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From: Lawrence Galea <9h9mhrlaw@gmail.com>
Date: Thu, 31 Jan 2019 20:56:56 +0100
Message-ID: <CAFDVXLk7BdfkZ0WxCoTbqAFmTc+4Oq8=NtGb42FFuoQvwUTQUQ@mail.gmail.com>
To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: TXing WSPR/MF on an indoor loop, part 2
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Hi
Thinking aloud, it is reasonable that the 6.8 nf capacitor heats more than
the 2.2 nf since its impedance will be approximately 1/3 of the 2.2 nf one
and when subjected to the same voltage will pass more current.
Regards

On Wed, Jan 30, 2019 at 6:25 PM 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 =C2=
=B0C
> 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 =C2=B0C, 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
>
>

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<div dir=3D"ltr"><div>Hi</div>Thinking aloud, it is reasonable that the 6.8=
 nf capacitor heats more than the 2.2 nf since its impedance will be approx=
imately 1/3 of the 2.2 nf one and when subjected to the same voltage will p=
ass more current.<br>Regards</div><br><div class=3D"gmail_quote"><div dir=
=3D"ltr" class=3D"gmail_attr">On Wed, Jan 30, 2019 at 6:25 PM DK7FC &lt;<a =
href=3D"mailto:selberdenken@posteo.de">selberdenken@posteo.de</a>&gt; wrote=
:<br></div><blockquote class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.=
8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Hi MF,<br>
<br>
Last night i&#39;ve been at home while transmitting on the indoor loop <br>
again. The TX power was doubled, i.e. about 2 mW ERP, based on Rik&#39;s <b=
r>
convincing calculation/estimation.<br>
The temperature dependency of the antenna current was much more <br>
expressed. I started at 70 % and moved to 100 %, then back to 70 % <br>
whithin less than one WSPR TX period. 100 % is equal to about 20 A <br>
antenna current.<br>
<br>
As mentioned, the tuning C consists out of 2.2 nF + 6.8 nF both <br>
WIMA-FKP-1 and a 1 nF variable capacitor.<br>
Yesterday, it was clearly noticable that the 6.8 nF cap becomes warmer <br>
than the 2.2 nF cap, although there were thermally coupled to each other.<b=
r>
<br>
The datasheet, <br>
<a href=3D"https://www.wima.de/wp-content/uploads/media/e_WIMA_FKP_1.pdf" r=
el=3D"noreferrer" target=3D"_blank">https://www.wima.de/wp-content/uploads/=
media/e_WIMA_FKP_1.pdf</a>, tells <br>
about the AC voltage derating curves, page 77, the 2000 VDC chart.<br>
But they say &quot;Permissible AC voltage in relation to frequency at 10 =
=C2=B0C <br>
internal temperature rise (general guide).&quot;<br>
<br>
I interprete this chart combined with the text beside it as follows: &quot;=
If <br>
you want to avoid a temperture rise of more than 10 =C2=B0C, due to life-ti=
me <br>
of capacity-deviation considerations, then you should not apply more <br>
than x volts at a certain frequency&quot;<br>
and NOT like &quot;In any way you must not apply more than x volts at a <br=
>
certain frequency because otherwise the capacitor will explode and the <br>
whole building will burn down immediately!&quot;<br>
<br>
Furthermore, the thermal connection of the leads to e.g. a ground layer <br=
>
will have an effect on the temperature at the working point. So, is the <br=
>
chart based on the worst-case or the best-case?<br>
All in all i see no risk at all to apply the full 700 V rms AC level, at <b=
r>
least for smaller capacity values and if they have a good connection to <br=
>
a heat sink that pulls the heat away from the internal plates.<br>
<br>
Another thing in the resonated loop arrangement is the fact that the <br>
current and voltage will drop as soon as the capacitor is getting <br>
damaged or becomes to warm, so this is a self-regulating circuit.<br>
<br>
I&#39;ve just built the arrangement shown here: <br>
<a href=3D"http://www.iup.uni-heidelberg.de/schaefer_vlf/MF/MFcaps.jpg" rel=
=3D"noreferrer" target=3D"_blank">http://www.iup.uni-heidelberg.de/schaefer=
_vlf/MF/MFcaps.jpg</a><br>
It also gives 9 nF, but with a better distribution and heat transfer. If <b=
r>
necessary, a fan could be used to keep the temperatures down.<br>
<br>
Will be tested tonite on MF. But 20 A at 475.7 kHz and 10 nF already <br>
means 670 V, so i can&#39;t do much more here.<br>
<br>
73, Stefan<br>
<br>
</blockquote></div>

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