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May 12, 19*3
RADIO DIGEST
First Steps for Beginners in Radio
Chapter II — Radio Waves and Their Propagation
By Thomas W. Benson, A. M. I. R. E.
THE USUAL, practise of using waves on the surface of water as an analogy in describing the action of Radio waves has nothing to recommend it but its simplicity. As a matter of fact it may lead to erroneous belief for several reasons. Radio waves travel through the mass of ether, if we may call it a mass, and not on the surface as the water analogy would indicate.
Substance In Which Radio Waves Travel In order that the Radio waves may travel we must assume that a certain medium exists in which they may move. Since they travel freely through all known matter that is not a conductor in the usual sense of the word and pass through a vacuum where physical matter is absent we must then assume the existence of some subtle substance permeating all matter and extending to the uttermost end of space. This substance we term ether and its existence is generally accepted by science and used to explain many and varied phenomena in the fields of light and electricity.
Ether possesses great elasticity, has no
BEGINNERS will find the accompanying series by Mr. Benson very helpful in learning the rudiments of the popular science of Radiophony. The first chapter of his series appeared in the May 5 issue. The articles yet to appear are:
Chapter III— Pointers about Aerials and Grounds.
Chapter IV — About Condensers and Inductances.
Chapter V — Tuners and How to Tune Your Set.
Chapter VI— About Crystal Detectors.
Chapter VII— Tube Detector Theory and Operation.
Chapter VIII— The Regenerative Detector.
Chapter IX — Radio Frequency Amplification.
Chapter X — Audio Frequency Amplification.
Chapter XI — How Super Regeneration Is Accomplished.
Chapter XII — Reflex Circuit Operation.
Chapter XIII — About Headsets and Loud Speakers.
Chapter XIV — Batteries Used in Radiophony.
constantly widening circle. It should be remembered that it is only the change in the lines of force that creates the wave. The waves increase in height as they
Figure 3 — How the waves are snapped off from the transmitting aerial
inertia and exists in and around all matter. This is the medium through which the electromagnetic and electrostatic waves known as Radio waves travel.
When the ether is put under electrical stress at any point and the stress released a wave motion is created in the ether that travels in all directions in an expanding circle. This action takes place in the manner shown in Figure 3.
Waves Thrown Out Prom. Aerial
Consider the aerial as shown, coupled inductively to a primary circuit excited
STRATA OF RAREFIED AIR
Figure 4 — The waves are reflected by upper conducting strata of atmosphere
with a high frequency current. The aerial is then charged alternately positively and negatively. At the instant the aerial is charged positive the condition shown in the illustration exists. There is an electrostatic field extending from the elevated wires to the ground. When the polarity changes the ends of the lines of force run down the aerial and a wave is snapped off that travels in all directions away from the aerial in a
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travel, following the surface of the earth. These waves travel at the speed of light, that is 185,000 miles per second and result in the ether in all directions, being put under electrical stress in a vertical direction at equal distances. This may be clearer by observing the arrows marked on the waves. At A the stress is down, at B up, C down and so on. We can represent this stress by a curve as shown below where F and G may represent positive halves and H negative halves of the waves. When undamped waves are emitted the intensity of the electrical strain in each succeeding waves is the same, hence the amplitude is constant.
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209 South State Street, Chicago
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The Wave length
The wave length of the wave is that distance between points where the strain is in the same direction. The wave length is then equal to the distance between humps in the wave as shown.
Bearing this in mind it is evident that the closer the waves are the shorter the wave length. Since the speed of the wave is constant they follow each other closer as the frequency of the exciting current increases. This should clearly demonstrate the relation between frequency and wave length. For instance, where a station is transmitting on a 400 meter wave length, and knowing that the waves travel with the spegd of light being 300,000,000 meters per second, we can readily realize at the end of one second the first wave would have traveled that distance and the waves between would be spaced 400 meters apart. Since one wave is emitted per cycle the frequency of the exciting circuit is equal to the total distance 300,000,000 meters divided by 400 meters or 750,000 cycles per second. Conversely, the wave length can be determined by dividing the frequency in cycles per second into the distance traveled by a wave in one second.
Magnetic Field of the Wave
In addition to the electrostatic strain or rather because of it there is a magnetic field in each wave parallel to the surface
of the earth. This magnetic field is equal to and identical with the electrostatic field and one cannot be emitted without the other.
As the waves travel over the surface of the earth there are several conditions acting to hinder and others to assist in their propagation. The ideal medium for their transference would be a perfect insulator for there would then be no absorption. However, they must pass through the air, and as this substance is not always a perfect insulator, due to the presence of moisture and dust which act to absorb some of the energy and dissipate it in heat, it weakens the wave. On the other hand the propagation is assisted by having a good conductor as a guide, which is naturally the earth.
Where the conductivity of the earth
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CURRENTS
CURRENT CHANGE AFFECTING 0ETECT1N6
Figure 5 — Current changes affecting the detecting Device
is low the waves are also hindered and energy absorbed in overcoming this resistance. This accounts for the fact that the range of a given set is much greater over water than land. Furthermore the upper strata of the atmosphere are much rarified and hence more conducting than the denser air near the earth's surface. (Continued on page 16)
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