The tuning coils in the portable outfit are mounted in a vertical position in back of the aerial switch.
*Loosely Coupled Tuning Coil.*—By the use of a loosely coupled receiving tuner or transformer, the range of a station is considerably increased, as is also the strength of the signals, and much finer tuning and selectivity made possible.
Fig. 127 illustrates the construction of such an oscillation transformer.
The base is wood and measures 14 x 5 1/2 x 1 inches. The primary winding is wound on a cardboard or fiber tube 4 1/4 inches long, having an internal diameter of 2 3/4 inches and an external diameter of 3 inches. The heads, _L_ and _M_, are the same size as those of the double slide tuning coil. The head, _L_, has a circular hole 2 3/4 inches in diameter cut in the center in order to permit the secondary coil to slide in and out of the primary.
[Illustration: Fig. 127. Oscillation Transformer.]
The secondary coil is a piece of round curtain pole 2 1/2 inches in diameter and 3 inches long. A 5/16-inch hole is bored through its axis. The head, _K_, of the secondary coil is 3 3/4 x 3 3/4 x 3/4 inches. A ten-point switch on _K_ is so connected that it divides the secondary into ten equal parts and permits any number of the divisions to be used as desired.
[Illustration: Fig. 128. United Wireless Receiving Transformer.]
A wooden post, _J_, 2 1/2 inches high and 1 1/4 inches wide, supports one end of a 1/4-inch brass rod upon which the secondary slides back and forth.
[Illustration: Fig. 129. Details of Receiving Transformer.]
No. 24 B. S. gauge copper wire may be used for winding both the primary and secondary. It is also the proper size to use on the double slide tuner. The best method is to use bare wire, wound with a thread so that a thread is interposed between adjacent turns of the winding. Give the whole winding one or two coats of thick shellac and allow it to harden. Then use a strip of sandpaper to remove the shellac in a long narrow path immediately below the sliders so that they may make contact with the wire.
[Illustration: Fig. 130. Slider for Loose Coupler.]
Some may prefer to make a loosely coupled tuner in which the inductance of both coils is adjustable by means of a sliding contact. In such a case the slider on the secondary coil must be constructed as illustrated in Fig. 130. The contact is long and narrow so that it can touch the innermost turns, when placed within the primary. By slipping the slider off the end of the rod and reversing it, the contact can be made to touch the turns next to the head. The square brass rod is set in a notch cut in the coil head so that the rod is flush with the top.