make the transformer, I first wound a primary winding of 10 turns on a 58 mm OD 3C85 core using insulated conductors
salvaged from some old 15 A mains flex. The primary winding was held in place with
two layers of PVC insulating tape.
The first 10 turns of the secondary winding were wound over the top of the primary
winding, and further turns added to provide the required transformation
ratios. Finally, the turns were held in place using cable ties.
Note that the transformer provides isolation between the antenna earth and the screen
of the feeder. I believe this helps to prevent an earth loop which might otherwise
introduce mains-borne noise on receive.