Page:Popular Science Monthly Volume 88.djvu/818

 What Radio Readers Want to Know

��Receiving Tuner; Sending Condenser

L, J. T., St. Louis, Mo., inquires:

Q. I. Some confusion exists in my mind regarding the designs for receiving tuners. Take for example the following: For a receiving tuner to be adjustable to wavelengths from 175 to 4,000 meters, is it preferable to construct two separate tuners or may two small-sized tuners be joined together (in the primary and secondary windings) to receive the longer wavelengths? Also, what is the most desirable size for the cylinder and the size of the wire in the primary and secondary? Is single cotton-covered wire better than enamel wire? In addition, approxi- mately how many taps are required on the secondary' winding?

A. I. A receiving tuner of this range is practical provided the precaution is taken to fit it with dead-end eliminating switches. If the dead-end losses are to be wholly eliminated, you are advised to construct two separate tuners. Assuming that the smaller tuner is to be used for amateur work It may have the following dimen- sions: The primary winding Is 3 J Ins. In diame- ter by 2 Ins. in length covered with from 80 to 85 turns of Xo. 28 D.S.C. wire. The secondary winding Is 3 Ins. in diameter by 2 Ins. In length covered with Xo. 30 D.S.C. wire. The secondary winding Is equally divided between the taps of a three-point switch, while the primary winding may be fitted with a slider. Connected to an aerial of the dimensions found at the usual amateur station, the following described receiving tuner will permit adjustments In both the antenna and detector circuits to a wavelength of 4,000 meters. The primary winding Is 4 ins. outside diameter by 7 Ins. In length and is wound closely with Xo. 24 S.S.C. wire. The secondary winding is 3^ Ins. In diameter by 6 ins. in length wound closely with Xo. 30 S.S.C. wire. The turns of the latter winding should be equally divided between the points of a ten-point switch. The primary windings may be fitted with a slider or preferably two lo-point switches, one of which takes in a single turn at a time and the second one connects in a number of turns In groups.

The secondary winding must be shunted by a condenser of 'small capacity; one of .0005 micro- farad capacity will permit the reception of wave- lengths In the vicinity of 4,000 meters.

If you are familiar with the construction of dead-end switches the windings of the long wave- length tuner may In this manner be broken up into groups and a small portion used for the reception of amateur signals, though the efficiency will probably not be so high as when two different tuners are used.

Q. 2. Please give the dimensions for a con- denser to be connected to a i-k.w. transformer regardless of the 200-meter wave.

��A. 2. The proper capacity of the condenser depends upon the secondary voltage of the trans- former and the frequency in cycles per second. Lacking this data we can not advise. If the potential of the transformer is 20,000 volts at a frequency of 60 cycles. It Is customary to fit it with a condenser having a capacity varying from 0.012 mfd. to 0.018 mfd. A single plate of glass -J In. in thickness with other dimensions 14 ins. by 14 ins. covered with foil 12 ins. by 12 Ins. will have a capacity of 0.002 mfd. XIne of these plates, connected In parallel, will total 0.018 mfd. If as assumed, the potential of the transformer is 20,000 volts, a series-parallel connection for the plates is required, that Is to say, 18 of these plates must be connected In parallel In each bank and the two banks connected In series.

The Use of Loading- Coils

E. C. T., Beaumont, Texas, inquires:

Q. I. Approximately what is the wave- length adjustment possible with a double-slide tuning-coil 18 Ins. In length, 3 tJ Ins. in diameter, wound with Xo. 22 S.C.C. wire?

A. I. Connected to the average amateur aerial this coil should permit adjustments to stations employing wavelengths as great as 3000 or 3,500 meters.

Q. 2. Would I secure better results If the coll were wound with bare wire?

A. 2. X'ot necessarily, since either bare or Insulated wire may be employed. This coil Is too large for the maximum degree of efficiency at wavelengths lying between 200 and 1,000 meters. For the ordinary aerial a single coil of wire, 6 Ins. In length by 3 Ins. In diameter wound with No. 26 S.S.C. wire, will be sufficient for the lower value of wavelength.

Q. 3. Will a pancake loadlng-coIl Increase the wavelength of the above tuning-coil?

A. 3. Yes, but we see no need for it.

Q. 4. How Is the loading-coil to be con- nected to the tuning-coil?

A. 4. It should be connected In series with slidlng-contact connected to the aerial wires.

Ground Connection P. V. D., Warwick, X\ D., Inquires: Q. What form of earth connection is con- sidered desirable where the sub-soil consists of hard and very dry clay?

A. If by digging to a depth of several feet moist earth cannot be reached, you are advised to Install what is known as a "surface ground." For your purposes this artificial earth connection may consist of several long copper wires spread out radially from the base of the mast and the greater portion placed directly underneath the flat top portion of the aerial. There should be at least as much wire in this "ground" as there is in your antenna.

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