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General info on the K9AY loop

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Make your own K9AY transformer

By George Maroti
K9AY web group, August 7, 2000

Regarding the magnetic matching transformer, some have ordered theirs from Minicircuits:

Or, here's how I built mine:
For the 9:1 matching transformer I used an FT140-43 toroid core from Amidon, and used solid #24 telephone wire for winding the primary and secondary.
As suggested in the Topband reflector for matching flag antennas, I didn't overlap the windings.
Rather, I wound the primary on one side of the toroid, and the secondary on the opposite side of the toroid. This type of transformer is a great asset in reducing local noise into the antenna.

My notes indicate that I wound 24 turns for the side connected to the loops, and 8 turns for the side connected to the coax feedline.

Keep the windings separated

By Johan Bodin
K9AY web group, November 13, 2000

"9:1" is the impedance transformation ratio (450/50 ohms). The actual turns ratio is simply the square root of this figure = 3.
I use a 1" toroid from Philips which is made of 3F3 material (blue). Primary is 45 turns, secondary 15 turns. This gives it enough inductance to work well down to 15 kHz or so.

I highly recommend using a "real" transformer (galvanically separated windings) instead of the autotransformer shown in the original K9AY article.
If you are using a "real" transformer with separate pri and sec windings, do not connect the coax shield to the antenna ground/counterpoise as this would allow noise from the shack ground to enter the antenna system. I went one step further and installed a 1:1 isolation transformer in the shack end of the coax. It helped a lot in my case. The control cable is also "floating" since it has no connection to the shack ground other than via the small pri/sec capacitance in the transformer in the power supply.
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