Wireless Telegraphy and Telephony/Chapter 4

by Walter W. Massie and Charles R. Underhill

 

Chapter IV

Method of Operation

16. Switching Device

While, as before stated, in some cases separate antennæ are used for sending and receiving, it is common practice to use the same antenna for both. This is accomplished by means of a switching device so arranged that when the transmitter is connected in, the receiver is cut out. This is absolutely necessary, as otherwise the delicate receiving apparatus would be destroyed should the high-power currents of the transmitter pass through it.

17. Sending the Message

In Fig. 18 are shown several types of keys used in wireless telegraphy. The operator sends the message by pressing the key lever downward, or allowing it to remain up for certain periods of time. Pressing the key lever downward for a brief interval represents a dot, and a longer period of depression, a dash. The proper arrangement of dots and dashes forms letters, and combinations of letters of course form words, etc. the letters being spaced by holding the key lever up for a given period of time, and the words being spaced by still longer periods.

The proper arrangement of the dots, dashes, and spaces constitutes a code. The Morse code is extensively used in America, while a modification of it, called the Continental code, is employed in England and on the

Fig. 17.—Types of Keys.

continent of Europe. These are shown in Fig. 19. When the key lever is depressed, two insulated pieces of platinum, called contacts, touch one another, closing the low-pressure circuit of the induction coil or transformer.

When an operator desires to communicate with another station, he first “listens in” by connecting his receiving system with the antenna and the ground, and placing the telephone receiver to his cars. He then adjusts his receiving circuits for various wave lengths and, if he hears no signals, he assumes that no one else is sending within his radius. He therefore “throws in” the transmitting apparatus, which action automatically disconnects the receiving side. He then sends the letters which constitute the “call” of the station desired, signing the letters designating his own station, after sending the call several times. He then listens in again, and if the operator at the desired station has heard his call, the latter answers, and regular telegraphic communication ensues.

Some wireless systems are so arranged that no switching is necessary. The operators can “break” one another, by keeping the telephone receivers over their ears all the time. If the receiving operator should wish to correct the sender, he does so by pressing his key; this is heard by the sending operator when his key is in normal position.

Tuning not only increases the radius of operation, but maintains secrecy as well. By means of careful tuning, two distant stations may be sending simultaneously, and if they employ different wave lengths, the operator at the receiving station may, by adjusting his apparatus to the wave length of the station with which he desires to communicate, “tune out” the other message, receiving only the one desired.

However, if the stations above referred to be quite

Letters	Morse	Continental
A	— ———	— ———
B	——— — — —	——— — — —
c	— ——	——— — ——— —
D	——— — —	——— — —
E	—	—
F	— ——— —	— — ——— —
G	——— ——— —	——— ——— —
H	— — — —	— — — —
I	— —	— —
J	——— — ——— —	— ——— ——— ———
K	——— — ———	——— — ———
L	——————	— ——— — —
M	——— ———	——— ———
N	——— —	——— —
O	— —	——— ——— ———
P	— — — —	— ——— ——— —
Q	— — ——— —	——— ——— — ———
R	—— —	— ——— —
S	— — —	— — —
T	———	———
U	— — ———	— — ———
V	— — — ———	— — — ———
W	— ——— ———	— ——— ———
X	— ——— — —	——— — — ———
Y	— —— —	——— — ——— ———
Z	— — ——	——— ——— — —

Numerals
1	— ——— ——— —	— ——— ——— ———
2	— — ——— — —	— — ——— ——— ———
3	— — — ——— —	— — — ——— ———
4	— — — — ——— —	— — — — ———
5	——— ——— ———	— — — — —
6	— — — — — —	——— — — — —
7	——— ——— — —	——— ——— — — —
8	——— — — — —	——— ——— ——— —
9	——— — — ———	——— ——— ——— ——— —
0	——————	——— ——— ——— ——— ———

Punctuations, etc.
. Period	— — ——— ——— — —	— —— —— —
, Comma	— ——— — ———	— ——— — ——— — ———
? Interrogation	——— — — ——— —	— — ——— ——— — —
! Exclamation	——— ——— ——— —	——— ——— — — ——— ———

Fig. 19—Wireless Telegraph Codes.

near one another, as in the case of two or more boats passing a shore station, the operator may not be able to tune out the undesired party, owing to their proximity, and the apparatus is then said to be operated by “forced oscillations.” Nevertheless an operator may receive the desired message by concentrating his mind upon the sound of the “spark” of the desired transmitting station, but this may only be done when the sounds of the “sparks” are dissimilar, or one is louder than another, owing to different strengths, or to varying distances.