Popular Science Monthly 



79S 



600, 1,000, 1,500, etc., meters. Make 

 final adjustments, using very loose coup- 

 ling. If a series primary-condenser is in 

 the circuit, leave it at the same setting for 

 all stations. Make a note of the number 

 of primary turns used for the best adjust- 

 ment for each of the stations. 



On a sheet of cross-section paper, scale 

 off the bottom in wavelengths the full 

 range of the loose coupler, beginning at the 

 lower left-hand corner. Scale off the pri- 

 mary turns at the left-hand side and from 

 the lower left-hand corner. Plot the points 

 at the intersection of the turns and the 

 wavelength, and draw through them a fair 

 curve, such as shown in the diagram. 



200 



160 



eo 



ZDO 400 600800 K)OOI20OMX)l60O lt00ai0022OO3(OO260O2S0O 

 WAVt UN6TH 

 Plot the points at the intersection of the 

 tiims and the wavelength, and draw the curve 



If the f)oints do not line up, see if the 

 plotting has been done correctly; if so, the 

 assumed wavelength is in error. 



Do not try to include the loading coil on 

 the same curve, but make a separate one. 



Only the primary is mentioned, because 

 the majority of amateurs use "untuned" 

 secondary systems. But if this is not the 

 case the secondary may be calibrated the 

 same way, if a fixed loose coupling is used. 

 The secondary shunt-condenser value must 

 remain constant. A set so calibrated may 

 be instantly adjusted for any station, even 

 though it has not been heard before, pro- 

 vided, of course, that its wave is known. 



The curve shown was drawn from results 

 obtained according to the foregoing explan- 

 ation, with a large loose coupler. Note 

 that the line is straight at the shorter wave- 

 length increases. If the loose coupler had 

 been much larger, the curve would have 

 reached a place where it would be nearly a 



vertical line. In other words, a place 

 would have been reached where a very 

 large amount of inductance would have to 

 be added to make a small increase in wave- 

 length. With a larger antenna, this place 

 would have been much farther out on the 

 wavelength scale. 



Making Small Slots in the Frame of 

 Pancake Helices 



IN the construction of the pancake type 

 of helices, the amateur is often puzzled 

 over the method of sawing the slots in the 

 frame of the helix in such a manner as to 

 insure a snug fit of the brass or copper rib- 

 bon used. In the construction of the 

 primary of such helices, where the ribbon is 

 usually about 11/16 in. thick, probably the 

 best method of sawing the slots for the 

 ribbon is to use an ordinary hack saw. 

 The blade generally runs pretty close to the 

 size of the ribbon, and this gives a snug fit, 

 without the necessity of "shims" — or small 

 pieces of copper, brass, or wood fitted in the 

 slots, which often result in a "messy" job. 



In the secondary, however, the ribbon 

 sometimes runs as small as gage 64. 

 Saw blades of this size are very difficult to 

 obtain, so the best way is to make your own 

 blade. This can be easily done by securing 

 a piece of an old clock-spring, which as a 

 rule is very thin. Cut off a piece as long as 

 your hack-saw blade, putting the two ends 

 in a flame for a few seconds to draw the 

 temper. A hole may then be bored in each 

 end to fit the hack-saw frame. Draw the 

 spring taut, taking care to hold it away 

 from the face, as it will sometimes snap. 

 Then press the "blade" against an emery- 

 wheel till a series of "teeth" are notched in 

 it. This does not make a very artistic 

 looking job, but it will be found that it cuts 

 into the frame — whether it be hard rubber 

 or any of the finer woods — rapidly and 

 with a clean stroke. Care should be taken to 

 keep the blade cutting straight, as it will 

 have a tendency to curve. If the right kind 

 of clock-spring is used, a snug fit for the 

 ribbon is assured. 



The pancake type of helix is probably the 

 easiest for the amateur to make up, pro- 

 vided he can secure the necessary ribbon. 

 Although fiber or hard rubber is best for 

 the frame, mahogany is very good, and is 

 easily handled. On the primary, it is 

 customary to space the ribbon ^ in. be- 

 tween turns, and on the secondary, where 

 finer ribbon is used, J^ in. — Paul Oard. 



