Fascinating indeed Stefan.  Keep up the good work!
73
Joe VO1NA

On Sat, 8 Oct 2016, DK7FC wrote:

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