WAVE TRAPS AND WAYS OF REDUCING INTERFERENCE

Radio Record, Volume I, Issue 6, 26 August 1927, Page 15

WAVE TRAPS AND WAYS OF REDUCING INTERFERENCE

LECTURE BY MR.

E. W.

MAHONEY

AT 1YA

The sensitivity of the set, emphasis-: ed Mr. Mahoney, had a great deal to do with the selectivity. If we have two receivers which have equal selectivity, but the one receiver is much more sensitive than the other, the more sen-' sitive one may seem to be broader! than the other, but if the volume control is adjusted that the volume output of the two sets is the same, the ability to Separate the stations may be the same for both sets. Again, if you have a very sensitive receiver, and are located in a place where the air is crowded with ‘statioris, you are not only likely to experience interference between; locals, but you will also encounter heterodyne whistles from distant stations in the background, This is impossible to remedy in the set itself. The only possible way of this trouble being eliminated is for the interfering stations ‘to have a' new wave-length allotted to tliem. . ‘ * Any or all of the following ideas and methods may be tried out by those in‘terested, and eventually you may be able to find a special method that Suits your receiver best, when it can be adopted as a permanent. feature for your set, It must be clearly understood, however, that wave traps, etc., are not suitable for sets under the fourvalve type and are really best when used with a more powerful set. ‘The reason is that when ‘you install. a trap of any type whatsocyer, or use a resistance in the circuit, a decrease of total amount of yolume, that is obtainable from the set under ordinary circumstances, is inevitable, and for that reason it is more advisable to try and use the following methods on Smaller sets. | The first method is:-Follow ont the wiring of the set and find out if the radio filaments are grounded, that is, connected to the ground connection, either by a wire soldered in place or by terminating the grid returns at the tuning condensers, whose rotor, or moving plates, are grounded to a metal panel, In many cases if this wire is removed an increase in selectivity will be .experienced. It may be found in some receivers, however, that this will materially decrease sensitivity, or in regenerative sets, may allow the receiver to oscillate, in which case it would not do to eliminate this connec. tion. You must use your own judgment in adopting the circnit that will suit your needs. | Second Method.-Although not now extensively used and amongst the | newer radio enthusiasts hardly known, a variometer can be used quite suc-. cessfully to tune the aerial circuit. Nearly all receivers now use the tun-. ed primary cirenit; that is, the antenna circuit has no variable element. in its makeup, but consists only of the antenna and ground and between them a fixed coil called the primary. Now the relative action between these two is as follows:-Radio high frequency currents flowing through the aerial circuit are induced or attracted by the fixed primary coil, although these coils are not touching one another. This is really a rough idea of the action between the aerial circuit and the primary, a longer description not now being desirable. Now to get back to the variometer. Where it is possible to use this type of circuit, it will be found to give not only add.ed selectivity, but sensitivity, the reason for this being that supposing you are using a set with two condensers of four or less, the method of tuning is always the same, You tune the secondary circuit until you have the maximum amount of current flowing through the receiver. Now it does not seem possible to increase the mavxiimum amount of current in face of this, but it is quite possible, by using the variometer, to tune the aerial, when an added gain in volume is possible and a larger degree of selectivity is also possible. If you are using the secondary method of tuning in your set, the aerial is untuned, or, in other words, simply acts as a piece of wire to pick up the high frequency currents that may be sent oat by the station. Now by tuning this aerial it is possible to make it respond to any wave-length, and to that one alone. So you can quite easily see that where the variometer is used it is quite possible to make it respond to any wavelength and to that one alone. So you can quite easily see that where the variometer is used it is quite possible to gain selectivity and sensitivity. Of course, it must be understood that any method of reducing interference or using a wave trap means an added control to the set except whiere a fixed condenser is used. This method will be described later on. Another meats of tuuing the aerial circuit is to use a variable condenser connected in the ground lead, but with this must be used a loading coil in the aerial circuit, The number of turns of this coil is roughly about the same as the secondary coils in your receiver, but it is quite simple to wind a coil of the solenoid type and just take a few turns until you find the best number. At least you should start off with 60 turns, and if you find that is too many just reduce the number of turns by taking off about three or four at a time. It will take a little time to determine the’ correct number, as it will depend on the type of receiyer you are using and the capacity of your aerial. If you are not particularly interested in making your set very sensitive and merely wish to add a little selectivity to it, you will find that if you install a fixed condenser of .00025mf in series with the ground lead, this will help to make ‘your set more selective. A condenser of less capacity may be

found even better, again depending | on the type of receiver, etc. , In connection with oscillating sets | Mr. Mahoney said it is a very hard task, and a very thankless one, to always be talking about the one thing all the time; but evidently, at last, public opinion is taking shape, andmany of you are beginning to look on this type of pest the same way as I am doing, and that is, he must be’ eliminated if we wish for the best reception possible. It is bad enough to have to endure the interference of static and fading, with consequent distortions, whilst listeiting to a station, withont having in addition the continnal howling and squalling of these parasites of the radio ether. The Post and Telegraph Department . has tried to assist by forwarding to all listeners a copy of a stage of neutralised high frequency to be installed before the detector, and also other .cireuits and advice, and the sum total of this kindly forebearance on the part of the Department has been nil They delight in’ making the concert: that you are listening to a hideous nightmare of growls, howls, and shrieks. They do not teceive the station, and apparently do not intend to let anyone else hear it and enjoy the programme. There is only one method with these disturbers of our radio programmes, and that is to ban them thoroughly and completely by compelling them to use a stage of neutralised radio frequency. ‘The time is past for talking kindly to them and trying to show them how it shonld be doue. They must be, as I said before, compelled to cease from making what should be an enjoyment into nothing less than a hideous row, I myself have never heard 2YA, Wellington, with satisfaction yet, there always being some howlers doing their best to turn music and musical items into a series of cat calls and pig grunts. My advice to these so-called ether hogs is, shut np or leave radio alone, and have a little thonght for those who wish to enjoy a programme ,and can do so, except for the efforts of these dear, kind, thoughtful squawkers.