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
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