TUNING CONDENSERS

Evening Post, Volume CX, Issue 50, 27 August 1925, Page 14

TUNING CONDENSERS

ADVANTAGE OF SMALL SIZES,

... Among the thing 3 that are not used by the best people nowadays is the ser'.v>i- parallel "switch for the aerial'timing condenser. "This switch ia commonly liibrQ of a nuisance than.an advantage. ■It is introduced «o that » given coil rathe aerial circuit can be made to cover » considerable range of wave-lengths, a short-wave band with the series connec-"j tion and a long-wave band with tha 'parallel connection. Unfortunately there is always a gap between these two bands. An examination of the switching system will show that it is capable of introducing losses, especially if it is not kept in first-class order. Actually it is much more satisfactory to use a condenser permanently connected in the parallel position, and to have it small in capacity. The New Zealand broadcasting band- extends from 230 metres to 375 metres. If only one coil is to be used to -cover this band, the total capacity needs ~to be variable over a range represented by the squares of these figures. The aerial itself and stray capacities may be "taken as about '.0002 mfd., and a parallel condenser of .0005 mfd. is more than -ample. Better results will be had, however, by using two. coils, one for the range from about 200 metres to 300 metres, and the other for the range from, gay, 270 metres up. 1 A tuning condenser of .00025 mfd. maximum capacity will then be liberally large. Besides giving, in most cases, better strength, such a condenser will be found remarkably easier to tune with.

'.. A .0005 mfd. condenser can be converted into one of half that maximum capacity by connecting it in series with a good fixed condenser of .0005 mfd. .This, contrary to an impression held by some people, will not noticeably reduce the minimum capacity of the condenser. "Its effect is to double the space between readings for the various wave- • lengths on the dial. In a receiver in "ivhich the aerial coil is directly connected to the valve or crystal there is some•thing to be said for the series condenser. It. provides, in certain circumstances, higher.voltages at the terminals of the aerials coil. But such circuits are not advised.. Crystal receivers are more efficient, all round, when two circuits are ,'used, and single circuit tuners for valve ■Bets are frowned upon by the authorities. In a coupled circuit the voltages across the ariel tuning coil ore unimportant; it iB the current in the coil that matters, and this is liable to be less when 'i series condenser is used than with 3he,/parallel;, connection. The remarks -already made about the size of the tuning "condenser apply also'to that used for the Secondary circuit. The smaller this is in maximum capacity, so long as it covers the desired tuning range, the easier the tuning is and the better (in most cases) '_the .".strength _of the signals. A range 3roni one wave-length to another twice HasT long" is"'provided if the tuning con-' rjlettSer "has a maximum capacity three jtimes that represented by the internal "capacities, of the circuit at the mini"ih'um reading of the condenser. This 7Syill."rai'ely be as high as .00005 mfd., so *tliat a condenser of .00015 mfd. is really enough'for all practical purposes in ilirbadc'a'st reception. Ane of ■ .00025 'mfd. allows plenty of margin and makes it a simple matter to select a suitably coil fur a given wave-length band. If ;a coil is chosen which tunes 2YK (285 -metres-) •■at -a. low- reading ■ say 10 detgrees7 such "a condenser, will tune'up'to about 570 or 600 metres, and will bring in 4YA (375 metres, with the condenser is'et. at about one-third of its full capacity. Thus a much smaller condenser jniglit be used to cover this wave-length jamre. -' . ■ «■• Trios* who find the dial readings fo.r various stations inconveniently close together will be agreeably surprised if they adopt a smaller condenser. As beforementioned, a large-condenser can be "rejduced" by connecting'a fixed condenser in series with it.. The value of two condensers in series ■is the "reciprocal of the'sum of the reciprocals" of the individual values. The simplest way to work this out is to multiply the-values together, find also a-dd them together, and then divide the product by the sum. Thus, taking one of .001 mfd. and one of .0005 mfd.: the first may be taken as 10. and the other as 5. The product is 50, t. and the sum 15, and the required quantity is 50 divided by 15, or 3 1-3. Kestoring the decimal value, the effective value of the two condensers in series is .00033...