12

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