The aerial or antenna ordinarily consists of a number of wires elevated
in the air to emit or intercept the Hertzian waves. In fitting up a
wireless station the location and erection of an aerial are of prime
importance, and the successful reception and transmission of wireless
messages will depend largely upon its condition.
A few years ago the wireless antenna consisted of a metal plate high in
the air and having a wire suspended from it, but to-day usually exists
in one of the forms illustrated in Plate II.
The higher an aerial is placed above the surface of the earth, the wider
will be its electrostatic field, and consequently more powerful
electrical waves will be developed. But after a height of 180-200 feet
is attained, the engineering difficulties and the expenses increase so
rapidly that few stations exceed it. Other things being equal, the
increased range in transmitting varies as the square of the height of
the radiating wires. For example, a 25-foot aerial capable of
transmitting one mile theoretically will send waves 16 miles if made 100
feet high. The actual ratio is often greater, but much is dependent upon
the many meteorological conditions.
After the limit in a vertical direction has been reached, the only
remaining possibilities are to increase the surface and spread out
horizontally.
The flat top aerials are used on shipboard or wherever it is an
advantage to suspend the wires between two masts.
They are especially recommended for amateur use, since they need not be
so high as the other aerials, to be efficient. The flat top aerials are
_directive_, that is, they receive or radiate waves better in certain
directions. The bent or inverted L type is one of these and exhibits a
preference for waves coming from a direction opposite to that in which
its free end points. This directive action of an inverted L antenna may
be somewhat lessened if the leads are taken off at the center and it is
made a T aerial. This is the most common form of flat top aerial in use
on ships.
The inverted U type is not used extensively because the two opposite
leads or rat-tails make a centrally located operating room necessary.
The loop aerial is used by the United Wireless Company, in both their
ship and land stations. This type of aerial is well adapted to long
waves and close tuning.
The Lodge-Muirhead capacity aerial does not make use of a ground and is
rarely seen in this country. Lately the United States Signal Corps have
applied it to their balloons where an earth connection would be
impossible. The upper part of the balloon is covered with a network of
wires which serves as the upper aerial, and a second system of wires is
suspended below the balloon to take the place of the ground. By this
means they have had little difficulty in establishing successful
communication between the balloons and the earth.
The pyramid aerial is the type used by Marconi in long-distance
ultra-powerful stations, but is debarred from extensive installation on
account of the large cost of erection.
The cage and grid aerials are of the vertical type and are excellent
where a high support to elevate them can be secured. They are at present
used principally by the Massie and Stone Companies.
[Illustration: *Plate II. Aerial Systems.*]
The desirable feature of an aerial is a quantity known as its
electrostatic capacity and is measured by the charge required to raise
its potential one unit. An increase in capacity enables more energy to
be accumulated in the antenna, and consequently greater radiation
results. The capacity of an aerial may be increased by adding wires, but
must not be carried too far or the transmitting apparatus will not be
able to raise its potential sufficiently. Owing to an effect caused by
mutual induction between the wires, the lines of strain are not
distributed symmetrically, and the capacity will not vary directly but
rather approximately as the square root of the number of wires. In order
to decrease this action and use the surface most efficiently, the wires
should not be placed nearer than one-fiftieth of their length and
preferably farther apart.
The materials used for the insulation and suspension of an aerial must
be reliable, so that in event of bad weather the station will not lose
energy or be put out of working order because the aerial blew down.
Porcelain cleats or a string of porcelain insulating knobs make
inexpensive insulators. The standard insulator for wireless telegraph
work is the "Electrose" insulator. These are made of a molded
composition, and have iron rings set firmly in the ends so that they can
withstand a very heavy strain. Hard rubber is undesirable for an aerial
insulator because it becomes carbonized and covered with a conducting
layer.
[Illustration: Fig. 18. Electrose Insulators.]
High frequency currents permeate copper wire only about one
three-hundredth of an inch, and so, in order to increase the surface and
decrease the resistance, it is best to make the aerial of stranded wire.
A phosphor bronze wire for this purpose which is very flexible but still
does not sag or stretch, is composed of 7 strands of No. 20 B. S. gauge.
Such a wire 150 feet long suspended vertically and insulated from the
earth will have a capacity of from 0.0003 to 0.0004 of a microfarad.
The aerial must receive very particular attention if the station is one
kilowatt or over in power. In that case stranded wire is necessary. The
insulation of the aerial must be as thorough as possible, and proximity
to large conductors such as smokestacks, telephone lines, etc., avoided.
Rope stays and guys are advisable in order to prevent dissipation of
energy. If wire stays must be used they should be divided up at frequent
intervals by insulators.
