[Lf] [Fwd: LF: Series loading coil]

[Andre Kesteloot]

Dick Rollema wrote:

> To All from PA0SE
>
> I also use the series coil system for tuning and matching the aerial
> to the 136kHz transmitter.
> The coil was made by bolting four 1m long and 12.5cm diameter PVC
> pipes together and putting on 200 turns of 1.4mm diameter copper wire
> with black PVC insulation as used for house wiring.
> Inductance is 4.4mH and unloaded Q  about 350.
> Starting from the top I made a tap at every 10 turns by putting a
> twist in the wire. Heating with a soldering iron melts the insulation
> that then easily comes off to  bare the wire.
> The bottom 10 turns have a tap at each turn. By short circuiting turns
> from the top and the bottom the coil can be adjusted in steps of one
> turn. For fine tuning a variable capacitor is in parallel with the
> coil. As this increases the current in the coil and thus the loss in
> it (proportional to current squared!) I adjust the coil so that only
> minimal capacitance is needed.
> [PA0SE Coil.jpg]
> The capacitor shown started to arc over at 60W of power. It was
> replaced by a vacuum capacitor rated at 5000V maximum.  When
> transmitter power was increased  from 120W to 320W I connected the
> capacitor over half the coil in order not to exceed  the maximum
> voltage.
>
> The transmitter is connected to one of the turns at the bottom end. I
> don't worry about SWR and simply select the tap giving maximum aerial
> current (3A in dry weather).
>
> A better picture of the coil above can be found on page 62 of G3LDO's
> The low frequency experimenter's handbook. The coil with the vacuum
> capacitor is shown on the cover of the book.
>
> 73, Dick, PA0SE



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