WIRELESS NEWS.

Press, Volume LXI, Issue 18419, 27 June 1925, Page 6

WIRELESS NEWS.

EXAGGERATED DX RECORDS. THE GRID LEAK. (SPKCIAIXT WRITTEN JOB "THE MISS.',') <By "Electra.") How many times have you been informed by owners of radio 6ets that on a certain nigHt they picked up some American broadcasting station such as KPO.—KD, KA, or some other station situated in the eastern portion of U.S.A., and that the reception was "real good?" It is becoming quite a common thing now to hear numerous radio set owners claming DX records, but from what I can see of it, it is not such a common thing to be able to actually hear these distant stations when one pays a visit to the receiving set in question. Time upon time I have been invited to come out and hear how the DX calls come in on certain sets and when I have been told that I will be able to hear certain American broadcasting stations on tlig loud speaker, I have certainly accepted the invitation. But never yet have I heard any American station on a loud talker, and so far as I am concerned, it is usually an evening wasted. I do not wish to imply that it is impossible to hear an American broadcasting station distinctly. through a loud speaker, but that more than half the DX records claimed are never confirmed by the. transmitting station — and, what is more, they never will be. Only the other week, a radio-owner informed me that he had heard wellknown U.S.A. broadcasting stations quite well, using only two valves. Being particularly interested in this station, I requested himto give me the names of some of the items heard, and it then came out that no distinguishable speech had been heard. The question was then naturally put> as to how he knew it to be the station he claimed to have heard, when the announcer's speech was not audible. I was coolly informed that one did not have to hear the fo know the location of the station. The time of transmission and wave-length tallied "exactly" with the above station. Imaginary DX Work. I think this is absolutely << hunk," and yet I know for a fact that this sort of thing is going on every day. Australian papers and magazines are full of imaginary DX work. In the case cited above, the claimant had no means whatever of cheoking up the wavelength of the carrier-wave received. No one should claim to have received such and such a station unless he has received from the said station confirmation of his reception report, or, better still, have on© or two witnesses of the reception. These exaggerations make a number of owners become discontented with their own sets, and they commence to pull them • to to pieces to try to .improve matters. My advice to anyone who is thinking of doing this, because someone has told him that on a set using the same number of valves he logs three or four American stations every evening, is to go to that person's place and hear for oneself. If all his claims are true, then by all means get busy and improve your set. Tew Exceptions. Some persons, I will admit, get exceptionally good results from one and two-valve sets using head 'phones only, but as for DX reception on broadcasting stations through, a loud talker, 1 have yet-to see this done. When I say satisfactory reception, I mean that every word spoken by announcer and artists can be heard , quito distinctly at least four feet from the loud talker, and I make no allowance for static. If space would permit I could cite numerous cases where these wild claims have been disproved, and in the majority of instances it has been shown that it was carrier-wave reception only, with a bit of unintelligible speech or music thrown in here and there, which was received. A NOVEL CATWHISKER. An original catwhisker can be made from the grid of a burnt-out valve. Grids are usually made of nickel or molybdenum, are springy, and, being inoxidisable, make efficient catwhiskers. The extremities of the grid must be cut slantwise ■in order to form an efficient point contact with the crystal. Any method of mounting may be applied to this type of catwhisker. Small Fixed, Condensers. A low-capacity fixed condenser suitable for use in grid circuits can be easily made by twisting together two lengths of insulated wire. One end of each wire is taken to the terminals. Care should be taken to see that the insulating covering is riot damaged at all, or no condenser effect will result. THE GRID LEAK. Tho writer has often met people who have said that they failed to see why a grid leak should bo insisted upon in a valve set, since their particular set; functioned just as well without. Many sets, admittedly, will do so; in fact, they aro often better behaved minus the grid leak, because quite a large amount of leakage has probably Ueon provided when the set was assembled, in the way of flux, grease, pencil lines, and fibre insulation. . The grid leak is there right onough but under other guise. Strictly speaking, tho grid leak is a most important factor in the control of a set from the. point of view of oscillation. In most circuits the value of the grid leak is shown as 1J or 2 megohms, and the happy-go-lucky home constructor buys a fixed leak of one of these values and places it before any kind of detector valve he may have decided upon. Sometimes excellent results are obtained this way, but this' would not be the case in a set using critical reaction. Useful Experiments. A few experiments will convince of' the truth of this statement. Take any ordinary valve in a non-regenerative receiver, and, using a reliable variable grid-leak, see what effect variation of tho grid-leak value has upon signal strength. It will be found that no perceptible difference will be produced over the whole range of the grid-leak. I feel sure that he will be contradicted upon this statement, but provided that no reaction, either intentional or otherwise, is used, I contend this is a fact. If reaction is used, however, a different story has to bo unfolded, and the value of the grid-leak, within certain limits, becomes fixed. If the value of the leak is too low, it will be found that as the reaction coil is moved up, before it is Rnywherc near what one might call moderately tight coupling, the set howls and the reaction coil has to be withdrawn several degrees to stop the oscillation. This is very annoying, since it should be possible to carry reaction to a much greater degree. If, however, the value of the grid-leak is too high, it will be found that oscillation commences before the signal strength has really reached its proper degree.

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Correct Grid-Leak Value. So between these two happenings, that of oscillation ■with comparative loos© coupling and oscillation before the ideal is attained, there exists a value of the grid-leak which ia the right one, and which, will produce neither of the defects above described. With the correct grid-leak value there should be no overlapping, as it were; as coupUng is tightened, signals should gain in stnength and keep. their purity until the oscillation point is reached; just inside this point is the spot to adopt. With a correct grid-leak, reaction can be pushed to its very limit without distortion. Thi3 ■value is much easier to write about than to find. It will vary, of course, with the valve that is used and with the strength of H.T. I strongly recommend a good variable grid-leak for all sets; but see that it is a. good one. ~

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