Radio Broadcast (May 1928-Apr 1929)

178 RADIO BROADCAST ADVERTISER Ionize H t?l 1 U 1X5. THE SECRET OF THE STANDARD REPLACEMENT TUBEA B ELIMINATORS The use of Ionized Helium, gives to the Raytheon "BH" Tube a superior ruggedness, a far longer life and a sustained voltage. Be sure you get a Raytheon BH for your eliminator when the tube needs replacing which is generally every nine to twelve months. Big, New Summer Radio Catalog Dealers — Set Builders — Agents Write for this Catalog. New Summer and Fall Book. Save money on newest hookups, sets, kits, parts, accessories at lowest prices. The "Big, Friendly Radio House" WESTERN RADIO MFG. COMPANY 126 W. Lake St. Dept. 47 Chicago, 111. Radio Convenience Outlets Wire your home for radio. These outlets fit any standard switch box. Full instructions with each outlet. No. 135— For Loud Speaker $1.00 No. 137 — For Battery Connections 2.50 No. 136 — For Aerial and Ground 1.00 With Bakelite Plates Now furnished with a rich satin brown Bakelite plate, with beautiful markings to harmonize, at 25 cents extra. See Illustration. At Your Dealers Yaxley Mfg. Company Dept. B, 9 So. Clinton St. Chicago, 111. No. 206 Radio Broadcast Laboratory Information Sheet July, 1928 A Screen-Grid Resistance-Coupled Amplifier ITS FREQUENCY CHARACTERISTIC ' T'HE frequency characteristic of a resistance coupled amplifier using screen-grid tubes is included on this sheet and indicates clearly the excellent quality which such an amplifier is capable of delivering. The screen-grid amplifier used in making this curve was described in the June, 1928, Laboratory Information Sheets Nos. 195 and 196. The frequency characteristic which is obtained from an amplifier of this type depends upon several factors. One of the most important is the voltages at which the screen-grid tubes are operated. The power unit supplying the amplifier should be capable of delivering 135 to 180 volts and the screen-grid voltage should generally be 22i although 45 volts is satisfactory if the 180 volts is used. This curve was made using 0.25-megohm plate resistors, 2.0megohm grid resistors and 0.01-mfd. coupling condensers. The high-frequency response of the amplifier would be poorer with higher values of coupling resistance because under such conditions the input 2500 2000 S 1500 LU O h 1000 500 100 and output capacities of the tubes, forming a shunt around the resistors, would produce a decrease in high-frequency response. The high-frequency response in this screen-grid amplifier is much better than is ordinarily obtained from a resistancecoupled amplifier using type 240 tubes. There is this to say, however, that the high-frequency response of this amplifier as indicated by the curve on this sheet is really better than it need be, for frequencies above 6000 or 7000 cycles do not contribute very much to the naturalness of the reproduction. The low frequency response of the amplifier is determined by the size of the coupling condenser; the smaller the capacity the poorer the low-frequency response. The value of 0.01 mfd. which was used is evidently satisfactory judging from the curve, and probably values considerably smaller than 0.01 mfd. would also be satisfactory. The high gain of this amplifier has some disadvantages which were pointed out in Laboratory Sheet No. 195 to which we refer the reader for further information. 1000 FREQUENCY 10,000 Radio Broadcast Laboratory Information Sheet Equalizing Wire Lines forBroadcasting July, 1928 VALUES INVOLVED TT IS obvious that the fidelity of reproduction obtained from a radio receiver cannot be any better than that transmitted by the broadcasting station and in discussing the subject it is therefore of value to know what frequencies are at present being transmitted by the better broadcasting stations. Some data on this subject was published in the article by C. E. Dean in the June, 1928, Radio Broadcast. What frequencies are transmitted by broadcasting stations depends, among other things, upon the audio-frequency characteristics of the apparatus — microphones, amplifiers, modulators — used at the broadcasting station and upon the characteristics of the wire lines used to connect the broadcasting studio with the transmitter. Many of the better transmitting stations are now located outside of cities and therefore must use a wire connection between the transmitter and the studios located within the city. At the present time the characteristic of the wire lines is very important in determining what audio frequencies will finally be impressed upon the carrier wave. The wire lines used with the broadcasting stations are at present equalized, that is made to transmit equally well, frequencies from 100 cycles to 5000. The characteristic of these lines below 100 cycles is probably quite,good, but in no case can it be certain that a station is actually transmitting any frequencies, at their proper amplitude, below about 100 or above about 5000. It is certain that as better loud speakers become available capable of reproducing frequencies below and above the limits given above that higher and lower frequencies will be included in the transmissions of broadcasting stations. In fact we may expect that the characteristics of the wire lines will be improved even before such loud speakers are generally available. The Telephone Company controlling the wire lines has always followed a policy of being prepared to furnish lines better than are actually essential at the time, considering the quality of the remainder of the apparatus included in the link between the microphone in the studio and the loud speaker at the listener's home. A wire line ordinarily tends to transmit the lower frequencies much better than the higher frequencies and it is therefore the function of the equalizer to lower the high-frequency response so that a flat characteristic is obtained over the entire band. Radio Broadcast Laboratory Information Sheet Power Values in Radio Receiving Antennas July, 1928 RELATION OF DISTANCE AND MEASURED RECEIVER VOLTAGE IT IS interesting to compare the amount of power ordinarily intercepted by a radio receiving antenna with the power which is required to operate an ordinary 60-watt incandescent lamp, for example, In Professor Morecroft's book, The Principles oj Radio Communication, some figures are given for the amount of current in a receiving antenna which had a resistance of about 60 ohms. In the figures which he gives for received antenna current, we find that when the receiver was located about a mile from the particular transmitter which was used (the power rating of the transmitter is not given) that the current in the receiving antenna was approximately 70 microamperes. If we square this current and multiply it by the resistance of the receiving antenna which is 60 ohms, we obtain the power in the receiving antenna, which proves to be approximately 3 X 10— 'watts. For those who do not realize what this exponent signifies, the power specified in the orinary way is 0.000000003 watts The power required to operate an ordinary electric light bulb is 60 watts. Therefore the power required by the electric light would be sufficient to supply antenna power to operate approximately twenty billion radio receivers each requiring 70 microamperes of current in the receiving antenna as specified above. The figures given at the end of this Laboratory Sheet, which have been taken from Morecrof t, also indicate that the amount of power in the receiving antenna varies approximately inversely with the distance between the transmitter and the receiver. At a distance of 100 feet the received current is twice as great as when the separation is 200 feet. The power is proportional to the square of the current and therefore a ratio of two in current means a ratio of four in power. Twice the distance therefore gave one fourth the power. DISTANCE IN FEET BETWEEN ANTENNAS 100 200 300 400 1260 2420 3700 4600 6220 CURRENT IN RECEIVING ANTENNAS (MICROAMPERES) 12320 6435 4548 3108 715 283.5 105 96.5 69.5