WIRELESS NEWS.

Press, Volume LXIII, Issue 18924, 12 February 1927, Page 10

WIRELESS NEWS.

RADIO INSULATION. USE OF MICA, (SrZCULLT KRnTEK rOR THE TR155.) (By "Eicctra.") Mien particles, which heretofore .•ouid not be used, arc now utilised as insulating material fur the production of radio high frequency insulator?. The now insulation, known as mycalex. is a composition of ground mica, and lead borate. Mica previously presented a difficult problem, because of the enormous amount of waste. Obtained in India and Canada, in large sheets, there was considerable waste at the mines, it being estimated that only about ~> per cent, of the material taken from the mines could be used. In manufacturing there was still further waste of small pieces of mica, but some years ago it was found that these mica flakes, mixed with a binding material, and compressed under heat made verv good insulation. And now a product lias been developed which uses mica particles, so that even more of the mica is utilised.

This tvpc of material has better insulating properties than has porcelain. The substance, light grey in colour, and with a metallic ring, is being used in the manufacture of bases for radio transmitting \nlvrs. for aerial insulators in high frequency work, and for numerous similar applications. Chief among its characteristics are that metal parts may be inserted or combined with mycalex during the process of moulding, and, althuitgh a hard and .stone-like product, it can be subjected to ordinary machinery methods. Mycalex is softened or made plastic by heating to dull redness and the plastic mass is then formed into the desired shape by compression in ;teel moulds closed under hydraulic pressure. Not onlv has it better electrical properties than porcelain, but it is stronger mechanically, except under pressure. It is not as heat resistant as porcelain or mica, but tests show that it is far superior in this respect to the ordinary moulded insulations made with phenolic resins, shellac, gtnns, pitches, and other materials. The, fact that metal parts eaTt be combined with mycalex during the moulding process means a tighter ami stronger combination. As a rule, metal inserts cannot be embedded m ceramic products during the process of manufacture, and this i* especially true ot porcelain, because of the considerable fchrinkugo and the high temperature, which causes melting or slagging ot meta! parts. Although niycalex can be drillcU, filed, sawn, polished, and similarly treated by usual machining methods, it is necessary to use special tools, 6incc the near when using mycalex is greater than when cutting metnl. The new compound has been recommended for use where the requirements demand an insulating material of high mechanical strength, high dielectric strength, and insulating resistance, low Joss undet high frequenciefl, heat resistance beyond the ordinary range of moulded or sheet material, and compact construction with metal inserts securely moulded in place.

By-Pais Condensers. If the stopping condensers in tlio grid circuits of resistance coupled amplifiers arc too small they defeat the very object sought in using resistance coupling—thai is, amplification of the low notes. The stopping condenser is a scries connected condenser through which the signal must pass, and a small condenser offers a very high impedance to currents of low frequencies, so that these currents will be depressed. Tho degree of suppression at any given, frequency depends, of course, on thb capacity of the condenser, and also on the plate resistance of the valve, on tho coupling resistance in tlio plutc circuit of the valve, and on the grid leak resistance of the valve following the stopping condenser. For the ordinary valves of these resistances, and for the valves of capacity of the condensers recommended by many designers of amplifiers, the suppression is by no means negligible. In some cases the small condensers used will introduce as much distortion an poor transformers. The ideal size of condenser in tho grid circuit in one of infinite capacity, or one which is as large as is practically possible. Hut larpe condensers arc both expensive and bulky, and for these reasons Hie larger sizes are ruled out of consideration. Perhaps the largest that need be used is a one m.f.d. unit. This condenser will introduce a distortion of less than one per cent, for the lowest audible notes.

A condenser as small as .Imfd will not introduce more than 8 per cent, suppression at 13 cycles a second, and much less at higher frequencies. But a condenser of .OOonifd. which is often recommended for resistance coupled amplifiers, will introduce as mucli as 36 per cent, suppression at 13 cycles, and quite notable percentages at the higher and more important frequencies. That is far from distortionless amplification. The smallest stopping condenser that should l>o used under any condition in an audio frequency amplifier may be taken as .Olmfd. and that should be used only for ultra selective sets. For receivers of ordinary selectivity the choice should be from the capacity range of .1 and lmfd. For the smaller of these the suppression at 16 cycles is less than 7 per cent. Objection h.is been raised to the use of the stopping condensers on the ground that it takes an appreciable time for these condensers to charge, and that as a consequence, distortion is introduced. That is not valid because the condenser does not charge, the voltage being the same on either side, as far as AC. is concerned. The

small condenser is charged quickly, and that i' just how the suppression enters. Use Thick Baseboards. Wood baseboards used in set building to mount transformers, sockets, and other parts on, should be at least half an inch thick if shorter than 14 or 15 inches, and about an inch thick if longer. The wood itself should be closegrained and well seasoned. Thin boards of inferior quality invariably warp, and in doing so sometimes crack the bases of valve sockets, panels, and resistor mountings. Brackets Make Strong Set. In fastening a wood baseboard ti> the bottom edge of a front bakclitc or hard rubber panel, do not rely ou a lew wood screws alone to make a strong job. Use. in addition, a pair of brass, aluminium, or hard rubber brackets iixed each end of the panel, and you will have a strong and rigid receiver. If the edge of ouc of your dials scrapes against the surface of the panel in certain positions, remove the dials, and place over the shaft two or three washers cut from ordinary note paper. Those should raise the dial sufficiently to make it clear the panel. Local News. By the time this appears in print aVA should be upon the air again, oper ating from the new studio. It is to be hoped tlia: with the discontinuation of the iand line between the studio and operating iuom improved aerial system, new studio, and operating-room, the transmission will be greatly improved. Quite a number have expressed the opinion this week that they will bo very pleased when the local station starts up again, as it may take some of the howlers olf the air. This last week it has been practically impossible to enjoy distant stations owing to the nuisance ot two-valve howlers. Another problem which will soon have to be taken up by ii-teiiers-in is that of interference from trams. Except on Cliristohurch and distant stations alter 11.10 p.m. it is impossible to listen to any concert with any «>gr-e of satisfaction. The noises picked up by a radio set in the vicinity of overhead wires and from a passing train itself must surely point to a loss of power in the first place and faulty motors in the second. Very shortly the Tramway Board will find it advisable to fall in line with the American tramway companies, and hunt the (rouble down, thereby not only cutting radio interference out, but saving on power losses, etc. A receiver is not complete without a SFERAYOX. Ask your dealer to demonstrate ono to you. It combines icnsit'vity and volume with a band- ■ •.•■.!(• :is"cni-:iiir-?. Ii.»iil!» built hi <\'-f colour, and makes a beautiful contrast in anv room.—

£5 Os. of all leading radio dealers.—New Zealand Heprcsentntivcs, Drown and Dureati, Wellington. —;i The man who has won success from his Crystal Set naturally turns to something bigger. The next step should be an "Atwater-Kcnt'' Valve Set. These '•an he heard at J. I. Smail's demonstration rooms at Carey's Buildings. Colombo street, between 3 and -1.00 p in. daily. —C