I have been using what appear to be similar polypropylene capacitors [data sheet specifications (see below) including spectral loss tangent seem very similar] for multi-turn indoor and outdoor (35 meter x 35 meter) single-turn VLF transmitting loops for about five years (when time is available) and have found the following:
1) Temperature rise has been roughly consistent with the spectral loss tangent table
2) No change noticed in capacitance after approximately 200 of hours of transmitter operation at capacitor case temperature of 115 F (46 C)
Used 80-PHE450HD6470JR6L2 (0.47 uF) and also used 0.1 uF through 1.0 uF from the same PHE450 product line; always series/parallel stacking capacitors of the same value to enable higher current and/or voltage at a given capacitance.
Data sheet:
https://www.mouser.com/datasheet/2/212/KEM_F3033_PHE450-1104182.pdf
Reached somewhat of a practical limit of series/parallel stacking of these capacitors, for indoor multi-turn loops from 1 meter diameter to ten square meters, and outdoor loop 35 m x 35 m; and have hoped to find capacitors that would allow more current and voltage. I thought I had investigated (through calculations, not on the workbench) silver micas and found these polypropylene capacitors to be better based on calculations, but based on the messages between Stefan, Dimitris and Marko I must have missed something (perhaps temperature rating?); thanks for the inputs on this topic; looking forward to silver micas.
73,
Jim AA5BW
From: [email protected] [mailto:[email protected]] On Behalf Of dhchurch
Sent: Thursday, January 31, 2019 5:55 AM
To: RSGB LF group
Subject: Re: Re: LF: TXing WSPR/MF on an indoor loop, part 2
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.
----- 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
>
