WIRELESS NEWS.

Press, Volume LXI, Issue 18431, 11 July 1925, Page 7

WIRELESS NEWS.

IMPROVED R.F. RECEIVERS. RECENT ALL ROUND DEVELOPMENTS. (SPECIALLT WBnTEN JOB "THE PSESS.") (By "Electra.") There is no disputing the influence of the neutrodyne type of circuit upon the design of modern radio receivers, particularly in regard to the construction of the tuning units, and air-core radio frequency transformers. Opinion as to the merits of neutralising the grid may vary, but the efficiency of the semi-aperiodic coupler and the tuned air-core transformer have never been questioned, and the parts have undoubtedly marked an epoch in radio history. In some way or other, the tuned semi-aperiodic coil is reflected -* nearly all circuits whether of radio frequency, regenerative or reflex type. Our old friend, the vario-coupler. ■which was so popular up to a year ago, has practically disappeared" and the majority of sets are now using the fixed coil type commonly known as "aperiodic." Doing away with the rotor gives one less control to worry over, and with multi-tube 6ets this simplification was a distinct advantage. Acting as radio frequency transformers, the tuned secondarv coil gave a pronounced increase in sensitivity and selectivity and.ui some form or another are found in the majority of radio frequency circuits.

Early Improvements. Eefinements of the early elementary solenoid coil with its barrel-shaped core have followed, of course, when their virtues were finally demonstrated. Tho first step in this direction was the elimination of useless dielectric material within the magnetic field, thus introducing the low-loss era of to-day Instead of having two or three pounds of hard rubber in some form or ah"other within the magnetic field, as with the old type vario-couplers, the modern fan is worried over the dielectric losses introduced by the silk or cotton coverings of the wire, and is introducing air instead of hard Tubber in his constructions. With all these improvements, however, the couplers and air core transformers still possessed several objectionable features, among which the back-coupling between stages, due to the stray magnetic field set up in the transformers, and" the "pick-up" pbility of the coils in acting as aerials fo r uridesired waves set up within, the receiver itself. The feed-back caused excessive regeneration with its whistling and howling while tho antenna tendency 1 educed tho selectivity of the receiver sometimes to such an extent that local stations could not be tuned out at all with the receiver standing in certain positions. ~ A Purther Step. The next step was, therefore, to., eliminate the stray fields set up,.by ■ : tlie transformers. This meant that the ' magnetic field 'must bo constrained to a path with the coils and that the turns must be so disposed thai a magnetic wave would cut positive and negative turns at the same'time,', thus creating opposing currents- within the coil. At first this did not seem to bs a simple problem, but-, like all other practical .arrangements,- it finally ■ work-ed-out-, so that no complications were introduced, either in the construction or application of tho coils. To illustrate the development we will start with the plain solenoid ceil. This field may extend for a foot _or more around the coil and everything within the field will be affected by it. Further, any stray magnetic field from an external source that passes even approximately along the direction of the coil axis will induce currents in the coil, and thus will cause serious interference. The first real attempt toward solving the problem was the tiro of differential coils placed side by sida with the magnetic, lines confined to; a much shorter path. Tho spread of the 'field" is shorter and the coil is less affected by outside stray fields, hut there is still pome outside the coils. The insulating tube is cut by the magnetic lines in both the above mentioned coils, and wo always have some dielectric loss, no matter how far we may place the insulating supports from the wire. Any coil with an external field cannot be a true "low-loss" type. A coil hent in a complete circle was finally adopted as being the one form in which the magnetic field is complete within'tho coil, as this type of coil has neither beginning nor end and it cannot create an external field when acting as a transformer. Further, the turns are so arranged that when they are cut by another stray field the induction is neutralised by v the opposing turns, and hence there will be no interference within the coil itself. This is a strong contrast to the common solenoid coil which, in many cases, can pick up signals for many miles without any separate aerial.

A Practical Measure. In practice, the absence of an external field can be proved by placing two of these circular type coils within one quarter inch of each other without causing the slightest- coupling be tween the two. One of the coils may be acting as the coupler while the other is serving as a transformer, but no feed-back nor regenerative squalls, will be produced within this distance. Tests made within a few hundred feet of a powerful broadcasting station have failed to show any tendency toward picking up signals by the coils under any conditions, and with the aerial disconnected the receiver was perfectly silent, something that _ had never been accomplished oefore 5d localities where the tests were made.

VARIABLE CONDENSER DESIGN. A prominent radio editor recently remarked "that tha more one knows of radio tho less he is inclined to .look for anv startling overnight changes.-" Advancement is gradual rather than sudden, and the result of a better application of old principles rather than the discovery of new ones. Mechanical correctness is vitally important and inability to secure it is the weakest point in many condensers, whose electrical features may follow commonly accepted practice. A few months ago large numbers seemed to think that any condenser with metal end plates was an improvement and, as usual, paid for their credulity. The radio public learns fast, however, if given tha opportunity, and at this time not so many are being dazzled by shiny end plates or intrigued by complex machinery. They are learning that counterweights and solid metal end plates prevent low minimum capacity, that half a dozen short <'leakage" paths axe worse than a few long ones, and that a dielectric has losses whether it be called an end plate or a gear. Likewise, to sacrifice mechanical stability and durability for the purpose of securing «• high ratio beyond a certain point is poor judgment rather than good design. An inductance of zero capacity (or one greatly lower than that of the inductance with which it is used) produces no advantages in a receiving set. 3LO MELBOURNE. Up to last week this station was working on a wave-length of 1720 metres, but it is now down on 371 metres, and is reported to be coming in with good volume. It works from 10.20 p.m. to midnight (New Zealand mean tune).