AERIAL SYSTEMS. EARTH CONNECTION.
Every radiotelegraphic station may be summed up as comprising these
elements: first of all, certain appliances collectively forming the
transmitter and serving to create the waves; secondly, the receiving
apparatus, whose function is to detect the signals of some far-distant
sending station, and lastly, an external organ called the aerial, or
antenna, consisting of a huge system of wires elevated high in the air
above all surrounding objects, either vertically or sloping, or partly
horizontal and partly vertical, which radiates or intercepts the
electromagnetic waves, accordingly as the station is transmitting or
receiving.
The antenna is at once both the mouth and the ear of the wireless
station. Its site and arrangement will greatly determine the efficiency
and range of the apparatus.
The site selected is preferably such that the aerial will not be in the
immediate neighborhood of any tall objects, such as trees, smokestacks,
telephone wires, etc., because such objects not only absorb an
appreciable amount of energy when the station is transmitting messages,
but also noticeably shield the aerial from the effects of incoming
signals and limit its range.
The nature of the ground over which the waves must travel also enters
into the question, and is always considered in locating a station. In
gliding over the surface of the earth, the waves generate weak currents
in the earth itself. If the ground is very stony or dry, these earth
currents encounter considerable resistance, and the possible distance of
transmission over soil of this sort is very much less than if it were
moist. Moist soil and water offer very little resistance, and the
difference in the results obtainable at the receiving station when the
waves travel over an area of this sort is very marked.
[Illustration: FIG. 11.—An amateur aerial and station.]
A station which can only send 100 miles over land can send messages
three or four hundred miles over the ocean.
Forests exert a very decided effect upon the electric waves. Each
individual tree acts as an antenna, reaching up into the air and
absorbing part of the energy. The difference in the range of a station
during the summer months and that of the same station in winter is
considerable. In summer the trees are full of sap and, being much better
conductors of electricity when in this condition, act in the capacity of
innumerable aerials rising in the air, and able to absorb appreciable
amounts of energy. During these same months the air becomes highly
ionized, in which state the air molecules carry an electric charge, and
are particularly opaque to the waves. This condition also usually exists
in the presence of sunlight, the result being that the most favorable
time for the wireless transmission of messages are the hours around
midnight.
[Illustration: FIG. 12.—The Army wireless station at Fort Gibbons,
Alaska, showing steel lattice work mast and aerial system.]
Locality is another factor which usually receives a fair share of
attention in selecting the site. Certain sections of the country, for
seemingly no apparent reason, are very hard to transmit messages, either
to or from. Wireless stations located on the Pacific Coast, for
instance, are more efficient than those situated along the Atlantic
seaboard, while those in the tropical regions are only able to send
short distances in comparison to those farther north or south. Messages
seem to travel better in the direction of the lines of longitude than
along the lines of latitude.
[Illustration: FIG. 13.—Lightning discharge near Montclair, N. J.]
"Static," that "bugbear" of the wireless operator, is very much more in
evidence in the eastern parts of the United States and in South America
than it is on the western coast of the country. If any one should ask a
wireless operator what "static" is, he would probably reply, "a
nuisance." In reality, it is caused by atmospheric electricity. When
atmospheric electricity "jumps," it is called "lightning." A lightning
discharge sets up very powerful waves in the ether, which strike the
aerial of the wireless station and produce a peculiar rumbling,
scratching sound in the telephone receivers, and sometimes seriously
interfere with a message. In fact, it is possible for a wireless
operator to predict a thunder shower by many hours from the sounds he is
able to hear in his telephone receivers.
The cause of lightning is the accumulation of electric charges in the
clouds. The electricity resides on the surface of the particles of water
in the cloud. These charges grow stronger as the particles of water
coalesce to form larger drops, because, as they unite, the surface
increases proportionally less than the volume and, being forced to lodge
on a smaller space, the electricity becomes more "concentrated," so to
speak. For this reason the combined charge on the surface of the larger
drop is more intense than were the charges on the separate particles,
and the "potential" is increased. As the countless multitudes of drops
grow larger and larger, in the process of forming rain, the cloud soon
becomes heavily charged.
Through the effects of a phenomenon called "induction," a charge of the
opposite kind is produced on a neighboring cloud or some object of the
earth beneath. These charges continually strive to burst across the
intervening air and neutralize each other. As soon as the potential
becomes sufficient to break down this layer of air, a lightning stroke
from one to ten miles long takes place. The heated air in the path of
the lightning expands with great force, but immediately other air rushes
in to fill the partial vacuum, thus producing atmospheric waves, which
impress the ear as the sound called thunder.
Wireless stations belonging to the United States navy and located on
land are usually housed in a small building in the immediate
neighborhood of the tall wooden mast which supports the aerial.
Commercial stations are usually situated on the top floor of a high
office building, or a hotel, and the aerials supported by a steel
lattice-work tower. Amateurs place a small pole on the roof of the
house, or in a tree, and locate their station in any convenient room
near the top of the house.
[Illustration: FIG. 14.—Photo of double lightning discharge passing
to earth near the First Orange Mountain, Montclair, N. J.]
[Illustration: FIG. 15.—Vertical aerials of the "grid," "fan"
and "inverted pyramid" types.]
Aerials are of numerous classes and forms, but the most prominent types
can be divided into two main groups, called respectively, the "Flat-top"
and "vertical" antenna.
The vertical aerials are the older form, and are usually employed for
long-distance work or ultra-powerful stations. The aerials intended for
transmission from Europe to America, installed by Marconi, consisted of
huge inverted pyramids, supported by four heavy lattice-work towers,
over 200 feet high. Vertical aerials also sometimes take the form of an
umbrella, or fan, where only one supporting pole is available. Iron pipe
masts may be employed for the purpose, by setting on an insulating base.
