May 1966 Electronics World
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
It would be more than a decade after the publishing of this
article before the first direct-to-home satellite television
broadcasts would be a reality, so it shows how long plans were
being made for such systems. Rural landscapes are still peppered
with the large vestigial C-band (~4 GHz)
satellite dishes, many with faded eyeballs and other clever
(and ugly) artwork on them. Before
coaxial cable was strung beyond suburbs, country dwellers who
either could not pull in over-the-air broadcasts from downtown
locations or just wanted more viewing options paid dearly for
satellite service. Equipment and installation costs on early
systems could run into the $30k realm. Today's satellite TV
systems use much smaller antennas operating in the Ku band
(~12 GHz), with equipment
and installation being free with a 2-year commitment. C-band
DBS (direct broadcast satellite)
systems are still available, BTW. This article is chock full
of good engineering information.
Home TV Via Satellite
By Bradford B. Underhill / Arthur D. Little, Inc.
Our author began his professional career
at the Harvard Underwater Sound laboratory in 1943. From the
end of the war until 1949 he was Assistant Professor of Engineering
Research at the Pennsylvania State Univ. and a Group leader
of Electronic Research at the Ionospheric Research Lab. From
1949 to 1959, he was with RCA engaged in the development of
advanced military systems using color television. Since 1959,
Mr. Underhill has been a Senior Staff Member of Arthur D. little,
Inc., an industrial research company with headquarters in Cambridge,
Massachusetts. There he has participated in and directed studies
aimed at assessing the technical-economic constraints on CATV,
pay television, and ETV, and has provided consulting services
in management, marketing, engineering, company policy, and corporate
assessment to many companies.
Can we use space satellites to broadcast TV directly to individual
homes? What are the problems and can they be solved in the immediate
future? Here are authoritative answers to these and other important
Editor's Note: We recently attended a financial seminar
presented by the National Community Television Association,
Inc. in New York City, During the all-day session over a dozen
papers were presented dealing mainly with the financial aspects
of CATV. Included were papers by Milton J. Shapp, president
of Jerrold Corp., on the history, size, and relationship of
CATV to the broadcasting industry; by Willard E. Walbridge,
representing the National Association of Broadcasters, on the
broadcaster's viewpoint; by Frederick W. Ford, former chairman
of the FCC and now president of NCTA, on industry relationship
with FCC and Congress; by E. William Henry, chairman of FCC,
on the FCC view of CATV; and others. Among the most interesting
and important was the talk on satellite broadcasting by Bradford
B. Underhill. A portion of Mr. Underhill's presentation dealt
with the possible use of satellites in connection with a CATV
system. Of more general interest, however, were his remarks
on the feasibility of using satellites to broadcast TV programs
directly to our homes. This is a subject of widespread and increasing
interest today, especially in view of the successful use of
Telstar, Syncom, Relay, and Early Bird communications satellites.
Because of its importance to our readers, we have excerpted
below a large portion of Mr. Underhill's remarks.
In discussing the impact of space satellites on the future
of world-wide television programming, we will consider the technical-economic
feasibility of a satellite system that could, conceivably, transmit
8 to 12 channels of TV to individual homes. (Ideally, we would,
of course, like to postulate over 100 channels which would allow
a wide choice for the individual television viewer; but a discussion
of 8 to 12 channels will, I believe, adequately demonstrate
the overall situation.)
Clearly, if a television set coupled with a relatively simple,
unobtrusive, and cheap antenna system at each home could, at
anyone time, pick up over 100 different TV programs ranging
from comedy through education, news, serious plays, and do-it-yourself
material, we would have reached a very advanced state of communications.
Even the ability to select 8 to 12 different and interesting
programs would provide a new dimension to television, particularly
if the selection were on a national or even international basis.
Let's consider the practicality of the idea of using space
satellites to broadcast TV directly to individual homes. Any
communications system requires a source or transmitter and a
receiver. In a satellite system, whether located on the ground
or in the air, the satellite performs two functions; that is,
it must receive signals from the primary transmitter or transmitters
and then amplify and repeat selected signals to selected audiences.
Conceptually, a space satellite can distribute signals on
a grand scale. Consider a high-altitude space satellite that
hovers over essentially the same spot on earth 24 hours per
day, 365 days per year. It could receive signals from anywhere
on almost one-half of the world and repeat this to the same
portion of the world. One such satellite could cover all of
North America, South America, and much of Europe and Africa.
