12
THE WIRELESS AGE
April, 1919
| cation and those which caused
but occasional interference and. could therefore be ignored. Static disturbances
due to local lightning and snowstorms were ignored, for the reason that
these types are of so infrequent occurrence ,as to be of negligible importance;
but there remained the three types, termed, "grinders," "clicks" and "hiss-ing."
The last named, which are due to an actual discharge from the aerial to
the earth, give no trouble in the ungrounded aerials used in the Weagant
system. Of the remaining two types, "grinders" and "clicks," the former
were found to constitute the major source of difficulty.
SOME FALLACIOUS IDEAS EXPOSED
The success of Weagant's endeavors
to eliminate from |
provided there is a marked difference
between the damp-ing of the signal and static currents ; but the relief
was by no means sufficient to be of any considerable value when working
over great distances.
The "interference preventer"
next came in for well deserved criticism and was proven by Weagant to be
ineffective. As many of our readers are aware, Fessenden coupled, differentially,
the two legs of a branched aerial, or the primary circuits of two separate
aerials, to a common detector circuit as shown in figure 2. He concluded
that if one branch, say A, be tuned to a trans-
mitting station and the other branch, B, be detuned,
static currents of equal intensity would be induced in both sides and would
be annulled, and that, as the signal in |
the receiver the most troublesome
forms of atmospheric electricity may be attributed primarily to his clear
recognition of the limitations of all so-called static elimination previously
evolved.
Take, as an example, the well
known receiving circuit in figure 1. His experiments and obser-vations
revealed that the static currents induced in the aerial system, A, L-2,
L-3, E had the frequency and the damping of the antenna circuit itself,
no matter what frequency of oscillation to which it happened to be adjusted.
It therefore became evident that if one were to separate, by any sort of
a device, the static currents in the antenna system from the signal currents,
he would be confronted with the proposition of separating two currents
of the same frequency in the same circuit.
Experimenters, heretofore, had
tried to get rid of static interference by detuning the antenna circuit,
by differentially combining two radio frequency receiving circuits, by
differentiall connection of two detectors of different characteristics,
by differentially combining |
Points of Interest Disclosed
by Weagant's
Experiments
THE static currents induced in a receiving aerial by static
"waves" are of the same frequency and of the same damping as the complete
receiving system. When the oscillation frequency of the antenna circuit
is altered by local tuning, the frequency of the static currents changes
in accordance.
For that reason, the differentially
connected, branched aerial system proposed by Fessenden is ineffective
in reducing static; for when one branch is detuned to the wireless signal,
static currents of different frequency exist in the two different branches.
Obviously, two currents of opposite phase but of different frequency cannot
be made to neutralize one another.
Mr. Weagant's researches
prove that all forms of static eliminators utilizing differentially connected
audio or radio frequency circuits, are of little or no value for continuous
long distance wireless reception.
The dominant type of static
waves, called "grinders," apparently is propagated vertically in respect
to the earth. Therefore the static "waves" resulting therefrom are at right
angles to the wireless waves. By the use of properly dis-posed aerials
advantage can be taken of this phenomenon to separate the static and the
signal currents.
Two closed circuit loop antennae,
spaced 1/2 wave length from center to center, the planes of which are in
the path of a passing wave, will be acted upon simultaneously by the vertically
propagated static waves, but at different times by the horizontally propagated
wireless waves.
Hence, when both loops are
correctly coupled to a common receiving set, the static currents will be
in phase and may be neutralized. The signal waves will be out of phase
and will not neutralize, but will add their E.M.F.'s vectorially.
By proper adjustment of the
phases of the currents in one loop, in Weagant's antennae system, uni-directional
reception is possible, signals of maximum intensity being secured from
waves arriving at one end of the loops, while interference from the other
end of the loops may be annulled.
Underground or surface-ground
aerials act as ordinary closed circuit loops erected above earth. By reason
of the capacity effects between the ends of the aerials and the earth,
a return path for the induced currents is afforded, which effectively closes
the circuit.
The greater the capacity
per unit length between the underground or surface-ground aerial and the
true underlying earth, the shorter is the maximum length which can be used
to advantage. This accounts for the fact that approximately 2500 feet is
the maximum length that can be employed for underground aerials placed
under brackish water. |
one branch had little or no opposition
from the other branch, it would be heard. He assumed the static currents
to be forced oscillations and, therefore, that their frequency and intensity
were unaffected by an amount of detuning that would greatly affect the
signal. This, as Weagant clearly pointed out, is an absolute fallacy.
The fact is, that when one branch
of the antenna circuit is detuned, the frequency of the static signals
changes accordingly, Ieaving static currents of one frequency in
one branch, and of different frequency in the other branch. It is,
obviously, not possible to balance out two opposing E.M.F.'s o f different
frequency. Moreover, the detuning of one branch affects the
intensity of both the signal and static currents in the secondary circuit
in the manner just explained.
It is important to note here
that two opposing E.M.F.'s can completely neutralize each other only when
they have the s a m e frequency, the same wave form, and opposite phase.
And in the case of damped oscillations, in addition |
two audio frequency circuits,
as in DeGroot's method,
and finally by the use of the Dieckmann shield.
The hoped for results in detuning
the antenna circuit could not be realized because such detuning did not
reduce the intensity of the static signal, but simply changed its frequency.
The loss, in the transfer of static energy to the secondary circuit when
tuned to the frequency of the incoming signal, is exactly the same as the
loss in intensity experienced by the signal currents through detuning the
antenna. This, of course, does not improve conditions in. the slightest,
for it reduces the static and the wireless signal in the same ratio.
Some improvement has resulted
from the use of loose couplings between the primary and secondary
circuits, |
to these requirements, the dampings
of the two E.M.F.'s
must be identical. The writer feels assured that readers
will at once recognize that the steps by which Mr. Wea-
gant eventually arrived at the result attained in his
receiving system, constitute one of the most original applications of engineering
principles ever made in radio telegraphy.
Continuing, Mr. Weagant said
that if any experimenter had secured worth-while results by means of a
differential audio frequency circuit, such results have been due to the
looseness of coupling involved in the circuits under test. He pointed
out also that the effectiveness of balanced detector circuits is due solely
to the protection against loud crashes afforded to the ear of the operator.
Re- |
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