Radio broadcast .. (1922-30)

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204 RADIO BROADCAST ADVERTISER Right here/ You can't go wrong when you use Polymet High Voltage Condensers in sets or power units. Built to withstand t,ooo volts permanently, and individually tested for this rating, their obvious superiority, both in workmanship and performance, definitely establishes Polymet's leadership as condenser manufacturers. Polymet condensers incorporate finest insulating paper, best foil and specially prepared impregnating compounds. An exclusive, new and improved process renders them non-inductive, with high dielectric re- sistance for long life. Obtainable as individual units or in blocks; in cans or unmounted; with fixed or flexible leads. Capacities .1 to 5. Mfd. . 60c to $4.50 Raytheon Circuit Blocks Tested by the Raytheon Laboratories, they have passed with highest honors and been given an envi- able raring. F1001 .l-C-.l Mfd. F1000 14 Mfd. . . $2.00 9.50 Polymet Products are used by over 125 high grade receiver and power unit manufacturers. There's a reason—Polymet Products have passed their exhaus- tive tests! Follow the manufacturers—specify Polymet Products—at all good dealers everywhere. I Illustrated descriptions sent FREE on request. Polymet Manufacturing Corporation 599 B Broadway - New York City World's Largest Manufacturers of Radio Essentials'* POLYMET The Relation! Between Wavelength and Frequency By CHARLES F. FELSTEAD r || "<HE relation between wavelength and fre- I quency is of a fixed and definite value, and •^ is such a very simple matter when it is once understood that it should offer no confusion to the beginner. A radio wave is an electric wave by means of which radio communication is car- ried on. Radio, light, heat, waves, and X-rays, are all forms of electric waves. The only differ- ence in these waves is their frequency; they all travel at the same speed. The to-and-fro mo- tion of the alternating electric current set up in the antenna and counterpoise (the latter often replaces the ground in a transmitting station) by the transmittingset, produces an alternating elec- tric pressure in the space between this antenna and counterpoise system. This causes a to- and-fro wave motion in the ether that travels out from the radio transmitting station in all directions with the speed of light, just as ripples run out in all directions when a stone is dropped in a pool of water. These electro-magnetic waves travel at the rate of 186,300 miles per second, or 300,000,000 meters per second (to be more exact, they travel at 299,820,000 meters per second). When the electro-magnetic waves sent out by a transmitter reach a receiving an- tenna, they produce a slight to-and-fro electric current in that antenna, which corresponds in frequency to the frequency of the transmitted electro-magnetic waves. This occurs only when the receiver is tuned to the same frequency as the transmitter. The slight current thiis set up in the receiving set is rectified by the detector and changed to a pulsating direct current, which actuates the diaphragms of the head phones, or is amplified by successive audio amplifier stages until it has enough strength to operate a loud speaker. "Wavelength," as the word itself explains, is the length of the waves sent out by the trans- mitting station, while "frequency" refers to the number of waves generated per second, or the number of to-and-fro electric oscillations per second. The wavelength, or length of a single wave, is the distance between two similar points on two successive electric waves, as shown in Fig. i, and this resembles a cross-section of the ripples caused by the stone in the pool of water. One wavelength is the distance from the crest of one wave to the crest cf the next adjacent wave We can speak of either the number of waves per second or the number of cycles per second sent out by a transmitting station. The wavelength can be measured in feet, meters, or any other linear unit of measure, though it has been cus- tomary to express wavelength in meters. Four hundred meters are equal to approximately 1300 feet; one meter equals about 3.25 feet. The metric system of linear measure (centimeters, meters, and kilometers) has the advantage that it is more easily adaptable to scientific measure- ments. With.the advent of congestion in the ether channels allotted for the use of broadcasting stations, it became necessary to decide upon a uniform figure by which stations should be sep- arated. For various reasons a ten-kilocycle separation was chosen, and nowadays it is be- coming customary to refer to a station by its requency instead of its wavelength. All that is necessary to convert meters to kilocycles (1000 cycles) is to divide the number of meters into 300,000. The reverse is also true; :o convert kilocycles to meters, 300,000 is di- Examined and approved by RADIO BROADCAST * vided by the number of kilocycles. The result is the corresponding wavelength in meters. The explanation of this relation between the wavelength in meters and the number of cycles per second follows. As previously mentioned, the velocity of electro-magnetic (radio) waves has been found by experiment to be the same as the velocity of light, which is approximately 300,000,000 meters per second. If a radio transmitter is adjusted—or tuned, as it is called —so that it sends out 100,000 waves per second, that is, 100,000 cycles per second, and since we know that each wave travels 300,000,000 meters in one second, then we know that in one second 100,000 waves have left the transmitting station, and that the first wave is 300,000,000 meters away from the station. Thus, the 100,000 waves are equally spaced over a distance of 300,000,000 meters. By dividing the 300,000,000 by the 100,000, we find that each wave is 3000 meters long. In other words, the frequency which corresponds to 3000 meters wavelength is i*0ne Wavelength^ «One Wavelength » or one Cycle >» One Wavelength^ FIG. I 100,000 cycles, or 100 kilocycles. The shorter the wavelength, the greater the number of waves that will pass a given point in one second; that is to say, the shorter the wavelength, the higher the frequency. The above is usually put into formula by let- ting V represent the velocity of electric waves, or 300,000,000 meters per second; N the fre- quency of oscillations (cycles); and the symbol X (the Greek letter "lambda") the wavelength in meters. The formula is now V = N X. Sub- stituting and dividing we get: . _ 300,000.000, ,. _ 300,000,000 N —x— A table of wavelengths, with the correspond- ing frequency, is given below. It will be seen from an inspection of this table that as the fre- quency becomes greater, the wavelength be- comes shorter, and vice versa. WAVELENGTH IN METERS 0.5 I 4 5 10 50 IOO 150 2OO 3OO 40O 50O 600 IOOO 2OOO 5OOO IO.OOO 2O,OOO FREQUENCY N KILOCYCLES 6oO,OOO 30O,OOO 75,ooo 60,000 30,000 6000 3000 2OOO 1500 IOOO 750 600 500 300 150 60 30 15