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B. Transmitters Used
(1) 72-Megacycle Radiosonde Transmitter (T-49)
The standard T-49 transmitter of the Ar.my Weather Service was
first used in project work, with a modified commutator bar
switching specially coded resistors into the circuit. as the
balloon passed from one critical pressure to another. The
operating characteristics of this transmitter may be found in
the following publications: T.B. Sig. 165, T.M. 11-2403,
T.M. ll-2404 and the Weather Equipment Technician's Manual.
The defects which were encountered in the use of this trans-
mitter were principally (1) relatively short range and (2)
unfitness for direction-finding using available equipment.
Our experience has been that reception from the T-49 trans-
mitter by standard equipment is not much above 80 miles under
good conditions. lihen flights were made which traveled many
times this distances, the inadequa~y of this vransmitter was
clearly demonstrated.
The problem of direction-finding is of major importance when
attempts are made to position e.nd track the balloon and its
equipment train. Since no standard directional receive~ equip-
ment is available for this use with the T-49, this transmitter
is of limited value.
(2) 400-Megacycle F.M Transmitter (T-69)
The T-49 transmitter was abandoned in favor of the T-69 400-mc
system as soon as ground receiving equipment for the latter was
available. By using the directional receiving set SCR-658
with the T-69 transmitter, the problem of direction-finding
and positioning was attacked. A second advantage enjoyed by
this system is the improved range attainable.
Our experience has been that an SCR-658 set in good condition
can receive a signal up to a range of 150 miles, providing
that the line-of-sight transmitter is high enough to preclude
blocking by intervening terrain. The use of two or more sets
to increase the area of a tracking net is discussed under
"Radio Direction-Finding" below.
The operating characteristics of the T-69 system and the SCR-658
may be found in these publications: T.B. Sig. 165, T.M. 11-ll58A.
Pressure indicators were obtained, as with the T-49, by use of
the modified commutator bar switching specially coded resistors
into the circuit as. the balloon passed from one fixed pressure
to another. A few special tests were made of a chronometric
system of pressure modulation. For a complete discussion
of pressure modulation methods, see Section Vl, A.
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