The Birmingham age-herald., 1922-06-25
OPERATION OF THE
 VAR|O-COUPLER IS
 FULLY DESCRIBED
Its Difference From Loose Coupler as Well as Func- tions of Both in Radio
 Circuit
------------- By JACK BINM@


 -Isve boen getting very good re- suits with 8 1oose-ooupier and g crystal detector. l have just made vario- o-coupier, but 1t doesn't work although l use the same circuit that 1 use with the loose-coupler. What Is the troublef'
 This query is typical e& many that reach me. axd bound up in 1t there is a Fundamental fact which puzzles many radio novicsA They cannot understand why 1t is that there Is such big difference between the two instruments, although their functions are practically the same. In view of this. I have come to the conclusion that it would be a good subject to discuss thoroughly this week.
 In the first place, In order to get complete understanding of the two instruments. we will consider just what kind of duties they perform in wireless circuit. It Is fact that both the loose coupler and the varto- coupler perform the same work, but they do 1t in a slightly different
 
 many of the fans are puzzled.
 Roth Arc OsciIIating Transformers
 Both of them are transformers, They act just in the same manner as any other transformer in an electri- cal circuit, by inducing a current in the secondary when an alternating current passes through the primary. In this case. however, they are deal ing with very high frequency alter nating currents which are in an os- cillating state. In fact, they were originally referred to as oscillating transformers, and still are in some quarters.
 The two instruments differ from ordinary transformers, however, in that they are made variable so that they can be placed into resonance with certain electric currents that are alternating at a certain definite frequency. a characteristic that is known as tuning. In addition to this. they also are made variable in such manner that the amount of in- duction taking place between tho primary and secondary can be va ried a characteristic that Is called "ooupling." Thus you speak of vary ing the coupling of the secondary. Such are the two characteristics of these two instruments. Before de- scribing the method by which they are achieved 1t would be we'll to in- quire into the reason why the char acteristics should be necessary. The loose coupler or vario- coupler are instruments that enable us to con- struct what is called a two-oircuit re- eeiver, wherein the first or primary circuit consist of the aerial. an aeri- A A Il. e4 connection. and the secondary cir cult consists of the secondary wind ing of the instrument and a variable condenser shunted across 1t.
 Cuts Down Interference
 The object of having a double-cir- ouit receiver Is to cut down as far as possible all interference caused by electro magnetic waves thtt are not of the same length as the wave of the station we desire to listen to. There Is not the slightest doubt about the two-circuit receivef doing quite a lot in this direction, and 1t Is a very valuable system in radio work.
 This can be readily seen when we consider just what takes place when we are tuning. In the first place we adjust the aerial tuning condenser and the primary of the particular type of coil we are using. The aeri all of course. will pick and pass to ground whatever waves pass over it. Tn theory none but those In tune with the aerial circuit would actuate the detector As matter of fact, quite a number Of waves out of tune do actuate the detector. and cause interference.
 Now if we add a secondary cir cult which is also attuned to the particular wave-1ength we desire to receive from, 1t will be readily seen that the possibility of the unwanted signals passing through both cir cults will be greatly decreased. In actuality this is the case, although, of course, all interference is not al- 1 ways tuned out.
 Variable Condenser Necessary | If we look upon the eIectro-mag- netic waves, which are surging 1 through the ether as though they | were burglars, and then look upon 1 the receiver as though t were safe, which the waves. like the burglars. l were trying to enter. the value of l the double circuit receiver can be 1 seen. It is an easy matter for a l burglar to pick a safe that is 1, equipped with lock consisting of I a single combination. but 1t Is not so easy to pick lock having two com 1 binations, although it can be done. | the task is harder. So 1t is with the 1 waves.
 This is quite Q long introduction to explain why a vario-coupler won't | work where a loose coupler would. | Well, in Investigating the complaints l such as that quoted in the first para l graph of this article, 1t invariably | turns out that the complainant was I not using variable condenser | across the secondary of his loose- l coupler. and of course tried to work
 the vario- o-coupler in the same man | ner.
 Secondary Must Re Variable l Now loose-coupler will work per- I fectly we'll in quite a lot of cases l without a condenser across the sec