Page:Popular Science Monthly Volume 90.djvu/170

 154

��Popular Science Monthly

��found that the corresponding wavelength is three hundred meters. The following table of frequencies and wavelengths will be helpful.

��TABLE I.

�Period in Seconds

�Frequency Per Second

�Wavelengths in Meters

�0.00000033 0.00000066

�3 000 000 I 500 000

�100 200

�O.OOOOOI

�I 000 000

�300

�0.000002

�500 000

�600

�0.0000033 0.0000066

�300 000 150 000

�1000 2000

�0.0000 1

�100 000

�3000

�0.00002

�50 000

�6000

�0.000033

�30 000

�1 0000

��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

��TABLE IL

�Height (length)

�Fundamental Wavelength |

�in Feet

�(in Feet)

�(in Meters)

�35

�147

�45

�50

�210

�64

�75

�315

�96

�100

�420

�128

�125

�525

�160

�150

�630

�192

��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.

�� �