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Date: Thu, 31 Jan 2019 08:32:42 +0100
From: DK7FC <selberdenken@posteo.de>
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Subject: LF: Silver mica caps...
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Hello Mike and Dimitris,

Hmmmm, ok, i admit i'm getting convinced by the idea. I just read a bit 
at https://en.wikipedia.org/wiki/Silver_mica_capacitor
Can you recommend a good source for them?
I just found that one 
https://de.rs-online.com/web/p/mica-kondensatoren-glimmerkondensatoren/8713189/ 
. The data sheet 
(https://docs-emea.rs-online.com/webdocs/13e5/0900766b813e58c0.pdf) 
tells that a 1 nF type will carry 0.94 A. So this is to small for me.

73, Stefan


Am 30.01.2019 22:57, schrieb Michael WALTERS:
> 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
>>
>>      
>