Hi Daniele,
Looking at those alligator clips: Did you ever remove the rubber
insulation, and check the connection between clip and cable, and
check the 'copper diameter' of these cables ?
I did, and found that...
a) they didn't even solder them, but used a trashy loose "crimp"
(well, not really crimped but crippled) connection, the copper
conductor folded back over the wire's insulation, sometimes only
slightly touching the chrome surface of the clip,
b) there's a ridiculously low amount of copper in those wire ! (lot
of PVC which makes you *think* the wire is thick, but it's crap)
Cure: Replaced the wire with other colourful cabling salvaged from a
PC power supply, and SOLDERED the wire to the clips.
Effect: DC resistance "from clip to clip" droppe to less than 25 %
of the original, for the same cable length.
Maybe, if those cables are part of your parallel resonant circuit on
the test bench, they are a show stopper - besides the close
aluminium shield, which should really be a bit closer away from the
rod / coil, or be slotted, as already suggested by Peter.
Cheers,
Wolf .
Am 28.09.2011 21:05, schrieb Daniele Tincani:
Hello Stefan,
this morning
I received the litz wire and tried another build, using a 16
mm PVC tube to enclose the 3 x ferrite rods. I wound 70
turns of litz (as good as I could) and went to the test
bench again to measure Q (please see the attached picture).
In fact, things have improved a bit. With the air varcap I'm
using for these tests, tuning can range from about 235 KHz
up to about 978 KHz. Using nominal min/max valued of
capacitor, this gives approx L = 670uH which is very good
for my purposes.
Unfortunately
Q - although improved - is still quite low: about 104 @ 235
KHz, about 60 @ 978 KHz.
As I don't
plan to try another build of the ferrite-cored coil, I can
only hope that Q will improve further with shorter and
better connections with the capacitor.
Many thanks
to you, Stefan, for the very helpful suggestions!
Best
regards
Daniele
Da: Stefan
Schäfer <[email protected]>
A:
[email protected]
Inviato:
Martedì 27 Settembre 2011 22:42
Oggetto: Re:
LF: Re: My first ferrite experiments
Hi Daniele,
Am 27.09.2011 17:15, schrieb Daniele Tincani:
Hello
Stefan,
>
but maybe the Q is reduced by the aluminium
shield!
Do
you think the aluminium could be the problem
or the distance between the coil and the core?
The wire shouldn't wound directly on the rod. I have a PVC
tube above the rod and so the wire is in a distance of
about 3 mm to the rod. I used litz wire and got a Q of
485...
I
could replace the aluminium tube with a
plastic tube, if shielding is not necessary,
but this would not improve the Q if the
problem is the gap between the coil and the
core.
Just experiment what is the best solution. But the
aluminium shield - if used - should be thin and can be
applied above the winding in some distance. I know you
have much QRM at home so probably an E field shield makes
sense here. Anyway you will be bothered by the local
magnetic QRM of course.
I personally think that a shield is not necessary if you
are /p and far away from man made noise. Then, a higher Q
helps more than the shield and a Q of 30!
> Are the rods
galvanically separated from each other by a
thin plastic tape? What type is the ferrite
material?
Yes
I put two layers of plastic tape on tips of
each rod. The ferrite material is russian
MU400HH.
Fine! I also use the same ferrite type and 55 turns. Your
cap is surely the best what you can use. So shorten the
wires between L and C and measure the Q on 137 kHz. If it
is still in the range of 30, i assume the shield causes
high losses.
Another idea: There is some conducting spray available.
Maybe this can help here? No experience with that, just a
spontaneous idea...
http://it.farnell.com/_/kontakt-chemie-emv-35-200ml/rivestimento-conductive-emv-35/dp/220115
73, Stefan/DK7FC
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