interesting that the cores seem to be more effective (larger effective µr) for a slim (large l/d) coil.
Just a (maybe stupid) thought: could it be that the cores are more effective in the coil center?
If you still have the 145mm coil available it might be interesting to check how the inductance (and thus the effective µr) changes if you gradually reduce the number of cores while keeping them centered inside the coil.
Another thing: as you noticed for a slim coil the inductance is almost linear with the number of windings (= coil length). This means that a bunch of 30 1m high coils (in series) will be as effective as one 30m high coil.
Athough you will need the same amount of wire and cores construction and placement is feasible that way.
73, Rik ON7YD - OR7T
the experiment with 5 cores has shown that the iron powder cores are not so efficient at a low l/d. But of course they help a bit, like seen in the current transmit coil configuration that i use.
I'm going to order the 3C-85 ferrites and continue with the experiments. These high ur ferrites are useful for several applications anyway..
Am 28.04.2016 22:23, schrieb Rik Strobbe:
what about a "fatter" (length/diameter = 1) coil filled with some core stacks in parallel?
For a slim coil (large length/diameter ratio) the inductance is more or less proportional to the number of windings while for a fat coil inductance is more or less proportional to the square of the number of windings.
But I don't know how effective the cores will be in such a fat coil.
73, Rik ON7YD - OR7T
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 :-(
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.
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
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.
Am 27.04.2016 20:20, schrieb DK7FC:
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.
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?!?