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- The Radio Amateur's Hand Book - 6/44 -
wireless telegraph messages and the _vacuum tube_ for sending wireless telephone messages.
The Spark Gap Wireless Telegraph Transmitter.--The simplest kind of a wireless telegraph transmitter consists of: (1) a _source of direct or alternating current_, (2) a _telegraph key_, (3) a _spark-coil_ or a _transformer_, (4) a _spark gap_, (5) an _adjustable condenser_ and (6) an _oscillation transformer_. Where _dry cells_ or a _storage battery_ must be used to supply the current for energizing the transmitter a spark-coil can be employed and these may be had in various sizes from a little fellow which gives 1/4-inch spark up to a larger one which gives a 6-inch spark. Where more energy is needed it is better practice to use a transformer and this can be worked on an alternating current of 110 volts, or if only a 110 volt direct current is available then an _electrolytic interrupter_ must be used to make and break the current. A simple transmitting set with an induction coil is shown in Fig. 2.
[Illustration: Fig 2.--Simple Transmitting Set.]
A wireless key is made like an ordinary telegraph key except that where large currents are to be used it is somewhat heavier and is provided with large silver contact points. Spark gaps for amateur work are usually of: (1) the _plain_ or _stationary type_, (2) the _rotating type_, and (3) the _quenched gap_ type. The plain spark-gap is more suitable for small spark-coil sets, and it is not so apt to break down the transformer and condenser of the larger sets as the rotary gap. The rotary gap on the other hand tends to prevent _arcing_ and so the break is quicker and there is less dragging of the spark. The quenched gap is more efficient than either the plain or rotary gap and moreover it is noiseless.
Condensers for spark telegraph transmitters can be ordinary Leyden jars or glass plates coated with tin or copper foil and set into a frame, or they can be built up of mica and sheet metal embedded in an insulating composition. The glass plate condensers are the cheapest and will serve your purpose well, especially if they are immersed in oil. Tuning coils, sometimes called _transmitting inductances_ and _oscillation transformers_, are of various types. The simplest kind is a transmitting inductance which consists of 25 or 30 turns of copper wire wound on an insulating tube or frame. An oscillation transformer is a loose coupled tuning coil and it consists of a primary coil formed of a number of turns of copper wire wound on a fixed insulating support, and a secondary coil of about twice the number of turns of copper wire which is likewise fixed in an insulating support, but the coils are relatively movable. An _oscillation transformer_ (instead of a _tuning coil_), is required by government regulations unless _inductively coupled_.
The Vacuum Tube Telegraph Transmitter.--This consists of: (1) a _source of direct or alternating current_, (2) a _telegraph key_, (3) a _vacuum tube oscillator_, (4) a _tuning coil_, and (5) a _condenser_. This kind of a transmitter sets up _sustained_ oscillations instead of _periodic_ oscillations which are produced by a spark gap set. The advantages of this kind of a system will be found explained in Chapter XVI.
The Wireless Telephone Transmitter.--Because a jump-spark sets up _periodic oscillations_, that is, the oscillations are discontinuous, it cannot be used for wireless telephony. An electric arc or a vacuum tube sets up _sustained_ oscillations, that is, oscillations which are continuous. As it is far easier to keep the oscillations going with a vacuum tube than it is with an arc the former means has all but supplanted the latter for wireless telephone transmitters. The apparatus required and the connections used for wireless telephone sets will be described in later chapters.
Useful Information.--It would be wise for the reader to turn to the Appendix, beginning with page 301 of this book, and familiarize himself with the information there set down in tabular and graphic form. For example, the first table gives abbreviations of electrical terms which are in general use in all works dealing with the subject. You will also find there brief definitions of electric and magnetic units, which it would be well to commit to memory; or, at least, to make so thoroughly your own that when any of these terms is mentioned, you will know instantly what is being talked about.
PUTTING UP YOUR AERIAL
As inferred in the first chapter, an aerial for receiving does not have to be nearly as well made or put up as one for sending. But this does not mean that you can slipshod the construction and installation of it, for however simple it is, the job must be done right and in this case it is as easy to do it right as wrong.
To send wireless telegraph and telephone messages to the greatest distances and to receive them as distinctly as possible from the greatest distances you must use for your aerial (1) copper or aluminum wire, (2) two or more wires, (3) have them the proper length, (4) have them as high in the air as you can, (5) have them well apart from each other, and (6) have them well insulated from their supports. If you live in a flat building or an apartment house you can string your aerial wires from one edge of the roof to the other and support them by wooden stays as high above it as may be convenient.
