20
THE WIRELESS AGE
April, 1919
it, constitutes, as
a whole, a uni-directional receiving antenna; that is, signals arriving
from one end of the loop can be tuned in, while an, interfering signal
from the opposite end can, by proper phase adjustments, be tuned out.
When the looped ,antenna system
is adjusted to annul static of the "grinders" type, the system has |
"Suppose now, the phases of all
currents in the left-hand loop are shifted forward 90 degrees; then the
currents due to the desired signal in this loop are shifted around until
they are in phase with those from the right-hand loop, while the phase
of the currents due to the interfering signal in this loop, and which were
previously 90 degrees ahead of those due to the right-hand loop, are |
a reception curve of the form
shown, in figure 16 its equation, is, v = V cosine2,
while that of the single loop is the cosine curve. The directional effect
in this case is materially greater than with the single loop.
By shifting the phases of the
currents in the loop antennae, the reception curve becomes that of figure
17. Between 0 = 0 and 0 = II, the curve is a cosine2 curve while
between the angles 0 =II and 0 = 2II the curve is a sine-cosine curve,
when the loops are 1/4 wave length apart. This curve indicates maximum
reception in one direction and zero reception in the opposite direction,
with a considerable reduction of signals in the third and fourth quadrants.
The line of zero reception can be swung at will through the third and fourth
quadrants by alteration of the phases of the currents in the two loops,
so that interference from any station arriving in either quadrant can be
annulled, while reception is maintained from signals arriving in the first
and second quadrants.
Mr. Weagant pointed out that
advantage can be taken of this property to eliminate static interference
if the static waves happen to come from a direction other than that from
which the signal arrives. This is of considerable help when a thunderstorm
is gathering in the vicinity of the station. Although the most effective
spacing of the loop to obtain this uni-directional characteristic is one-quarter
length, a general order of the result is obtainable with any spacing between
the loops. The process of adjustment for obtaining one-way reception
is quoted from Mr. Weagant's paper as follows
"Suppose that the two loops
of the system are one-quarter wave length apart and that the desired signal
arrives from right to left; then the currents in the left-hand loop are
90 degrees behind those of the right-hand loop, if the circuits are accurately
tuned, and they will add in quadrature. Next, suppose .a signal arrives
from left to right ; then the currents due to this signal in the left-hand
loop are 90 degrees ahead of those in the right-hand loop and therefore
also combine in quadrature. Then currents due to both signals exist
in the common receiving circuit. |
now 180 degrees ahead of those
in the right-hand loop, so that they oppose and neutralize. Because
of the unusual characteristics of the aerial used, this shift in phase
is readily accomplished by a small adjustment of the condenser in the loop
circuit. If the interfering signal is not in line the right amount
of phase shifting can be made to take care of it, and this general order
of result is obtainable to some extent with any spacing between the loops,
although one-quarter wave length is best. The reception of Carnarvon's
signal, 14,200 meters, through
the powerful interference of the 200 kw. Alexanderson
alternator at New Brunswick, only 25 miles away, working at 13,600 meters,
has been an everyday performance of the system, while at the same time
preserving a good static balance. All forms of the arrangement described
have capabilities of reception through interference, these capabilities
varying with the type of aerial employed, the loop aerials and the horizontal
aerials giving similar curves."
(To be continued)
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