Page:Popular Science Monthly Volume 88.djvu/500

 What Radio Readers Want to Know

��Range of Station

S. D., Glendale, Cal., inquires :

Q. 1. With a four-wire aerial, 100 feet in length by 55 feet in height at one end and 70 feet at the other, connected with a "1500- meter" tuning coil, galena detector, 1000-ohm receivers, 43-plate variable condenser and a fixed condenser, how far should I be able to receive ?

A, 1. The daylight receiving range of this apparatus is perhaps 250 miles, while the night range may be 1000 miles, depending largely upon the power of the transmitting station from which it is desired to receive.

Q. 2. With the foregoing aerial, -^/i K. W. transformer connected to the proper con- denser and oscillation transformer, how far can I transmit and approximately what will be the wavelength emitted?

A. 2. The natural wavelength of the an- tenna system is about 300 meters, and ra- diated, it win be above that value by an amount depending on the number of turns and the general over-all dimensions of the second- ary winding. If your station is located so that the Government Authorities will allow it to be operated at a wavelength of 300 meters, the daylight range will be approximately 50 miles. At a wavelength of 200 meters its probable range will be from 20 to 30 miles.

Condenser for Transmitter

LeR. D., Milwaukee, Wis., inquires :

Q. 1. How many plates of glass, 8 inches by 10 inches covered with tinfoil 6 inches by 8 inches, are required to make a suitable con- denser or a % K. W. Thordarson trans- former?

A. 1. Assuming that this condenser is to be operated at a wavelength of 200 meters, its maximum capacity in any case cannot exceed 0.01 Mfds. With the dimensions given, the capacity of each plate is approximately 0.0000 Mfds. For a value of 0.01 Mfds. approxi- mately 16 plates should be connected in par- allel. If the potential of the transformer is 20,000 volts, the condenser should be split into two banks. You then require 32 plates con- nected in parallel in each bank and two such banks connected in series.

Q. 2. Please give the construction of a 0.5 Microfarad condenser.

A. 2. We infer that this condenser is to be somewhat similar in construction to the type used in telephone work and operated at low potentials. If so, two strips of foil, 6 inches in width by about 90 feet in length, are separated by a similar thin strip of paraffin

��paper. A second sheet of paraffin paper is then placed over one of the tinfoil strips and the entire unit wound up in circular form. The connections from each strip may be brought out to a binding post.

Q. 3. How many electrodes should be em- ployed in connection with a rotary spark-gap having a disk 6 inches in diameter? The mo- tor has a no-load speed of 6000 R. P. M. This gap is to be used with a % K. W. trans- mitting set.

A. 3. With the transformer operated from a 60-cycle source of current supply, it is not advisable to produce more than 300 to 400 spark discharges a second. Assuming the load speed of the motor to be about 4000 R. P. M., it is recommended that the disk be fitted with 6 discharge electrodes equally spaced about the circumference. Excessive speeds are undesirable and unnecessary. The average commercial, non-synchronous, rotary spark discharger operates at a speed of 2400 R. P. M. and has 10 discharge electrodes mounted on the disk.

Q. 4. What are the names of the cities corresponding to the abbreviations sent out from Arlington in the weather forecasts, such as M, C, U?

A. 4. These abbreviations refer to impor- tant weather observation points. An interpre- tation follows : T, Nantucket ; S, Sidney ; A, Atlantic City ; H, Hatteras ; C, Charleston ; K, Key West; P, Pensacola; B, Bermuda. For the Great Lakes the designations are as fol- lows : DU, Duluth ; M, Marquette ; U, Saulte St. Marie ; G, Green Bay ; CH, Chicago ; L, Alpina ; D, Detroit ; V, Cleveland ; F, Buffalo.

Inductively Coupled Tuner

W. M. K., Windsor, Ontario, writes : Q. 1. I have an inductively-coupled re- ceiving tuner with a primary winding iVz inches in diameter by 6 inches in length. It is covered for 5 inches with No. 18 enamel wire. The secondary is 6 inches in length by 3V2 inches in diameter covered for 5 inches with No. 24 single cotton wire. Kindly ad- vise the range of wavelength.

A. 1. The range of wavelength to which this apparatus is responsive depends upon the size of the variable condenser employed in shunt to the secondary winding, but with one of very small capacity it .should be adjustable to about 2500 meters. The present winding does not represent the best design for an ef- ficient tuner, since No. 24 wire is preferred for the primary winding and No. 30 or 32 for the secondary winding.

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