Two more could cover 95% of the populated world. Compare this
with the 35- to 40-mile television reception available today.
The comparison is staggering.
Such a satellite, operating at an altitude of about 22,300
nautical miles, is called "synchronous" since it keeps pace
with the earth's rotation. Another possibility imagines nonsynchronous
satellites operating at a much lower altitude say, 300 miles
- which move with respect to the transmitter and receiving stations.
In order to keep each satellite in view of its neighbors and
the ground, at least ten such satellites would be required.
Each would have to be tracked by appropriate ground stations
and each would have to transmit to the ground as well as to
its neighbors. In addition, each receiving station would have
to track the satellites as they move across the zenith and acquire
the next one as it becomes visible. We do not believe that such
a system will prove feasible for home use, but concede that
COMSAT or some such organization might be able to develop a
base for non-synchronous satellite global communications with
heavy government support.
Assuming you will accept the loosely supported contentions
as far as non-synchronous satellites are concerned, let us examine
the case for synchronous satellites. Remember that this is a
satellite that remains over essentially the same spot on earth
and can receive signals from anywhere in its field of view and
retransmit to that same wide area; roughly one-half of the world
can be seen from such a satellite. A synchronous orbit is operationally
desirable in order to avoid problems of pointing antennas toward
the satellite and making contact with (acquiring) the satellite
as it rises over the horizon.
The satellite must contain a source of electrical power. At
the present time, photovoltaic cells which draw power from sunlight
are the commonest source of satellite power. Conventional batteries
are nearly useless for long-time activity. In the future, nuclear
devices capable of generating a substantial amount of electric
power for long periods of time may be generally available for
communications satellites. Until nuclear power supplies are
available, transmitted power from a satellite will be severely
limited, for a solar-battery power supply weighs about one pound
per watt of continuous available power.
The satellite platform will contain the receiving antenna
and amplifier, modulator, a rebroadcasting transmitter, and
a transmitting antenna. As these antennas must have high gain
and correspondingly narrow beams, the attitude
control of the platform must be sufficiently precise to aim
the antennas within appropriate tolerances. This problem is
not insurmountable, but every sharpening of the attitude tolerances
increases the complexity of the attitude sensor and control
system and adds to the amount of energy which must be available
for attitude control.
The receiving ground complex consists of receiving antennas
and receivers, that is, an amplifier and demodulator to convert
the signal back to the preferred usable form. With the satellite
in a practically stationary orbit, a complex control system
is not needed to aim the ground antenna.
where: dB = 10 log10 P1/P2 or 2x power = 3 dB;
10X power = 10 dB; 100x power = 20 dB.
Down-Link Power Computations
At first sight, the transmission properties of the link from
the ground to the satellite look very much like those of the
link from the satellite to the ground. The principal asymmetry
is that in the up-link the transmitter is on the ground and
the receiver is in orbit, whereas in the down-link the transmitter
is in orbit and the receiver is on the ground. The state of
our technology is such that we can install more powerful elements
on the ground than we can in orbit. Practically speaking, this
usually makes up-link performance easier to achieve than down-link
performance, for a ground transmitter can be vastly more powerful
than an orbiting transmitter, whereas a ground receiver can
only be a little more sensitive than an orbiting receiver. Consequently,
we shall examine the power budget for the down-link only.
Frequency Assumptions, We will confine our discussion primarily
to the existing television broadcast frequency bands at 54 to
88, 174 to 216, and 470 to 890 MHz. For specific numerical examples,
we chose frequencies of 60, 200, and 600 MHz as representative
of these three bands. Still higher frequencies, i.e., 2000-7000
MHz might be used but these would require expensive conversion
equipment at each home.
Radiated Power Assumptions: Where there is dependence on
solar power, it is likely that the maximum continuous radiated
power will not exceed a few watts, perhaps 10 watts at most.
When nuclear or other power supplies are available, the primary
power limitation is removed and the significant limit will arise
from constraints on dissipation and life-time in the power output
stages of the transmitter. We believe 100 watts radiated power
will represent a likely maximum for the next decade. Under extreme
conditions, power of 1000 watts may be attained. However, even
with ground-based equipment, current technology does not attempt
to achieve long life-time and high reliability in electronic
circuit elements operating at power levels of even one kilowatt;
reliability relies on replacement. The ordinary techniques of
enhancing reliability by overdesign and derating are ineffective
where high power throughout is required. These suggested limits
are consistent with the guidelines cited by G.M. Northrup in
a paper prepared for the Rand Corporation in 1963. Accordingly,
I shall assume a total radiated power from the satellite of
100 watts except where otherwise stated.
