Popular Science Monthly 



637 



Umbrella Antenna; Variometer; 

 Loose Coupled Tuner 



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



Q. 7. Please state the dimensions for an 

 umbrella aerial to have a natural wavelength of 

 165 meters. The pole for support of same is to 

 be 55 ft. in height and the antenna proper is to 

 consist of from 12 to 20 wires. The lead-in wire 

 will not be over 15 ft. in length. 



A. I. We know of no formula by which the 

 natural wavelength of an umbrella aerial can be 

 computed exactly but as an approximation we 

 should say that if the ribs of the umbrella are ex- 

 tended to a distance of 20 to 30 ft. from the top 

 of the mast, the natural wavelength will be about 

 the value you require. 



Q. 2. Can a rotary variometer be made to 

 tune from 200 to 1500 meters? If so, please 

 state the correct dimensions. 



A. 2. We are not quite sure as to the type of 

 apparatus you refer to. Some misunderstanding 

 seems to e.xist among amateur experimenters 

 regarding the action and the use of a variometer. 

 Ordinarily the term variometer is applied to a 

 variable inductance the value of which may be 

 varied from nearly zero to maximum by means 

 of two concentric inductance coils of fixed value 

 which are connected in series. The inner coil usu- 

 ally rotates on an axis and is constructed so that 

 it can be turned completely around. In one position 

 the magnetic fluxes of the two coils are opposite 

 with an approximately zero value of inductance. 

 In the other position the magnetic fields of the 

 two coils are accumulative and the inductance 

 value is at a maximum. The variometer for 

 amateur purposes may have the following 

 dimensions: The outer coil may be a cardboard 

 tube 6 in. in diameter wound with a single 

 layer of No. 24 S.S.C. wire to a width of 2 in. 

 The inner coil is 5 in. in diameter wound to 

 about 2i in. with No. 26 S.S.C. wire. The inner 

 and the outer coils are of course connected in 

 scries. 



Q. 3. In the case of a receiving apparatus 

 where a distant transmitting station can be 

 tuned to by means of inductance alone, will the 

 use of a. variable condenser intensify or assist in 

 any manner in securing a higher degree of 

 sensibility? 



A. 3. In an instance where the audion is 

 employed as a receiving detector and the in- 

 ductance is of such value as to permit the 

 receiving apparatus to be tuned to the distant 

 transmitting station, a variable condenser is of 

 little use, but where the secondary winding of a 

 receiving tuner is fitted with a multipoint switch, 

 the variable condenser gives a closeness of 

 adjustment between the taps of the switch 

 which cannot otherwise he obtained. The 

 variable condenser in shunt to the secondary 

 winding of a receiving tuner is only of value under 

 conditions of loose coupling iK'twecn the primary 

 and secondary circuits. 



Q. 4. From the standpoint of long range and 

 efficiency, which is the better, an inductively 

 receiving tuner or an ordinary straight-coupled 

 tuning coil? 



A. 4. Practically equal degrees of signal 

 loudness can be obtained with either type, but 

 the inductively coupled tuner is preferred on 

 account of the ease with which the mutual 

 inductance between the primary and secondary 

 windings can be regulated and the sharper tuning 

 which results. Similar ciTects of course can be 

 obtained in a simple tuning coil by the use of 

 three sliding contacts, but the operation is more 

 complicated. 



Photographs of Marconi Apparatus 

 and Books on Radio Topics 



H. P. B., Chicago, 111., inquires: 



Q. I. Where may photographs be obtained 

 of Marconi apparatus cither on board ship or 

 inland stations? 



A. I. Application for such photographs may 

 be made to Mr. George W. Hayes, superintendent 

 of the factory, the Marconi Wireless Telegraph 

 Company of America, Aldene, N. J., Underwood 

 & Underwood, New York City, also have in 

 stock photographs of wireless telegraph equip- 

 ment. 



Q. 2. Are there any books on the market 

 which cover the quenched spark system of 

 radio telegraphy in detail? 



A. 2. No; there is no publication that covers 

 distinctly the quenched spark system in detail, 

 but practically all modern books of wireless 

 telegraphy cover the subject partially. Perhaps 

 Zenneck's "Wireless Telegraphy" gives about as 

 complete a description of the action of the 

 quenched spark discharger as can be obtained. 



Q. 3. Kindly tell me in what publication the 

 following subjects are treated in a simple 

 manner and in a way that may be understood by 

 the beginner, viz.: Damping; Logarithmic 

 Decrement; Inductance. 



A. 3. No simpler description of the Logarith- 

 mic Decrement and the effects of Damping can 

 be obtained than that given in the .May, 1916, 

 issue of the Popi'i..\R Science Mo.sthlv. 



Copper-clad Antenna Wire and 

 Thickness of Copper 



P. L. D., Grants Pass, Ore., inquires: 



O. I. Is copper clad wire with a core of iron 

 wire suitable for wireless telegraph purposes? 



A. I. Yes; it has been used on commercial 

 installations for a numlx-r of years. 



Q. 2. How thick should be the copper 

 coat ing? 



A. 2. .-K coating of i-f)4th of an inch is 

 <|uite sufficient. 



