[Amps] Russian power L-coupler

[Peter Chadwick]

Tom said:
>They are sometimes called variometers.

They have terrible Q at minimum inductance, especially at 
higher frequencies. Of all the common methods, this is the 
least efficient way to vary inductance over a wide range.<
Strictly, a variometer is where the two coils are in series, and the inductance can be adjusted from L1 + L2 + 2M to L1 + L2 - 2M, where M is the mutual inductance. They do give a wide inductance range, but as Tom says, the low Q at minimum inductance is a problem. Historically, they were used a lot in long wave tx's, especially in the marine band at 410 - 516 kHz, as well as in early (pre 1930) broadcast radios. The roller coaster type of variable inductor gets a bit impractical when you want a 500 or 1000 microhenry inductance change............ There was a German WW2 military radio that used iron cored ones in the antenna tuning, and I believe that with the core removed, they were popular with German amateurs in the 1950's because the construction was such that they were very stable when used in VFOs. The Collins ART13 airborne tx of WW2 used one in the output pi network as well.
For variable coupling between tuned circuits, a lot would depend on the actual inductance value as to how useful such a device would be on any particular band.
73
Peter G3RZP