Hi ULF,

This night i have completed the 4th stack of my large ULF loading coil.
An image: https://dl.dropboxusercontent.com/u/19882028/ULF/20161008_ULF_TX_coil.jpg

The exciting question was, what is the new resonance frequency together with my antenna. In the lab i put an equivalent C in parallel and found the resonance peak at 3.67 kHz. So the L seems to be 3.8 H now. This is still not enough. The l/d of the coil dimension becomes larger now, so i need more and more wire to reach the final resonance frequency of 2.97 kHz at about 500 pF.

I will do a next test on the actual resonance frequency in some days, when the WX permits. I will also check where the resonance can be found when switching the small coil in series that i use for 6.47 kHz now. It could be quite close to 2.97 kHz then! So maybe this allows a next QRO step on 2.97 kHz: 100 mA antenna current must be reached for the planned experiment in 65 km distance.

BTW the 4 stack coil is wound out of almost 4 kg of enameled Cu wire with 0.4 mm diameter. The layers are isolated by Kapton tape. My assumption is that it will hold 20 kV now.
Ah and the DC resistance is 445 Ohm now. When connecting 275 V AC / 50 Hz, i can measure 269 mA. So the actual inductance is 2.93 H (beeing farer away from the self resonance frequency)

I can tell you it's quite fascinating to operate down on these low frequencies :-) To be continued...

73, Stefan/DK7FC