Fig. 19 illustrates a guy insulator used by the United Wireless
Telegraph Company. It is made of two strips of well paraffined wood
separated by two porcelain knob insulators.
[Illustration: Fig. 19. Guy Insulator.]
Copper wire is the most desirable for an aerial. Iron wire must never be
used unless it is very heavily galvanized, and even then it is not to be
recommended. Aluminum wire is undesirable except for kite sustained
aerials. When used on an aerial and exposed to smoke and other fumes it
becomes quickly coated with a layer of oxide. All connections made in
aluminum wire must be soldered. This necessity may be better understood
when it is explained that electro-magnets on dynamos, etc., are
sometimes wound with bare aluminum wire and that the natural coating of
oxide on the wire is sufficient insulation to separate the turns.
[Illustration: Fig. 20. Insulating Tube.]
Where the aerial enters the building in which the instruments are
located it must be very carefully insulated. The simplest method is to
bore a hole through the wall and push a porcelain tube through it. The
rat-tail or leading-in wire is then passed through and the interstices
between it and the tube poured full of melted paraffin.
The best method is to bore a hole in the window pane and pass the wire
through a hard rubber insulating tube. Fig. 20 shows such a tube. It is
three-quarters of an inch in outside diameter and has an internal bore
of three-eighths of an inch. The tube is threaded throughout its entire
length. Two hard rubber flanges inch thick and 2 inches in diameter are
threaded to screw on the tube. The tube is inserted in the hole in the
window pane and the flanges screwed on either side. If a soft rubber
washer is placed between the hard rubber flanges and the pane there will
be less likelihood of cracking the glass. The leading-in wire is then
passed through the tube. The hole in the window pane may be bored by
using a copper tube having an external diameter equal to that of the
required hole. The tube is set in a brace and used like an ordinary bit,
but must be kept well smeared with emery and oil or else it will not
cut.
This method of leading in the rat-tail is the only one to be recommended
if the transmitter is one-quarter kilowatt or over in power.
[Illustration: Fig. 21. High-tension Cable and Insulator.]
The lead-in should be anchored just outside of the window so as to
relieve the glass pane and the tube from all strain. Pirelli cable or
the high-tension cable which is used for the secondary wiring of an
automobile is the best conductor to use for the aerial in the interior
of a building. The way to lead it over the ceiling is to support it on a
porcelain cleat similar to that shown in Fig. 21.
Many are under the erroneous impression that four times the length of
the aerial is the wave length which the station will emit. This is only
at the best a very rough approximation, for many undeterminable factors
such as the nature and location of surrounding objects, trees, etc., so
affect the capacity and inductance of the aerial that the wave length
must be determined empirically after the aerial is in operation.
The standard wave length of the United States Navy for ship
installations is 425 meters. An inverted L aerial, calculated before
erection to have a wave length as near as possible to this, has the
following dimensions: Four horizontal stranded phosphor bronze wires (7
strands No. 20 B. S.) each 160 feet long and spaced 5 feet apart, four
vertical wires 85 feet long and a 35-foot rat-tail.
It is always desirable that the wave length should be as long as
possible, for the waves will then travel farther and are not absorbed to
such an extent by trees, etc. The absorption due to trees is said to
vary as the fourth power of the frequency.
It is sometimes very convenient to calculate the strain on insulators or
masts caused by a horizontal antenna. This is easily found by the
following equation:
_P_ equals L² x W/8S
where _P_ is the required strain in lbs., _W_ the weight in lbs. per
foot of aerial, _L_ the length of the aerial and _S_ the sag of the wire
in feet.
When erecting an aerial, it is best to fasten a pulley at the top of the
supporting mast and hoist the aerial up after the pole is in position.
Then in case the wires become twisted or broken they may be lowered and
repaired without any difficulty.
*Erection of an Aerial.*—The average amateur aerial is generally from 40
to 60 feet high and supported at one end by a short pole placed on the
house and at the other end by a mast set in the ground or lashed to a
tree. Fig. 22 illustrates such an arrangement whereby a flat-top T
aerial is supported at one end by a short pole fastened to the house and
at the other end by a pole set in the ground.
[Illustration: Fig. 22. Flat-top T Aerial.]
The flat-top loop aerial is preferred by some amateurs and it is to be
recommended for receiving but is an inefficient radiator. When this type
of aerial is used the two leading-in wires should be connected to a
switch so that when the switch is closed they are connected. The aerial
may then be used as a straight-away aerial for transmitting, and by
opening the switch, as a loop aerial for receiving. This precaution is
advised when a loop aerial is to be used with a low-powered induction
coil as a transmitter, for otherwise there will be a loss of energy at
the anchor gap.
