Evening star. [volume] (Washington, D.C.) 1854-1972, 1927-06-05
SUPERVISORS PLAN
radio AREA MAPS.
Work Designed to Relieve
Puzzles in Broadcast
and Reception.

Some of the problems relating to
broadcast transmission and reception.
which have been defying both broad
cast station directors and puZzling
the radio listeners, will be solved this
Fall by the field strength measure
ments and the mapping Of service
areas of stations by the Department
of Commerce radio supervisors. If
they get suitable radio-equipped test
cars. The Federal Radio Commission
is anxious to have data of this sort
SO that 1t can more efficiently allocate
broadcasting channels and assign ade-
quate transmitting power.
Experimental surveys relating to
to the strength of several broadcast
stations signals at different distances
from their tranSmitters have been
made by Supervisor S. w. Edwards of
the eighth radio district in his radio
test car. In many cases it has been
found that the transmitters were not
well located. The results showed that
the areas these stations wanted to
serve with programs were not as regu-
lar nor as great as desired. The ma
iority of stations were found to be To-
Gated on buildings within the metro
politan areas of cities varying in size,
while most of them were in or on
steel buildings, generally much too
close to networks of electric wires.
That such conditions produce undo
Sirable effects on broadcast transmits
sion, has been demonstrated by tests
made in several widely separated
cities east OF the Mississippi River, on
frequencies between $40 and ITO kilo-
cycles. Some of the best broadcast
stations were found undesirably To-
/ Gated, despite the use of from $50 to
$500 watts, their service areas were
were found limited and Indispensable.
Two Serious Factors.
Two of the most serious factors dis
covered were absorption of power by
/ surrounding masses, such as build
| ings, and "shadowings" by either
these or other bodies of non-conduct-
ing materials. Masses which absorb
the radiated energy from a broadcast
transmitter are usually grounded, and.
therefore, part of the energy trans
mitted is lost to the listeners. Nom
conducting masses which cast radio
l"shadow" or dead spot. seem to reflect
the broadcast transmissions in an
other direction. This limits the area
served, throwing a sort of shadow |
over it and preventing a perfect radio
field around the station.
Absorption is propably the worse OF
these two enemies of the broadcast |
crs and the listeners. Absorption in |
the surrounding territory of one sta |
ton measured. prevented its broad |
casts from being heard at a distance |
of more than IS miles in one direc- |
ton, whereas in others it was con-
Sistently reported from far greater |
distances.
Another transmitter, the troubles of |
which were definitely determined by |
tests, was located on the shores or |
lake. The transmitter was on top OF |
a large steel building, which absorbed |
energy from the circuit on its normal
wave length, but radiated very strong |
" on a higher wave length to such |
degree that it was audible fer more /
than IS miles. This double transmits !
sion had to be corrected. Such diff !
, culties as these two cited produce low |
efficiency in transmission in the first |
case and inferior service to listeners |
supposed to be we'll served in the |
second.
Locations Cause Trouble.
Tn some cities there are centrally To- /
Gated two or more stations with equal
a)ower operating on the same channel. |
Frequently they use the same sype OF |
apparatus, but in certain directions |
one station shows much better SlgnaI I
strength than the other. In some in |
stances the broadcast programs from |
one of them are much stronger than |
Shey are from the other. Assuming
chat they are efficiently operated. then
<he radical differences in results indi-
Gates that their locations are the |
causes. One may be on or near a |
large metallic body. Almost univer- |
sally it has been found that if one of |
two similar stations is in the country |
and the other in q city, the former |
! proves far superior in serving its lis |
teners. To compensate this the trans |
I mitter in the city has to increase its
input power materially to compete
with the one outside, which is unaf-
fected by metropolitan absorption. to
furnish signals of the same strength.
Increased power in cities is undesir-
able to nearby listeners. This proves
that equivalent power does not neces-
sarily produce similar results.
1t is for this reason that the meas
IIrement of the output or signal power
of a station at different distances and
in several directions is necessary, in
stead of computing or measuring the
input power. The basis of such meas
urements in microvolts per meter vary
ing with the wave lengths. Results
thus found are comparable, and the
regulation of a stations power, based
only fair method to the competing sta
tions and the radio fans. Such meas
urements show instantly whether or
not broadcasting station is using too
much or too little power from its loca-
ton.
Observations indicate that where
transmission is difficult from a station
located on a steel building on certain
frequency, conditions may be improved
by ascertaining on what frequency
broadcasting can be carried on there
so as to avoid the resonant effects of
the building. This research work has
also demonstrated that q signal
strength of about 10000 microvolts. or
10 millevolts, is ordinarily necessary
for reliable reception on the average
receiver, although on occasions lower
field strength makes programs audible.