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.
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.
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"Over the Horizon" Transmission
Super-powered transmitters beam TV and phone signals up to 200
miles without help from relay stations
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
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.
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 telephone service.
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
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.
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
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.