Is Stratovision the Answer?
January 1950 Radio & Television
sounds about as serious as 'Wonkavision,' but unlike the candy maker's
fictional machine that transported chocolate bars across the room,
Stratovision was in fact a serious proposal. It was an early form
of satellite television. Since orbiting communications satellites
would not be practical for another decade, Westinghouse devised
a system in 1945 using aircraft flying at high altitude to relay
television signals. Engineers calculated that 14 airplanes circling
at 25,000 feet could provide coverage to 78% of the country. By
1950 they had a working system, but needless to say (because nobody
has ever heard of it... except you, now), the concept ultimately
did not pan out. It was not because the system failed to work as
designed, but because there was not enough demand for coast-to-coast
broadcasts at the time.
Terry W. for providing this article.
Is Stratovision the
By Kenneth Boord
Fourteen planes would provide adequate coverage for approximately
78% of the people in the U.S.A.
The experimental Stratovision station (a modified B-29).
flying at 25,000 feet over the Pittsburgh area, rebroadcast
telecast picked up from the east coast television network
over an estimated area 525 miles in diameter. Programs from
the ground stations are picked up on the antenna projecting
above the tail. and rebroadcast from the long. mast-like
antenna extending down from the nose of the plane.
Although Stratovision experiments
in the United States have been suspended temporarily, there are
many TV authorities who believe it is the only answer to truly adequate
"nationwide" video coverage - including the small towns and rural
areas as well as the large metropolitan centers of the country.
Five years ago (1945), the Westinghouse Electric Corporation, in
cooperation with The Glenn L. Martin Co.) announced the intention
of developing this system in the United States.
What is Stratovision?
It is airborne television-transmission
relay from planes in motion at high altitudes. By raising the broadcasting
antenna to greater heights, relatively large improvements in v.h.f.
(very-high frequency) and u.h.f. (ultra-high frequency) transmission
range may be obtained. "Radio" horizon distances, of course, are
largely dependent upon atmospheric conditions. A transmitting antenna
30,000 feet above the earth's surface has a "line of sight" distance
to the horizon of 211 miles, and this distance roughly represents
the service area of such a v.h.f. or u.h.f, transmitting antenna.
Assuming true spherical earth conditions, calculations indicate
that television coverage over this large distance is attainable
with powers equivalent to those used in present-day commercial television
A transmitter operating at 600 megacycles, airborne
30,000 feet above the surface of the earth, with an effective radiated
power of 25 kw. assumed, will achieve a field intensity of 2 millivolts
per meter for a 3D-foot high receiving antenna, out to 238 miles.
This distance represents the coverage from an airborne station over
Minimum values of field intensity occur at several
points closer to the transmitter and fall below the value of two
mv/m. required for service. These minimums will not occur at the
same distances for a different receiving antenna height. This makes
it possible to increase the field intensity at any location by changing
(either raising or lowering) the receiving antenna by a few feet.
Energy at the receiving antenna arrives from the transmitting antenna
over two paths - a direct path and a reflected path. Thus, the net
energy at the receiving antenna is a vectorial addition of these
two components. As the distance between transmitter and receiver
is changed (or the antenna heights are changed), the changing vectorial
relationship produces nulls and maximums. There are as many nulls
between the transmitter and its horizon as the number of half-wavelengths
that the receiving antenna is above ground. It is desirable to reduce
the receiving antenna height as the receiver is moved closer to
the transmitter, even to the point of laying it close to the ground
when the location is near the transmitter.
factors influence the coverage obtained, and these factors have
a greater attenuative effect in the u.h.f. television band than
in the v.h.f. band.
Westinghouse and Glenn Martin companies undertook two series of
flight tests to study the propagation and plane motion effects.
The first of these studies was made with a twin-engine PV-2 aircraft,
capable of operation at 25,000 feet. (This ship was by no means
of the type suitable for day-to-day broadcasting but was quite suitable
for propagation measurements.)
Two transmitters were installed
in the unpressurized cabin of this ship. A 250-watt transmitter,
operating on 107.5 mc., and a 5 kw. (peak of pulse) transmitter
operating at 514 mc., were flown at various altitudes away from
Baltimore (Maryland), and continuous recordings of field intensity
were made at locations ranging from Norfolk (Virginia), to Pittsburgh
(Pennsylvania), and Boston (Massachusetts).
During the one-year
flight-test period using PV-2, measured field intensities were compared
with calculated values and effects of motion of the plane upon the
pulsed waveform of the 500 mc. transmitter were studied. No effects
of the motion of the plane were found except for the expected variation
of field intensity as the distance changed. Measured field intensities
agreed with calculated values reasonably well when the receiving
antenna site approached a true spherical earth condition. Departure
below predicted values was noted when the receiving antenna was
down behind a hill. In a comparison study of measured and calculated
values when the receiving antenna was set up on a beach at Norfolk
(Virginia) - the total path between transmitter and receiver being
over the waters of Chesapeake Bay - it was noted that even with
this ideal "spherical earth" location, the minimums were never as
deep as calculated.
