...indeed! Since more than 72 hours i'm running the WSPR transmission
in 25% duty cycle (each 8 minutes). First i tuned to 20 A antenna
current and the temperature / antenna current changes were much
smaller. But there was a certain click, or krrrrkkrt audible from the
Cs. It was likely that there were indeed internal voltage
breakthroughs, which were self-healing. Now after 3 days the current
dropped again to 14 A, meaning that the capacity dropped, so the loop
became detuned and so the RF current and voltages dropped to a level
which is accepted by the capacitors.
So my conclusion is, the derating curves actually should be considered.
I wonder if the effect is still happening if one keeps the temperatures
down at 25 C or even lower. When placing the capacitors in de-ionised
iced water, it should be possible to keep the temperatures down, also
in the internal of the capacitor.
But all in all this is not a good solution. Fortunately i have another
antenna which is more efficient :-)
73, Stefan
Am 31.01.2019 11:54, schrieb dhchurch:
Hello Dimitrios
Polypropylene capacitors can actually self-heal to such an
extent that there's little left of them. The melting point of
polypropylene is not very high and they can end up as just a liitle
blob of plastic.
Don't worry though, I still use them suitably rated for the
frequency in use and most of them are 600Vac or more.
73, Hugh, M0DSZ
-----
Receiving the following content -----
Time:
2019-01-31, 00:25:55
Subject:
Re: LF: TXing WSPR/MF on an indoor loop, part 2
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
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