Radio Broadcast (May 1929-Apr 1930)

coupling. The suggestion made above that magnetic coupling gives a better signalnoise ratio than the usual amateur method of capacity coupling to the end of the antenna was confirmed on some previous experiments on higher wavelengths by enclosing the primary in a metal box with a 2" hole facing the secondary coil. This permitted magnetic coupling but static coupling was prevented by an ordinary static screen of insulated wire hanging vertically across the hole. The upper end of each wire was secured to the metal box and the lower end hung free. Doubtless the same result would have been secured at 10 meters. COMMENTARY It will be seen that Mr. Hart's set departs widely from current short-wave practice of using a large ratio of C/L in the tuned system. For his particular purpose a wide tuning range was desired but the very great merits of a high C/L ratio can be retained while securing a more open tuning scale by merely putting most of the capacity into a fixed form as was suggested in a previous "Armchair" discussion. A 16-megacycle tuning range without variation of the regeneration control is far beyond that ordinarily obtained in short-wave receivers which rarely manage to go a tenth that far and, there 28 \ % !•> => _ z> -7 t fr t ( C ) 10 Meter j amateur hand J*"-' • 100 90 80 70 60 50 40 30 20 10 0 TUNING C0N0ENSER SCALE DIVISIONS Fig. 2 — Calibration curve of Mr. Hart's 30-megacycle receiver. Insert diagram shows lengths of connecting leads. fore, rarely are fit for calibration. The signal strength for a well-made oscillating receiver seems (and measures) unchanged by large alterations in C/L. The choke used has undoubtedly a capacity reactance at 10 meters which would be very unfortunate in circuits of conventional proportions but in this case is of distinct advantage. The absence of shielding is intentional. Since frequent alterations were to be made in the set, a shielding system would have been troublesome during the first stages. Most builders of short-wave receivers entirely fail to appreciate that shielding is useful only if there are no circulating r.f. currents in the shield and if the wires emerging from the shield are effectively bypassed and choked. This means that the apparatus inside the shield must connect to the latter at one point only, and that much thought must be given to the location of that point, the bypasses, and wires going to it. Frequently the easiest solution is to place a metal sheet inside the shield to which these things are connected and which, in turn, touches the shield at only one point. This is a rudimentary double shield. At very short waves bypasses are not to be judged by their capacity but by their inductive reactance. Paper condensers of 1 microfarad capacity are frequently much less effective than a small mica "postage-stamp" unit of a hundredth the capacity. Chokes, if placed outside the shield, have missed a part of their function and if placed inside may easily reverse it and collect r.f. power which the wire alone would not have intercepted. Again double shielding is indicated as the most desirable solution to the problem. SKIP-DISTANCE AND RANGE TABLE For frequencies between 1500 and 30,000 kc. (This table was prepared especially for Radio Broadcast by L. C. Young, Naval Research Laboratory) Frequency in Kilocycles Approximate Wavelength in meters Range of Ground Wave SKIP DISTANCE MAXIMUM RELIABLE RANGE SERVICES (International Radiotelegraph Convention) REMARKS Summer Winter Summer Winter Day Night ^Day Night Day Night Day [ Night 1500—1575 200—175 100 100 100 150 300 Mobile 1604 Experimental — 1600-1652-1664 1680-1704-1712 Portable. 