January 2011 The previously constructed “pyramid” wound coil for 136kHz produced a more compact coil and worked well but the basket weave section needed more wire to produce the required inductance and was considerably larger. I got an idea from an e-mail sent to the LF Group by Marcus DF6NM regarding the making of his VLF (9kHz band) loading coil. The construction consisted of the coils being wound onto buckets and inserted into each other to create the large inductance needed for the new 9kHz VLF band. The buckets could be moved in respect to each other to vary the total inductance. I retained my “pyramid” coil and in place of the basket weave sections I made a coil on a large paint tub, this was tapered like a bucket and fitted inside the larger cooking oil drum. This method used about 20 metres less wire. I now needed a method of adjusting the total coil inductance and I finally made 3 plastic brackets with an 8mm nylon nut and bolt in each. With these I could raise and lower the large outer coil by small amounts over the inner coil to vary the setting.
New inner coil
Both coils mated
In practice the new compact coil worked very well until recently I needed to increase the transmitter output power to try to contact a special event station who was not receiving my signals. The turns on the inner bucket like tapered former came too close to the outer winding and there was a flash-over. There was no damage to my transmitting equipment but the coil insulation was badly burned.
New narrower inner coil
Outer coil adjuster supports
March 2011 I carried out a repair using a slightly smaller diameter but longer straight internal former which has proved to be fine even at my highest power levels. May 2011 Having used temporary supports for the outer coil I needed to make the outer coil adjustable. The final result is shown above with the new supports which not only adjust the height but maintain the spacing between the coils.