May 2011 Due to the changes to the coil structure I had to abandon the nylon bolt adjustment method as shown previously so I tried a ferrite block just resting on the top of the coil former and it has worked OK initially. The top being the high voltage end of the coil I need to find a less vulnerable position for the ferrite. Another problem with this method is that only about 70% of the band is covered with the variometer and that would mean if I needed to transmit in that part of the band not in the tuning range I would have to go outside to the coil box down the garden and move the ferrite. My next test is to insert the ferrite into the variometer coil to see if it will increase the tuning range of the variable inductance and therefore cover the whole band remotely. The material I am using is ferrite core from a PC monitor line output transformer. I will also need to test this tuning method at higher power and longer transmit periods.
The Ferrite material used and shown inserted in the variometer coil
I have now tested the cores at higher power levels and cannot see any adverse effect or heating. I am now using 3 of these cores with a useful increase in variometer tuning range.