Satellite Transmitting Antenna Directivity: The effectve
radiated power from the satellites can be enhanced 80 times
(19 dB) with a directional antenna whose beam just illuminates
the near-hemisphere visible from the satellite. An antenna 10
times as large will illuminate, in the temperate zone, an oval
about 2500 x 4000 nautical miles, and will have a gain of 800
(29 dB). An antenna 10 times larger yet will have a gain of
8000 (39 dB) and illuminate an oval about 800 x 1200 nautical
miles. At the frequencies under consideration, however, these
antennas are quite large, as shown in Table 1.
Table 1 - Diameter of various high-gain transmitting
Although very large antennas have been considered, we have
as yet no experience in the erection and operational use of
antennas even as large as 60 feet, to say nothing of antennas
10 times this diameter. Furthermore, a 29-dB antenna has a beamwidth
of about 6°, and a 39-dB antenna has a beam width of less
than 2°. The pointing of such a sharp beam adds a burden
to the attitude control system of the platform.
For the moment, let us assume the 19-dB antenna on 1 satellite
and defer consideration of higher gain antennas until later.
Path Loss: Because of the way certain standards of television
signal strength have been developed, comparisons are facilitated
by computing signal strength at the earth's surface directly
in terms of received power density per unit aea for one channel.
Let us assume that the 100 watts which is radiated through a
19-dB gain antenna is equally divided among 12 channels, each
having a bandwidth of 6 MHz. The slant range from the satellite
to the temperate zones is approximately 45,000 kilometers. On
the ground the power density is: PD = 2.6 x 10-14
watt per square meter or 26/1000th of a micromicrowatt/meter2.
Receiver Antenna Directivity: The effective capture area
of an omnidirectional antenna is shown in Table 2 for the various
frequencies under consideration. A half-wave dipole antenna
has 1.6 times (2 dB) more directivity, provided it is appropriately
aimed. Combinations of reflectors and multiple dipoles can give
gains of several decibels, but more substantial gains require
structures whose size is comparable to those shown in Table
Table 2 - Effective capture area of omnidirectional
An antenna with more than a few dB gain is likely to be large
and expensive. Such antennas will probably be ruled out in applications
where one would be required for each home television user. For
the sake of comparison, further computations are based on the
use of a non-directional receiving antenna. This factor can
easily be modified by assuming some other antenna gain figures.
From: Television Engineering Handbook,"
Donald G. Fink, ed.; McGraw-Hill, N.Y. 1957, pp. 2-35.
Noise assumptions: We have two bases for estimating noise
limitation. On the one hand, we can use the grades of television
service from the FCC regulations, as quoted in Table 3.
Table 3 - Grades of television service and
the FCC regulations governing them.
For convenience, the power density in watts per square meter
has been computed and exhibited alongside of the field strength.
On the other hand, we can base signal-to-noise relationships
on thermal noise and an assumption about the noise temperatures
of the receiver and the associated environment. A set of such
assumptions is shown in Table 4.
Table 4 - Equivalent-noise assumptions for
In order for the television receiver to function properly,
the signal strength must exceed the noise by a substantial margin.
The exact amount depends on the grade of service required and
on the character of the noise. It will be assumed that a signal-to-noise
ratio of 26 dB is required for satisfactory service and that
5 dB additional margin is required to allow for system degradation
other than that arising in the down-link from the satellite.
If the noise is white thermal noise this will result in a high-quality
black-and-white television picture but one that is not totally
free from visible noise effects. The sensitivity of color television
to noise interference is somewhat greater.
Resulting Margins: Table 5 compares the received signal strength,
as computed above, with the FCC standards and with the performance
of a low-noise receiver with a 100°K effective noise temperature
connected through a non-directional antenna. The resulting figures
are expressed in decibels and are all negative, showing the
deficit in dB between the signal as it would be received from
the satellite and the desired grade of service indicated at
the head of the column.
