Date: Wed, 30 Jan 2019 21:57:56 +0000 (GMT)
From: Michael WALTERS <michael.walters26@ntlworld.com>
To: rsgb_lf_group@blacksheep.org, DK7FC <selberdenken@posteo.de>
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Subject: Re: LF: TXing WSPR/MF on an indoor loop, part 2
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Hi I Still recommend hi value S/Mica crimped.Voltage ability very good too. They do not heat noticeably so remain stable. I found a large box of multiple values and mixed as required. Used at 137 also.   Mike G3JVL

> On 30 January 2019 at 17:22 DK7FC <selberdenken@posteo.de> wrote:
> 
> 
> 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
>