Radio Broadcast (May 1928-Apr 1929)

220 RADIO BROADCAST AUGUST, 1928 THE R. C. A. 3OO-WATT AIRPLANE SET volt circuit knows what a terrible shock can be sustained on such potentials. Broadcast technicians, used to handling potentials of the order of thousands of volts, become contemptuous of lower voltages. They are making a mistake. As one gets older in this business one becomes more and more reluctant to fool with electricity on any voltage at all, when power is known to be behind the terminals. As a young man, just out of college, I used to test for a live circuit on 1 10 and 220 volts without hesitation, using my fingers. On one occasion 1 actually touched my thumb and forefinger to a 440-volt line feeding a telegraph quadruplex to see what it would feel like. I did it in such a way that the movement would disengage me, and there were spectators present, but it was a pretty idiotic stunt. Nowadays, I won't touch the metal of a 110-volt circuit if I can help it. There are plenty of test-lamps to be had for twenty cents. And, incidentally, these warnings hold for receiving set rectifiers as much as for broadcast transmitter apparatus. One of these days some enterprising broadcast experimenter is going to be burned up on the 500 volts from his power rectifier, when he forgets to turn off the electricity before putting his hands into the case. A few electrocutions are imminent. All I can do is to point out the fact and wait for the newspaper reports. Commercial Radio Publications Radio Corporation of America: "Aircraft Radio Equipment" THIS is a beautifully printed pamphlet of specifications and illustrations of three types of airplane radio stations marketed by R.C.A. It is issued by the Sales Department of this company, at 233 Broadway, New York City, which will supply quotations on request. The three types of transmitters and receivers available for planes of different sizes are described as follows: ET-3652 — Weight: 865 pounds. A light weight transmitting and receiving equipment for installation on the smaller types of "single seater" airplanes. Rated at 10 watts, it will, under favorable conditions, transmit via radiotelephony a distance of approximately 25 miles, or by radiotelegraphy (c.w.) a distance of approximately 75 miles. ET-3653 — Weight: 132! pounds. A medium weight transmitting and receiving equipment for installation on practically any type of airplane. Rated at 100 watts, it will, under favorable conditions, transmit via radiotelephony a distance of approximately 75 miles, or by radiotelegraphy (c.w.) a distance of approximately 300 miles. ET-3654 — Weight: 202 pounds. This transmitting and receiving equipment is designed for use on large aircraft where long distance communication to ground stations and other aircraft is desired. Rated at 300 watts, it will, under favorable conditions, transmit via radiotelephony a distance of approximately 200 miles, or by radiotelegraphy (c.w.) a distance of approximately 500 miles. The above ranges are stated by the manufactures to be approximate, yet conservative, so that at times a considerable increase in range may be secured. The following major units are included in the 10-watt Model ET-3652 equipment: 1 transmitter, including 3 ux-210 radiotrons; 1 control unit; 1 terminal box; 1 receiver; 1 filter unit; 1 antenna with wire (300 ft.); 2 antenna weights; 1 fairlead (an insulating tube in which the antenna lead is carried out through the fuselage of the plane); 1 aircraft anti-noise microphone; 1 winddriven double-current generator; 1 Deslauriers air propeller; 1 antenna ammeter; 1 flame-proof key; 1 helmet and phones, cord and plug; 1 set inter-connecting cable. The rating of these airplane transmitters is based on power in the antenna, so that the Model ET-3652 will deliver 10 watts of r. f. power to an antenna of the trailing type, 80-130 feet long. The frequency range available is 22502750 kilocycles (133-109 meters). Three ux-210 tubes are used, one as a master oscillator, one as a radio power amplifier, the remaining one as a modulator. With voice input a 50-60 percentage modulation is reached, the Heising system of modulation being used. The quality of reproduction is stated to be very good, but no figures as to the actual audio-frequency range are given. The receiver is a simple and compact 5-tube affair, with two stages of neutralized radiofrequency amplification, a regenerative detector, and two stages of transformer-coupled amplification. The receiver derives its power supply from the same generator which feeds the transmitter, suitable filters being interposed. The larger transmitters (100 and 300 watts in the antenna for telegraphy) carry substantially the same parts as the 10-watt outfit, with the addition of spares, owing to the greater weight permissible, and, of course, in proportionately larger sizes where the power requirements are greater. The 100-watt (ET-3653 )model uses a ux-210 tube in a Hartley master oscillator circuit, followed by a neutralized power amplifier in the form of a uv-21 1 tube. This power amplifier is modulated for telephony by the Heising system. The receiver for this set covers a 3750-2200 kilocycle (79.95-136.3 meter) range. It is slightly heavier than the receiver provided with the 10-watt outfit. The antenna wire, carried on an insulated reel, is copper-clad steel. Ten antenna weights are provided. In the case of this transmitter and receiver the wind-driven generator supplies voltages of 1000 for the plates and 10 for the filaments. The io-volt winding also feeds an inter-communicating telephone system between the pilot and operator. The maximum total output of the generator is 800 watts. Normally it provides 450 milliamperes at 1000 volts for the plates and 9 amperes at 1 1 volts for the filaments and excitation circuits. The housing is of stream-line design. As long as the plane is in the air the generator maintains a speed of 4000 r.p.m., regardless of wind speed or load, the regulation being automatic through a centrifugal governor which changes the pitch of the propeller blade to compensate changes in speed. The 300-watt (ET-3654) outfit utilizes 1 uv-21 1 tube as a master oscillator, 3 uv-21 i's as power amplifiers, 1 uv-211 as a speech amplifier, and 3 uv-21 i's as modulators. Presumably the object of employing such a large tube as a 211 as a microphone amplifier was to confine the set to one kind of transmitting tube. The master oscillator is a modified Colpitts circuit, with its tank circuit exciting the power amplifier, which in turn feeds the antenna through a coupling transformer. Part of the d.c. voltage across the power amplifier grid leak is used to bias the speech amplifier and modulators. One novel design feature of this set is the inclusion, in the microphone transformer, of a side-tone winding which is connected across the headphones of the operator and pilot and across the receiver output. The headphones are thus used for interphone communication, monitoring radio telephone transmission from the plane, and for reception of incoming signals, whether telegraph or telephone. Operation is possible from either the pilot's or radio operator's seats in the plane, a magnetically controlled send-receive relay being provided. The general features of the 300-watt equipment do not differ, except in size, from those of the 100-watt model. In all airplane installations adequate electrostatic shielding of the aircraft engine is essential if any kind of reception is to be expected. Another precaution which cannot safely be neglected is the bonding of all metal parts of the plane by means of electrical conductors, to eliminate the danger of sparking between surfaces.