EXPERIMENTAL CAPACITIES

Hawke's Bay Tribune, Volume XIX, Issue 282, 13 November 1929, Page 9

EXPERIMENTAL CAPACITIES

In testing a newly built receiver containing a stage of high-frequency amplification, it is often necessary to connect the aerial temporarily to a tuned circuit that was designed to tunc 'over the desired range without any such addition (says an exchange).

If, for example, a set fails to give the signals expected of it, tho first test would probably consist in attaching the aerial to tho secondary of the radio-frequency transformer, coupling the first valve to tho detector, and connecting telephones in the plate circuit of this valve to find out whether the local station can be heard at the expected strength for a one valve receiver. If all seems well at this point the aerial and the telephones are moved, valve by valve, out from tho detector, so checking the performance of each valve separately. If such a scheme of test is proposed—and it is a very informative one—it is annoy ing, to say the least of it, to find that the'addition of. the aerial capacity to the grid circuit of the detector has altered the tuning range of the circuit to such an extent that the stations to which it is desired to tune for test purposes cannot be tuned in. If this discovery is made it is not necessary, as at first might appear, to abondon the test, for a condenser of small capacity in series with the aerial will speedily circumvent the difficulty. Pro bably the test is being made in the evening no condenser of suitable capacity is at hand, and the shops are shut. But there is still no need for despair, for any two conductors placed near to one another, but not in electrical contact, form a condenser of sorts. A foot of twin flex, for example, may be commandeered, one wire feeing connected to the set and tho other to tho aerial, both connections being made at the same end of tho flex. The capacity between the two wires, though not as free from losses as tho purist might desire, will at least servo to pass the high-fre-quency currents through from aerial to set. Although there is no slowmotion dial on this improvised condenser, it is variable. If less capacity is required, one can always cut the flex, or. if in an economical mood, it need only be untwisted. To experienced wireless men, these suggestions will be familiar enough, but there will bo many experimenters to whom the idea of using a pair of insulated wires twisted together to act as an emergency small condenser will come as a novelty.