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

["marcocadeddu@tin.it" <marcocadeddu@tin.it>]

so you are planning the old fashion indoor frame antenna ;-)
For a longer life of the caps, WIMA has detailed datasheet and you can find series with specified current limits.
Also CD has caps with high current.
Remaining in argoument of small antennas wish to post the almost incredible resoults of a friend of mine: he has a D2T antenna which is sold as broadband antenna (in fact it is a terminated transmission line.. something like a folded elevated beverage ;-).
Although the very small footprint, after adding a second impedance transformer (the original one because of the core is limited to 160m), he tested D2T on MF and even LF. The gain (loss) of the antenna is a bit more than a dummy load, but he got wspr spot from several hundred km distance!

73, Marco IK1HSS

----Messaggio originale----
Da: selberdenken@posteo.de
Data: 26-gen-2019 13.57
A: <rsgb_lf_group@blacksheep.org>
Ogg: Re: R: LF: TXing WSPR/MF on an indoor loop

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
>
>