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updated November 2014
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LF and MF Experiments etc - 10
Pyramid winding
136kHz coil
October 2009
As previously mentioned I wanted to be able to quickly change bands between 136kHz and 500kHz so I set about trying out the “pyramid” (or “bank”) method of coil winding to produce a more compact coil.

I constructed a couple of small coils to find out the requirements and found out the hard way that some form of adhesive is needed to hold the wire in the pyramid form. After some thought I decided to use double sided self adhesive tape. I wound a few centimetres of 25mm tape onto the former to hold the first few turns and after three or four I then made a two layer pyramid and wound a turn back over previous two. Now that didn’t hold in place very well so using a layer of tape over the previous two turns I found that it held OK, this tape layer was wide enough to overlap down onto the former surface to hold the next turn in place.

Continuing on I wound one turn on the coil former surface and the following turn back on the previous two. It was rather messy using the tape and difficult to wind but over a couple of days I managed to fill the cooking oil drum I used as a former. This produced a value of 3.4mH on one drum compared with 2.8mH using two drums and conventional windings. Therefore I had reduced the physical size by half. I made the coil adjustable by constructing basket weave coils (similar to my earlier experiments) for the additional inductance I needed for resonance on the 136kHz band. By moving adjacent coils up or down their former I can vary the inductance and fine tuning using the variometer in the lower (500kHz) coil .

Changing from 500 to 136kHz and back would be a simple matter of moving two connections.

I have fitted the new coil in my coil cupboard on a shelf to separate the 136kHz unit from the 500kHz unit to reduce any proximity effect when only the 500kHz coil is in use.
New top load
During 2010 gale force winds caused my antenna top load to collapse. I have now replaced the damaged part with a new larger construction of approximately 10 square metres (above right). A substantial increase in top capacity has noticeably improved the performance of the antenna
LF & MF experiments 11 LF & MF experiments 9