A Diagnosis of Radio

Radio Record, Volume IV, Issue 51, 3 July 1931, Page 18

A Diagnosis of Radio

Descriptive Non-technical Talks ~~ on the Elements: of Radio _ by The Technical Editor. This. week we are starting a series of illustrated articles on the . ‘fundamentals of radio. We propose to take all.the common parts of a wireless sét-and show both, by. pie torial and theoretical diagrams, what. . they are, and in a few brief, non-technical sentences, to define them and .*. then: to:say something about the working of each. It might be. added ‘that this series of articles is the outcome of several requests that we go back to the more elementary stages of home-construction, and ‘we ean-. not do this until we have told constructors-to-be what it is all about. But these articles are not written up solely from.a constructional point ~ of view. They are intended to interest everyone: who is interested in radio. We would like the expressed views of cortespondents concerning them; whether they are too technical or not technical enough. If there is anything we appear to miss, and if the interested reader will merely. write. in and ask for it, we will give every consideration to its being included in this series. And now we start.

The Aerial: HE word "aerial" implies something : to do with. the air, so that. the: proper definition of an aerial would be "a wire or collection of wires in* the air to collect waves that come from the broadcasting station." . ; Generally speaking tHere are. two types of aerials-an inside one and an outside one.:: An inside one may consist of wires slung .ahout the room,

underneath the carpet or in the :attic. but no matter how good an indoor aerial may be, it is not nearly as efficient as a good outdoor aerial. Of the outdoor aerials, there are two main types. the "IT" aerial, the lead-in of which comes from the centre, and the "TZ," aerial, the lead-in of which comes from one end. The lead-in connects the aerial with the set, and when measuring it must be counted as part of the aerial. Here are some facts about aerials :- 1, The best aerial is vertical without any horizontal portion whatever. — 2. The next best is the "L" shape as high as possible, with the top portion short in comparison with the lead. Altogether the total amount of’ aerial should not exceed 100 feet, although for small sets, it is sometimes advisable to have 200 feet or even more. 8. The more aerial you use the less selective’ your set becomes-in other words, you will find difficulty in separating one station from another.

4, If the "T" aerial is used the effective length is that between the set and one mast, i.e; the flat top is equal to half the length of ‘actual aerial minus the lead-in, ° » The aerial must be as clear from trees and buildings ,as possible. Trees have a sereening effect upon the radio ‘Waves. 6. If there must be joints, see that ed are soldered. . Have the aerial so that it can be let for cleaning without lowering the masts. 8. Use plenty of insulators and ar-

range so that the aerial ‘proper is well away from the mast. 9. The most satisfactory: wire is 7 7/22 enamelled, , ‘ The Earth HE Americans call it "the ground." The aerial may be regarded as a set of condenser plates and the earth as the other‘set. Therefore, it is as important to get-as good a connection to the ground as to the aerial, Many modern sets automatically make their ground connections through the a.e. mains or through an_ eliminator. Sometimes you will find that by grounding an a.c. set you will get less strength

than if that connection is left off. Sometimes by putting the ground wire on the aerial terminal, better results are obtained. Here are some facts about the ground :- ; 1. The ground can be a water-pipe. a series of pipes or any metal. buried in the ground. 2. In fillings, refuse, etc., is the best place to make an earth connection. gravel is the worst. 3. The earth connection must be kept damp and fairly warm. Throw a bucket of salt water on it every few weeks. 4, The diameter of the pipe used for ground. connections is not important, but see that its connection with the earth wire is a good one, It is difficult to solder to q galvanised pipe, so

use a clean saatip. Take it off and polish it'up occasionally. _ ,/There is, little. to. be by driv‘ing 4 pipe more than 6 feet into the ground, or by filling a copper or any ‘hollow conductor with charcoal or any carbon. it iga bad’ conductor. The Frame "Antenna ‘HE frame. antesin: is. a device used ‘‘jndoors for picking . -up the. waves from. the wireless station. ‘It.is a kind of aerial," but it works -on.a totally (different principle ‘from the other types. It is really an énlarged coil. o* the set, and for this reason *t cannot } _ connected between. the earth and ‘th S earth terminals. It must supplant the

