Radio mirror (Jan-Oct 1923)

RADIO DIGEST ILLUSTRATED 11 A. B. C. Lessons for Radio Beginners By Arthur G. Mohaupt CHAPTER IV IT IS generally said that Radio messages travel through space in the form of a wave motion, or that certain concerts come in on a 40i)-nieter wave length. Now as to the meaning of all these terms, wave motion, wave length, frequency, etc. Probably it can be explained and made more clear to all if we use the old familiar illustration of water waves as they exist on the surface of a pond or lake. If a stone is dropped into a pond of still water, the entire surface of the pond at one complete wave length consisting of a crest and a hollow. The horizontal line OX represents the position of rest, and the distance from this line to the highest or lowest part of a wave is known as the amplitude. Being acquainted now with the nature of a wave and a wave motion in a medium, we are ready to consider the methods by which Radio messages are carried through space. A complete Radio communication sys y"i -^---CREST* 1 AMPLl \ TUOE \ i \ 0 / HOL X LOW 1 \,i[\\/f i rt-iPTi i 1 Figure 15 once becomes disturbed. At the instant the stone strikes the surface, a circular ridge or crest at once forms and expands outward in all directions. Following the crest is a depression or trough in the water. Such a disturbance consisting of a crest and a hollow is called a wave. One important point to remember at this time is that the water does not actually move from the point at which the stone struck the surface, but the disturbance, only, moves outward until it strikes some object or dies out altogether. Water Is Medium of Transmission If the stone were tied to a string and pulled up and down so that it struck the water at regular intervals, the entire surface would soon be disturbed with a series of crests and hollows, all starting out at the point at which the stone struck and all spreading outward in all directions. Such a disturbance is called a wave motion, and the water in which the disturbance takes place is called the "medium of transmission.'' The rate at which the stone is moved up and down determines the rapidity with which the waves are sent out, or the frequency. The frequency may thus be defined as the number of waves occurring per second. The greater the frequency, the shorter will be each wave; while the less the frequency, the longer will be each wave. Now by the length of a wave, ot the wave length, we mean the distance tem consists of three essential parts, namely, a sending station, a medium through which the messages are carried, and the receiving station at which the messages are intercepted. The Radio sending station can be compared to the stone which was moved up and down so as to cause a disturbance or series of waves in the surface of the water. The Radio receiving station, on the other hand, can be compared to the cork, which floated on the water. The receiving station, like the cork, responds in all respects to the disturbance of the medium in which it is placed. What Ether Is Like From the very nature of Radio communication we know that it is accomplished by means of some form of wave motion. Now our common sense tells us that it is impossible to have a wave motion or disturbance without also having some medium in which it can take place. We know it is not the air which carries Radio messages, for they will travel through a vacuum in which no air exists. Consequently it has been assumed that there is present throughout all space and permeating all objects a medium called the ether, and that it is this ether through which Radio messages are carried. The ether permeates (passes through) all matter like water permeates a sponge. This explains why Radio messages can travel through walls of solid material which physically appear A-l A-Z Figure 16 y^$ from one point on a wave to a corresponds 'ing point on the next wave. Height of Wave Is Amplitude Furthermore, if the stone is raised higher so that it strikes the water with a harder impact, then a more severe disturbance will be caused; that is, the waves will still be of the same length, but the crests will be higher and the troughs deeper. This brings in another new term, namely, amplitude. By the amplitude of a wave or wave motion is meant the maximum distance from the position of rest to highest or lowest point on a wave. The amplitude of a wave, it must be remembered, always depends upon the force or intensity with which the object strikes the medium. If a cork is placed on the water at some point, it will be caused to move up and down in exact accordance with the nature of the wave or disturbance that is passing. The cork will not move toward the shore, but will always remain at the same spot and merely move up and down. This shows that the water does not move toward the shore, but that the wave disturbance only moves through it. The cork, however, will in all respects, respond to the wave motion in the water on which it floats. Thus in Figure 15 we have illustrated impenetrable. Although no one has as yet in any way come into physical contact with the ether, still for want of a better explanation we will assume that this ether exists and that it is the medium through which Radio messages travel. Extent of Ether Various properties or qualities have been assigned to the ether in order to enable us to use it for explaining the various existing conditions. The ether extends throughout all space, even beyond the farthest stars. The ether exists around and through all objects much like water fills a sponge. It cannot be removed from any portion of space, nor