Should you live in a detached house in the city you can usually get your next-door neighbor to let you fasten one end of the aerial to his house and this will give you a good stretch and a fairly high aerial. In the country you can stretch your wires between the house and barn or the windmill. From this you will see that no matter where you live you can nearly always find ways and means of putting up an aerial that will serve your needs without going to the expense of erecting a mast.
Kinds of Aerial Wire Systems.--An amateur wireless aerial can be anywhere from 25 feet to 100 feet long and if you can get a stretch of the latter length and a height of from 30 to 75 feet you will have one with which you can receive a thousand miles or more and send out as much energy as the government will allow you to send.
The kind of an aerial that gives the best results is one whose wire, or wires, are _horizontal_, that is, parallel with the earth under it as shown at A in Fig. 3. If only one end can be fixed to some elevated support then you can secure the other end to a post in the ground, but the slope of the aerial should not be more than 30 or 35 degrees from the horizontal at most as shown at B.
[Illustration: (A) Fig. 3.--Flat top, or Horizontal Aerial.]
[Illustration: (B) Fig. 3.--Inclined Aerial.]
The _leading-in wire_, that is, the wire that leads from and joins the aerial wire with your sending and receiving set, can be connected to the aerial anywhere it is most convenient to do so, but the best results are had when it is connected to one end as shown at A in Fig. 4, in which case it is called an _inverted L aerial_, or when it is connected to it at the middle as shown at B, when it is called a _T aerial_. The leading-in wire must be carefully insulated from the outside of the building and also where it passes through it to the inside. This is done by means of an insulating tube known as a _leading-in insulator_, or _bulkhead insulator_ as it is sometimes called.
[Illustration: (A) Fig. 4.--Inverted L Aerial.]
[Illustration: (B) Fig. 4.--T Aerial.]
As a protection against lightning burning out your instruments you can use either: (1) an _air-gap lightning arrester,_ (2) a _vacuum tube protector_, or (3) a _lightning switch_, which is better. Whichever of these devices is used it is connected in between the aerial and an outside ground wire so that a direct circuit to the earth will be provided at all times except when you are sending or receiving. So your aerial instead of being a menace really acts during an electrical storm like a lightning rod and it is therefore a real protection. The air-gap and vacuum tube lightning arresters are little devices that can be used only where you are going to receive, while the lightning switch must be used where you are going to send; indeed, in some localities the _Fire Underwriters_ require a large lightning switch to be used for receiving sets as well as sending sets.
How to Put Up a Cheap Receiving Aerial.--The kind of an aerial wire system you put up will depend, chiefly, on two things, and these are: (1) your pocketbook, and (2) the place where you live.
A Single Wire Aerial.--This is the simplest and cheapest kind of a receiving aerial that can be put up. The first thing to do is to find out the length of wire you need by measuring the span between the two points of support; then add a sufficient length for the leading-in wire and enough more to connect your receiving set with the radiator or water pipe.
You can use any size of copper or aluminum wire that is not smaller than _No. 16 Brown and Sharpe gauge._ When you buy the wire get also the following material: (1) two _porcelain insulators_ as shown at A in Fig. 5; (2) three or four _porcelain knob insulators_, see B; (3) either (a) an _air gap lightning arrester,_ see C, or (b) a _lightning switch_ see D; (4) a _leading-in porcelain tube insulator,_ see E, and (5) a _ground clamp_, see F.
[Illustration: Fig. 5.--Material for a Simple Aerial Wire System.]
To make the aerial slip each end of the wire through a hole in each insulator and twist it fast; next cut off and slip two more pieces of wire through the other holes in the insulators and twist them fast and then secure these to the supports at the ends of the building. Take the piece you are going to use for the leading-in wire, twist it around the aerial wire and solder it there when it will look like A in Fig. 6. Now if you intend to use the _air gap lightning arrester_ fasten it to the wall of the building outside of your window, and bring the leading-in wire from the aerial to the top binding post of your arrester and keep it clear of everything as shown at B. If your aerial is on the roof and you have to bring the leading-in wire over the cornice or around a corner fix a porcelain knob insulator to the one or the other and fasten the wire to it.
[Illustration: (A) Fig. 6.--Single Wire Aerial for Receiving.]
[Illustration: (B) Fig. 6.--Receiving Aerial with Air Gap Lightning Arrester.]
[Illustration: (C) Fig. 6.--Aerial with Lightning Switch.]
Next bore a hole through the frame of the window at a point nearest your receiving set and push a porcelain tube 5/8 inch in diameter and
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