Hello Mike,
for my K9AY I used the version I found in Devoldere's "Low-band DXing" (a
recommendation from W8Ji).
I glued 3 BN73-202 together, and used 3 turns primary and 1turn secondary (
500Ohm:50Ohm)
For 500:75 it should be 2.5 turns:1 turn.
For the insulation I didn't used the trick with the straw, but heated a heat
shrink tube that was a little to wide until it fit into the holes.
For me the transformer worked fine.
For a comparison of different transformer I also found Mark Conelly's (WA1ION)
article "Broadband receiver antenna matching" (2003) also very usefull. J.
Briant's, B.Bowers, N Hall-Patch's "Impedance matching transformer for
receiving antennas at mediaum ans lower shortwave frequencies" (2003) also
contains a wealth of information.
Good luck
Kurt-Erich
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