250 - 400 ÁH Variometer.

Tuning an LF antenna can be tricky.  An antenna that's much shorter than a quarter wavelength will have a high Q and consequently a narrow bandwidth. A loading coil with a series of taps can be used, but a constantly adjustable inductor like a variometer will make tuning a lot easier.
A variometer only contributes a small percentage of the total loading inductance necessary to tune the antenna. 5 - 10% is typical, a variometer is used in series with a conventional coil. Finally, it's possible to use remote controll by using a small motor. e.g. a radio modelers servo. Being able to tune the antenna from the comfort of your warm shack is a real treat.

 

                  

Mechanical design by Otto, G0RKA

The variometer is a inductor which has an outer fixed winding and an inner section that can be rotated. The total inductance is at a maximum when the inner section is in a position that puts its magnetic field in phase with the field of the outer coils. As the inner section is rotated through 180 degrees the fields are in opposition and the overall inductance is at a minimum. For maximum Q the variometer should be used at a value near maximum inductance where the windings are in series not in opposition

With this variometer the range is 250 to 400ÁH.

Construction.
Hopefully the pictures show the general arrangement. It's not necessary for every aspect of the design to be reproduced, but the details here may prove helpful in designing a similar unit more suited to your specific application.
The two fibreglass tubes are standard sizes and are detailed below. The shaft is a plastic rod drilled so the wires can pass through to connect the outer and inner coils. Small collars with grub screws hold the shaft in place.
The wire passed through the shaft is PTFE insulated with additional PVC sleeving over each of the two wires. The external coils are wound with standard PVC insulated stranded 'equipment' wire.

Outer Tube                          110mm OD  255mm long 102mm ID
Inner Tube                          82mm OD    58mm long
Top winding (ext)                33t  16/0.2mm   55mm long
Lower winding (ext)            33t  16/0.2mm   55mm long
Distance between windings  48mm
Internal windings                 2  off  9t  19/0.1mm with PTFE insulation covered with self-amalgamating tape

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