A large aerial is of no advantage when used with a small transformer or
induction coil because it cannot become properly charged.
To erect a flat-top T aerial, first select its location. If possible
take advantage of two trees and lash a short pole in the top of each, so
that the aerial may be raised up clear of the leaves. Another good plan
is to erect a pole at each end of the house. In any case, the distance
separating the poles must not be greater than three times the height
above ground or the directive action of the aerial will be very
pronounced. An aerial 50 to 60 feet high should have a length of from 80
to 100 feet. Stranded wire is no advantage for receiving, but must be
used if the transmitter is other than a small induction coil.
Secure two spruce sticks about 2 inches in diameter and 10 feet long.
Fasten an insulator 6 inches from each end of the spars and two more
each 3 J feet from the ends. This arrangement will separate each of the
four wires which compose the aerial by three feet.
The two spars are then laid on the ground at a distance apart equal to
the desired length of the aerial. Four wires, either stranded or No. 12
B. S. gauge copper, are cut to equal lengths and fastened to the
corresponding insulators. The middle of each wire is found and a long
copper wire soldered to it. These four wires constitute the rat-tail or
lead-in. They should be of the same length, and are not connected
together until they are about to enter the building. A short rope tied
to each end of the spars and fastened to the rope which passes over the
pulley on the top of the pole, serves as a bridle to prevent the aerial
from twisting. The aerial is then hoisted up but allowed to hang
slightly slack.
There is considerable difference of opinion among experts as to whether
or not the ends of the horizontal wires should be connected, and it is
impossible to say with good reason which method is better. However, when
erecting a flat top aerial, exercise every care to make all the wires of
exactly the same length.
An aerial of the size and type just described will send and receive the
following distances.
These distances are only approximate and will vary with the efficiency
of the operator and the location of the station.
When any of the transformers described later are used, the aerial should
be 80 to 100 feet high. In this case the last named receiving outfit
will have a range of from 500 to 1,000 miles.
*Ground Connections.*—The importance of a good earth or ground
connection can hardly be overestimated. Whenever possible commercial
stations are located on moist ground or near a body of water so that a
good ground may be secured by imbedding zinc or copper plates in the
earth or water. A ground on shipboard is easily secured by fastening a
conductor to one of the ship’s plates.
If the ground connection is poor, the natural period of the oscillation
circuit is made irregular and short, so that the currents are choked in
passing in and out of the earth. The result is an undesirable rise of
potential at the lower end of the aerial and often harmful sparking at
the ground connection. The transmitting and receiving ranges of a
station are very considerably reduced through a poor earth.
Ground connection can often be obtained in the country by immersing
metal plates in a well or a cistern. Where connection is made to a water
supply pipe some sort of a ground clamp should be used to insure a good
contact.
[Illustration: Fig. 23. Ground Clamp.]
An efficient earth for portable outfits may be quickly formed by
spreading a large area of wire netting over the ground.
Proper precautions for protection against lightning by grounding the
aerial outside of the building should be taken.
The wisest plan is to install a heavy single pole double throw switch
outside of the building where the rat-tail enters. The knife of the
switch should be connected to the aerial, one contact to the house lead
and the other to a heavy wire grounded on the outside of the building as
in Fig. 24. When the apparatus is not in use the aerial should be
grounded by throwing the switch on the grounded contact.
[Illustration: Fig. 24. Switch for Lightning Protection.]
The rulings of the National Board of Fire Underwriters governing this
class of work are appended below.
"1. Aerial conductors to be permanently and effectively grounded at all
times when the station is not in operation by a conductor not smaller
than No. 4 B. S. gauge copper wire, run in a direct line as possible to
water pipe on street side of said water pipe within the premises or to
some other equally satisfactory earth connection.
"2. Aerial conductors when grounded as above specified must be
effectually cut off from all apparatus within the building.
"3. Or the aerial to be permanently connected at all times to earth in
the manner specified above, through a short gap lightning arrester; said
arrester to have a gap of not over .015 of an inch between brass or
copper plates not less than 2 1/2 inches in length, parallel to the gap,
and 1 1/2 inches the other way, with a thickness of not less than
one-eighth of an inch, mounted on non-combustible, non-absorptive
insulating material of such dimensions as to give ample strength. Other
approved arresters of equally low resistance and equally substantial
construction may be used.
"4. In cases where the aerial is grounded as specified in paragraph 1,
the switch employed to join the aerial to the ground connection shall
not be smaller than a standard 100-ampere jack-knife switch.
"Notice of wiring done for these installations should be sent to the
Board, the same as for all other work."