aerial coil and, like-it, -be tuned by a condenser, One side of the frame goes to the grid of the first valve and the other side to.earth. When regeneration is ‘used, that is, a system whereby energy is fed back into the detector oil. to get greater strength, the frame an- X" tenna is tapped as. is shown in tile theoretical sketch alongside.. A frame antenna is strongly directional and must point in the direction from which the radio waves are coming. Coil Tuning ‘THE tuning coil may take one of very many forms, but the most common are those illustrated.. The one on the left igs usually known as a three-coil tuner, while the one on the right is a valve base coil: Two others are shown, a spiderweb eoil-and ‘a tapped coil. Of course there are many others, and often there are only two coils, or eyen one, on the one former. One of the functions of coils is fo © connect two circuits together electric-

ally, in such a way that there is no mechanical connection, Thus we have tL primary and secondary, and energy is transferred from the primary to the secondary without any connection between the. two. It sounds rather wonderful, but it is one of the things that makes radio possible. To get the most out of a set, a-coil, which possesses "‘induetance," and the condenser, which possesses. "capacity," ‘must. be combined, When a coil of known induct-

— ance, or, if you like, of a certain size, is connected. with a condenser of a certain capacity, that coil and condenser can receive only one frequency, and a frequency is what is used by broadcasting stations when they transmit. If either the coil or the condenser is altered that combination will be ready to receive a wave of a different frequency, and that is how a set is tuned. We = generally tune the secondary and its proximity to the primary causes that coil to come under the influence of the secondary and be tuned. There are occasions ‘when, to gét greater strength, we feed back energy from the detector valve into the

secondary coil. Here is the way the current goes. From the top of the secondary it goes to the grid of the yalve. From there it is amplified in the valve and goes into the plate. Now, from the plate, a certain amount of that energy is fed back to the tickler coil (found in "regenerative" circuits), where it-is picked up by the secondary and passed through the valve again, : To regulate the amount of current that is. fed back to the tickler we must have some means of control. We can alter the distance between the tickler and the secondary-called induction control (the familiar swinging coil of the Browning Drake)-resistance control, which controls the amount of voltage on the plate of the detector, or, what is more usual, the condenser, Lately the differential condenser has become very popular with constructors, for it is an admirable means of controlling the amount of energy fed back to the tickler coil. On the valve base coil illustrated fhere are only two windings. This particular coil can be used either for a get that.does not have any amplification before the detector and one in which the aerial is brought straight in on to the secondary eoil (in which case the other coil would be the tickler) or it could be used for a stage that goes before the detector (radio stage), in

eee . which case there is no need to use the tickler coil. The coils would be a primary and a secondary. The spider-web coils are wound in that peculiar fashion.to reduce what is known as self-capacity. They are, however, going out of date. The tapped coil is used generally in experimental sets and sometimes in erystal sets. It provides a means of connecting two circuits so that the optimum coupling between the two can be obtained by varying the tapping. Thus, if this coil is used as the first in the set, it might be found that it will be better with the aerial brought into "9" than if brought anywhere else. Generally speaking, in commercialwound tuning-coils, enamelled wire is used, and the sizes are kept very small In amateur construction, it is usual to use (.8.¢. or 4.¢c.c. Wire, of a gauge varying between 24 and 30 and on formers ranging from lin. to 3in. Choke Coil A CHOKE coil is a winding of wire which will allow low frequency current to pass, but will block the high frequency. It is the reserve of the condenser, so that the reader can see there are many possibilities with condenser and choke coil combinations. Technically speaking, a choke coil is one of great reactance or impedance. Its purpose is to limit the flow of alternating current at certain frequencies through the part of circuit in which the choke is placed. It is really a valve (not a wireless one). There are two types of chokes, radio (R.¥.C.) and audio (A.F.C.). The radio choke ean pass the highest frequency pulsations, but the audio choke will allow the lower frequency ones to pass. The high frequency or radio frequency chokes are usually of an air core type, that is to say, they are wound on a former like an ordinary coil and have no iron about them. For broadcast purposes a choke coil has about 1000 turns on a small former of about .lin.,