can it be pumped out of a container. The ether also offers no opposition to objects moving through it, for the action would be similar to moving a sieve through the air. It is also of an extremely elastic nature, for a disturbance set up at any point in it immediately spreads outward in all directions. Waves Emanate in All Directions The Radio sending or transmitting station consists of some form of electromagnetic machinery by means of which a disturbance or wave motion is set up in this medium which we called the ether. These waves emanate or are sent out from the transmitting station by means of a group of parallel stretched wires known as the transmitting antenna or aerial. These waves represent a certain amount of electrical energy sent out "broadcast" through space. In order that electrical energy can be sent out, through space in the form of a wave motion as was just explained, it is necessary that they be of very high frequency. Consequently, the apparatus In the transmitting station consists of a combination of oscillating circuits by means of which high frequency electric currents are generated, and these high frequency oscillating currents then excite the antenna and cause the electromagnetic waves to be radiated through all space. Travel 186,000 Miles Per Second The Radio waves leave the antenna and spread out in the form of expanding spheres all of which have the antenna as a center. They travel through space at the enormous speed of 186,000 miles (300,000,000 meters) per second. In other words, a Radio message would travel approximately eight times around the earth in one second if there were enough energy behind it to keep it going. In Radio measurements the metric system is very extensively used. In the metric system the unit of length is the meter, which is equivalent to about 39.37 inches. Consequently a speed of 186,000 miles per second, if expressed in the metric, system, would be 300,000,000 meters per second. A meter originally was chosen as one-ten millionth part of the distance between the equator and either pole of the earth. Waves in Commercial Practice The waves used in Radio communication range in length from 75 meters (about 244 feet) to 25,000 meters (about 15% miles). Since Radio waves travel through space at the enormous speed of 300,000,000 meters p^r second, the above wave lengths represent frequencies ranging from 4,000,000 to 12,000 per second, respectively. The frequency can always be calculated by dividing the speed of propagation (300,000,000 meters per second) by the wave length. If it is desired to calculate the wave length when the frequency is known, it is only necessary to divide the speed by the frequency. According to Government regulations, all amateur transmitting stations must operate at wave lengths not exceeding 200 meters. This corresponds to a frequency of about 1,500,000 cycles per second. The shortest wave lengths used in commercial practice are 300 meters, which are the wave lengths used for communicating between ships at sea. Practically all broadcasting in this country at the present timo is being done at wave lengths ranging from 360 to 485 meters, the correspond ing frequencies being 833,000 and 618,000 oscillations per second. For reliable long distance communication such as transoceanic service, longer wave lengths are in general use, ranging from 10,000 to 25,000 meters. These longer wave lengths are more advantageous to use because they are less affected by atmospheric conditions and changes, and hence are more dependable for long distance transmission. Another important item is that when large quantities of power must be handled such as is necessary for long distance transmission, the sending apparatus is less costly with the longer wave lengths than with the shorter wave lengths. Heat and Light Cut Bange Radio waves in their passage through space are subject to some very peculiar conditions, most of which are as yet not satisfactorily explained. For example. Radio messages can be transmitted over longer distance and with greater ease during the night than during the daytime. (Continued on page 12) B' BATTERIES .A \ J NOTE THE_INSULATED_BXND1NG POSTS AND 7m. WIRE" CONNECTOR OVO TVTF'G co. W. 33 r* ST. E'W YORK To Yearly Subscribers Only Limited Number of BOUND VOLUME TWO RADIO DIGEST Volume Two 13 Numbers 2.00 July 15 — October 7, 1922 Now Ready for Delivery. Parcel Post Prepaid, Only On account of the limited supply, the sale of these bound volumes No. 2 is limited strictly to those readers of the Radio Digest who have subscribed for a year. TO NEW SUBSCRIBERS One year subscription to Radio Digest and fi* ^7 <T^^\ both bound volumes 1 and 2 *P » %\J\Jr By subscribing now for one yew to the Radio Digest you get both BOUND VOLUMES 1 AND 2, 26 numbers, with one year's subscription for $7.00. or either volume 1 or 2, FREE with a year's subscription to Radio Digest . . ALL ORDERS MUST BE SENT DIRECT— No orders on this offer will be accepted, through subscription agencies, news dealers or other sources. Check, money or express orders (no stamps) must accompany all orders. $5.00 Supply Limited Better Hurry TEAR HERE— MAIL COUPON PUBLISHER, RADIO DIGEST ILLUSTRATED. 123 West Madison St., Chicago, Illinois. I have already subscribed to the Radio Digest for one year, so jilease reserve me one bound volume Number Two for which I am enclosing check — M. 0. for Two Dollars. Please reserve me Bound Volume Number One and also Number Two and one year's subscrijrtion to the Radio Digest. Illustrated, for which I am enclosing check — M. O. for Seven Dollars. Name Address City