From: "Clemens Paul" <cpaul@gmx.net>
To: <rsgb_lf_group@blacksheep.org>
References: <D98C1CCC49324084BCF225E22F521809@DELL4> <1UTCTNxG9H.5T3bfQF3xD0@optiplex980-pc> <9C9C8088-AD45-4DDF-BD7E-66D13F0D835C@md.metrocast.net> <CANA3B6VBpQ0G92wmOniO3GvedBbaOoBuMuFPW1Fs743B8VCvpw@mail.gmail.com> <5C4B1BD5.6010404@posteo.de> <5C51DD4B.1070205@posteo.de> <CAKvcm2f7mY5Z1B5z99Aspv6cTjLeYMymBpB6rJ-VcpzDGvtJzg@mail.gmail.com>
In-Reply-To: <CAKvcm2f7mY5Z1B5z99Aspv6cTjLeYMymBpB6rJ-VcpzDGvtJzg@mail.gmail.com>
Date: Thu, 31 Jan 2019 07:35:31 +0100
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Subject: RE: LF: TXing WSPR/MF on an indoor loop, part 2
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 Hi Dimitris,

reactive near field: lambda/ (2*pi)

73
Clemens
DL4RAJ 

>-----Original Message-----
>From: owner-rsgb_lf_group@blacksheep.org 
>[mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of 
>Dimitrios Tsifakis
>Sent: Thursday, January 31, 2019 1:26 AM
>To: rsgb_lf_group@blacksheep.org
>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
>
>Στις Πέμ, 31 Ιαν 2019 στις 4:25 π.μ., ο/η DK7FC 
><selberdenken@posteo.de> έγραψε:
>>
>> 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
>>
>
>
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