Radio broadcast .. (1922-30)

tivity of the receiver are also better with plate rectification. The Transmitter The type of transmitter used depends naturally upon many factors. Roughly, a radio-phone will cover about one-third the distance of a radio-telegraph trans- mitter of the same power, but will weigh about 20 per cent. more. However, these figures are only rela- tive, since range vs. weight varies consider- ably with equipment. The transmitter is governed by the same requirements as the re- ceiver in regard to rug- ged construction, flex- ible mounting of parts, accessibility, d i m e n - sions, and frequency stability. In the latter case crystal control is a great aid. A range of at least 100 miles of consistent com- munication is accep- table for commercial aircraft flying in standard airways, since stations are located every 200 miles, and beacon marker stations with auxiliary equip- ment every 100 miles. A combined cw and radiophone transmitter seems to be the desirable thing, because few pilots have the time or pa- tience to learn the code sufficiently for expert operation of>a straight cw transmitter. As planes become larger and the pay-load ques- tion is of less import- ance than at present, sufficient weight may be allowed for a radio- phone of sufficient power to communicate over fairly long dis- tances. The wind-driven generator is giving way to the more recent type which is direct coupled or geared to one of the plane motors. A voltage regulator keeps the output practically constant. These generators vary in output from 500 watts to 2 kw, and both the filament and plate supply currents are delivered by them. They are available for both a.c. and d.c. out- put. In some cases the high voltage de- livered is 500 cycles or so. Possibly the power supply of greatest value lies in a separate small gas engine- driven generator. This could be used for ordinary communication service, and would be invaluable as an auxiliary. Several types have been developed, but a great deal of experimental work is still to be done in this field. A small two-cycle, two-cylinder motor of about 2 H.P. is about right for most commercial purposes. A battery-driven dynamotor may also HINGED COVER OVER THREE JACKS 1 - OUTPUT 2 - VOLTAGE 3 - S.G.CURRENT WOVEN TINNED COPPER COVERING (THESE TWO CABLES CONNECT TO ONE PLUG WHICH MAKES POSSIBLE DETACHMENT OF THESE CABLES FROM RECEIVER CHASSIS) 14 TERMINALS ON JACKS COVERS HELD BY SCREWS COVERING LINE-UP ADJUSTMENT ANTENNA POST SHOCK ABSORBERS (sec DETAIL) REMOVABLE BOX COVER HELD BY FASTENED WING^UT SCREWS DETACHABLE DENTAL CABLE DRIVE-10 FT. LONG- WOVEN CLOTH COVERED. 3 - UX-222 2 - UX -112A 1 - UX - 171 A AV. SENsmviTY-e 6V.-5TORAGE BATTERY 22JV. TAPPED'C" BATT. 135V."B* BATTERY WITH METAL SUPPORT ENCIRCLING RUBBER DETAIL OF SHOCK ABSORBERS 1-100 fc 90 °60 tj TO I 60 30 10 7- TYPICAL OVERALL FIDELITY GRAPH OF AN AIR BEACON RECEIVER 60 100 3 6 10OO CYCLES The above curve indicates the overall fidelity of the average air- beacon receiver. A mechanical description of aircraft beacon receiver type AR1286. be used to advantage for an auxiliary power supply, and will furnish power as long as the lighting battery lasts. Frequency Choice The standard allocated frequency band for radio beacon service is from 285 to 315 kc., and that for aircraft radio com- munication is from 315 to 350 kc. The design of a receiver to cover this compara- tively narrow band of frequencies is com- paratively easy and most commercial aircraft receivers are designed for this use. The 500-kc. band may be used for dis- tress or emergency signals. Frequencies around 3000 to 4000 kc. are good for fairly long-distance low-power communication and are not much affected by fading, skip- distance, directional effects, type of an- tenna, and other phenomena characteristic of frequencies higher than this. These frequencies will undoubtedly prove of great value in the future of aircraft radio. Frequencies as high as 11,000 kc. have been used with very good success for long-dis- tance, low-power communication on trans- oceanic flights and expeditionary flights. However, they were naturally affected by skip-distance, etc. Radio Beacons The radio beacon of to-day is as prac- tical and perfect as the ordinary magnetic compass. High noise level caused by the ignition and mechanical noises made the cross-coil beacon necessary. This beacon consists of two coil antennas disposed in two vertical planes fixed at an angle from each other. In a simple form the two-coil system is free to be rotated about a vertical axis. When the two coils are similarly excited with a modulated radio-frequency current, signals of equal intensity will be heard on a receiving set when situated along either one of the two vertical planes bisecting the angles be- tween the planes of the coils. At other points, the signal intensity from each coil will be different. A mechanical device can transmit the letter N ( ) on one loop and the letter A (—) on the other, and they are so interlocked that a con- tinual buzz is heard along the equisignal zone. A deviation of from 1 to 3 degrees from the course will result in one or the other letter becoming distinctly pre- dominant. The advantages of this type of beacon are: 1. No zone of max- imum or minimum sig- nal strength. 2. Location of course found by the compar- ison of two signals. 3. A plane may be guided along a given airway without regard to wind drift. 4. When considerably off course, the beacon furnishes a definite sig- nal allowing plane to return to former course. Valuable when plane must make detours or is forced off course. 5. Used on national airways. 6. Ordinary receiver and antenna may be used. The disadvantages are: 1. "Plane effect" or angle of plane causes slight error. 2. "Night effect" or shifting of equisig- nal zone at night makes it undesirable for night flying. (Continued on page 358) TYPICAL PERFORMANCE GRAPH OF AN AIR BEACON RECEIVER 295 3O5 315 325 335 FREQUENCY IN KILOCYCLES 7000 The curve "HN" gives sensitivity in microvolts per meter for 50 mW. output. The other curves give band width in kc.for inputs of 10, 100, and 1000 HN. 332 • • RADIO BROADCAST FOR APRIL •