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Re: LF: TXing WSPR/MF on an indoor loop, part 2

To: [email protected]
Subject: Re: LF: TXing WSPR/MF on an indoor loop, part 2
From: DK7FC <[email protected]>
Date: Sun, 03 Feb 2019 14:58:04 +0100
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...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 -----
Receiver: rsgb_lf_group
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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