Radio Digest (Oct 1923-July 1924)

March 29, W2.{ RADIO DIGEST— Illustrated 25 Clever Modification of the Flewelling Circuit Sharp Tuning Brings Maximum Volume By Warren S. Higgins THE coil marked P is the primary; that marked S, the secondary, and the coil marked T is the tickler. The other apparatus and connections are those of the regular Flewelling circuit. The action of the circuit"is as follows: It operates as a Flewelling which is inductively coupled to the antenna instead of conductively coupled. As is well known, a loose coupling decreases interference, but also decreases the signal strength. If, then, we connect the Flewelling circuit with a loose coupling, making coil S the secondary of the coupler, instead of connecting coil S to the antenna, as in the Flewelling, we would naturally expect less interference and weaker signals. However, on trying out the circuit, we find that although we get much sharper tuning, as expected, we get somewhat stronger instead of weaker signals. Maximum Loudness One explanation of this seems to be this: With the Flewelling circuit, it is usually found that when it is adjusted to give the maximum loudness, it will suddenly start squealing without any apparent reason. If a little searching is done, it will generally be discovered that some other station, on very nearly the same wave length, is causing interference which, while it may not show up in the phones, seems to upset the balance of the set. This is particularly true of ship spark sets — for obvious reasons; The increase in the signal strength may therefore be due to the fact that the set can be adjusted to a more critical point. It will be found that the secondary coil must be reversed with respect to the primary and tickler coils. This is what we should expect. Since the tickler coil and the secondary coil together form a lightly loaded air core transformer with a variable leakage, in which transformer the S coil is the primary and the T coil the secondary, the induced E. M. F. of the tickler coil lags the E. M. F. of the S coil by a phase angle of almost 180 degrees. Now, since this is a feedback circuit, the two E. M. F.'s should neutralize each other if they are of the same amplitude and the two coils are in the same direction. This is because the trough of one wave is exactly in time with the crest of the preceding wave and therefore the resultant is zero. Doubling Signal Strength It is very often assumed that if regeneration is carried to the limit, it would exactly double the signal strength, and a super-set is supposed to use the full regeneration. Therefore, with the two coils in the same direction, no signal should be heard, and this is just what occurs. However, if coil S is reversed, the two E. M. F.'s should add, giving a wave of double amplitude. There is also another matter to be considered; coils P and S taken together form Only Variable Grid Leak to fit all sets DURHAM Variables Cartridge style, having onefinger plunger control, snaps into the place of present fixed leak. No panel-drilling and no wire-changing. Only 75c at dealers, or 80c postpaid No. 101 for storage battery tubes No. 201A for dry cell tubes Typical sets improved with DURHAM Variable Grid teak: RADIOLAS, WESTINGHOUSE ACE, CROSLEY, FEDERAL, EAGLE, MURAD, GREBE, KENNEDY, PARAGON and all Neutrodynes. Free folder *'D" gives details. Get yours from dealer or write. I DEALERS . ■ Every detector tube needs a DURHAM % — the easiest to install, use — and sell Sa$is£kction Guaranteed DUKHAM& COJnc. I936 Market St., Philadelphia ) Hr-g another transformer, and therefore the E. M. F. in S lags the E. M. F. in P 180 degrees; and the E. M. F. in the tickler lags the E. M. F. in coil P almost exactly 360 degrees, or is in phase with it. It should be noted here that it is a given wave and the preceding wave which are in time with each other. Now the currents in both T and P lag their respective E. M. F.'s almost 90 degrees, and since the E. M. F.'s are in phase, the currents will be in phase also. The magnetic flux is in phase with the current generating it and therefore the fluxes of coils P and T will add together. Due to this, coil S will have this combined flux to induce an E. M. F. in it. This also sheds more light on the loudness of the reception. Distortion Dodged Still another point that should be considered is that some of the flux induced by P links directly with T. The E. M. F. induced by this flux lags the E. M. F. of P 180 degrees. Since the E. M. F. in T induced by the flux from S lags the E. M. F. of P 360 degrees, these two E. M. F.'s induced in the T coil have a phase difference of 180 degrees and oppose each other. WACNART VARIABLE GRID RESISTANCE i^y --t [oK The Walnart Grid Resistance ^ilrfU' Varies Zero to Six Meg. £%1V/> Walnart Fixed Condenser %S 25c extra Ask to WALNART QUALITY see RADIO PRODUCTS SEND FOR FOLDER Jobbers, Dealers, Manufacturers Write for Discounts