1715—2000 175—150 90 120 175 170 600 Mobile — Fixed — U. S. Entirely Ama Amateur teur. 2000—2250 150—133 85 130 250 200 750 Mobile — Fixed. U. S. 2002 to 2300 Exp. visual Broadcasting 2250—2750 133—109 80 150 350 220 1500 Mobile 2398 Experimental 2750—2850 109—105 70 170 500 300 2500 Fixed 2750 to 2950 Exp. Visual Broadcast 2850—3500 105—85 65 200 900 350 3000 Mobile — Fixed 3088 Experimental 3500—4000 85—75 60 250 1500 400 4500 Mobile — Fixed — U. S. Entirely Ama Amateur teur. 4000—5500 75—54 55 300 4000 500 7000 Mobile— Fixed 4795 Experimental. 5500—5700 54.0 —52 7 50 400 4000 600 8000 Mobile 5700 — 6000 52.7 —50 0 50 50 50 50 60 450 5000 650 8000 Fixed 6000—6150 50.0 —48 8 50 60 70 60 90 500 5500 700 8000 Broadcast 6150—6675 48.8 — 45 0 45 70 115 80 175 550 6500 750 8000 Mobile 6675—7000 45.0 —42 8 45 80 185 100 290 650 7000 820 8000 Fixed 7000—7300 42.8 —41 0 45 90 220 115 360 700 7500 900 8000 Amateurs 7300 — 8200 41 . 0 —36 6 40 140 290 175 465 750 8000 1100 8000 Fixed 8200—8550 36.6 —35 1 40 160 370 200 570 800 8000 1300 8000 Mobile 8550—8900 35.1 —33 7 40 170 420 230 630 900 8000 1460 8000 Mobile — Fixed 8900—9500 33.7 —31 6 40 200 485 270 710 950 8000 1680 8000 Fixed 9500—9600 31.6 —31 2 40 220 530 280 740 1000 8000 1820 8000 Broadcast 9600—11000 31.2 —27 3 35 260 625 325 860 1100 8000 2140 8000 Fixed 11000—11400 27.3 —26 3 35 300 750 380 1000 1200 8000 2460 8000 Mobile 11400—11700 26.3 —25 6 35 315 800 400 1080 1300 8000 2700 Broadcast 11700—11900 25.6 —25 2 35 335 835 420 1120 1500 8000 2800 Fixed 11900—12300 25.2 —24 4 30 350 870 430 1170 1550 8000 3000 Fixed 12300—12825 24.4 —23 4 30 370 940 460 1240 1600 8000 3200 Mobile 12825—13350 23.4 —22 4 30 390 1000 485 1700 8000 3440 Mobile— Fixed 13350—14000 22.4 —21 4 30 420 1075 510 1800 3660 Fixed 14000—14400 21 . 4 —20 8 30 440 1150 545 1950 4060 Amateurs. 14400—15100 20.80—19 85 30 460 1230 580 c 2200 4360 Fixed 15100—15350 19 . 85—19 55 30 475 1300 610 w 2300 4640 P Broadcast 15350—16400 19.55—18 30 30 500 1370 640 H 2500 ►J 5060 fa fa Fixed 16400—17100 18.30—17 50 25 550 Q 700 U 3000 5 5600 CO Mobile 17100—17750 17.50—16 90 25 580 fa 740 fa ►J 3500 fa 6200 P Mobile — Fixed 17750—17800 16.90—16 85 25 600 H 755 fa 4000 w co 6450 H Broadcast 17800—21450 16.85—14 00 20 660 u 835 « 5000 P 7000 O Fixed 21450—21550 14.00—13 90 20 750 w J 1050 A 6000 H 7000 Broadcast 21550—22300 13.90—13 45 20 780 fa 1090 H 7000 O 7000 Mobile 22300—23000 13 . 45—13 10 20 835 1130 O 7000 7000 Mobile— Fixed Good only for few 23000—28000 13.10—10 70 15 900 « 1200 Z unknown unknown Not reserved H hours during daylight. 28000—30000 10.70—10 00 10 1000 O 2 1400 unknown unknown Amateurs NOTES: Mobile: Ships and Coastal Stations, Aircraft, Barlroad Stock, etc. Fixed: Permanent stations handling point to point traffic. Skip Distance: Shortest distance beyond the ground wave at which communication is possible, or the point where the sky wave lirst comes to earth. On certain frequencies and at certain seasons communication is possible within the skip distance due to echoes and around-the-world signals. The above table was obtained from the general average of a large number of observations. For the night ranges given it is assumed that the greater part of the path between the transmitting and receiving stations is in darkness. As the distances given in this table are general averages many discrepancies may be found in practice due to seasonal changes, suu spot activities, geographical location, local weather conditions, eta 166 • / • JULY • 1 929 •