Re: R: LF: TXing WSPR/MF on an indoor loop

[N1BUG <paul@n1bug.com>]

Hi Stefan,

Yes I am sure that is true about the capacitors. I did destroy a few
operating with applied voltage about half way between the datasheet
derated maximum for the frequency and the full AC voltage rating.
They went into a rapid runaway condition under load. Subsequently
reducing power to be well within ratings did not restore normal
operation so they must have been internally damaged. In fact I do
not know if they were harmed by too much voltage or too much current...

The multi-turn TX loop sounds interesting. Let us know how it works
after you build and test it. ;-)

Well, it's into the work shop for me. I have to start work on a new
LF exciter today.

73,
Paul


On 1/26/19 7:57 AM, DK7FC wrote:
> Hi Paul,
> 
> Oh yes, you are right, i forgot that. In the same way they could limit 
> the maximum current at a given frequency and capacity...
> But this is probably to keep a certain life time and guarantee a certain 
> deviation of the nominal capacity, for commercial applications...
> 
> I'm just thinking about a multi-turn TX loop that uses no capacitor at 
> all. Imagine a loop with say 15 turns of the same wire i am using now. 
> It will have 45 Ohm then? Now one turn results in 0.2 Ohm. When touching 
> the capaciors during the transmission, i notice they heat up by 15 °C 
> maybe, not to much. So i estimate the losses are about 2 W, i.e. most of 
> the losses are in the wire. A loop of 15 turns will have an internal 
> capacity and a self resonance frequency. Maybe it can be tuned to 475 
> kHz, then you can close the loop without a serial C! and inject a RF 
> current with a ferrite core just like i do now. The residual impedance 
> will have no reactive component. And then, the limiting factor will be 
> the maximum temperature of the wire and its mechanical expansion when it 
> becomes warm, which will detune the loop like the upheating capacitor 
> does. But that time constant will be higher, so you tune it out for a 
> given average power when TXing WSPR.
> 
> 73, Stefan
> 
> Am 26.01.2019 13:02, schrieb N1BUG:
>> Hi Stefan,
>>
>> Congrats on your results with the small indoor loop!
>>
>>    
>>> The limiting factor in that system is the capacitor! I use 2.2 nF +6.8
>>> nF + 1090 pF variable C. Maybe it can be optimised by using 10 x 1 nF
>>> WIMA-FKP-1-2000. The voltage is about 450 V, which is well below the 700
>>> V limit.
>>>      
>> You may want to take a close look into the WIMA data sheet. There
>> are charts showing maximum permitted Vrms vs. frequency. I think you
>> may find the 1 nF 2000VDC / 700VAC FKP is derated to around 300 Vrms
>> at 475 kHz. I had some experience destroying these things... :-)
>>
>> 73,
>> Paul