CHAPTER III. AERIALS AND EARTH CONNECTIONS.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 aredirective, 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.Plate II. Aerial Systems.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.Fig. 18. Electrose Insulators.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.Fig. 19. Guy Insulator.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.Fig. 20. Insulating Tube.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.Fig. 21. High-tension Cable and Insulator.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:Pequals L² x W/8SwherePis the required strain in lbs.,Wthe weight in lbs. per foot of aerial,Lthe length of the aerial andSthe 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.Fig. 22. Flat-top T Aerial.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.Sending and Receiving Range TablesThese 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.Fig. 23. Ground Clamp.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.Fig. 24. Switch for Lightning Protection.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."
CHAPTER III. AERIALS AND EARTH CONNECTIONS.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 aredirective, 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.Plate II. Aerial Systems.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.Fig. 18. Electrose Insulators.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.Fig. 19. Guy Insulator.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.Fig. 20. Insulating Tube.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.Fig. 21. High-tension Cable and Insulator.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:Pequals L² x W/8SwherePis the required strain in lbs.,Wthe weight in lbs. per foot of aerial,Lthe length of the aerial andSthe 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.Fig. 22. Flat-top T Aerial.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.Sending and Receiving Range TablesThese 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.Fig. 23. Ground Clamp.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.Fig. 24. Switch for Lightning Protection.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."
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 aredirective, 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.
Plate II. Aerial Systems.Plate II. Aerial Systems.
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.
Fig. 18. Electrose Insulators.Fig. 18. Electrose Insulators.
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.
Fig. 19. Guy Insulator.Fig. 19. Guy Insulator.
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.
Fig. 20. Insulating Tube.Fig. 20. Insulating Tube.
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.
Fig. 21. High-tension Cable and Insulator.Fig. 21. High-tension Cable and Insulator.
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:
Pequals L² x W/8S
wherePis the required strain in lbs.,Wthe weight in lbs. per foot of aerial,Lthe length of the aerial andSthe 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.
Fig. 22. Flat-top T Aerial.Fig. 22. Flat-top T Aerial.
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.
Sending and Receiving Range Tables
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.
Fig. 23. Ground Clamp.Fig. 23. Ground Clamp.
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.
Fig. 24. Switch for Lightning Protection.Fig. 24. Switch for Lightning Protection.
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."