As a result of these tests, the two
companies were encouraged to equip a .more suitable plane for actual
picture transmission in accordance with commercial television standards.
A complete set of modern TV transmission equipment was built
and installed in a B-29 "Super Fortress" with all military armament
removed to make room for the TV equipment. This aircraft is a pressurized,
four-engine, high-altitude airplane capable of operation at 30,000
1 C. E. Nobles, originator of the Stratovision
system for airborne television and frequency modulation transmission,
monitors the television signal received and transmitted from the
Stratovision airplane. Ben Carroll (shown at right), Martin Stratovision
project engineer, listens at the sound monitoring position.
2 C. E. Nobles at the twin video monitoring boards in the experimental
Stratovision airplane. The boards are so arranged that each can
be used separately to monitor the video signal coming to the plane
or the signal being transmitted by the plane, or they may be used
together so that the signal picked up by the plane appears on one
screen while the plane's broadcasted video signal is monitored on
the other. The TV picture is viewed on the large screen.
3 Angus A. Macdonald and Larry Smith, Westinghouse engineers,
record current and voltage readings of the 1000 watt transmitter
used to rebroadcast frequency modulation sound signals from the
airplane. This transmitter plus the TV transmitter and the other
equipment aboard the plane generated so much heat in early flight
tests that a refrigeration system, capable of air conditioning a
seven room house, was installed promptly.
equipment installed in this ship consisted of receivers capable
of receiving standard commercial television stations or a special
547.5 mc. relay link from the Westinghouse studios in Baltimore
(Maryland). Transmitting equipment capable of emitting 5 kw. of
video and 1 kw. of sound on TV Channel 6 (82-88 mc.) and monitoring
and control equipment were also installed. In addition, other transmitters
operating on 250 mc. (with 200 watts c.w.) 750 mc: (with 200 watts
c.w.), and 3300 mc. (with 50 kw. peak of pulse) were installed to
carry out further propagation measurements.
circular dipoles were located on an eight-foot streamlined mast
atop the vertical tail fin. These antennas were used to receive
standard ground stations for rebroadcast. The vertical tail fin
was chosen as a location for the receiving array because it was
the spot most isolated from the transmitting array, thereby reducing
the problems of receiving weak signals in close proximity with the
5 kw. video and the sound transmitters. This location plus the use
of filters made it possible to receive any commercial channel except
Channel 5, where the lower sideband components of the Channel 6
transmitter were relatively high.
In broadcasting position,
the main transmitting antenna mast hung vertically downwards. This
mast carried a two-element turnstile array for the Channel 6 video
carrier, a single-element circular dipole sound antenna, and a 547.5
mc. relay link receiving array. The mast is 28 feet long and induced
some 600 h.p. of additional drag to the plane in spite of its being
streamlined. This array, of course, was retractable for take-offs
for all equipment was obtained from three 15 kva., 500-cycle alternators
coupled directly to the plane's engines. The plane used a 70,000
BTU-per-hour air conditioning system in order to dissipate the heat
from the operating compartment. Without air conditioning, the compartment
temperatures reached 134 degrees Fahrenheit in spite of the fact
that outside air temperatures were 25 degrees Fahrenheit.
This equipment was flown more or less regularly from June of
1948 to February of 1949. Usual flight procedure after take-off
from Baltimore (Maryland) was to climb to operating altitude at
Baltimore. During the hour required to climb, equipment was brought
on the air and checked out. Upon reaching altitude, the plane was
flown away from Baltimore while continuous propagation measurements
and range measurements were being made. The programs of WMAR-TV
of Baltimore usually were broadcast on Channel 6.
each flight, many TV fans wrote in to describe results of the test
at their particular location. These reports from the public were
numerous; several hundred letters were received on some flights
and proved to be a valuable source of information. Area covered
by reports agreed very well with predicted coverage.
angle of five degrees or greater is obtained for only the first
mile from a 500-foot high antenna compared with 64 miles from the
30,000-foot antenna. An angle of one degree or greater is obtained
for only 5 miles from a 500-foot high antenna as compared with 168
miles from the 30,000-foot antenna.
estimate that it would require only eight planes in "relay" to provide
a transcontinental network and six additional planes to give adequate
coverage to 78 percent of the 149,000,000 people in the United States.
As to the future of airborne TV, C. E. Nobles, head of Stratovision
work for Westinghouse, says "The major technical problems of the
system have been solved, and the commercial development awaits only
the crystallization of public demand for the expanded services offered
by airborne broadcasting, application of the system by the radio
industry to meet this demand, and the clarification of channel facilities
available to make possible this application."