WIRELESS NEWS.

Press, Volume LXI, Issue 18407, 13 June 1925, Page 6

WIRELESS NEWS.

RESONANCE IN RADIO. A SIMPLE EXPLANATION. ' J (SPECIALIST WRITTEN FOE "THE PBESS.") j (By "Elcctra.") When ire tune a radio set we are adjusting the circuits so that they "will be in resonance with each other and the transmitting station. A clear understanding of this phenomenon is ncces- j sary for the intelligent operation and construction of a radio set. • Resonance is dependent upon inductance and capacity. These two elements are necessary to establish, in a circuit/ wave-length or resonance to frequency. l All circuits have resistance, which is present in the inductances, conductors, and condensers. There are, therefore, three quantities—inductance, resistance, and capacity in a circuit. The amount of current that will flow in a circuit depends upon the strength of the voltage and the resistance of the circuit. These two forces will establish a condition that could be easily handled if the third quantity, frequency, was negligible. Voltage is that energy which forces current through a circuit. Frequency is the number of times per second that force is applied. Current flow,' therefore, is controlled not only j by the force, but.also upon the number of times "per second that force is applied. If a constant voltage is applied to a circuit and the frequency is steadily increased from a low to a high frequency, it is found' that thecurrent. value increases to a certain point and then decreases. Resistance. The frequency at which maximum current passes will be found to be the natural frequency or wave-length of the circuit. At this point the frequency of the energy applied is the same as the. natural wave-length or frequency of the circuit. This condition is known as resonance. In addition to the resistance natural to the materials, composing a circuit, inductance and capacity cause what is knoA -as re-' actance. At certain frequency in a circuit having inductance and- capacity, this reactanco becomes zero, the opposition of the inductance and capacity cease and we have resonance. . But if the resistance in the circuit is high it affords a constant opposition to the current flow. . This condition results in broad tuning. If this resistance is high, the tuning becomes broad, and long-distance reception impossible.' Remove this resistance and the tuning becomes sharp and interference is ; reduced if, not eliminated. SELECTING CONDENSERS. There is probably nothing more important in a receiver thaii the variable condenser. Therefore, when selecting one bo sure it is ot' good design. • The fir&t thing to examine is the dielectric or end plates. Those with a good grade of hard rubber insulation axe passable, but those built . on the lowloss style with a minimum\of dielectric are by far the best. Beware of condensers employing materials that are 'likely to absorb moisture, as this will increase the resistance of the circuit and reduce your signal strength. The . second thing to look for. is good mechanical construction. Make sure the plates are made of some standard material, namely, brass ,or aluminium. The plates should be Soldered- together 'so'as-to ensure perfect contact between each plate. The bearings and also the means of adjustment of the moviiig plates should be of' good del sign. '

TUNED- AND UNTUNED PRIMARY.

One of tbe commonest questions asked to-day. by radio experimenters is: Why is it that'.it is easier to pick up, long distance stations ; when an untuned primary consisting of two or three turns is used, in place of a tuned primary circuit? Which arrangement is better? Is ifi possible to obtain the same signal intensity with. an untuned primary" receiver as witli a. tuned pnniary receiver? : Is the discarding of a tuned primary circuit for an "untuned" justified? _ The increased simplicity or I>X reception with the "untuned" primary is due directly to the elimination of _a variable control and the necessity of bringing two circuits to resonance. In the reception of long-distance (DX) signals with a receiver which utilises - a separate tuned primary and _ another separate tuned, secondary circuit it is necessary,; , before any response is obtained, to tune the second--arv circuit iiito resonance with the print ary circuit, in order that the energy 'flowing in the latter circuit, due to the received signal, be induced in the secondary circuit, and passed into, the detector. Invariably the tuning of primary circuit is different from that of the secondary, that is,-the setting is different, hence the bringing to resonance of the two circuits, with an infinitesimal weak signal .as the guide, is quite a proposition, "Untuned Primary" Circuit. On the other hand, when an "untuned primary" circuit is. used the tuning is all done with the secondary circuit control. In this instance it is unnecessary to bring the two circuits into resonance by means of independent controls; (the. couplingbwtween tlip "untuned primary" and the tuned secondary is sufficiently close to. allow tha secondary circuit to display a reaction ary effect hpon t-iie primary, and practically tunc it to resonance with itself by .means of its own action.. It is -for this reason ' that/relatively close coupling is necessary when "untuned primaries" are used. ' Wit.li the "untuned primary" the secondary absorbs the energy of the same frequency .which is flowing in the primary circuit,

whereas -with the tuned primary the epergy is induced into the secondary circuit. Better for the Beginner. , For the beginner • the "untuned" arrangement is superior to the tuned, in' that simplicity is the paramount feature. In addition, it is possible to hear as many stations as Avith the tuned arrangement, but, of course, the signal intensity -will not be so great. This is logical, as the maximum amount of current will flow in a circuit when it is tuned to the same frequency as tho source of energy. This condition is obtainable , only when the primary- circuit of the receiver is tuned to the same wave-" length as the primary circuit of the transmitter. Experiments along this line showed the variation in received signal intensity with an "untuned" primary to be from 10 to 25 per cent, less on DX work and from 20 to 45 per cent, on loeal signals. Advantages of Tuned Primary. Very many radio experimenters entertain fallacious ideas appertaining to the degree of selectivity obtainable with an "untuned primary." This seemingly greater selectivity is only imaginary, and is attributable to the fact that a smaller amount of energy is fed in the secondary. With the proper degree and means of coupling, a tuned primary receiver will at all times afford as great a degree of selectivity as any "untuned" primary receiver. In view of the fact , that the primary circuit is untuned, we must keep the resistance low, iu order to allow the maximum amount of current flow, hence the wire constituting the primary coil should be of low resistance. JTo. 14 or 16 insulated wire should be used. For all-round work, signal intensity, selectivity, sensitivity, the tuned primary is invincible.