# Page:Popular Science Monthly Volume 89.djvu/972

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What Radio Readers Want to Know

Interesting and Instructive Questions and Answers

��Dimensions for a Receiving Tuner; Effect of Variometer on Wavelength

E. C. S., Deer Lodge, Montana, writes:

Q. 1. Please give the dimensions for a 4,000- meter inductively coupled receiving tuner. The secondary winding is to be shunted by a conden- ser of .0005 microfarads and the primary by one of .001 microfarads. The aerial has a natural wavelength of 450 meters,

A. 1. The fact that the aerial has a natural wavelength of 450 meters does not gi\ e us suffi- cient basis to compute accurately the dimensions of the primary' winding. We must know the inductance and capacity of the aerial system to work out the problem. The secondary winding may be wound on a form 4 in. in diameter, 7 in. in length with No. 32 S.S.C. wire. The primary winding should be 42 in. in diameter, 6 in. in length, wound closely with No. 24 S.S.C. wire.

Q. 2. How many meters will the variometer described on page 539 of the October, 1915, issue of the PoPUL.\R Science Monthly add to the wavelength of a receiving circuit?

A. 2. We would require more details of the particular circuit in which it is to be employed to answer this question definitely; but off-hand we advise that with the No. 20 wire recommended in that issue it will have but a slight effect on the tuning of a circuit. To be eflectual it should be wound with No. 30 S.S.C. wire and will then alter the wavelength of a small set about 250 meters.

Making a Transmitter for an Amateur Station

C. F. L., Galveston, Texas, writes:

Q. 1. Please give the data for the construc- tion of a J-K.W. open core transformer to be operated on 300 volts alternating current at a frequency of 500 cycles. The secondary winding is to deliver 20,000 volts.

A. 1. Data for an open core transformer is not available at this writing but it may be pos- sible to supply it at a later date. The following, however, is applicable to a closed core trans- former. The core is 9 in. in length, 2 1/16 in. in width and ij in. in thickness. The ends are 5} in. in length and of the same thickness. The primary winding has 98 turns of No. 10 D.C.C. wire wound in two layers. The second- ary win<ling is made in sections and has totally 4<x)0 turns of No. 20 D.C.C. wire. The secondary winding should be split into 5 sections wound cither in the form of pancakes or multilayered units of 36 layers each. Appropriate insulation between the windings and the core is required.

Q. 2. Give the dimensions for an oil- immersed condenser to be used in connection

��with the above transformer. I prefer to use photographic plates 8 in. by 10 in. if possible, and should like to have two sections of con- densers in series. I propose to use a syn- chronous rotary spark-gap with this set.

A. 2. We presume that you desire to operate the station at the wavelength of 200 meters and consequently the capacity of this condenser can- not exceed .01 microfarads. If the 8 in. by 10 in. photographic plates are covered with tinfoil 6 in. by 8 in. each plate will have an approximate capacity of .00066 microfarads and therefore 16 plates connected in parallel will give about the required value of capacity. Since you prefer a series parallel connection, you must connect 32 plates in parallel in each bank and then connect the two banks in series.

Q. 3. In view of the fact that my aerial is so small, would not a high voltage set of this character carry further than a j-K.W. set using a voltage of 7,000 and a quenched spark-gap of poor design?

A. 3. Yes, by all means. The higher poten- tial will enable you to use a greater amount of power with the restricted condenser which the 2(K)-meter wave requires. The fundamental wavelength of your antenna system is about 215 meters and can be reduced to 200 meters by con- necting a "short wave condenser" in series with the antenna system, or, preferably, by attaching the lead-in wires to the center of the flat top portion.

A Long-Wave Tuner

J. L., Scranton, Pa.

Q. 1. Where can I obtain the parts and full directions for constructing a 15000-meter in- ductive coupler of the Navy type, including all the blueprints and necessary diagram of connections ?

A. 1. We know of no concern which supplies such data and parts. If you ha\e become fann'liar with the construction and operation of smaller receiving transformers you should have no dilViculty in building an apparatus of the sort you wish. The exact dimensions will of course depend upon the size of antenna to be used. For a good-sized aerial, your primary-coil should be of No. 28 wire on a cardboard tube 8 in. in diameter and 18 in. long, taps being taken out at each fifty turns. The secondary may he a 6-in. tube of the same length, wound with No. 36 wire an<l tapped at each 100 turns. A variable condenser should be placed in shunt to the sec- ondary terminals, for tuning, and a finely- variable loading coil, or variometer, should be placed in series with the primary.

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