Page:Popular Science Monthly Volume 91.djvu/803

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�I Amateur - Electrician

��-And Wireless Operator

��Increasing the Efficiency of the One-Inch Spark Coil

THE more advanced amateur usually turns his attention to power transmit- ters, and consequently there is a lack of relia- ble data applying to the small spark-coil set. This necessarily works a hardship on the beginner, whose initial equipment gen- erally includes a spark coil. Therefore, a few practical an- swers to various problems encoun- tered in the opera- tion of the popular i-in. coil transmitter should prove help- ful not only to many newcomers, but also to more advanced operators who are troubled by inter- ference from badly- tuned small stations. The writer has raised the efficiency of his i-in. coil transmitter to a point where he can consistently outdis- tance by more than

���The compact arrangement of the one-inch spark coil when set upon the operator's table

��two miles any 2-in. coil in his vicinity, on a power input of only 15 Watts. Specific working directions for duplicating this set will be given; but a few generalizations must first be considered.

It is a fallacy to suppose that the prin- ciples governing the installation and tuning of large sets do not apply to spark-coil transmitters. The author is aware that many beginners possessing small coils have attempted to tune their stations but have abandoned condenser and helix in disgust after noting a considerable decrease in sending radius. Little, if any, advantage as to distance is to be gained by tuning sets using coils smaller than ^ in., but larger

��transmitters become vastly more efficient when properly adjusted. I have visited many small stations that were not giving satisfaction, and usually located the trouble in the design or capacity of the trans- mitting condenser, or discovered that closed and open circuits were hopelessly out of resonance. In one instance a station was found to be using a condenser consisting of sixteen 12 by 14-in. plates across the secondary terminals of a little ij^-in. spark coil. The spark discharge was spectacular, yet the range of the sta- tion was less than two miles. The er- ror of regarding the oscillation- trans- former, or helix, as a device to raise the voltage of the high- frequency currents traversing the open circuit seemed espe- cially widespread. The owners of the stations had tuned them by varying the positions of thehelix clips until sparks 3 or 4 in. long jumped a gap inserted in the lead-in. Such evidence should convince the skeptic that haphazard application of the principle, not the princi- ple itself, is responsible for the poor results sometimes obtained by "tuning" a station of this type.

Tests have convinced me that in con- nection with a spark-coil transmitter it is best to use a conductively-coupled pancake helix having a number of inductive turns in both the primary and the secondary. This necessitates the use of a transmitting aerial not more than 40 ft. in length (T- aerials excepted) with a lead-in not longer than 30 ft., and a very small condenser in

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