"OVER THE HORIZON" TRANSMISSION

Direct TV Transmission Through Space
at Distances up to 200 Miles is Now Possible Without Relays

THE direct transmission of television signals for distances up to 200 miles, without relay stations and at ultra-high frequencies, has been accomplished by the Bell Telephone Laboratories and the Massachusetts Institute of Technology.

The new technique is an extension of the transmission methods recently applied to the continental defense system. The principal virtue of the new “over-the-horizon” transmission is that longer communications bridges are possible over water and rugged terrain. In the present microwave relay network across the United States, relay stations are 30 miles apart.

For many years, line-of-sight transmission between antennas placed on towers on the horizon was thought to be the only practical means of transmitting by radio the wide bands needed for television and multi-channel telephone service.

This has now been disproved after years of research at M.I.T. and Bell Labs. The Bell research stemmed from the Laboratories’ 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 day-in-and-day-out applications.

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. M.I.T. and Bell engineers did this by erecting larger antennas and using higher power than is employed in conventional microwave systems. Thus, they put to use the weaker signals that drop off from a straight radio beam beyond the horizon and are reflected or scattered to distant points by the atmosphere.

In order to make use of over-the horizon transmission, 10 kw. transmitters and 60-foot diameter antennas are being used. This is 20,008 times the power and 30 times the antenna area used in the present transcontinental 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.

The scientists emphasize that this success with over-the-horizon transmission will probably result in a supplement to, rather than a replacement of the line-of-sight radio relay systems presently in use.

The new technique, unlike ionospheric scatter, provides signals that are useful for the wide bandwidths required for TV transmission.

Radio and Television News/June 1955