[Boatanchors] Historic Firsts: Airplane Radio Telephony

[William L Howard]

>From Mindy at Fort Monmouth:
Historic Firsts: Airplane Radio Telephony

Although the airplane was scarcely out of the experimental stage at the
beginning of World War I, its great possibilities for military and naval
use were obvious. To bring out the full advantages of these
possibilities, however, radio communication to and from the plane seemed
essential. During the early years of that war, radio-telegraph equipment
was extensively tried. Radiotelephone communication would have been more
desirable, but no suitable equipment or circuits were available. The few
trials of the radiotelephone made prior to this employed low efficiency
apparatus operating at low efficiencies and requiring a large amount of
space, while for the airplane use it was essential that weight and space
be kept to a minimum.

Foreseeing the need for such apparatus, Bell Telephone Laboratories-then
the Engineering Department of the Western Electric Company- had already
carried on considerable experimental work, and had fairly well surveyed
the possibilities. When Major General Squier, Chief Signal Officer of
the Army, called a conference late in May, 1917, to consider the
feasibility of radio communication for planes in flight, it was thus in
a position to outline what seemed possible. As a result of conclusions
reached at this conference, the Signal Corps placed an order on the
Western Electric Company to develop radiotelephone apparatus for
airplane service. Within less then six weeks experimental apparatus was
ready, and a series of tests was carried out at Langley Field, Va., from
June 30 to July 5, 1917. On July 2 speech was successfully transmitted
from plane to ground, and on July 5 good transmission was obtained from
ground to plane. Further tests, using modified apparatus were made
between August 14 and 24, and on August 18, successful two-way
transmission was attained between plane and ground, and two days later,
between plane and plane.

Many factors contributed to the promptness with which the Laboratories
overcame the many obstacles to radio airplane telephony. Probably the
basic contribution was the Heising constant current modulator. Of the
previous modulation systems, one required amplifier stages beyond the
modulator, which gave low efficiency and added greatly to weight and
bulk, while another, using grid modulation of an oscillator, gave only 5
or 10 percent modulation, and was difficult to adjust for its best
performance. With the Heising circuit only two tubes were required
altogether, and modulation was greater than 50 per cent. It was these
great improvements in operating characteristics that made operating
characteristics that made airplane radiotelephony practicable.