Earhart radio communications

[h.a.c. van asten]

ChVar. The 13C PA band coil sets were delivered with a condenser , p.e. "4000 mmF . The available schematic does not show this capacity. 1. Was the condenser connected parallel ovver the coil poles, parallel to a part of the coil , or how ? 2. Was the condenser a single double pole , or 3 pole design with the central pole earthed ?. I do btw not think that a 100-50 W PA dissipation played a major role for range. illustration : during the Battle of Britain , Aug- Sep 1940 , the entire communications circuit over southern England was handled by TR9-D HF transceivers installed in all fighter airplanes (Spitfire , Hurricane) and all interceptors (Beaufort etc.) . TR9-D RF output was 0.5 Watts (PA anode current 25 mA) from dry battery powered valves (Xosc-PA mode , AM) and the system worked flawless day and night. Aerials were long single wire , frequencies 4.3 - 6.9 mc/s . The receiver was  of Det- 2 x AF fashion. The HF system became obsolete , only after the introduction of TR 1143 VHF radios (USA SCR 522) later in the war. The WE radio o/b NR 16020 was comparable to the BC 375-191 (MO-PA) in powerful installations of  WW II USA B-17´s etc. Probably the laesio enormis of the Earhart crew was to rank DR-Celestial in the primary , and RDF in the secondary row of importance from the beginning , not against the end of the flight.

[Chuck Varney]

ChVar. The 13C PA band coil sets were delivered with a condenser , p.e. "4000 mmF . The available schematic does not show this capacity. 1. Was the condenser connected parallel ovver the coil poles, parallel to a part of the coil , or how ? 2. Was the condenser a single double pole , or 3 pole design with the central pole earthed ?

h.a.c. van asten ,

I have to assume that you're asking about the capacitor labeled "C (C10, C11 or C12)" in this diagram. The value of this capacitor was 4000 pF when used with the 7B coil for 3000-4000 kHz (and with the 7C coil for 2000-3000 kHz.) The diagram should make clear that it was a 2-terminal component, series-connected between a tap on the coil and the antenna connection point.

(Capacitor C13 in the diagram was also 4000 pF.)

Chuck

[h.a.c. van asten]

Ch.Var.  In the schematics I saw so far I couldn´t  identify this condenser (the captions unreadable). The PA layout is very much alike the one of the British TR9 (HF) transceiver of 1934-36 as used in many early WW-II aircraft , evidently a designer´s fashion of the era , also found in the English 38 and 22 transceivers , probably to turn the output circuit to more broad band characteristics for easier adaptation to a variety of antenna lengths and possibly to avoid harmonics break through.

[h.a.c. van asten]

General . The ´Donut´ theory / practice , why did it apply to 6,210 kc/s , not to 3,105 kc/s when A/c was in the roads of Howland ?