but for shortwave 100 is ample. The main thing to see in purchasing a choke coil is that it has a sufticiently high inductance; 2500 microhenries is not too little for a good radio frequency choke. Audio Frequency Choke UDIO frequency chokes always have an iron core. They are generally formed of layer windings of enamelled wire, although other wire is suitable.

The gauge of wire is determined by the current to he passed. Their inductance yaries between 25/500 henries according to their use. When built up they generally resemble an audio transformer. A smoothing choke, used in power packs, is usually made with

heavy wire, and has an inductance ranging between 25/100 henries. The iron used for the core is not continuous; in other words, it has a "gap" which varies in thickness between 1-32in, and 3-16in. Its function, like that of the "Radio Choke," is to keep the a.c. out of certain portions of the circuit, Fixed Condensers CONDENSER is a piece of apparatus that can receive and hold an electrical charge. It is the only apparatus that can do so. A battery cannot, it merely changes the form of the electricity. A condenser consists of two sets of plates arranged so that they cannot touch one another. The set connected with the power are. the positive plates, and the others, which usually go to the ground or some other similar point, are known as the negative plates. In ordinary fixed ‘condensers, such as those illustrated, the plates are really long strips of tinfoil, and to prevent them from touching, waxed paper is placed in between them. All four (‘the

two plates and the two lots of waxed paper) are wound spirally on a machine and then placed in the case pictured. The amount of plate determines capacity-in other words, the capacity is an indication of the amount of current that a condenser can hold. The distance the plates are apart, or the material used for separating them, determines the voltage at which the condenser breaks down. By "breaking down" we mean the puncturing: of the insulating substance (called the dielectric), and consequently the ruination of the condenser. To be on the safe side condensers are always tested

for a higher voltage than they actually have to work with. Here are some of the uses for condensers :- _ 1. Blocking condensers. " Although this name does not strictly apply to one class, it usually means condensers of a fairly big capacity, that is, .601 to perhaps 2 mfds., used for preventing the flow of direct current in any. portion of a receiver. It is a remarkable fact that though a condenser will stop a flow of. ordinary electric current (D.C. or low-frequency A.C.), it will allow radio waves to pass or, as we ‘eall them, . high-frequency currents. Thus, if we put a condenser in a stream of direct or low-frequency A.C. current in which is present high-frequency current, the high-frequency will go through the blocking condenser, but the ordinary current will be blocked. Thus we call this type of condenser a blocking condenser, although it is really no different from the others. Hxamples: Filter circuits, a.f. coupling condensers in r.¢..amplifiers. ; 2. Smoothing condensers. These are usually large capacity condensers, from 1 to 2000 or more mfds. used in power apparatus such as eliminators, to smooth out the ripple and make the supply pure. These have usually a high break-down voltage, often’ many thousands of volts. ' 3. By-pass condensers. These may be of any capacity from .0005 up, and usually do notshave a very high breakdown voltage. They are used in many: different places in the circuit to provide a channel for unwanted. highfrequency current to pass round some obstacle such as a choke or. battery. Examples: Across batteries, transformers, resistances, ete. ..° , 4, Grid condensers. A condenser of suitable capacity usually between .0001 and .0008. If one of these is-placed in the grid circuit of a valve and a grid leak is associated with it that valve becomes a detector and will pass eurrent in one way only. All condensers are shown by. the one theoretical symbol-two thick lines. When sets are hard to stabilise bypass condensers placéd in the right place will do much to steady them.