Continental Bakelite Dilecto XX is used as a base for this instrument to insure positive dielectric Insulation. WALNART El UC MFC. CO. Dept. 428, 1249 W. Van Buren St., Chicago^Jll. However, a closed circuit coil forms a pretty good mirror for electro-magnetic waves of Radio frequency, so that not very much of the direct flux from P reaches T, especially when the coupling is rather loose, since P has a weak field at all times. Due to the above-mentioned fact that it is a given wave and the preceding wave which combine, it would seem that there would be distortion if the carrier wave was very long. This theory seems to check up in so far as the set seems to work much better on waves around 300 to 360 meters than it does on those around 500 meters. There is also another factor affecting this, as will now be explained. A clever modification of the Flewelling circuit by R. D. appeared in the November 3 issue of Radio Digest, as Question and Answer Number 5104. In the regular Flewelling circuit any current passing from the grid around through the grid return and back to the filament has two possible paths, one through the condenser C in this diagram, and the other through the coil S. The current therefore will SELECTIVITY Add a Ferbend Wave Trap to Your Set YOU can get "Distance." But can you keep it? Stop the interference. Don't let every Tom, Dick and Harry spoil the concert you are enjoying. Don't lose that DX Station so long "dialed for." Trap out the interfering Station with a Ferbend Wave Trap. Our guarantee plainly tells the story: We guarantee that the Ferbend Wave Trap, when properly connected to any workable receiving set, will tune out any interfering station. Mr. John F. Parsons, 109 North Avenue 19, Los Angeles, California, writes: "I .have installed the FERBEND WAVE TRAP I purchased from you some time ago and given it a thorough try-out. There are five broadcasting stations in this city, four of them I can trap out by moving dial ten points, the other KFI, the most powerful one, I can silence by changing the dial twenty points. This station is heard on the Atlantic coast and Hawaiian Islands." Make Every Night "Silent Night" The Ferbend Wave Trap is a valuable addition to any set. It is designed and manufactured complete by us, after years of careful experimenting. It is not to be confused with imitations hastily assembled from ordinary parts. The price is $8.50. Shipment made parcel post C. O. D. plus a few cents postage. If you prefer, send cash in full with order and we will ship postage prepaid. Send us your order today. Ferbend Electric Company Dept. 3, 19 E. South Water St., Chicago Descriptive Folder on Request FERBEND divide, and the currents passing through i Ik: two portions of the circuit will be w.-r.seiy proportional to their respective lances. It will therefore be seen that the less condenser C we have, the greater will be the amount of current passing through S, and therefore the greater flux set up by coil S. Now in R. D.'s circuit the coil S is split Into two separate ( and the second part, which he refers to as the middle coil, Is connected into the circuit between A and B. The result of this is that he uses the current passing through the condenser C to generate flux. It will be noticed in his circuit that condenser C can only tune the 35 DL coil, and it would seem, therefore, that his set would a rather limited range of wave lentThis modification will strengthen signals considerably, especially on the longer ■ lengths, when most of condenser C must be used. Changing Inductance of Colls As regards the slight movement of the middle coil affecting the strength of signals which he mentions, this Is due to a double action. Changing the coupling between the oil and the 25 DL coil changes the inductance of these two coils, which are in series, and therefore chai the tuning of the set, and in addition it also makes a vernier adjustment of the coupling with the tickler coil. However, it would seem that the experimenters who wind their own DL coils might try winding a coil composed of two 25 DL coils, wound one outside of the other, and connecting one winding in the usual way and the other between A and B, if they wished to try out this modification of the circuit without changing from a two coil mounting to a three coil mounting. If it is de(Continued on page 31) Now you can UNDERSTAND RADIO! Take the mystery out of it — build and repair sett — explain the vacuum tube — operate a transmitter — be a radio expert! \ RAVHO \ SSdboo* 1 VOLUME 514 PAGES Compiled by HARRY F. DART E.E. Formerly with ibe Western Electric Co., aod U. S. Army Instructor of Radio. Technically Edited by F. H. Doane 40,000 ALREADY SOLD This practical and authoritative Handbook is considered the biggest dollar's worth in radio to-day. Over 40,000 homes rely on the I. C. S. Radio Handbook to take the mystery out of radio. 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