If µr tends to 6.5 and L/l to 22 mH/m for 0.5mm wire for l -> infinity, and if i want to keep the coil length at 0.8m, which would be 17.6 mH for 0.5mm wire. However i need 788 mH! So i need a 6.7 times smaller wire, i.e. 0.074mm diameter. This is impossible and i wouldn't carry the antenna current.
A new coil would only make sense if i can rise the signal by at least 6 dB. I just measured 260 mA. So let's say 600 mA would be fine. And i like the 0.4mm wire and would like to use that.
In the moment i see no chance for a single layer coil using this technique :-(

73, Stefan

Am 28.04.2016 20:02, schrieb DK7FC:
Hmm, well, ok, after some discussions, the show ehm the experiments must go on.

I'm continuing with a higher l/d ratio. 13 of these cores are available, the other ones are parts of my transmit coil now. I like to get 3 measurements to approximate a curve showing L/l and µr(eff) over the ratio l/d. So a useful number of cores is 5 (already done, see below), 9 and 13.

5 core stack (yesterdays measurement):
The ratio coil diameter / coil length, l/d = 49/33 = 1.48.
Effective µr = 3.75
L/l = 452 µH / 49 mm = 9.22 mH/m

9 core stack:
As a resonance C i use 0.3 uF (measured C = 306 nF)
The resonance is at 7.26 kHz. The bandwidth is (7.36 - 7.18) kHz = 180 Hz. Q = 40. L = 1.75 mH
Without the cores inside, the resonance is found at f = 17.1 kHz. BW is (17.6 - 16.69) kHz = 1.05 kHz. Q = 16.8. L = 283 µH.
https://dl.dropboxusercontent.com/u/19882028/VLF/9%20cores.jpg
l/d = 96/33 = 2.91
So now the effective µr is 6.18
L/l = 1.75 mH / 96 mm = 18.23 mH/m
L/l (9) / L/l (5) = 1.98

13 core stack:
C= 202 nF
Resonance (with cores) at 6.485 kHz. BW = (6.56 - 6.42) kHz = 140 Hz. Q = 46. L = 2.98 mH
https://dl.dropboxusercontent.com/u/19882028/VLF/13cores.jpg
Resonance (without cores) at 16.78 kHz. BW = (17.26 - 16.38) kHz = 880 Hz. Q = 19. L = 445 µH.
l/d = 145/33 = 4.39
The effective µr = 6.70
L/l = 2.98 mH / 145 mm = 20.55 mH/m

OK, now, this tends to a certain value for L/l, maybe 22 mH/m (see attachment) for a 0.5mm diameter wire. Hmm, so my coil would be just 35m high, about as high as the feed point of the antenna :-)
So a thinner wire is needed or a tube with 3 or more cores in parallel.
More soon...

73, Stefan

Am 27.04.2016 20:20, schrieb DK7FC:
Hi VLF,

I've done a quick experiment with the T106-52 cores which could give some more ideas regarding these cores for a compact VLF coil.

I wound a coil with 0.5mm enameled cu wire, 85 turns at 33mm diameter. Inside the coil there are 5 of these cores stacked on another. In parallel there is a suitable C of 1 uF.
https://dl.dropboxusercontent.com/u/19882028/VLF/20160427_195530.jpg

The resonance was found at 7.49 kHz. The 3 dB bandwidth is (7.64-7.36) kHz = 280 Hz. Q = 27. L = 452 uH.
Without the cores inside, the resonance frequency rises to 14.54 kHz and the bandwidth is (15.12-14.07) kHz = 1.05 kHz. Q = 14. L = 120 uH.

Hmm, so in this configuration, the effective µr (ur) seems to be just 3.75! :-/
That means i still need half of the number of turns for a single layer VLF transmit coil?!?

73, Stefan

--------------090008050601020203090904--