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"Bank" or "Pyramid" winding of 136 kHz coils
One method of producing coils for LF use seems not to be known by many LF amateurs. This is the case in spite of the fact that this method is probably the way of coil winding that renders the best compromise between compactness, Q-requirements, low winding capacitance etc. The "Bank" og "Pyramid" winding method was used for decades when coils and variometers were required for marine transmitters for 400-500 kHz, for aero- and marine beacon transmitters for the 250 to 400 kHz range and probably, too, in the very old days, when 143 kHz was a maritime calling frequency. The said method of coil winding does not allow machine production of coils, it is a largely manual procedure winding such coils. I think a slowly turning lathe or a similar type winding machine was used for turning the coil former, the hands and maybe small auxillary tools were used for placing the coil wire. And probably some sticky stuff was used, too. No much is to be found on these coils in the radio litterature. Terman does, however, (1943 Handbook, pg. 84) briefly mention it. The coils made by this method are multi-layer coils, I
have seen four-and five-layer Coils. I have, however, apart from
a few not too successful trials, only ventured making two-layer coils. This
sketch shows how the coil turns are placed:
The advantages of this method are obvious: You get a compact coil, you save wire (which means that you get a higher Q), you have an interwinding capacitance mainly between electrically close turns and also relatively low voltage between adjacent turns The losses due to the proximity effect have increased, no doubt. Nevertheless, the positive net effect is great! As for the coil wire, professionals normally used RF Litz ; I have, however, seen old coils wound with double-cotton-covered solid wire. I do myself use RF Litz wire, I was lucky to find some 320 x 0,071 and 600 x 0,063 Litz. wire for my coils and variometers. There is no doubt that it is difficult to find such wire; yet one can sometimes find rare things at odd places. For instance, there might be companies that once made marine transmitters and the like, that could have such unused wire. Or maybe the manufacturers of RF Litz could have surplus stock (manufacturers known to me were Daetwyler and Isola-Werke, Switzerland and Leonische Drahtwerke, Germany). Other types of wire could without any doubt be used; there is plenty of space for experiments and development! Examples of "bank" or "Pyramid" wound variometers: |
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Variometer for use in the output P-filter of a 2 kW valve power amplifier. L = 390 to 900 mH; Q = 360; 2 layers, wound with 320 x 0.071 Litz wire; Dimensions 110 mm diameter, 132 mm high.
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"Coaxial" antenna variometer, L = 1.4 to 2.7 mH, Q > 600 600 x 0.063 Litz wire. The outer coil is a professional ex-beacon transmitter coil, I wound the inner coil. |
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The beacon transmitter coil. Dimensions are: 87 mm mean diameter and 210 mm length.. August 2003 OZ8NJ
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Copyright © 2000-2013.
by Niels Rudberg.
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