The umbrella aerial is used extensively in the army and portable sets.
The flat-top aerials are gradually coming into very extended use. They
are used to the exclusion of all others on shipboard. They need not be
so high as a vertical type aerial in order to be as efficient. Flat-top
aerials consist of a vertical portion and a nearly horizontal portion.
The horizontal portion is practically useless, as far as its work in
radiating waves is concerned, it being used for the purpose of
increasing the capacity of the aerial. An increase in capacity in an
aerial means that more energy can be stored and radiated. Flat-top
aerials have the objection, however, of possessing a directive action;
that is, they receive, or radiate waves, better in one direction than in
the other. A flat-top aerial always receives or transmits better in the
direction that the ends point than in a direction at right angles to the
wires.
[Illustration: FIG. 16.—A diagram showing pyramid aerial.]
The accompanying diagram is an illustration to show the effects of the
directive action of a flat-top aerial. The black lines marked A B, and
appearing very much like a little grating, represent an aerial of the
inverted "L" type, looking down on it from above. B is the free end of
the aerial, and A the closed end, or end to which the wires leading down
to the station are attached. If a snapshot of the lines of strain
produced in the ether as the waves move away from the aerial could be
taken, they would appear like the curved lines in the illustration. It
can be readily seen that those passing outward from the aerial in a
direction opposite to that in which the free end points are the
strongest, and that the radiation in that direction is the best.
[Illustration: FIG. 17.–A diagram illustrating the directive action
of a flat-top aerial.]
[Illustration: FIG. 18.—Aerials of the "V" and inverted "L" types.]
The "V" aerial and also the inverted "L" type both receive waves much
better when they come from a direction opposite to that in which the
free end points.
[Illustration: FIG. 19.—A diagram showing the arrangement of a "T"
aerial.]
Probably the most interesting feature of the directive action of aerials
lies in the fact that a land station is able to determine the
approximate bearing of a ship signaling with a horizontal aerial.
[Illustration: FIG. 20.—Flat top aerials of the inverted "U" and "T"
types.]
It is beyond the scope of the book to enter into all of the engineering
details pertaining to the installation of a wireless station, but a few
remarks and instructions for the benefit of those who may be interested
in this phase of the subject may be appreciated.
The flat-top "T" aerial gives the best "all around" results. The
vertical and umbrella forms are close seconds.
[Illustration: FIG. 21.—Umbrella aerial.]
For the very best results, the top or horizontal portion of a "T" aerial
should be slightly shorter than the vertical section.
The umbrella type of antenna is very efficient. Instead of a wooden
mast, an iron pipe terminating above in a system of wires, inclining
downward and serving both as part of the aerial and as guys to support
the pole, is often used. The bottom of the pole is placed on an
insulating base, protected from the rain by a small shelter. The wires
are insulated near the lower ends by strain-insulators. The action of
the wires is to serve as a capacity extension to the aerial.
[Illustration: FIG. 22.—An amateur aerial (flat-top).]
Vertical aerials are not as efficient as either of those forms just
mentioned. They require to be 50 per cent. higher than a flat-top
aerial, in order to be of the same value.
The "L" and "V" types are somewhat directional. They are used where the
highest point must be near the station, with a lower point some distance
away. It is possible to secure excellent results with either type.
The terms straightaway and loop denote the method of connecting the
aerial wires. In the first form the upper or free ends of the wires
terminate at the insulators. In the loop form they are all connected
together, and divided into two sections, each of which is led separately
into the operating room.
[Illustration: FIG. 23.—Diagram showing the difference between "loop" and
"straightaway" aerials.]
The straightaway aerial is the most efficient in most cases, but
wherever great height cannot be obtained, or the aerial is necessarily
short, the loop aerial will give the best results.
Bare copper wire is the best, and is generally used for aerials.
Wherever the stretch is 100 feet or over, however, so that the wires are
subjected to considerable strain from their own weight, phosphor bronze
is used because of its greater tensile strength. Commercial and navy
stations employ stranded wire. High frequency currents have the peculiar
property of traveling near the surface of wires and conductors. They do
not permeate to the center of the wire, as do normal currents. The
surface of a stranded wire is greater in comparison to its cross-section
than a solid conductor of the same diameter, and therefore is often
employed because it offers less resistance to currents of this sort.
[Illustration: FIG. 24.—Showing how wires are arranged and insulated.]
Aluminum wire is very light, and causes very little strain on the pole
or cross-arms. It offers more resistance than copper, but some of the
larger sizes may be used with equally good results.
Iron wire must never be used, even if galvanized or tinned. It possesses
a certain reactance tending to choke off the high frequency currents.
[Illustration: FIG. 25.—Aerial insulator.]
The aerial is always very carefully insulated from its supports and
surrounding objects by special insulators, capable of withstanding
severe strains, made of a moulded material having an iron ring imbedded
in each end.
[Illustration: FIG. 26.—Leading-in insulator.]
The wires leading from the aerial to the operating room are called the
"rat-tail," or "lead-in." They must be very carefully insulated by
leading through a bushing placed in the wall or window of the operating
room.
One of the most important factors in a wireless station is the proper
earthing arrangement. The usual method is to use large copper plates
buried in moist earth, or thrown in the sea. On shipboard it is merely
necessary to connect the earth wire to the metallic plates of which the
hull of the vessel is built. Amateurs employ the water or gas pipes in
the house, the former being preferred. Connections are established by
means of a ground clamp.
In the country, where water-pipes are not available, the best way is to
bury a sheet of copper three or four feet deep in moist earth.
A very efficient earth can be formed by spreading a large area of
chicken wire netting over the ground. This method is the best where the
earth is very dry or sandy, and no other way is readily convenient.
[Illustration: FIG. 27.—A side view of the aerial shown in Fig. 22.]