154 



Popular Science Monthly 



found that the corresponding wavelength 

 is three hundred meters. The following 

 table of frequencies and wavelengths 

 will be helpful. 



As the frequency of the sending current varies, 

 the wavelength increases or decreases with it 



From an examination of this table it 

 becomes very clear that as the period is 

 increased, the frequency decreases and the 

 wavelength increases. Remembering that 

 the longer the antenna wire, the more time 

 it takes for the charge to pass from the top 

 to the bottom and back again, or the longer 

 the period, it is easy to see that the longer 

 the aerial wire (including the coils connected 

 in series with it) , the greater the wavelength 

 will be. As a matter of fact, the fundament- 

 al wavelength of a simple aerial, which is 

 its wavelength without any coils in series, 

 is about 4.2 times its actual length measured 

 from ground to top end. To use this rule, 

 both height and wavelength must be 

 measured in the same unit. A table 

 showing the fundamental wavelengths of 

 several heights of plain vertical antenna 

 wires is given in the next column. 



The table is strictly applicable only to 

 plain vertical antennas without any loading 

 coils, but it may be used for the approxi- 

 mate fundamental wavelengths of L-shaped 

 antennas if the total length of a single wire 

 is used instead of that of the vertical lead 

 alone. If the antenna is T-shaped, the 

 length from the ground to the center of the 

 flat-top and from there to one end should 

 be used. The two parts of the flat-top 

 should have the same length, as measured 

 from their junction with the vertical lead. 

 Where several wires are used in parallel, 

 whether in a horizontal or vertical antenna, 

 the length is taken as that of one of the 

 wires, and not of the total amount of wire 

 in the aerial system. Neither Table 

 No. II, nor the simple rule must be used 

 when loading coils are connected; for the 



wire on such coils is much more effective 

 in increasing the apparent length of the 

 antenna than is the straight-away portion. 



Tuning 



The bearing of the foregoing discussion 

 upon the adjustment of the receiver's 

 tuning to get the greatest distance becomes 

 clear on considering that, for this to be 

 obtained, the receiving set must be adjusted 

 ''in tune'' with the wavelength it is desired 

 to receive. In the simple wireless set 

 described last September, some small degree 

 of tuning was secured by making the 

 antennas and the tuning-coils alike at the 

 sending and receiving station. This receiver, 

 as shown in Fig. 3, has the detector right 

 in series between the aerial and ground 

 connection. The result of this arrangement 

 as regards tuning is that the high resistance 

 somewhat spoils the sharpness of adjust- 

 ment. If one makes material changes in 

 the length of the aerial wire, or in the 

 number of turns of coil used, a weakening 

 of signals is noticed. The tuning is neither 

 critical nor "sharp," however, and even 

 approximate adjustments will give about 

 as good results as exact ones. 



When the receiver described in the 

 December article is used, as shown in Fig. 4. 

 the adjustment is much more accurate. 

 Here the detector is removed from the 

 antenna circuit, and the aerial is connected 

 directly to ground through a loading-coil. 

 As a result the effect of the coil is con- 

 siderably increased. It was pointed out 

 that the coils and antennas at sender and 

 receiver should be made exactly alike, if 

 possible, but that if there should be any 



The fundamental wavelength of a plain 

 vertical antenna wire increases with its length 



difference in the aerials the shorter one 

 should have more turns of coil connectec 

 in series with it. This was, of course, tc 

 make the effective lengths of the two] 

 antennas the same, so that they would bej 

 tuned to the same wavelength. 



