August 1955 Popular Electronics
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
Here is a brief article about
implementing "over the horizon" transmissions for television and phone signals.
Doing so eliminates numerous relay towers in-between which are not only expensive,
but often are difficult to locate due to property acquisition issues.
Bell Telephone Laboratories - Over-the-Horizon Communications full-page advertisement
in the October 1955 issues of Radio & Television News magazine.
"Over the Horizon" Transmission
Super-powered transmitters beam TV and phone
signals up to 200 miles without help from relay stations
Direct television and multichannel telephone transmission through space for as
much as 200 miles - without relay stations and at ultra-high frequencies - is now
a reality, according to announcements by Bell Telephone Laboratories and the Massachusetts
Institute of Technology. Video and audio information can both be sent "over the
horizon" on u.h.f, channels in an extension of a transmission technique recently
applied to the continental defense system.
Over-the-horizon transmission means that longer communications bridges are possible
over water and rugged terrain. In the present microwave radio relay network across
the United States, relay stations are only about 30 miles apart.
Standard AM radio broadcasting employs waves that follow the earth's curvature.
But waves used in television and telephone relays were presumed to travel in a straight
line. For many years, "line of sight" transmission between antennas placed on towers
on the horizon (about 30 miles apart) was thought to be the only practical means
of transmitting by radio the wide bandwidth needed for television and multichannel
This was disproved after years of research at M. I. T. and Bell Telephone Laboratories.
The Bell Laboratories' research stemmed from Bell's success with transcontinental
microwave systems for carrying telephone conversations, radio and television programs
from coast to coast, and their continued interest in radio propagation. The M. I.
T. interest was stimulated by work for the Government in radar and overseas broadcasting.
Scientists knew that ultra-high frequencies traveled "over the horizon" under
certain conditions but believed them to be too weak and undependable for practical
use. In the course of investigating occasional interference attributed to these
waves, however, the scientists discovered that many actually overshot the relay
towers they were aimed at and arrived at farther points with remarkable consistency.
The next step was to provide reliable long-distance transmission "over the horizon."
Engineers did this by erecting larger antennas and using higher power than is employed
in the conventional microwave system. Thus, they put to use the weaker signals that
drop off a straight radio beam beyond the horizon and are reflected or scattered
to distant points by the atmosphere.
The effect of the new system is very much like that of a powerful searchlight
which casts a beam in a straight line. A searchlight aimed at the sky can be seen
from the ground miles away, even when the searchlight is behind a hill. This is
possible because some of the light is reflected and scattered by the atmosphere.
In order to make use of over-the-horizon transmission, 10-kw transmitters and
60'-diameter antennas are being employed, representing 20,000 times the power and
30 times the antenna area used in the present transcontinental microwave system.
It was found necessary to employ the lower frequencies (in the u.h.f. band) to develop
with available equipment sufficient power to attain a satisfactory degree of reliability.
Even after scientists learned that transmission was possible "over the horizon,"
they were not certain that this medium would support the broad band of frequencies
needed for multichannel telephone or television transmission. In the fall of 1953,
they found that they could transmit 12 voice channels "over the horizon." Television
was first successfully transmitted this way in 1954 between Bell's Holmdel, N. J.,
laboratory and the M. I. T. Round Hill Research Station near New Bedford, Mass.,
a distance of 188 miles.
Bell and M. I. T. scientists emphasize that this success with over-the-horizon
transmission will probably result in a supplement to - rather than a replacement
of line-of-sight radio relay systems.
Over-the-horizon signals are not to be confused with a similar type of transmission
known as "ionospheric scatter," which is useful in long-distance transmission of
telegraph signals at relatively low frequencies. Unlike ionospheric signals, the
over the-horizon technique provides signals that are useful for the wide bandwidths
required for a television picture or by many telephone channels.
Posted August 24, 2011