OK, after a break in the sunset i thought:
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
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