Table 5 - Deficit received power from a satellite
system. Received power density in dB relative to various service
It is easy to see that the FCC Grade B Service is closely
parallel in signal strength requirements to that required by
a low-noise 1000°K receiver with an isotropic antenna. Those
of you who have ever tried to operate a television receiver
in an area with Grade B Service, so-called fringe-area reception
know that with off-the-shelf home television receivers, the
performance is very unsatisfactory without a directional antenna.
In one way or another, the deficit of 33 to 70 dB must be
made good. If a 1000-watt transmitter can be used aboard the
satellite 10 dB can be gained at once.
The transmitting antenna gain may possibly be increased 20
dB at 600 MHz and 10 dB at 200 MHz. At 60 MHz it is hard to
see how this could be increased at all.
A receiving antenna six feet in diameter will have a gain
of 20 dB at 600 MHz and 10 dB at 200 MHz. At 60 MHz it is fruitless
to attempt antenna directivity because galactic noise at 60
MHz is greatly in excess of 1000°K noise assumed for the
receiver. The highly directive antenna would focus on sources
of high galactic noise periodically during the earth's rotation
and nullify the desired improvement.
The receiver noise figure could be reduced somewhat provided
a high-gain, low-side-lobe receiving antenna were used. This
leads to a complex and expensive system, and would probably
be useful only at the 600-MHz frequency; for even at 200 MHz,
galactic noise is a significant source of interference. Let
us assume a possible 5-dB improvement at 600 MHz. Table 6 shows
the total improvement achievable by these means. It is clear
that success is possible only at 600 MHz and that the margin
is slim. Even this success is bought at the cost of a 1000-watt
transmitter, a 60-ft diameter antenna on the satellite, a 6-ft
diameter receiving antenna with low side lobes, and a receiving
system operating at an effective noise temperature of 300°K
on the ground. We believe that such a low-noise receiver antenna
system would add a minimum of $200 to the homeowner's cost.
Even if we are overly conservative, a home conversion cost of
at least $6 billion is indicated to convert current TV households
to satellite receivers. Without further analysis, it appears
that this is hopelessly expensive for the ultimate users of
such television signals.
Table 6 - Additional system improvements
that are available.
Sources: "A Study of the Sources of Noise in Centimeter-Wave
Antennas," D. C. Hogg, 1961. "Aids for the Gross Design of Satellite
Communication Systems," G. M. Northrop, Rand Corporation.
Synchronous-altitude satellite radiating 100 watts divided
equally among 12 channels through a 19-dB gain (full-earth coverage)
antenna. (assumes 2.6 x 10-14watt/m2
available; Table 3 power required, and Table 2 antenna gain)
*19-dB transmitting antenna -26 dB S/N Into 1000°K receiver,
Although the technical problem is made easier by going to
frequencies above 2000 MHz, this would add the burden of conversion
cost to the individual home owner and would still provide only
marginal signal strength unless only a small portion of the
earth were covered by the satellite.
The 9-dB deficit at 600 MHz could be made up by reducing
the number of channels transmitted to only one or by increasing
the power to 10,000 watts. The first is not attractive and the
second is beyond our capability at this time.
There is no doubt that television transmission via satellite
relay is technically feasible with a larger permanent ground
antenna and receiver installation, but such installations cost
many millions of dollars to build and hundreds of thousands
of dollars per year to staff and maintain.
Such installations would allow economic world-wide transmission
and reception of a single TV channel and several radio channels
and would be attractive to the broadcasters. Retransmission
to homes would be achieved by relaying the received information
to the individual homes via cable or local transmitters, Certainly
governments might find such a system valuable for transporting
information to other world centers or to developing nations.
To sum up, we at Arthur D. Little, Inc. believe that:
• Non-synchronous satellites would require trackers
at each home. Large-scale CATV operators could conceivably develop
a receiving system to track multiple satellites, but this would
be an expensive undertaking.
• Synchronous satellites broadcasting twelve channels
to the home are not technically feasible below 200 MHz and are
only marginally feasible at 600 MHz. Here expensive receiving
equipment is required.
• At frequencies between 2000 and 7000 MHz, trunking
is possible but this would require expensive ground equipment
that only the most affluent CATV systems could afford.
• A feasible system would require at least a 20,000-watt
continuous power source (more probably 30,000 to 40,000 watts
of prime power would be required) and the development of high-power
transmitting equipment that could operate reliably for many
years. Neither development appears possible for many, many years.
Posted December 4, 2014