Hi Stefan,
In my experience these polypropylene capacitors when they fail, they
just lose some capacitance. I think they call it 'self healing', maybe
they imply that when they fail there is no short circuit?
I have some really nice silver mica capacitors (ex MW broadcast stuff)
that would be perfect for this task...
Anyway, I am slowly gearing up for some VLF action over here. The
other day I did some tests at 16.72 kHz into an ugly loop (very
roughly 120 metres by 3 metres, single turn) and I was able to receive
my signals using an unoptimise ferrite antenna at a bit over 2 km from
the TX. I don't think I am in the near field at that distance, am I?
:-)
73, Dimitris VK2COW
Στις Πέμ, 31 Ιαν 2019 στις 4:25 π.μ., ο/η DK7FC <[email protected]> έγραψε:
>
> Hi MF,
>
> Last night i've been at home while transmitting on the indoor loop
> again. The TX power was doubled, i.e. about 2 mW ERP, based on Rik's
> convincing calculation/estimation.
> The temperature dependency of the antenna current was much more
> expressed. I started at 70 % and moved to 100 %, then back to 70 %
> whithin less than one WSPR TX period. 100 % is equal to about 20 A
> antenna current.
>
> As mentioned, the tuning C consists out of 2.2 nF + 6.8 nF both
> WIMA-FKP-1 and a 1 nF variable capacitor.
> Yesterday, it was clearly noticable that the 6.8 nF cap becomes warmer
> than the 2.2 nF cap, although there were thermally coupled to each other.
>
> The datasheet,
> https://www.wima.de/wp-content/uploads/media/e_WIMA_FKP_1.pdf, tells
> about the AC voltage derating curves, page 77, the 2000 VDC chart.
> But they say "Permissible AC voltage in relation to frequency at 10 °C
> internal temperature rise (general guide)."
>
> I interprete this chart combined with the text beside it as follows: "If
> you want to avoid a temperture rise of more than 10 °C, due to life-time
> of capacity-deviation considerations, then you should not apply more
> than x volts at a certain frequency"
> and NOT like "In any way you must not apply more than x volts at a
> certain frequency because otherwise the capacitor will explode and the
> whole building will burn down immediately!"
>
> Furthermore, the thermal connection of the leads to e.g. a ground layer
> will have an effect on the temperature at the working point. So, is the
> chart based on the worst-case or the best-case?
> All in all i see no risk at all to apply the full 700 V rms AC level, at
> least for smaller capacity values and if they have a good connection to
> a heat sink that pulls the heat away from the internal plates.
>
> Another thing in the resonated loop arrangement is the fact that the
> current and voltage will drop as soon as the capacitor is getting
> damaged or becomes to warm, so this is a self-regulating circuit.
>
> I've just built the arrangement shown here:
> http://www.iup.uni-heidelberg.de/schaefer_vlf/MF/MFcaps.jpg
> It also gives 9 nF, but with a better distribution and heat transfer. If
> necessary, a fan could be used to keep the temperatures down.
>
> Will be tested tonite on MF. But 20 A at 475.7 kHz and 10 nF already
> means 670 V, so i can't do much more here.
>
> 73, Stefan
>
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