Loading Coil Upgrade At the same time as experimenting with the antenna I had also been working on improvements to the loading coil. I decided that I would make the loading coil more compact and frequency versatile. I tried to achieve this by constructing the main inductance using basket weave method to produce pie type coils. The picture (bottom left) shows the construction with one fixed and three adjustable by sliding up or down making the inductance variable due to the proximity of the the coils. In the finished assembly there are 4 of these, one is fixed and the other three are adjustable and easily removable to make quick changes to the total inductance, the construction also includes a remote controlled variometer using a geared motor. The adjustments would include tapping points, disconnection points and coil to coil proximity. I found that I could use the variometer on its own to tune the antenna to the 500 kHz band. During practical tests it was revealed that I had not produced sufficient basket weave pie coils and also with the much wider coil assembly and limited space there was a danger of arcing over to the sides of the coil box with it’s limited internal and external space. As the inductance was much lower with this type of construction I would require four or more coils, this coupled with the danger of arcing I decided to return to my original conventional type of coil.
July 2008 My next move was to make two individual coil assemblies one for 500 kHz and one for 136 kHz by physically adding a second coil on top of the other, this is shown below. My reasoning for this is to maintain best efficiency in each coil unit rather than compromise by using tapping points to make one unit cover both bands. During 500 kHz operation I found that tapping the coil towards the bottom but leaving the rest of the coil in close proximity did in fact reduce the output on 500 kHz and removing the surplus part of the coils made a dramatic improvement in output which required much less PA power for increased antenna current, therefore the unused part of the coil was absorbing energy.
In the near future I hope to have a try at constructing a new 136 kHz loading coil using the pyramid method. The advantages of this method are that you get a compact coil, you save wire (which means that you get a higher Q), you have an inter-winding capacitance mainly between electrically close turns and also relatively low voltage between adjacent turns, see links page.