Wireless World

Evening Post, Volume CVI, Issue 48, 25 August 1923, Page 19

Wireless World

The. notes in this column will give items of interest regarding wireless developments in New Zealand and elsewhere, and itemß calculated to be helpful to amateurs, particularly beginners. Questions are invited, and the replies will be published when they are likely to be of general interest. -

Honeycomb coils, when used with the three-circuit type of tuner, including primary, secondary, .and tickler are worked- in sets of three—one for each circuit. For short-wave work the number of turns should be in the ratio of about 1-2-3, in which the smallest is the primary', the next size the secondary, and the third size the tickler. For instance,. for broadcasting work, a good combination witfi an antenna of from 75 to 100 feet-in length is 35-50-75. Where long distance is desired, a larger tickler may be used and the combination will be 35-50-100. This ratio applies only to the shorter wave lengths. As the coils increase in size, the effect of the tickler become more pronounced, until with coils of 400 turns and upward a tickler of the same number of turns as in the secondary may be used, as in the combination 500-600-600. Oh the larger coil combinations an even smaller tickler may be used, the combination 1250-1500-1000 being used for 25,000 metre receptions. The secret of long distanco work with honeycomb coils, says an American paper, lies in keeping the primary wide open and the tickler close in., With this established, feed up the vacuum tube until the whistle of the distant station is -located on the secondary condenser. Then cut down the filament current below the whistling point and bring the primary in slightly and the faraway stations should begin to leap in. To cut interference with honeycombs, such as avoiding nearby stations, while working distant ones, follow the same procedure. The wider the separation between primary and secondary the less interference. Reference was made in last week's notes to the use of capacity coupling between primary and secondary circuits a* effective in tuning out the strong signals of VLW when listening to telephony in Wellington. The Wellington radio signals are so strong' that many amateurs are unable to get rid of them by means of loose coupling. Something can l^e done to reduce the interference by installing: a filter circuit. This can be don» in a number of ways, {f a- closed circuit like the ordinary secondary circuit is placed near the primary inductance, or coupled to the aerial circuit by means of a special coil in the latter, and is tuned to 800 metres, it will absorb a great part of the interfering energy without seriously affecting that of the shorter wave-length. Or a precisely similar "rejector" circuit (inductance and capacity in parallel) can be connected between the tuner and the earth.. Another alternative .is to use a "drain," which is an inductance and capacity in series, connected between the aerial and earth terminals of the tuner, and ajso tuned to the interfering signals. Both the first and the third methods can be used if desired. There is a good field for experiment in.the use of filters in Wellington, owing to the high power of the interference, but too much must not be expected from simple devices. Practically- nothing can be done to cut out all interference from a near-by spark transmitter owing to thesmixture of wavelengths to be dealt with.

What appears to be: the most success^ ful attempt _up_ to date to get rid of interference, especially by "jamming"' in the reception of long-distance telegraphy, has been worked out by an English expert, Mr. -N. P. Hinton, B.Sc, of the British Post Office. It is based on the use of rejector circuits like those described in the preceding paragraph.Such a circuit offers a very high impedance to oscillations to which it ie tuned, but its selectivity is less as itsresistance ii!' higher. If -'the resistance could be zero, its impedance to oscillations of its own frequency would be infinite. The theory of the phenomenon is that" the self-induction of the circuit causes an oscillation exactly equal and opposed to that impressed upon it. .On the other hand, such/a circuit permits otfier ifrequencies vto pass freely, -and thus theoretically it can be used to select one frequency from among many. Unfortunately the rejector circuit, like every other, has resistance: Mr. Bin. ton's method is to couple the coil of the rejector circuit to'a reaction coil in such a" way that oscillations are generated in it sufficient to make good the resistance losses; with the effect in.. the practical sense of making its resistance zero, 6 o that it operates with- very high efficiency. This principle can be applied on the high frequency side, and also on the low frequency side. The hy;h frequency 'rejector filters out what is not of the' right wave-length, and the low frequency rejector purifies the 'audible note, and helps to get rid of atmospherics. The rejector circuit coupled in parallel with the primary inductance or with the secondary is tuned to the wave-length it is desired to receive. It may also be connected in series with the aerial, between the aerial, inductance and the earth, and adjusted to the frequency or wave-length of - a signal which it is- desired to exclude, and in that arrangement may be used to avoid disturbance by a neighbouring transmitter. The Hinton system is reported to have been used with great success by British Government stations. Its effectiveness in reducing atmospheric troubles is indicated by the fact that long-distance .reception has been made thoroughly reliable at Qairo. where the neighbourhood of the Sahara Desert —a vast atmospheric factory—is otherwise a great handicap. Fixed condensers can be made up to any definite value within a reasonably small margin of error, by using mica of known thickness, and smooth metal foil. Tinfoil-is satisfactory,,but is not so easily handled as copper or aluminium. The standard thickness for mica for use in condensers is two mils., or .002 inch. Using such mica, of good quality, the capacity of a condenser is .0001 microfarad for each square centimetre of overlap. When using several foils, multiply the overlap not by the number of foils, but by the number of separators. The value given is correct enough for practical purposes, but the condenser must be clamped well'together so as to exclude air as much as possible. The presence of air to the same thickness as the mica reduces the capacity to about one-sixth of what it would be if the elements are in contact. For this reason it is easier to make condensers with thicker mica and larger foils. The thickness of the mica should be accurately measured; this can be done with a micrometer, and somebody can usually be founds who owns one of these useful instruments. ' ■

The old idea that radio antennae create a "lightning hazard" is^ dead. Great annoyance was caused to American amateurs by the early insurance rules that compelled every aerial to be connected to the ground lead, when idle, by means of a heavy switch. This rule has been abandoned, and all that is required now is a small spark gap, enclosed. The aerial is probably not nearly so dangerous a proposition as a "telephone wire; and the most suspicious'person has no'ready examples of the telephone blowing places up. . - "